Estimation of Fiber Fragility and Digestibility of Corn Silages and Cool Season Pastures

Abstract

Fiber fragility is defined as the particle size reduction rate during chewing and can help to explain the effects on feed intake and animal performance of different fiber sources. This study aimed to estimate the fiber fragility of corn silage and cool-season pasture based on their chemical composition. Between June and December 2022, 25 samples of corn silage and 25 samples of cool-season pasture were collected from dairy farms in the state of Rio Grande do Sul. The samples were analyzed for particle size distribution, chemical composition, and fiber fragility. Contents of dry matter (DM), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and fiber fragility were greater in corn silages compared to cool-season pasture. However, the ADF–NDF ratio was similar in forages. Crude protein (CP) content and the in situ degradation of DM and NDF were greater in cool-season pasture than corn silage. Dry matter and NDF in situ degradation were negatively correlated with increased contents of ADF, NDF, and ADL and the ADF–NDF ratio in forages. Fiber fragility was negatively correlated with DM degradation and positively correlated with contents of ADF, NDF, ADL, and DM. Fiber fragility decreased as CP content increased. Thus, greater fiber fragility may jeopardize nutrient degradation, and levels of fiber fragility are directly associated with fiber content.

1. Introduction

Fibrous carbohydrates are essential components in ruminant diets, providing energy and stimulating rumination despite the possible fiber-negative effects on feed intake due to rumen filling [1]. Since Van Soest’s detergent system of fiber analysis [2], several guides have proposed improvements in fiber recommendation levels [3,4] and adequate diet particle size [5,6,7]. However, the rumen-filling effect of fiber has not been consistent across forage sources.

The last version of the North American guide for dairy cows’ nutrition [7] proposed fiber fragility as a predictor of feed intake. Grant [8] defines fiber fragility as the particle size reduction rate during chewing. This reduction affects ruminal retention and chewing activity and indirectly impacts feed intake [9]. Grasses typically have lower lignin contents and higher fiber digestibility [10] when compared to legume forages, resulting in a greater filling effect attributed to their lower fiber fragility [7]. As a result, fiber fragility can elucidate the varying relationship between neutral detergent fiber (NDF) digestibility and feed intake observed in grasses and legumes [6]. However, fragility evaluation is laborious and not standardized across laboratories.

White et al. [6] proposed the acid detergent fiber (ADF) to NDF ratio as the primary predictor of fiber fragility, considering the lower fragility and ADF–NDF ratio observed in grasses compared to legumes. However, in subtropical regions of Brazil, corn silage and cool-season forages are the main forage sources. Regrettably, no studies have been conducted to evaluate the differences between these materials or propose models that predict fiber fragility in corn silage and cool-season forages. Therefore, we hypothesized that the fiber fragility of corn silage is greater than in cool-season pastures and that the ADF–NDF ratio is the primary predictor of fiber fragility, regardless of forage.

2. Materials and Methods

2.1. Sample Collections

Twenty-five samples of corn silage and twenty-five samples of cool-season pasture (including oats, ryegrass, and wheat) were collected from June to December 2022 from dairy farms (n = 32) in Rio Grande do Sul State. Cool-season pasture samples (~2 kg) were collected by grazing simulation at random points [11]. Samples were collected from pastures after observing animals grazing at a minimum of 10 different sites. Samples were placed in plastic bags and frozen until they were shipped to the laboratory. The cool-season pasture samples were representative of the most common properties in the southern region and included 10 pastures of ryegrass, 10 pastures of ryegrass and oats in a consortium, 3 pastures of oats, and 2 pastures of wheat. Corn silage samples (~2 kg) were collected from various locations on the silo panels using a sample probe to create a representative sample. Immediately after sampling, corn silage was vacuum-sealed in plastic bags and frozen until it was shipped to the laboratory. Corn silage samples were collected from 21 trench silos and 4 bag silos. Sampling was conducted over time to capture pastures at different stages of development and corn silage stages in various periods.

2.2. Particle Size Distribution, Chemical Composition, and In Situ Degradation

The samples were dried in a forced-air ventilation oven at 60 °C for 72 h. A sub-sample was then utilized to determine the particle size distribution using a Penn State Separator [12]. Then, samples (n = 50) were placed in a ball mill (13 cm diameter × 13 cm length) loaded with 2.5 to 3.0 mm ceramic balls and ground at 80 rotations per minute for 15 min, following the Miner Institute method to access fragility [13]. Subsequently, the particle size of the ground sample was evaluated, and fragility was determined using the following equation:

$$Fragility\left({\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}\right)={\displaystyle \frac{{PEF}_b-{PEF}_a}{{PEF}_b}}\times 1000{\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}$$

(1)

where ${PEF}_b$ represents the proportion of particles larger than 8 mm before grinding and ${PEF}_a$ represents the proportion of particles larger than 8 mm after grinding.

Another sub-sample was processed in a knife rotary mill using 2 mm sieves for in situ assays [14] and 1 mm sieves for chemical analysis. Samples were analyzed for contents of dry matter (DM, method 950.15), crude protein (CP, 6.25 × N, Kjeldahl method 984.13), acid detergent fiber (ADF), and acid detergent lignin (ADL, method 973.18) [15]. In addition, neutral detergent fiber (NDF) analysis was performed according to Van Soest et al. [16]. For silage samples, amylase was used during the analysis.

The samples processed through the 2 mm sieves were placed into non-woven tissue bags (5 × 5 cm) and subjected to in situ incubation for 48 h [16] in a rumen cannulated steer (Brangus, 800 kg body weight, fed a forage diet of Tifton—Cynodon spp.). After removal from the rumen, the bags were washed with running water, and the residual NDF content (uNDF) was analyzed, as described previously. This uNDF content was then used to calculate the degradation of DM [17] and NDF using the following equations:

$$DMdegradation\left({\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}\right)=1000-uNDF$$

(2)

$$NDFdegradation\left({\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}\right)=\left({\displaystyle \frac{NDF-uNDF}{NDF}}\right)\times 1000{\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}$$

(3)

2.3. Statistical Analysis

Data analysis was conducted using the SAS statistical program (SAS Inc., University Edition, Cary, NC, USA). Corn silage and cool-season pasture samples were compared using the following statistical model:

$$Y_{ijk}=\mathsf{\mu }+M_i+e_{ijk}$$

(4)

with $e_{ij}$ ≈ N (0,${\mathsf{\sigma }}_i^2$), where $Y_{ijk}$ is the observed value of the dependent variable, μ is the overall average, $M_i$ is the fixed effect of the material (I 1 and 2, $e_{ijk}$ is the random residual error, and ${\mathsf{\sigma }}_i^2$ is the random residual error for each material.

Spearman correlation analysis was conducted to identify variables more strongly associated with fragility and in situ degradation. Additionally, the PROC REG was employed to develop models for estimating fragility, DM degradation, and NDF degradation. These models were derived using all possible predictors or exclusively chemical analysis variables. Correlation and regression analyses were performed for the following subsets of samples: I. all samples; II. only corn silage samples; or III. only cool-season samples.

3. Results

3.1. Chemical Composition, Fragility, and In Situ Degradation of Corn Silage and Cool-Season Pastures

Contents of DM, NDF, ADF, and ADL were higher (p ≤ 0.013) in corn silage samples compared to cool-season pasture samples (

Table 1. Chemical composition, fiber fragility, and in situ degradation of dry matter (DM) and neutral detergent fiber (NDF).

Item	CS 1	PAS 2	p 3
Fragility, g/kg	$80.6\pm$ 11.79	$24.0\pm 4.02$	<0.001
Chemical composition, g/kg
DM 4, g/kg natural matter	$278\pm 13.2$	$174\pm$ 7.2	<0.001
NDF 5	$523\pm 14.1$	$436\pm$ 14.6	<0.001
ADF 6	$311\pm 11.5$	$250\pm 8.7$	0.001
ADL 7	$39.7\pm 5.08$	$24.3\pm 3.00$	0.013
CP 8	$66.3\pm$ 2.22	$237\pm 12.0$	<0.001
ADF–NDF ratio	$0.592\pm$ 0.01	$0.574\pm$ 0.005	0.100
In situ degradation, g/kg
DM 4	$672\pm$ 17.6	$849\pm$ 20.6	<0.001
NDF 5	$375\pm 25.7$	$676\pm$ 34.6	<0.001

¹ Corn silage; ² cool-season pasture; ³ probability for CS and PAS comparison; ⁴ dry matter; ⁵ neutral detergent fiber; ⁶ acid detergent fiber; ⁷ lignin in acid detergent; ⁸ crude protein.

). On the other hand, cool-season pasture samples showed higher (p < 0.001) CP contents and in situ degradation of DM and NDF than corn silage. The ADF–NDF ratio was similar (p = 0.100) between corn silage and cool-season pasture, whereas the fragility was higher (p < 0.001) for corn silage compared to cool-season pasture.

3.2. Correlation between In Situ Degradation, Fragility, and Chemical Composition

In general, fiber fragility showed (p ≤ 0.020) a positive Spearman correlation with ADL (R = 0.62), ADF (R = 0.60), NDF (R = 0.58), DM (R = 0.45), and the ADF–NDF ratio (R = 0.329,

Table 2. Spearman correlation among in situ degradation, fragility, and chemical composition of both forage sources, corn silage, or cool-season pasture [R (p)].

Item	DM 1	NDF 2	ADF 3	ADF–NDF 4	ADL 5	CP 6	Fragility
General
Fragility	0.45 (0.001)	0.58 (<0.001)	0.60 (<0.001)	0.33 (0.020)	0.62 (<0.001)	−0.56 (<0.001)	-
DMD 7	−0.62 (<0.001)	−0.80 (<0.001)	−0.87 (<0.001)	−0.52 (<0.001)	−0.80 (<0.001)	0.70 (<0.001)	−0.68 (<0.001)
NDFD 8	−0.65 (<0.001)	−0.67 (<0.001)	−0.78 (<0.001)	−0.50 (<0.001)	−0.74 (<0.001)	0.71 (<0.001)	−0.67 (<0.001)
Corn silage
Fragility	−0.09 (0.669)	0.32 (0.122)	0.35 (0.834)	0.32 (0.117)	0.45 (0.023)	−0.17 (0.430)	-
DMD 7	0.38 (0.061)	−0.59 (0.035)	−0.73 (<0.001)	−0.60 (0.002)	−0.73 (<0.001)	−0.01 (0.959)	−0.43 (0.029)
NDFD 8	0.27 (0.193)	−0.14 (0.521)	−0.43 (0.0337)	−0.47 (0.019)	−0.50 (0.011)	−0.04 (0.849)	−0.38 (0.060)
Cool-season pasture
Fragility	0.12 (0.548)	0.37 (0.069)	0.35 (0.081)	0.06 (0.795)	0.42 (0.036)	−0.10 (0.649)	-
DMD 7	−0.74 (<0.001)	−0.82 (<0.001)	−0.86 (<0.001)	−0.30 (0.160)	−0.59 (0.002)	0.60 (0.002)	−0.36 (0.078)
NDFD 8	−0.76 (<0.001)	−0.76 (<0.001)	−0.81 (<0.001)	−0.34 (0.096)	−0.06 (0.004)	0.61 (0.001)	−0.38 (0.065)

¹ Dry matter; ² neutral detergent fiber; ³ ciad detergent fiber; ⁴ ADF–NDF ratio; ⁵ acid detergent lignin; ⁶ crude protein; ⁷ DM degradation; ⁸ NDF degradation.

). On the other hand, fragility was negatively correlated (p < 0.001, R = −0.560) with CP content. In situ degradation of DM and NDF was (p < 0.001) negatively correlated with contents of ADF (R = −0.869 and −0.776), NDF (R = −0.799 and −0.674), ADL (R = −0.797 and −0.742), and DM (R = −0.621 and −0.659) and the ADF–NDF ratio (R = −0.683 and −0.496). In situ degradation of DM and NDF exhibited a positive correlation (p < 0.001, R = 0.696 and 0.709, respectively) with CP content. Fragility and the degradation of DM and NDF were negatively associated (R ≤ −0.67, p < 0.001).

The fragility of corn silage exhibited (p = 0.023, R = 0.452) a positive correlation with ADL content. There was no correlation (p ≥ 0.083) between corn silage fragility and contents of DM, NDF, and ADF or the ADF–NDF ratio. The DM degradation of corn silage showed a negative correlation (p ≤ 0.035) with contents of NDF (R = −0.591), ADF (R = −0.733), and ADL (R = −0.725) and the ADF–NDF ratio (R = −0.592). DM degradation had a negative correlation (p = 0.029, R = −0.437) with fiber fragility. NDF degradation exhibited a negative correlation (p ≤ 0.034) with contents of ADL (R = −0.498) and ADF (R = −0.426) and the ADF–NDF ratio (R = −0.467). The NDF degradation of corn silage did not show a correlation (p ≥ 0.060) with contents of DM, NDF, and CP or fiber fragility.

For cool-season pastures, ADL content was the variable that was the most correlated (p = 0.036, R = 0.422) with fragility. The fragility of cool-season pastures did not correlate (p ≥ 0.069) with contents of DM, NDF, ADF, and CP or the ADF–NDF ratio. The DM degradation of cool-season pastures showed a negative correlation (p ≤ 0.002) with contents of ADF (R = −0.857), NDF (R = −0.818), DM (R = −0.736), and ADL (R= −0.590) and a positive correlation (R = 0.60, p = 0.002) with CP content. There was no correlation (p ≥ 0.078) between DM degradation and NDF degradation or the ADF–NDF ratio and fiber fragility.

3.3. Estimation of Fiber Fragility

The model describing fiber fragility in both forage sources identified DM degradation as the primary predictor (Model 1,

Table 3. Models to estimate fiber fragility from all variables and chemical composition in both forage sources, corn silage, or cool-season pasture samples.

Response Variable	Intercept		Variable 1			Variable 2			R²	RQME 4	p 5
	EST 1	EPM 2	VAR 3	EST 1	EPM 2	VAR 3	EST 1	EPM 2
Model 1	242	34.8	DMD 6	−0.249	0.045	-	-	-	0.388	41.2	<0.001
Model 2	60.4	17.59	CP 7	−0.214	0.068	ADL 8	0.764	0.298	0.358	42.7	<0.001
Model 3	157	33.9	NDFD 9	−0.205	0.086	-	-	-	0.200	53.0	0.030
Model 4	46.7	41.10	ADL 9	0.854	0.450	-	-	-	0.135	56.0	0.071
Model 5	270	66.6	DM 10	−0.376	0.148	DMD 6	−0.213	0.052	0.458	15.5	0.001
Model 6	−32.3	21.37	ADF 11	0.225	0.084	-	-	-	0.237	17.9	0.014

¹ Estimate; ² standard error of the mean; ³ variable; ⁴ root of the mean square error; ⁵ probability; ⁶ dry matter degradation; ⁷ crude protein; ⁸ acid detergent lignin; ⁹ neutral detergent fiber degradation; ¹⁰ dry matter; ¹¹ acid detergent fiber. Models—1: general model considering all predictor variables; 2: general model considering only predictors related to chemical composition; 3: corn silage model considering all predictor variables; 4: corn silage model considering only predictors related to chemical composition; 5: pasture model considering all predictor variables; 6: pasture model considering only predictors related to chemical composition.

, Figure 1). When considering only chemical composition variables, multiple regression enabled the estimation of fragility based on CP and ADL levels (Model 2, Figure 1). The variable that best explained the fragility of silage fiber was the in situ degradation of NDF (Model 3). For estimating pasture fragility, the primary predictors were DM content and DM degradation (Model 5). When considering only the chemical composition variables, the fragility of corn silage and pasture was predicted based on the ADL content for corn silage and ADF content for pasture (Models 4 and 6).

4. Discussion

This study hypothesized that the fiber fragility of corn silage is higher than that of cool-season pastures, with the ADF–NDF ratio being the primary predictor of fragility across different forages. Although corn silage showed greater fragility than cool-season pasture, the ADF–NDF ratio was similar between materials. Fragility was influenced by fiber content, with an inverse relationship with degradation. Degradation coefficients emerged as the best predictors of fragility for cool-season pasture and corn silage.

In general, C₄ grasses have lower DM digestibility and higher cell wall concentration compared to C₃ grasses [18]. In this study, corn silage (C₄ grasses) showed higher contents of DM, NDF, ADF, and ADL and lower degradation of DM and NDF. In addition, cool-season pastures had a higher protein content than corn silage, which has already been documented by other authors [19]. It was expected that differences would be found in all determined parameters of chemical composition when comparing a high-energy fermented forage with a high-protein fresh forage. Tropical grasses store carbohydrates as starch, which contributes to their low protein content. In tropical grasses, vascular bundles and sheaths with thicker walls are commonly observed. This characteristic leads to higher lignin levels and greater cell density in the central leaf tissue compared to grasses thriving in temperate climates [18]. The aforementioned differences may explain the greater tensile strength of tropical grasses compared to cool-season grasses.

The composition and arrangement of fibers (cellulose, hemicellulose, and lignin) in the cellular structure of plants influence fragility, serving as indicators of their resistance during the chewing process [18]. Considering this situation, White et al. [6] proposed the ADF–NDF ratio as a marker of fiber fragility. In the present study, the ADF–NDF ratio was similar between corn silage and cool-season pasture. However, the fragility of corn silage was greater than the fragility of grasses. Other authors [20] found greater fragility for corn silage when compared to wheat straw and orchard grass hay. A study evaluating the particle size reduction of tropical and temperate grass leaves reported that tropical grass leaves have a thick-walled tissue cross-sectional area that is twice that found in the leaves of temperate grasses [21]. Furthermore, the authors found a higher density of vascular bundles per unit of leaf width and a smaller amount of mesophyll in tropical grasses [21]. Still, they are more densely compacted than temperate grasses’ leaves. Although these characteristics contribute to greater leaf rigidity, tropical grasses are more easily broken down by chewing than temperate grasses.

The fragility of the fiber is associated with the plant cell wall, and the concentrations of NDF and ADF may reflect the material’s resistance to breaking during chewing and rumination [22]. However, in the present study, the fiber fragility of the materials was positively correlated with the contents of ADL, ADF, and NDF and the ADF–NDF ratio. Weinberg et al. [23] observed results similar to those of the present study. The authors observed a positive correlation between fragility and contents of NDF, ADF, and ADL (0.64, 0.73, and 0.41, respectively). Similarly, when evaluating the correlation in a single material throughout the day, Gregorini et al. [22] reported that forage tenacity was positively correlated with ADF and NDF contents (0.53 and 0.58, respectively). These authors suggested that the increase in non-structural carbohydrates likely diluted the concentrations of ADF and NDF, reducing the tenacity (i.e., increasing fragility) of Festuca [22].

Forage NDF has a prolonged ruminal retention time due to the larger particle size and fluctuation in the rumen. Although most studies indicate a significant decrease in DM intake rate as the amount of NDF in the forage increases, the response of DM intake may vary depending on the degree to which intake is limited by the filling effect caused by forage NDF [24]. In cool-season pastures or corn silages, the ADL content was the component most positively correlated with fragility. Lignin is renowned for its high tensile strength [18]. However, Wilson and Kennedy [25] suggest that greater lignification of the cell wall would result in greater fragility, causing the particles to become more brittle during chewing, in line with the results obtained in this study.

Considering the data from both materials, fiber fragility was positively correlated with DM content. According to McDonald et al. [18], the growth stage of forage affects their composition and nutritional value. As plants mature, the DM content increases, along with the fiber content [7]. This increase in DM content makes forage crops more fragile [22]. These authors observed a positive correlation between fragility and DM content (0.63) [22]. The results of the present study indicate that the DM content has little influence on fragility and highlight the differences between the materials: corn silage is both more fragile and drier.

As the CP content increased, the fiber brittleness of the materials decreased. On the other hand, there was a positive correlation between the DM and NDF degradation and the CP content of the materials, indicating that higher CP content is associated with greater ease of DM and NDF degradation. According to McDonald et al. [18], the growth stage is an important factor that can influence the composition and nutritional value of forages. The degradation of NDF varies depending on the type of forage and decreases as plants mature and become richer in lignin [7]. As plants grow, DM content increases, fiber content increases, and protein content reduces [7]. Samples of cool-season pastures were collected at both the beginning of the grazing cycle and the end of the grazing cycle, with care taken to avoid collecting plants in the flowering stage. This selection accounts for the high protein content observed in the materials.

The present study showed negative correlations of DM and NDF degradation with contents of ADF, NDF, ADL, and DM and the ADF: NDF ratio. The ADL, ADF, and NDF fractions are more resistant to microbial degradation in the rumen relative to other feed components, such as starch and proteins. The ADF–NDF ratio indicates the proportion of lignin in the fiber comprising the plant cell wall. The higher the ADF–NDF ratio, the greater the presence of lignin in the fibers and, consequently, the lower the DM digestibility. Jung and Lamb [26] associated lignin as the primary factor limiting the in vitro digestibility of NDF. However, the impact of lignin on fiber digestibility is more significant in grasses compared to legumes. Lignin serves as a crucial anti-nutritional component in grasses, with its negative effects being more pronounced in grasses than in legumes due to the localized deposition of lignin in legumes [27].

The fragility of forage particles influences the filling effect caused by NDF [7] and impacts the rate of particle breakdown and the rate of rumen renewal of feed. There is a positive association between NDF digestion capacity and forage fragility [28]. The fragility of forages can be influenced by the lignin content, digestibility, and anatomical characteristics of different plant species, such as cell wall thickness. Therefore, the digestibility of the forage cell wall may be an indicator of its fragility, as highlighted by Grant [8]. However, in the present study, a negative correlation was observed between fiber fragility and degradation coefficients. As discussed earlier, forages with higher lignin content exhibit lower digestibility and greater fiber fragility.

In general, the most effective equations for predicting fiber fragility incorporate digestibility coefficients with negative estimates as the primary predictor, as previously described. When focusing solely on chemical composition variables, fragility estimates are derived from CP and ADL contents. Specifically, for cool-season pasture, this includes ADF, while, for con silage, it includes ADL.

5. Conclusions

Corn silage showed greater fragility compared to cool-season pastures. However, the ADF–NDF ratio did not differ between the materials. Fragility is directly related to fiber content and inversely related to digestibility. In this study, degradation coefficients were the most reliable predictors of forage fragility. More samples and experiments are needed to validate the findings observed in this study.