4.2. Weed Species Composition during the Research
The usual composition of maize weed flora for the researched agroecological areas was recorded during the experiment [
66]. Grassy annual weed species,
E. crus galli and
S. viridis, detected at both experimental sites are defined as the most common and competitive weeds in maize, with a high adaptability level to different environmental conditions [
66]. The pronounced presence of the broadleaf perennial species
C. sepium, frequent in maize, was noticeable in both experimental areas, which is probably a consequence of the suitable soil conditions for the development of this weed. Moist soil with clay silt and silty loam soil texture favors
C. sepium, which is resistant to dry periods at the same time [
67,
68]. Of numerous weed species,
S. officinale was specific only to the experimental site Križevci (ES 2), which is probably the result of favorable soil texture (silt). The mentioned weed species is an indicator of moderately moist soil, tolerates occasional stronger wetting and drying, and is widespread in soils of a lighter texture with variable humidity [
62].
F. convolvulus appeared only at the experimental site Čačinci (ES 1) in soil with a higher clay content, which is consistent with [
69], and the site-specific occurrence of certain weed species was also confirmed by Pätzold et al. [
70].
Ecological indices for soil acidity, which represent a significant factor for weed growth, classify the majority of identified weed species for both experimental sites in a weekly acidic or neutral soil class (pH 4.5–7.5) with wide ecological amplitude [
62]. Although specific soil reactions can be a limiting factor for weed development [
64], warm season weeds can appear in a relatively wide pH range (4.8–6.4) [
71], and the optimal pH for many weed species is about 5.5 [
72]. The occurrence of certain weed species is probably conditioned by pedoclimatic specificities of experimental sites and the high ability of weeds to adapt to various environmental factors.
4.3. Maize Weediness
Different pedoclimatic conditions of the investigated areas had a significant influence on the maize weediness indicators. The greater average weed occurrence in terms of weed density and weed coverage for the experimental site Križevci (ES 2) in the first observation (V7 maize growth stage) was a possible result of the present weather conditions. Weather conditions were highlighted as the dominant factor affecting weed emergence [
73], and the variation in environmental factors was cited as more important than tillage systems in terms of the impact on weed species diversity and weed density [
74]. The optimal amount of precipitation at the very beginning of maize vegetation enabled the emergence and development of weeds, whose competition was more pronounced in later dry conditions, especially in June with a marked lack of precipitation with above-average temperatures. Although unfavorable weather conditions have a negative influence on weed species development [
65], drought stress during maize vegetation increased weed competition, despite requiring similar abiotic conditions as agricultural crops [
75]. Weeds show better physiological resistance to drought with the excessive use of water that allows them to grow in conditions of pronounced lack of water in the soil [
76].
Maize weediness had a dynamic increase among two observations, with the exception of weed density, which was higher for ES 2 during the first sampling, and different weed densities of various soil types were also confirmed by Ervio [
69], while Salonen [
77] found variation in weed biomass between other soil types. Excessive amounts of precipitation in July at the experimental site Čačinci (ES 1) led to a significantly higher weed biomass (WB) and weed coverage development (WC) compared to ES 2. This is probably the result of soil waterlogging caused by weak infiltration because of the common impermeable layer on poorly draining Stagnosol, which leads to a lack of oxygen in the soil [
78]. In the specified environmental stress conditions, weed competition is more pronounced compared to crops [
76] due to their marked adaptability to different unfavorable conditions [
79].
Tillage intensity is an important factor that influences the level of weediness, which, in conservation and conventional tillage systems, depends on the plant production system, soil, and climatic conditions. Tillage had a significant effect on maize weediness throughout the vegetation season, with the exception of weed density in the R5 maize growth stage. A significantly higher average weed density with conventional tillage in the V7 maize growth stage may be the result of the weed seed incorporation into deeper soil layers that provide more moisture for weed germination and development when insufficient soil moisture is present [
80], as in the case of this research when there was a lack of precipitation in May. In conventional tillage systems, the seeds are distributed more or less evenly throughout the soil layer, while in reduced systems, a large part of the seed is concentrated on the surface of the soil [
81].
The influence of soil tillage on weed density is variable, which has been confirmed by various studies [
29,
81,
82], and it is also visible from this research where the influence of soil tillage on average weed density in the second sampling (R5) was not recorded. Higher weed densities using conventional tillage were confirmed by Shrestha et al. [
83]. Onwards, plant residues on the soil surface in conservation tillage can have a suppressive effect on weed emergence by changing the physical environment at the very beginning of the maize vegetation, while later in the growing season, by conserving soil moisture and nutrient release, they can stimulate the growth of weeds [
84]. Shallow conservation tillage (CTS) resulted in the highest average weed aboveground biomass (WB), weed coverage (WC), and weed species number (WSN) compared to conventional tillage in maize vegetation, with a noticeable trend of increasing the level of weediness during the growing season, which is consistent with [
23,
24,
28]. Significantly higher weed biomass in reduced tillage compared to conventional tillage, with an increasing trend from the beginning of the vegetation season to the later stages, was also proven by Hofmeijer [
28], while the increase in weed coverage and weed species number was also confirmed with previous research [
31,
35,
85]. Reduced tillage can lead to increased incidence and the development of perennial weeds [
28,
30]. The average occurrence of weeds with respect to the life cycle differed among soil tillage systems in this research. By reducing the intensity of soil tillage, the average number of perennial weeds increased through maize vegetation. Shallow conservation tillage (CTS) had the highest average incidence of perennial weeds, and a greater perennial weed density using reduced soil tillage compared to conventional was confirmed with previous research in maize [
29,
86]. More pronounced changes in weed composition and the appearance of perennial weeds can be expected with longer conservation tillage implementation [
24], which also applies to the increasing biodiversity [
34].
Significant experimental site and soil tillage interactions were present in the case of weed density (WD) and weed coverage (WC), which were significantly higher using conventional soil tillage for the experimental site Križevci (ES 2). According to Derksen [
10], agroecological conditions and soil tillage are important factors that influence weed occurrence with emphasis on annual grasses. Greater weediness using conventional soil tillage for ES 2 was probably conditioned by the soil type (Gleysol) and a lighter soil texture (silt), which enabled the better emergence and development of weeds. Additionally, the most numerous weed species for ES 2 using conventional tillage was
S. viridis, and a greater incidence of annual grasses associated with plowing that are site-specific has been previously reported [
25,
70].
Weed occurrence in agricultural fields is mainly affected by anthropogenic activity, and various soil improvement management is directly aimed at reducing weediness. The liming of acidic soils, along with a positive effect on the chemical, physical, and biological soil properties can also affect the reduction of weediness of agricultural crops [
48] and thus can have a positive effect on increasing crop productivity [
87]. In this research, liming performed on moderately acidic (ES 1) and weakly acidic (ES 2) soils had a positive influence on average weediness decreasing. All investigated weediness indicators were, on average, lower with the liming application with the existence of an increasing trend in the level of weediness during the growing season. The statistically significant influence of liming on the reduction of weediness in the V7 maize growth stage was determined for weed density (WD) and weed coverage (WC), while later in maize vegetation (R5), liming had a significant effect on weed aboveground biomass (WB), weed coverage (WC), and weed species number (WSN). It can be assumed that liming positively influenced the competitive ability of maize, which led to an average reduction in weediness similar to the findings of Skuodiene [
47], in which weed species number, weed density in the earlier vegetation crop growth stage, and weed biomass in the maturity stage of the crop were significantly lower in limed soils. Contrary to the above, few research studies have reported a greater diversity of weed species and a higher weed density and biomass caused by different liming treatments [
88,
89]. Significant experimental site and liming interactions were recorded during maize vegetation, where lower average weed coverage was determined for the experimental site Čačinci (ES 1) with liming application, as well as lower weed biomass, but only at the R5 maize growth stage. The positive influence of liming on the soil property improvement was more pronounced in moderate acidic Stagnosol, which led to a decrease in the competitive ability of weeds compared to maize. Liming can have a contrasting effect on weediness, which may depend on the specific weed species and soil properties. By reducing soil acidity, liming can make it less suitable for some weed species development, which was confirmed by Stephenson et al. [
72], indicating that the optimal pH for many weed species is about 5.5. Liming led to lower weediness using the shallow conservation tillage system (CTS), which significantly decreased weed coverage in the V7 maize growth stage and weed density in R5. This was probably caused by the positive impact of conservation treatment in acid soil management [
41], since it has a positive effect on the soil structure and biological activity, while plant residues additionally protect the soil and reduce weediness, which is additionally expressed with the liming application and its positive influence of chemical, physical, and biological soil properties. The positive influence of conservation tillage on the soil pH reaction was proven by the research of Ligowe et al. [
42], which reported higher soil pH values when conservation tillage was applied, which is why the positive influence of liming was additionally pronounced for CTS soil tillage treatment and reducing weed competition.
4.4. Maize Yield
Agroecological conditions, agricultural management, genetic background, and other often limiting factors in different agroecological regions are key factors for maize yield formation [
90]. The experimental sites had a significant impact on the final average maize yield in this research. Maize vegetation during the research was followed with variable weather conditions with unfavorable rainfall and temperature patterns. A pronounced lack of precipitation with above-average high temperatures was present in the critical early growth stage of maize for both experimental sites (ES 1 and ES 2) and continued until the late vegetative growth stage (tasselling/silking), in which the optimal temperature and available water are of great importance for the grain formation [
90]. However, sufficient soil moisture in Križevci (ES 2) led to better conditions for maize sowing due to the water supply from April, and the emergence and initial growth of maize was better for ES 2. Adverse weather conditions continued through July for the experimental site Čačinci (ES 1), with an excessive amount of rain that resulted in soil waterlogging. This was caused by weak infiltration due to the impermeable layer present in poorly draining Stagnosol, which leads to a lack of oxygen in the soil [
78]. Waterlogging can cause a significant stagnation in growth and reduction in yield depending on the duration [
91], in particular, to the sensitivity of maize with insufficient oxygen concentrations, which prevent optimal root function. The amount of precipitation at the experimental site Križevci (ES 2) in July was slightly above average, and in August, the lack of precipitation was expressed at both investigated sites. Variable environmental conditions at the research sites, along with specific soil properties, resulted in a significantly lower average maize yield for the experimental site Čačinci (ES 1). Despite varying weather conditions, maize yields were satisfactory for ES 1 and above average for ES 2, which was probably conditioned with luxury fertilization that was conducted for both poor supplied soils. Optimal nutrient supply (N, P, K) strongly affects the maize yield when drought is present [
92,
93] through water-use efficiency increasing, stomatal regulation, and photosynthesis activity [
94,
95]. Moreover, it is likely that weed pressure during maize vegetation did not interfere significantly with the final maize yield.
Yield differences with respect to soil tillage treatments confirm maize sensitivity to changes caused by soil tillage [
96]. The reduction in soil tillage intensity and depth resulted in the highest average maize yield for the shallow conservation tillage (CTS), which is in line with Sime et al. [
97], who reported a 14–19% higher maize yield under a conservation tillage system compared to a conventional system, and similar was cited by much other research [
39,
40,
41].
Conservation tillage has a broad effect on the soil water regime and water availability of plants [
98], which had a positive effect on the maize yield increase in adverse weather conditions during the research. Residue management in conservation tillage systems stands out as an important factor affecting root development, water and nutrient availability, and yield increase, with pronounced positive effects in drier environmental conditions [
99,
100].
At the experimental site Čačinci (ES 1), the highest maize yield was obtained using CTS soil tillage, with a significant difference (
p < 0.05) compared to ST, while at Križevci (ES 2), there were no significant differences among CTS and ST. This is probably a consequence of site-specific soil properties, where the positive impact of conservation treatment in acid soil management [
41] was pronounced for ES 1, which had higher soil acidity. Higher average maize yields for both experimental sites using CTS compared to CTD indicated the importance of higher soil cover (50% for CTS), which significantly affects water conservation and greater water use efficiency while reducing water losses [
101], especially in drier conditions on heavy or light soils. Deep conservation tillage CTD had the lowest average yield, and a maize yield decrease in deep reduced tillage has been confirmed before [
98].
Liming application led to an increase in the average maize yield due to many positive effects on soil properties, which allowed more favorable conditions for crop growth [
44,
45,
46]. The proven liming effect refers to improving the rooting systems, availability and uptake of nutrients, the water supply, and thus, better drought resistance, which were probably reflected by the increase of maize yield in this research. Crops showed better drought resistance under liming treatment, and higher maize yields with the liming application were also reported by several researchers [
41,
53,
102,
103].
Although the changes in the weed flora in conservation systems were already recorded after the first year of research in some previous studies [
104], as well as an increased corn yield with the application of liming [
53], future research will certainly contribute to a better understanding of the conservation system influence on crop productivity and the implementation of adapted and specific measures in weed control that should be based on the reduced use of herbicides.
Furthermore, the comparability of the results of the one-year experiment in different agroecological conditions with long-term research can contribute to a better understanding and greater acceptance of the transition to CA by farmers.