Performance of Mombaça Grass Under Irrigation and Doses of Biodegradable Hydroretentive Polymer

Abstract

Biodegradable hydroretentive polymers, such as UPDT^®, have emerged as promising alternatives to synthetic hydrogels, particularly in pasture systems where sustainable water management is crucial. These materials enhance subsurface drip irrigation by maintaining soil moisture, which supports germination and early root development until roots access deeper water reserves. However, their degradation dynamics in tropical forage systems remain poorly characterized, posing a challenge to long-term application strategies. This study aimed to evaluate the effects of different UPDT^® doses (0, 7.5, 15, 22.5, and 30 kg ha⁻¹) on the morphological and agronomic traits of Mombaça grass under controlled conditions. After a uniformity cycle, treatments were evaluated across four cultivation cycles with monitored irrigation to avoid water deficits. Morphogenetic traits such as number of live leaves (NLL), final number of emerging leaves (NEmL), leaf appearance rate (LAR), and stem elongation rate (SER), as well as shoot dry mass (SDM), were analyzed. Results showed that morphological variables responded quadratically to polymer doses during the initial and intermediate cycles. In the final cycle, reductions in these traits and in water productivity suggested the onset of polymer degradation and loss of hydroretentive capacity. Agronomic traits were influenced throughout all cycles, with the fourth cycle showing the highest SDM due to elevated temperatures. These findings highlight the need to better understand the degradation kinetics of biodegradable hydrogels such as UPDT^® in tropical pastures. Field trials are recommended to define optimal reapplication intervals and integrate degradation monitoring into irrigation planning, ensuring long-term sustainability in pasture management.

1. Introduction

Cattle ranching in Brazil, especially forage production, faces challenges related to climatic seasonality, with factors such as air temperature and rainfall impacting the availability of water for plants [1,2]. Mombaça grass (Megathyrsus maximus (Jacq.) BK Simon and SWL Jacobs), a C4 forage species, is widely used in tropical and subtropical regions, with its productivity influenced by management practices and environmental conditions such as temperature and nutrients [3,4]. Studies indicate that irrigation, as demonstrated with Mombaça grass in Minas Gerais, can increase forage production, especially in seasons with lower precipitation [5,6].

In this context, subsurface drip irrigation has proven to be an effective alternative for increased water use efficiency, especially in intensive grazing areas. This system allows for more precise and frequent water application directly to the plant’s root zone, which is essential for optimizing the growth of forage crops [5,7]. However, challenges related to the spacing and depth of the emitters still need to be better understood, as the placement of the emitters influences the water distribution in the soil. It is crucial to adjust these parameters to ensure that water reaches the surface layers, particularly during the initial development phase of the plants [8,9].

Installing drip tubes at greater depths can reduce seed germination in grass, as it reduces soil moisture and increases soil temperature, negatively affecting seed performance [10]. Gonçalves et al. [11] identified higher moisture contents in the layers from 0.20 m to 0.60 m for drip tubes installed at a depth of 0.20 m. Additionally, they observed greater variations in water content in the 0 to 0.10 m layer of soil cultivated with Megathyrsus maximus irrigated by subsurface drip irrigation [7]. Therefore, it is essential to apply techniques that increase soil water availability, especially in the early stages of pasture formation, when the root system is still shallow.

A strategy to improve water availability in the substrate and ensure greater water security in pasture formations is the use of hydroretentive polymers. The application of the water-retaining polymer at the initial stage of pasture cultivation can be carried out either by broadcasting or in rows, aiming to increase soil water retention and promote plant establishment, especially in sandy soils or under irregular rainfall conditions. In broadcasting, the dry polymer should be evenly distributed over the soil and lightly incorporated before seeding. In row planting, localized application of the polymer in the furrow, close to the seeds, is recommended for more efficient water use. For planting with cuttings, such as in the case of Tifton grass (Cynodon dactylon (L.) Pers.) and Star grass (Cynodon plectostachyus (K. Schum.) Pilg.), which are propagated vegetatively, the polymer can be applied in its hydrated form directly into the planting holes, helping to retain moisture and support proper root development.

Petroleum-derived polymers have a high water absorption rate but their slow degradation can cause adverse environmental effects and affect plant growth, as well as reducing the soil’s matric potential, making the retained water less available to plants [12,13,14]. As an alternative, biodegradable hydroretentive polymers, which degrade naturally, have the advantage of improving the soil structure and ensuring that the retained water is more accessible to plants, promoting both environmental and agricultural benefits [15,16].

These biodegradable polymers, produced from renewable sources such as polysaccharides, have advantageous characteristics such as high water absorption, mechanical resistance, and a high biodegradation capacity, making them innovative and sustainable biomaterials [17,18]. Furthermore, biodegradable hydroretentive polymers respond to environmental changes such as temperature, pH, and electrical stimuli. They can be formulated as water-dispersible granules, such as UPDT^®, which is starch-based. This type of formulation facilitates handling and application while minimizing the suspension of breathable particles [19,20].

Although UPDT^® technology has shown potential for improving water retention in soil, its degradation dynamics and long-term effectiveness have not yet been studied in pasture systems. This knowledge gap is particularly relevant in perennial tropical grasses such as Mombaça, which develop extensive root systems that may influence polymer behavior over time. Understanding how UPDT^® degrades in the soil and affects its water-holding capacity is essential for sustainable land management and ecological restoration. In degraded pastures, hydroretentive polymers could support recovery by improving soil moisture conditions and reducing compaction and erosion. For example, Han et al. [21] found that applying 0.50% of a superabsorbent polymer improved water retention, plant growth, and physiological performance in Lathyrus sativus under drought, while also reducing oxidative stress and harmful compounds in plant tissues. However, their degradation must be synchronized with plant regrowth cycles to avoid the accumulation of residual material in the soil [20,22]. These knowledge gaps highlight the importance of studying biodegradable polymers in the context of pasture establishment and soil recovery. Additionally, remote sensing tools such as the Normalized Difference Vegetation Index (NDVI) may contribute to large-scale monitoring of the interactions among soils, plants, and polymers in pasture systems under different management conditions.

Given the above, it is hypothesized that the application of the biodegradable hydroretentive polymer UPDT^® to the substrate increases water retention and improves the morphogenic and agronomic performance of Mombaça grass during the initial cultivation cycles. Thus, the aim of this study is two-fold: first, to establish the extent of persistence and rate of biodegradability of the hydroretentive polymer UPDT^® to facilitate water retention in a soil substrate; second, to apply the polymer in order to evaluate its capacity to improve the morphogenic and agronomic performance of Mombaça grass during the initial cultivation cycles.

2. Materials and Methods

2.1. Experimental Area

The study was conducted at the Center for Water Resources Reference (CRRH) of the Federal University of Viçosa (UFV). The CRRH is located in the municipality of Viçosa, Minas Gerais, at a latitude of 20°46′19″ S, longitude of 42°52′28″ W, and altitude of 650 m above sea level. According to Köppen’s classification [23], the region’s climate is Aw, meaning Tropical Savanna climate with a dry winter. According to the Climatological Normal data from the National Institute of Meteorology [24], Viçosa-MG records an average annual rainfall of 1289 mm, with monthly average air temperatures ranging between 20.3 °C and 22.3 °C during the rainy season and between 15.4 °C and 18.3 °C during the dry season. Regarding the minimum temperature, the month of July records the lowest values, with an average of 10.1 °C, while the maximum temperatures vary between 25.0 °C and 35.0 °C, and in winter these values range between 23.0 °C and 25.0 °C.

2.2. Water Retention by the Hydroretentive Polymer

Prior to conducting the experiments with different application rates of the hydroretentive polymer UPDT^® (UPL, Campinas, Brazil) on Mombaça grass, a preliminary study was carried out to evaluate the polymer’s biodegradability over time, based on its water retention capacity. In this experiment, the water retention capacity of the hydroretentive polymer was evaluated over a period of 223 days. Four product concentrations were tested, corresponding to doses of 0, 0.1 g kg⁻¹, 0.2 g kg⁻¹, and 0.3 g kg⁻¹ of UPDT^®. The experiment was conducted with a completely randomized design, with four replicates per treatment, totaling 16 experimental units. Initially, the polymer was hydrated and incorporated into 2 kg of dry soil, according to the dose assigned to each treatment. The mixture was placed in 2 L plastic bottles made of polyethylene terephthalate (PET). These bottles were capped and perforated at the base to allow for drainage.

Soil saturation was performed via capillary rise, with the bottles placed in plastic reservoirs and the water level gradually raised until complete saturation of the soil was achieved. During saturation, the caps were temporarily removed to allow air displacement, preventing the formation of air pockets. After full saturation, the caps were replaced at the beginning of the drainage process to prevent water evaporation, and consequently the underestimation of the field capacity. The units were then left to drain freely until runoff ceased, simulating field capacity conditions.

After drainage, each unit was weighed to determine the retained moisture, i.e., the maximum water retention capacity of the soil with the hydroretentive polymer. Subsequently, the caps were removed to allow soil moisture evaporation until the next evaluation cycle. This procedure was repeated at 19, 35, 57, 87, 116, 147, and 223 days after setup, enabling the monitoring of the hydroretentive polymer’s efficiency over time.

2.3. Mombaça Grass Performance

After the preliminary assessment described in Section 2.2, the main experiment was carried out to evaluate the effects of different application rates of the hydroretentive polymer UPDT^® on the performance of Mombaça grass under greenhouse conditions, as described in the subsequent sections. The experiment was conducted in a protected environment to ensure that water from rainfall did not affect the results. The structure had an area of 30 m² (5 m in width and 6 m in length). The sides were protected with a 2-m-high polyethylene mesh, leaving open spaces for ventilation. The structure had two openings on opposite sides with removable curtains to allow air circulation for natural ventilation. The roof of the protected environment was covered with 150 micron low-density polyethylene (LDPE) film, which was UV-additivated.

For the experiment, 20 pots were installed, each with dimensions of 21 cm in top diameter, 12 cm in bottom diameter, and 21 cm in height, resulting in a volume of 4.8 L. The pots were filled with soil collected from a slope on the UFV campus. The soil used in the experiment was classified as a dystrophic red–yellow Latosol [25] or Oxisol [26]. The physical–hydric and chemical characteristics of the soil are presented in

Table 1. Physical–hydric and chemical characterization of the soil used to fill the pots. Viçosa-MG, DEA-UFV, 2023.

Coarse	Fine sand		Silt		Clay		FC		PWP		Bd		K0		Textural class
---------------------------------- kg kg−1 ----------------------------------											g dm−3		cm h−1
0.307	0.137		0.128		0.427		0.247		0.137		1.132		0.054		Clay
pH	pH		P		K+		Na+		Ca2+		Mg2+		Al3+		H + Al
H2O	KCl		------------ mg dm−3 ------------						------------------ cmolc dm−3 ------------------
6.30	5.69		117.3		46.0		6.60		5.68		0.55		0.00		1.90
SB	t		T		V		m		SSI		OM		N–total		P–rem
---------- cmolc dm−3 ----------					---------------- % ----------------						------ dag kg−1 ------				mg L−1
6.38	6.38		8.28		77.1		0.00		0.35		2.96		0.115		37.0
S	B	Cu		Mn		Fe		Zn		Cr		Ni		Cd	Pb
-------------------------------------------------------- mg dm−3 --------------------------------------------------------
1.90	0.26	2.54		40.5		54.1		12.93		0.00		0.78		0.42	1.26

FC = field capacity; PWP = permanent wilting point; Bd = soil bulk density; K₀ = saturated hydraulic conductivity; P, Na, K, Fe, Zn, Mn, Cu, Cd, Pb, Ni, and Cr–Mehlich–1 extractant; Ca²⁺, Mg²⁺, and Al³⁺–1 mol L⁻¹ KCl extractant; H + Al–0.5 mol L⁻¹ calcium acetate extractant at pH 7.0; SB = sum of exchangeable bases; t = effective cation exchange capacity; T = cation exchange capacity at pH 7.0; V = base saturation index; m = aluminum saturation index; SSI = sodium saturation index; OM = organic matter (C. Org × 1.724–Walkley–Black); P–rem = remaining phosphorus; N–total–sulfuric digestion and Kjeldahl distillation; S–extracted with monocalcium phosphate in acetic acid; B–hot water extractant.

.

Based on the results of the soil analysis and the crop requirements, soil fertility correction was performed at the time of experiment implementation, following the recommendations of Ribeiro et al. [27]. The fertilizers used for fertilization were urea (0.105 g of CO(NH₂)₂ per pot) (Agropecuária Nô da Silva, Viçosa, Brazil), monoammonium phosphate (MAP) (0.068 g of NH₄H₂PO₄ per pot) (Agropecuária Nô da Silva, Viçosa, Brazil), and potassium chloride (KCl) (0.068 g of K₂O per pot) (Agropecuária Nô da Silva, Viçosa, Brazil). These fertilizers were dissolved in 1.5 L of water and homogenized, and then each pot received 50 mL of the solution. This fertigation was carried out before sowing and on the same day as the respective cuts, continuing for the following four days of irrigation of Mombaça grass, without the need for soil acidity correction.

The forage used in the study was Mombaça grass (Megathyrsus maximus cv. Mombaça). The seeds had a minimum purity of 95% and were treated with fungicide (carboxin + thiram) (Agropecuária Nô da Silva, Viçosa, Brazil) and insecticide (imidacloprid) (Agropecuária Nô da Silva, Viçosa, Brazil). For sowing, a circular furrow 3 to 5 cm deep was made in each pot, and 2 g of seeds was evenly distributed along the furrow. Sowing was carried out together with the application of the UPDT^® hydroretentive polymer, which requires two subsequent irrigations to ensure full hydration [28]. The UPDT^® product was purchased from the company UPL, whose responsible unit is located in Campinas, São Paulo state, Brazil. Additional information about the product and the company can be found on the official website: http://www.upl-ltd.com/br (accessed on 29 July 2025).

The experimental period lasted from 9 May 2023 to 15 December 2023, with evaluation cycles conducted from May to December 2023. Five cuts were performed; the first, for uniformization, occurred 98 days after sowing, and the subsequent forage collection cycles took place on 15 September 2023 (31 days), 15 October 2023 (30 days), 14 November 2023 (30 days), and 14 December 2023 (30 days), maintaining a residue of 15 cm above the soil surface. This height favors forage regrowth by preserving growth buds located near the base of the plants. Both the cutting height and defoliation interval followed the seed supplier’s recommendation, which establishes a 30 day cultivation cycle for the grass.

The standardization cycle was conducted to uniformize the forage during the first cycle, ensuring consistent initial conditions and minimizing possible variability that could interfere with the results. By interrupting the free and disorderly growth of the plants, which is common in forage species [29,30], a homogeneous starting point was created.

2.4. Meteorological Elements

The climatic data recorded during the experimental period are presented in Figure 1. The average values of the meteorological variables for cycles 1, 2, 3, and 4 were 19.5, 22.7, 22.8, and 24.4 °C for air temperature; 81.2, 75.5, 77.8, and 75.4% for relative humidity; and 11.3, 13.1, 14.6, and 16.6 MJ m⁻² d⁻¹ for solar radiation, respectively.

In the literature, there is disagreement regarding the specific lower and upper base air temperatures for Mombaça grass. Therefore, to calculate the accumulated degree-day (GDD) values, average data for Mombaça grass were adopted. The lower and upper base air temperatures considered were 12 °C and 35 °C, respectively [31,32].

2.5. Experimental Design

The experiment was arranged in a randomized block design (RBD) with four replications. The experimental setup followed a split plot arrangement, with four cultivation cycles in the main plots and five doses of the hydroretentive polymer in the subplots. Four cultivation cycles of Mombaça grass were conducted, excluding the standardization cycle from the analysis.

The subplots consisted of five experimental treatments of the hydroretentive polymer UPDT^® (0%, 50%, 100%, 150%, and 200%), based on the reference dose of 15 kg ha⁻¹ recommended for sugarcane by the product’s manufacturer [28]. Thus, the tested polymer doses were 0, 7.5, 15, 22.5, and 30 kg ha⁻¹. In each pot, 0, 18, 36, 54, and 72 mg of the polymer were added, corresponding to the doses of 0, 7.5, 15, 22.5, and 30 kg ha⁻¹, respectively. The polymer amounts were previously calculated based on the target dose for each treatment and individually weighed using a high-precision digital scale (accuracy: 0.001 g; precision: ±0.003 g). After weighing, the product was hydrated using municipal supply water and then thoroughly mixed into the soil before sowing. This procedure ensured the exact application of the intended dose to each pot, as well as uniform distribution of the polymer within the soil volume—especially in the upper 10 cm—simulating real field application conditions.

Thus, the experiment consisted of 20 experimental units, organized into four blocks, each containing five treatments corresponding to the different doses of the water-retaining polymer. The units were uniformly arranged on a bench inside a greenhouse under controlled conditions. This layout enabled proper comparisons between treatments and minimized potential environmental variation within the protected environment. The block arrangement also enhanced the repeatability of the experimental conditions, while the use of split plots allowed for the assessment of dose effects over the four cultivation cycles, ensuring greater statistical robustness of the obtained results.

2.6. Experiment Management

To prevent significant variations in soil water storage and maintain moisture near field capacity, the Mombaça grass was irrigated every two days. Irrigation management was conducted based on the mass difference between two irrigations. The pots functioned similarly to weighing lysimeters, allowing for the measurement of crop evapotranspiration (ETc). Equation (1) was used to calculate ETc values and determine the amount of water applied to the soil-filled pots.

$$\mathrm{E}\mathrm{T}\mathrm{c}={\mathrm{M}}_{\mathrm{p}\mathrm{o}\mathrm{t} \mathrm{i}}-{\mathrm{M}}_{\mathrm{p}\mathrm{o}\mathrm{t} \mathrm{i}+1}-\mathrm{D}$$

(1)

where ETc—crop evapotranspiration, L; M_{pot i}—pot mass on day i, kg; D—drained water, L.

With the ETc value, it was possible to apply an amount of water to raise the soil to field capacity during each irrigation event. The pots were weighed using a scale with a precision of 0.01 g. Irrigation was performed manually in the afternoon (17:00). The following morning (07:00), drainage water was collected. It is important to highlight that the entire drained volume from each pot was reintroduced along with the irrigation water into the same pot, ensuring the balance of salts and nutrients in the soil. The irrigation water, collected from the redistribution network near the greenhouse, was analyzed. The maximum redox potential (e max1+) was measured, and the recorded pH values ranged between 7.2 and 7.9. Electrical conductivity was measured on three different occasions, with an average value of 107 µS cm⁻¹ at 25 °C.

2.7. Experimental Evaluations

After the standardization cycle, two tillers were randomly selected from each pot to evaluate the morphogenesis of Mombaça grass. The tillers were identified using plastic clips (Figure 2) in different colors. The total lengths of both expanded and emerging leaf blades were measured, along with the pseudostem length, which was taken as the distance from the last exposed ligule to the base of the tiller. Data collection occurred in the morning, starting at 07:00, twice a week (Tuesdays and Fridays).

With the aid of a graduated ruler, measurements were taken of the length of the leaf blades and pseudostem of the tillers previously identified. The lengths of the expanded leaves were measured from the tip of the leaf to its ligule. In the case of expanding leaves, the same procedure was followed but the ligule of the last expanded leaf was considered as the measurement reference. For senescent leaves, the length corresponded to the distance from the leaf ligule to the point where the senescence process had advanced. The pseudostem length was measured as the distance from the soil surface to the ligule of the youngest fully expanded leaf. Based on the data obtained from the leaf growth study, the following variables were calculated according to Gomide and Gomide [29]:

-

Number of emerging leaves (NEmL): Obtained immediately before harvest, considering as emerging or expanding leaves those that did not show an exposed ligule.

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Number of expanded leaves (NEpL): Obtained immediately before harvest, considering the number of fully expanded leaves of each tiller, i.e., with an exposed ligule.

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Number of live leaves (NLL): Obtained immediately before harvest, summing the number of expanding and expanded leaves of the tiller.

-

Leaf appearance rate (LAR, leaves tiller⁻¹ d⁻¹): Obtained by subtracting the number of new leaves per tiller from the initial leaves, divided by the number of days involved.

-

Leaf elongation rate (LER, cm tiller⁻¹ d⁻¹): Obtained by subtracting the initial and final leaf blade lengths, dividing the difference by the number of days in the evaluation, and multiplying the result by the number of tillers considered.

-

Stem elongation rate (SER, cm tiller⁻¹ d⁻¹): Obtained by subtracting the initial and final stem lengths, dividing the difference by the number of days in the evaluation, and multiplying the result by the number of tillers considered.

-

Leaf senescence rate (LSR, cm tiller⁻¹ d⁻¹): Calculated by dividing the total final length of senescent tissue by the number of days involved.

The agronomic characteristics of shoot fresh mass (SFM) and shoot dry mass (SDM) were evaluated at the end of each cultivation cycle of the Mombaça grass. SFM corresponded to all above-ground parts (leaves and stem) of the plant with a height greater than 15 cm, and it was weighed using a digital precision scale (0.01 g). Later, the leaves and stems of each pot were stored in paper bags and dried in a forced-air oven at 65 °C for 72 h, then weighed again with a digital precision scale (0.01 g) to determine the SDM.

The water productivity (WP) was calculated using Equation (2) as the ratio between SDM and the volume of water applied to the pots with Mombaça grass.

$$\mathrm{W}\mathrm{P}={\displaystyle \frac{\mathrm{S}\mathrm{D}\mathrm{M}}{{\mathrm{V}}_{\mathrm{W}}}}$$

(2)

where WP—water productivity, kg m⁻³; SDM—shoot dry mass, kg pot⁻¹; V_W—total volume of water applied, m³ pot⁻¹.

2.8. Statistical Analysis

The data were subjected to an analysis of variance (ANAVA) using a 0.05 significance level in the F-test. Regardless of the significance of the interactions between the factors, the breakdowns were performed considering the focus of the study. To check the assumptions of homogeneity of variances and normality, the Bartlett and Shapiro–Wilk tests were used, respectively, with a 0.05 significance level for both. For qualitative factors, the means were compared using the Tukey test at a 0.05 significance level. For quantitative factors, linear and quadratic models were tested. The model selection was based on the significance of the regression coefficients, using the t-test at a 0.05 significance level, the coefficient of determination (R²), and the biological phenomenon. The statistical analyses were performed using the Experimental Designs package in the software “R”, version 4.4.2 [33].

3. Results and Discussion

3.1. Preliminary Experiment: Water Retention and Polymer Degradability

The results presented in this section refer to a preliminary experiment conducted prior to the main Mombaça grass trial, with the specific objective of evaluating the water retention dynamics and the biodegradability of the hydroretentive polymer UPDT^® over time. Although this test was performed under different conditions, with no plants, distinct doses, and a different experimental setup, it provides important complementary information that supports the interpretation of results from the later grass-growing experiment.

Figure 3 presents the soil moisture values over time as a function of the different doses of hydroretentive polymer applied. At the initial time point (0 days), it was observed that water retention was proportional to the applied doses; the higher the polymer concentration, the greater the moisture retained in the soil. This highlights the polymer’s initial ability to significantly increase soil water retention, especially at doses of 0.2 and 0.3 g kg⁻¹.

However, over the 223 days of evaluation, a progressive reduction in the polymer’s efficiency was observed. The treatments that received the hydroretentive polymer showed gradual losses in water retention capacity, reflecting the material’s degradation over time. This is due to the fact that the polymer used is corn starch-based—a natural biopolymer with high biodegradability—which favors its decomposition by soil microorganisms [15,28]. By the end of the experiment, the reductions in soil moisture compared to the initial values were 6.0%, 13.7%, and 19.4% for the doses of 0.1, 0.2, and 0.3 g kg⁻¹, respectively.

On the other hand, the control treatment, which did not receive polymer application, maintained relatively constant moisture levels throughout the experimental period, indicating that the variations observed in the other treatments are due to the gradual loss of the polymer’s efficiency. These results reinforce the importance of considering the stability and functional lifespan of biodegradable materials when planning water management strategies.

Additionally, the biodegradability of the polymer, although environmentally desirable, imposes limitations on its prolonged use, especially in long-cycle crops or permanent production systems. Therefore, the use of natural polymers must be planned considering not only their initial efficiency but also the necessary frequency of reapplication to ensure the desired effects over time. In regions with water scarcity or in crops established in soils with low water retention capacity, such as sandy soils, the application of this type of polymer can represent an important tool for mitigating water stress, provided that its replenishment is technically feasible and economically justifiable.

In this context, the use of the corn-starch-based hydroretentive polymer is especially recommended for pastures irrigated with subsurface irrigation systems. This is because the increase in water retention promoted by the polymer is crucial in the early stages of pasture development when the root system is still shallow and depends on moisture in the upper soil layer. After this initial period, with root growth and deepening, the plant becomes more efficient at accessing water applied at depth, reducing its dependence on the polymer.

Moreover, the hydroretentive polymer presents significant environmental advantages compared to petroleum-derived synthetic polymers, which although more durable can leave toxic and persistent residues in the soil, compromising their quality over time [14,34,35]. Thus, the use of biodegradable polymers such as the hydroretentive polymer represents a more sustainable alternative aligned with good agroenvironmental practices.

Although the results of this study demonstrated a gradual loss of the hydroretentive polymer’s efficacy in water retention over time, there are limitations that need to be addressed in future research, particularly regarding the biodegradability of the polymer. The main limitation lies in the lack of a more in-depth analysis of the polymer’s degradation process and its actual influence on soil water retention capacity. Since degradation occurs gradually, it is essential to investigate the efficiency loss dynamics in more detail, using methodologies such as Fourier transform infrared spectroscopy (FTIR) or a thermogravimetric analysis (TGA). Additionally, future studies could explore combinations of the polymer with other materials, such as controlled-release fertilizers, to further enhance the water management efficiency. Analyzing additional variables, such as microbial activity in the soil and the polymer’s effects on different soil types and cultivation systems, could also provide a more comprehensive understanding of the potential and limitations of biodegradable polymers in agriculture.

3.2. Standardization Cycle

The standardization cycle lasted from sowing on 9 May 2023 until the uniformity cut on 15 August 2023. The first uniformity cut in a pasture study is an essential practice to ensure homogeneous conditions before the start of treatments or data collection. This procedure eliminates initial variations, such as differences in height, age, or plant vigor, which could interfere with the results, and establishes a baseline for the experiment. Additionally, the cut stimulates regrowth, especially in grasses, making the pasture more representative of the management to be studied. Standardization is particularly important in scientific studies, as it allows for the isolation and control of variables, facilitating the analysis of the effects of the planned interventions in the experiment [10]. Thus, the uniformity cut improves result accuracy by minimizing unwanted variations and increasing the reliability of conclusions.

Figure 4 shows that increasing doses of the hydroretentive polymer led to a reduction in the water consumption of Mombaça grass. This reduction can be explained by the fact that the hydroretentive polymer has the ability to retain water in the soil, gradually releasing it to the plants. As a result, Mombaça grass can access the stored water more efficiently, reducing the need for constant irrigation. Thus, by increasing the polymer dose, the soil maintains a more stable moisture level, which decreases evaporation and the need for water consumption for plant growth. On the other hand, the other evaluated characteristics were not affected by the doses of the hydroretentive polymer.

The fresh and dry shoot masses at the end of the standardization cycle had average values of 15.15 and 3.27 g pot⁻¹, respectively (Figure 4), resulting in an average dry matter content of 21.6%. Water productivity had an average value of 0.41 kg m⁻³, meaning that in this experiment 2.42 m³ of water was required to produce 1 kg of dry mass of Mombaça grass. Although it was not possible to fit a regression equation, it was observed that the application of 15 kg ha⁻¹ of hydroretentive polymer resulted in higher fresh and dry shoot mass production rates, as well as lower water consumption, which contributed to greater water productivity. These results suggest that a application of 15 kg ha⁻¹ may be a promising recommendation for the establishment of Mombaça grass pastures. However, it is important to emphasize that this conclusion cannot be confirmed based solely on this variable, requiring the consideration of other factors and complementary studies.

It should also be noted that the experiment was conducted under controlled pot conditions, which allows for more precise control of irrigation volumes, soil type, and plant spacing. While these conditions reduce experimental variability, they also introduce certain limitations when extrapolating results to field scenarios. In natural field conditions, factors such as soil heterogeneity, unrestricted root growth, rainfall variability, and environmental interactions may influence both the effectiveness of the hydroretentive polymer and plant development. Additionally, root confinement in pots may alter the dynamics of water absorption compared to open soil systems. Therefore, although the results indicate a potential for water savings and productivity improvements, future trials under field conditions are essential to validate the practical applicability of this technology in real pasture systems.

3.3. Water Consumption

Figure 5 presents the evolution of crop evapotranspiration (ETc) and reference evapotranspiration (ETo), calculated according to Allen et al. [36], over the evaluated cycles of Mombaça grass. A progressive increase in water consumption was observed as the crop cycles advanced from the 1st to the 4th cycle, accompanied by a gradual rise in ETo and consequently ETc. This behavior can be attributed to the typical climatic conditions of spring, such as increased solar radiation and air temperatures, as evidenced in Figure 1. These variables directly influence the vapor pressure deficit, intensifying the evaporative demand of the atmosphere. Additionally, during the warmer months, greater vegetative vigor and leaf area expansion in Mombaça grass are commonly observed [37,38], which also contribute to increased transpiration. Comparatively, in the period of June–August (winter), lower temperatures, reduced solar radiation, and shorter days decrease the evapotranspiration rate, explaining the lower values observed in cycle 1.

It is important to highlight some specific microclimatic effects inherent to pot experiments that influence evapotranspiration measurements. In pots, the “bouquet effect” occurs, where the leaf area often exceeds the soil surface area of the pot, creating an intensified microenvironment around the plant canopy. This leads to higher transpiration rates per unit soil area compared to field conditions [14,39]. Additionally, the “oasis effect” or micro-advection may develop due to localized humidity gradients around the pots, further increasing the vapor pressure deficit near the plant and enhancing evapotranspiration [38,39].

As a consequence, crop evapotranspiration (ETc) measured in pots tends to be higher per soil surface area than under natural field conditions. For this reason, we present the water consumption data in terms of volume per pot (L pot⁻¹) rather than as a depth of water (mm), which is more suitable for field measurements. Despite these microclimatic differences, this approach does not impair the experimental objectives, since the study focuses on comparing relative effects of hydroretentive polymer doses within the controlled pot system, where all treatments were subjected to the same conditions. Thus, the internal validity and the comparative conclusions regarding polymer efficiency remain robust.

3.4. Morphogenetic Characteristics

There was an interaction between the factors crop cycles and hydroretentive polymer for all the morphogenetic characteristics evaluated (

Table 2. Mean squares, significance of the F-test (ANAVA), and average values of morphogenetic characteristics including number of emerging leaves (NEmL), number of expanded leaves (NEpL), number of live leaves (NLL), stem elongation rate (SER), leaf appearance rate (LAR), leaf elongation rate (LER), and leaf senescence rate (LSR) in different cycles of Mombaça grass and as a function of hydroretentive polymer (HP) doses. Viçosa-MG, DEA-UFV, 2023.

Variable	Mean Square			CV (%)	HP (kg ha−1)	Cultivation Cycles
	Cycle	HP	C × HP			1	2	3	4
NEmL	1.845 × 100 **	5.002 × 10−2 ns	2.563 × 10−1 *	27.09	0	0.875 a	1.250 a	1.250 a	1.125 a
					7.5	1.125 a	1.500 a	1.375 a	0.500 b
					15	1.125 b	1.875 a	1.125 b	0.750 b
					22.5	1.125 b	1.750 a	1.125 b	0.750 b
					30	1.250 ab	1.625 a	0.875 b	1.250 ab
NEpL	4.365 × 10−1 **	1.687 × 10−1 **	4.583 × 10−2 **	81.26	0	0.500 a	0.000 b	0.000 b	0.000 b
					7.5	0.125 a	0.000 a	0.000 a	0.125 a
					15	0.375 a	0.125 ab	0.000 b	0.125 ab
					22.5	0.625 a	0.125 b	0.125 b	0.375 ab
					30	0.125 a	0.000 a	0.000 a	0.125 a
NLL	1.558 × 100 **	2.312 × 10−1 ns	2.458 × 10−1 *	24.64	0	1.375 a	1.250 a	1.250 a	1.125 a
					7.5	1.250 a	1.500 a	1.375 a	0.625 b
					15	1.500 ab	2.000 a	1.125 bc	0.875 c
					22.5	1.750 ab	1.875 a	1.250 bc	1.125 c
					30	1.375 ab	1.625 a	0.875 b	1.375 ab
SER (cm tiller−1 d−1)	1.980 × 10−2 **	6.329 × 10−4 *	9.521 × 10−4 **	37.11	0	0.104 a	0.008 c	0.036 b	0.020 bc
					7.5	0.076 a	0.013 b	0.018 b	0.013 b
					15	0.089 a	0.024 b	0.000 b	0.020 b
					22.5	0.069 a	0.037 b	0.020 b	0.020 b
					30	0.081 a	0.062 a	0.008 b	0.030 b
LAR (leaves tiller−1 d−1)	6.070 × 10−3 **	6.102 × 10−4 *	4.514 × 10−4 **	24.16	0	0.073 a	0.040 b	0.040 b	0.050 ab
					7.5	0.073 a	0.054 ab	0.040 bc	0.020 c
					15	0.073 a	0.054 ab	0.040 b	0.040 b
					22.5	0.085 a	0.049 bc	0.036 c	0.065 ab
					30	0.089 a	0.067 ab	0.031 c	0.060 b
LER (cm tiller−1 d−1)	1.251 × 100 *	1.916 × 10−1 ns	8.122 × 10−1 **	15.42	0	2.906 ab	2.193 b	3.135 a	2.809 ab
					7.5	2.889 a	2.715 a	3.080 a	1.873 b
					15	3.050 a	2.661 a	3.192 a	2.680 a
					22.5	2.756 a	2.684 a	3.119 a	2.613 a
					30	2.834 b	2.092 b	2.796 b	3.903 a
LSR (cm tiller−1 d−1)	3.936 × 100 **	6.137 × 10−1 **	3.163 × 10−1 **	23.12	0	1.513 a	0.538 b	1.567 a	1.825 a
					7.5	1.538 a	0.667 b	1.227 ab	1.249 ab
					15	1.427 a	0.470 b	1.442 a	1.143 a
					22.5	1.364 ab	0.894 b	1.638 a	1.339 ab
					30	1.625 b	0.709 c	1.622 b	2.520 a

C × HP: interaction between crop cycles and hydroretentive polymer; CV: coefficient of variation; * and **: significance at 5% and 1% probability, respectively, using the F-test; ns: not significant; means followed by the same lowercase letters in the row do not differ significantly using Tukey’s test (p < 0.01).

). It can also be observed in Table 2 that the results for the number of emerging leaves (NEmL) varied significantly between the crop cycles. In the first cycle, the averages were lower compared to the other cycles. The second cycle presented the highest values for most doses. In the third cycle, there was stability in the treatments with lower doses of hydroretentive polymer and a reduction in the higher doses, while in the fourth cycle, a general decline was observed, especially in the intermediate doses. The differences in NEmL between the cycles reflect the varying climatic conditions throughout the experiment. The first cycle, with lower temperatures and less solar radiation (Figure 1), may have limited the initial leaf development. In the second cycle, the more favorable conditions, such as increases in the average temperature (22.7 °C) and solar radiation (13.12 MJ m⁻² d⁻¹), boosted leaf emergence, resulting in the highest values being observed. In the third cycle, the stability or slight reduction may be associated with competition for resources due to the higher biomass accumulation of the plants. In the fourth cycle, the increased water demand (average reference evapotranspiration of 3.39 mm d⁻¹) and the reduced average relative humidity (75.37%) likely intensified the water stress, negatively affecting leaf emergence, particularly at intermediate and lower doses. These results indicate that plant performance is directly related to the climatic conditions and water management in each cycle.

In the first crop cycle of Mombaça grass, the plants showed the highest numbers of expanded leaves (NEpL), regardless of the hydroretentive polymer dose used (Table 2). In subsequent cycles, the values progressively decreased, approaching zero in some polymer doses. This decrease is associated with more demanding climatic conditions in the later cycles, such as increased evapotranspiration and solar radiation (Figure 1), which intensified water stress and limited leaf development. These results highlight the impact of seasonal variations on plant growth, supporting studies that emphasize the role of climatic conditions in leaf emergence and expansion of grasses [10,40,41,42].

It is also observed in Table 2 that the numbers of live leaves (NLL) were higher in the first and second cycles across all doses of hydroretentive polymer, followed by a reduction in the third and fourth cycles, reflecting the decrease in leaf growth over the course of the experiment. This reduction follows the trends observed for NEmL and NEpL, highlighting the difficulty of plants in sustaining leaf development and maintenance in later cycles due to increased water stress and reduced efficiency in utilizing available water and nutrients. Studies in grasses under similar conditions highlight that limitations in leaf expansion directly impact NLL, reducing the photosynthetic area available to support plant growth [43,44].

The stem elongation rate (SER) showed significant variations between crop cycles (Table 2). In the first cycle, higher SER values were observed for Mombaça grass, probably due to milder climatic conditions (Figure 1) and the initial plant development. In subsequent cycles, SER progressively decreased, which may be associated with increased water stress, with higher evapotranspiration and reduced relative humidity, limiting stem growth. These results support the idea that stem elongation is highly influenced by climatic factors [10,45,46].

As seen in Table 2, the leaf appearance rate (LAR) was higher in the first cycle of Mombaça grass, with reductions in the following cycles. This pattern follows the behavior observed for SER. The justifications for these results are the same as those mentioned for SER.

The leaf elongation rate (LER) showed significant variations between the crop cycles (Table 2). In general, the values were higher in the first cycle, followed by a decrease in the second cycle, which recorded the lowest average value for LER. The third cycle saw an increase in LER values, reaching the highest value among the cycles, while the fourth cycle showed some recovery, although still lower than the third cycle. These results suggest that climatic conditions influenced leaf elongation, with greater growth in the third cycle, characterized by higher temperatures and increased solar radiation (Figure 1). However, water stress and reduced relative humidity in subsequent cycles led to a decrease in LER, especially in the second cycle. In the fourth cycle, despite some recovery in LER, the impact of climatic conditions and water management continued to limit leaf elongation compared to the third cycle.

It was observed that Mombaça grass had the lowest leaf senescence rate (LSR) in cycle 2 (Table 2). In the other cycles, there were no significant differences within most doses of hydroretentive polymer, with LSR values being higher and similar across these cycles. These results suggest that cycle 2 was characterized by more favorable climatic conditions, possibly due to increased relative humidity and moderate air temperatures (Figure 1), which favor the maintenance of leaves for longer. As a result, Mombaça grass in cycle 2 benefited from better leaf performance, leading to greater sustainability of vegetative growth [47,48].

Figure 6 presents the variations in morphogenic characteristics as a function of hydroretentive polymer doses in the different crop cycles of Mombaça grass. In cycle 1, a linear increase in the number of emerging leaves (NEmL) was observed with increasing doses of hydroretentive polymer. In cycles 2 and 3, the doses of hydroretentive polymer showed a quadratic effect on NEmL. Based on the regression equations and applying the partial derivative, the polymer doses that maximized NEmL were 19.2 and 4.5 kg ha⁻¹, resulting in 1.81 and 1.29 emerging leaves for cycles 2 and 3, respectively. This result aligns with studies suggesting that the use of hydroretentive polymers can improve water retention in the soil, increasing water availability for plants and stimulating leaf development [14]. In cycle 4, it was not possible to fit any regression model to the NEmL data. This suggests that during this period, the polymer doses did not contribute to water retention, not affecting the leaf development of Mombaça grass. The polymer was probably already losing its water retention capacity, as observed in Figure 3.

The results for NEpL were opposite to those observed for NEmL. As shown in Figure 6, in cycle 4, hydroretentive polymer doses caused a quadratic effect on NEpL. According to the regression equation, the polymer dose that maximized NEpL was 20.3 kg ha⁻¹, resulting in 0.28 expanded leaves. In contrast, in the other cultivation cycles, Mombaça grass did not show changes in NEpL due to polymer doses. This result suggests that in the first cultivation cycles, climatic conditions and water management were sufficient to allow normal growth of expanded leaves, without the need for additional water retention provided by the polymer to promote improvements.

NLL is the sum of NEpL and NEmL and is highly correlated with forage productivity [40,47]. In cycles 1 and 4, it was not possible to adjust any regression model to the NLL data of Mombaça grass. In cycle 1, the initial climatic conditions, with lower air temperatures, may have limited leaf growth, while in cycle 4, increased water stress may have restricted the polymer’s efficiency. In cycles 2 and 3, hydroretentive polymer doses caused a quadratic effect on NLL. Based on the regression equations and applying the partial derivative, the polymer doses that maximized NEmL were 18.6 and 7.7 kg ha⁻¹, resulting in 1.91 and 1.31 live leaves for cycles 2 and 3, respectively. These results indicate that the increased water demand due to hotter weather and higher evapotranspiration in cycles 2 and 3 may have intensified the need for water retention in the soil. In this context, the hydroretentive polymer appears to have contributed to maintaining water availability for the plants, maximizing NLL values up to an optimal dose. However, beyond this point, higher doses may have caused soil saturation, reducing the positive effect on preserving leaf structure. Studies show that excess water in the soil can compromise aeration and cause root stress, negatively affecting leaf preservation [49,50].

For SER, it can be seen in Figure 6 that hydroretentive polymer doses provided a linear increase in cycle 2. This effect can be attributed to the higher water availability in the soil, provided by the polymer, which favored the growth of the stems under possibly more favorable climatic conditions during this period. In the other cycles, no tested model fit the SER data.

Hydroretentive polymer doses promoted a linear increase in leaf appearance rate (LAR) during cycles 1 and 2 of Mombaça grass (Figure 6). In cycle 3, LAR’s response to hydroretentive polymer followed a quadratic pattern, with the application of 6.25 kg ha⁻¹ maximizing LAR at 0.04 leaves tiller⁻¹ d⁻¹. This increase in LAR is directly related to tillering, as each new leaf stimulates the development of a bud with the potential to form a new tiller [51]. On the other hand, in cycle 4, it was not possible to adjust the data to any regression model, possibly due to factors such as reduced polymer efficacy (Figure 3) or changes in cultivation conditions.

In cycles 2 and 3, hydroretentive polymer doses caused a quadratic effect on the leaf elongation rate (LER) of Mombaça grass (Figure 6). The regression equations indicated that doses of 14.4 and 10.3 kg ha⁻¹ maximized LER, reaching values of 2.78 and 3.19 cm tiller⁻¹ d⁻¹ for cycles 2 and 3, respectively. These results highlight the potential of hydroretentive polymer to increase water availability in the soil, promoting greater leaf growth in cultivation cycles with favorable climatic conditions. In cycles 1 and 4, however, no regression model fit the LER data, suggesting that other factors, such as climatic limitations or soil conditions, may have restricted the plant’s response to the hydroretentive polymer during these periods.

No regression model could be fitted to the LSR data for Mombaça grass (Figure 6), indicating that the different doses of hydroretentive polymer did not influence this morphogenic characteristic in any of the cultivation cycles. This result suggests that the plants did not experience water deficit or excess sufficient to limit their development, as all pots received equal volumes of water in each irrigation. The uniformity in water supply likely met the plants’ demands, reducing the need for differentiated responses to the polymer doses. Previous studies highlight that the effect of the hydroretentive polymer is more evident under conditions of greater water variability or significant water deficit [40,47], which was not the case in this experiment.

In cycle 4, as shown in Figure 6, most of the morphogenic characteristics of Mombaça grass did not show a significant response to the hydroretentive polymer doses applied at sowing. This lack of effect may be associated with the advanced degradation of the polymer, considering that approximately 190 days had passed since its initial application. The soil moisture results presented in Figure 3 support this hypothesis, indicating a progressive decline in water retention capacity over time. The literature also highlights that hydroretentive polymers, especially those based on organic materials, tend to lose effectiveness due to natural biodegradation, particularly under continuous cultivation systems and adverse environmental conditions [22,52,53].

Considering that Mombaça grass is commonly cultivated for animal feeding, either through intensive grazing or a cut-and-carry system, it is important to interpret the morphogenic traits not only in terms of plant development but also regarding their nutritional implications post-defoliation. Traits such as the number of live leaves (NLL), stem elongation rate (SER), and leaf elongation rate (LER) influence the leaf-to-stem ratio, which is a critical determinant of forage quality. A higher proportion of leaves is generally associated with greater palatability, digestibility, and crude protein content, while excessive stem growth may reduce forage quality. Therefore, the morphogenic responses to hydroretentive polymer doses observed in this study, especially those promoting leaf development, may enhance the nutritional value of the forage and its utilization efficiency by ruminants after grazing or harvest.

3.5. Agronomic Characteristics

As shown in

Table 3. Mean squares, significance of the F-test (ANAVA), and mean values of the agronomic characteristics water consumption (WC), shoot fresh mass (SFM), shoot dry mass (SDM), and water productivity (WP) for different cycles of Mombaça grass and based on doses of hydroretentive polymer (HP). Viçosa-MG, DEA-UFV, 2023.

Variable	Mean Square			CV (%)	HP (kg ha−1)	Cultivation Cycles
	Cycle	HP	C × HP			1	2	3	4
WC (L pot−1)	3.940 × 101 **	1.462 × 10−1 **	1.995 × 10−2 ns	6.32	0	4.420 d	5.913 c	6.491 b	7.716 a
					7.5	4.260 d	5.876 c	6.395 b	7.668 a
					15	4.037 d	5.705 c	6.377 b	7.608 a
					22.5	4.169 d	5.746 c	6.364 b	7.513 a
					30	4.181 c	5.802 b	6.163 b	7.483 a
SFM (g pot−1)	1.018 × 103 **	2.611 × 101 *	4.181 × 100 ns	8.16	0	41.289 c	47.772 b	42.625 bc	55.301 a
					7.5	39.130 c	48.965 b	43.681 bc	55.562 a
					15	39.192 c	48.944 b	44.589 bc	57.383 a
					22.5	37.118 c	47.173 b	43.520 b	54.548 a
					30	35.951 c	44.767 b	42.337 b	54.059 a
SDM (g pot−1)	8.307 × 101 **	3.209 × 100 **	3.426 × 10−1 ns	6.48	0	8.358 c	9.729 b	10.336 b	12.535 a
					7.5	7.594 c	9.782 b	10.860 b	12.446 a
					15	7.447 c	9.427 b	10.391 b	12.653 a
					22.5	7.354 c	9.873 b	10.358 b	12.234 a
					30	6.439 c	8.565 b	9.542 b	11.942 a
WP (kg m−3)	8.307 × 101 **	3.209 × 100 **	3.426 × 10−1 ns	6.53	0	1.886 a	1.642 b	1.591 b	1.625 b
					7.5	1.769 a	1.668 a	1.700 a	1.624 a
					15	1.842 a	1.660 a	1.630 a	1.663 a
					22.5	1.759 a	1.723 a	1.630 a	1.629 a
					30	1.524 a	1.485 a	1.548 a	1.596 a

C × HP: interaction between crop cycles and hydroretentive polymer; CV: coefficient of variation; * and **: significance at 5% and 1% probability, respectively, using the F-test; ns: not significant; means followed by the same lowercase letters in the row do not differ significantly using Tukey’s test (p < 0.01).

, there was no interaction between the factors of cultivation cycles and hydroretentive polymer for any of the agronomic characteristics evaluated. The water consumption of Mombaça grass increased significantly throughout the cultivation cycles, with the highest demand recorded in cycle 4. The variation observed between the cycles can be attributed to climatic conditions, such as air temperature and solar radiation, which were higher in the later cycles, resulting in greater reference evapotranspiration (Figure 1) and higher water consumption [10,36].

For the shoot fresh mass (SFM) and shoot dry mass (SDM) of Mombaça grass, significant variations between the cultivation cycles can be observed in Table 3. Regardless of the dose of hydroretentive polymer used, the highest biomass production rates were recorded in the later cultivation periods, indicating that the plant benefits from more favorable climatic conditions for its development, such as higher air temperatures and solar radiation [10,40,41,42,45,46]. The gradual increase in the shoot biomass production of Mombaça grass is also related to its water consumption. As the biomass increases, the leaf area also increases, resulting in higher water consumption by the plant. According to Doorenbos and Kassam [54], this correlation allows the assessment of the impact of irrigation management on the economic production of crops and helps in choosing more efficient strategies.

However, it is important to consider that the increase in biomass observed in cycle 4 cannot be attributed solely to more favorable climatic conditions, such as temperature and radiation. Other adaptive mechanisms may have contributed to this performance, especially in a scenario of possible reduction in the effectiveness of the hydroretentive polymer over time. The thermal plasticity of Mombaça grass, for instance, may have favored increased stomatal conductance under higher temperatures, resulting in enhanced photosynthesis even under less favorable soil moisture conditions, as discussed by Franco et al. [38]. Furthermore, although UPDT^® showed a decline in efficiency after approximately 190 days, as reported by Araújo et al. [14], other biodegradable polymers, such as starch–polyacrylate blends, may retain their functionality for longer periods. This suggests that degradation rates can vary depending on the crop type and microbial activity in the rhizosphere. In pastures with high microbial activity, such as tropical grasslands, degradation may be accelerated. Therefore, the reapplication of polymers such as UPDT^® during reseeding cycles of degraded pastures could be a viable strategy, provided that field studies carefully assess the impacts of degradation byproducts on soil pH and the establishment of native species.

Water productivity (WP) in the treatment without the application of hydroretentive polymer showed a continuous reduction throughout the cultivation cycles, with the highest values recorded in the first cycle (Table 3). This pattern can be mainly attributed to the more favorable climatic conditions, such as higher solar radiation and air temperature, which favored initial production (Figure 1).

In the treatments with hydroretentive polymer, no significant differences were observed between the cultivation cycles. This behavior suggests that the use of the polymer contributed to greater efficiency in converting the water consumed into biomass by Mombaça grass, promoting more optimized water use over time. This adaptation is related to more efficient regulation of CO₂ and leaf temperature, as well as reduced water loss due to changes in stomatal conductance [55]. In the polymer treatments, UPDT^® helped maintain water availability in the soil over the cycles, allowing Mombaça grass to maintain good performance even during periods of climatic variation, such as changes in air temperature, photoperiod, and solar radiation [56]. This resulted in the absence of statistical differences in water productivity between the cultivation cycles.

In addition, WP values in the treatments with hydroretentive polymer ranged from 1.49 to 1.84 kg m⁻³. These values surpass those found for Mombaça grass without polymer treatment, which ranged from 0.28 to 0.85 kg m⁻³ under similar irrigation conditions, according to Rocha et al. [10]. This improvement in water productivity indicates the potential of UPDT^® to optimize water use, promoting greater water efficiency in biomass production.

Figure 7 presents the variations in agronomic characteristics as a function of hydroretentive polymer doses across different cultivation cycles of Mombaça grass. Increasing hydroretentive polymer doses resulted in a linear reduction in water consumption in cycles 3 and 4, suggesting that the presence of the polymer may have contributed to greater soil water retention, reducing irrigation needs. However, in cycles 1 and 2, it was not possible to fit regression models to the water consumption data, which may indicate that in these early growth stages the polymer effect was less pronounced or that climatic and management factors did not yet allow for a clear trend of reduced water consumption. These results reinforce the idea that the polymer effect may be more evident during periods of higher evapotranspiration rates.

The hydroretentive polymer doses promoted a linear increase in SFM during cycle 1 of Mombaça grass (Figure 7), indicating that in this initial cycle the polymer had a direct and positive effect on plant productivity. In the subsequent cycles, hydroretentive polymer doses caused a quadratic effect on SFM, suggesting that the response of Mombaça grass to the polymer was more complex, showing a quadratic trend in each cycle and depending on the applied dose. Based on the regression equations and applying the partial derivative, the hydroretentive polymer doses that maximized SFM were 10.4, 14.4, and 12.0 kg ha⁻¹, resulting in productivity rates of 49.0, 44.3, and 56.3 g pot⁻¹ for cycles 2, 3, and 4, respectively. These results indicate that although higher polymer doses increase productivity, there is an optimal dose beyond which the polymer’s effect starts to decline. This pattern can be explained by the potential reduction in soil free porosity at higher polymer doses, which may occur due to increased soil water retention. Consequently, the soil’s ability to allow aeration and drainage may be compromised, negatively affecting root growth and plant productivity at higher polymer doses [10,14].

The productivity pattern of SDM followed the same trend observed for SFM productivity, with a linear increase in the first cycle and a quadratic response in the subsequent cycles to hydroretentive polymer doses (Figure 7). Based on the regression equations, the hydroretentive polymer doses that maximized SDM were 8.9, 10.1, and 7.9 kg ha⁻¹, resulting in productivity rates of 9.8, 10.7, and 12.6 g pot⁻¹ for cycles 2, 3, and 4, respectively. The explanations for these results are the same as previously discussed for SFM, related to the interaction between the polymer dose, soil porosity, climatic conditions, and crop development stage.

In cycles 1, 2, 3, and 4, hydroretentive polymer doses had a quadratic effect on water productivity of Mombaça grass. Through regression equations, the optimal polymer doses to maximize WP were determined as 5.2, 12.0, 12.4, and 13.0 kg ha⁻¹, resulting in values of 1.86, 1.71, 1.67, and 1.65 kg m⁻³ for cycles 1, 2, 3, and 4, respectively. The use of hydroretentive polymers improves water retention in the substrate, increasing its availability to plants, which promotes leaf growth and enhances water use efficiency [14,57]. However, doses above the optimal level may indeed inhibit root aeration, counteracting the benefits of enhanced water retention. Therefore, it is clear that agricultural practices that optimize water use not only promote higher productivity in agricultural systems but also contribute to water resource conservation, reducing pressure on ecosystems and supporting more sustainable agriculture.

These findings partially support the initial hypothesis that the application of the biodegradable hydroretentive polymer UPDT^® would increase water retention and improve the morphogenic and agronomic performance of Mombaça grass during the initial cultivation cycles. Although the polymer’s influence was less evident in early cycles, likely due to lower evapotranspiration demand or limited root development, it became more pronounced in cycles 3 and 4, especially regarding water consumption reductions and increased water productivity. The observed improvements in shoot biomass and water use efficiency with optimized polymer doses confirm that UPDT^® contributes positively to plant performance under conditions of higher water demand.

In addition to the results obtained in this study, it is important to consider that the effectiveness of hydrogel polymers can vary significantly depending on soil type. Previous studies indicate that sandy soils, for example, show greater benefits from the use of such polymers due to their low water-holding capacity, whereas clayey soils may show a less pronounced response due to their already high water retention [58]. Research conducted on other grasses under different edaphic conditions, such as in lateritic soils and Oxisols, has shown that the use of similar polymers also led to improvements in water use efficiency and plant growth, supporting the findings of this study [21,57,59]. Therefore, the technology presented here has potential for application in various production systems but it requires specific adjustments and evaluations for each soil type to maximize its benefits.

Although the results demonstrate that the application of UPDT^® improves water productivity and water use efficiency in Mombaça grass, some limitations must be considered. This study did not directly evaluate the effects of polymer reapplication over multiple cycles, nor the potential ecological impacts of its continuous use. In this regard, future research is recommended to explore reapplication strategies for UPDT^® every 90 days, aligned with the regrowth cycles of the forage, using soil moisture sensors to optimize the timing of applications. Moreover, it is essential to assess the long-term effects on the soil, especially possible changes in microbial communities, which can be monitored through metagenomic analyses and remote sensing techniques, such as hyperspectral imaging focused on soil organic carbon. There is also a need for studies on the life-cycle sustainability of starch-based polymers, considering their potential impact on land use for industrial versus agricultural purposes.

4. Conclusions

The results indicate that the water retention capacity in the soil provided by the UPDT^® hydroretentive polymer decreases over time, likely due to its biodegradation. Despite this reduction, the study demonstrates that an application of 15 kg ha⁻¹ of UPDT^® is the most efficient for establishing Mombaça grass pasture, ensuring better initial plant performance when cultivated in a controlled greenhouse environment. Water consumption progressively increased from cycle 1 to cycle 4, reflecting the influence of more favorable climatic conditions toward the end of the experimental period. Morphogenic traits were affected by both the cultivation cycles and polymer doses, although without a consistent response pattern. In contrast, agronomic traits showed a progressive improvement across cycles, with polymer doses ranging from 7.5 to 15 kg ha⁻¹ maximizing production. These results highlight the potential of hydroretentive polymers to optimize water use and enhance productivity under controlled conditions, contributing to more sustainable management systems.