3.2. Estimation of Harvest Date, Weather and Location
The DM content, averaged across all hybrids, all years and both locations, was 371 ± 42.3 g.kg
−1. Comparing this with the literature recommendation [
28] for the appropriate harvest time, harvest was slightly delayed. Bal et al. [
28] determined that for maize, the optimum stage for silage production was at the two-thirds milk line stage of the grain (with some flexibility to vary between the one-quarter and two-thirds milk line), i.e., between 324 and 350 g.kg
−1 DM. Carpentier and Cabon [
2] and Peyrat et al. [
29] recommended a similar interface, namely DM content of 300 to 350 g.kg
−1. Maize was only harvested in this DM range in 2020 (
Table 5). The higher DM content of whole maize plants (371 g.kg
−1) measured by us indicates harvesting in two-thirds of the milk line, which corresponds to the achievement of average starch (294 g.kg
−1 DM) and aNDF (448 g.kg
−1 DM) content. Bal et al. [
28] reported similar average values for starch (287 to 372 g.kg
−1 DM) and for aNDF (444 to 405 g.kg
−1 DM).
Agriculture is vulnerable to climate change at the global level. Climate change is a major concern and has adverse impacts on food production, food quality and food security [
13]. The growing season temperature record (
Table 2) was higher than the normal long-term weather range from 1981 to 2010 at both sites, L1 and L2. The growing season temperature trends were similar at both sites, but with the difference that temperatures were lower at location L1 than at L2 each year, even within the normal long-term weather range from 1981 to 2010. Growing season temperatures were higher in both locations during the drier years of 2018 and 2019 than in 2020 and 2021. The fact that 2021 was an exceptionally cool spring (April and May) may have influenced the results.
While L2 has always been warmer than L1, less precipitation has fallen there (
Table 3), even in the normal long-term weather range from 1981 to 2010. For the first two years of the experiments (2018 and 2019), extremely little precipitation fell at both sites, and even less at L2 than L1. Compared to the normal long-term weather range of 1981 to 2010, very little precipitation fell in April in all four years at both sites. It turned out that each year, the weather pattern had a slightly different effect on the chemical composition of the hybrids (
Table 5). In 2018, there was significantly lower starch content but higher aNDF and CP content. In 2019, there was significantly lower ash content but higher DM and CP content. In 2020, there was significantly lower DM and ash content but higher starch, aNDF and CP content. In 2021, there was a significantly lower aNDF, CP and ash content but higher DM and starch content. In 2018 and 2019, years with higher average temperature and lower rainfall, the CP content was significantly higher when compared to 2020 and 2021. The influence of climatic events, i.e., mainly drought, is highlighted by, e.g., Golbashy et al. [
30] and Kumar et al. [
31].
The fact that weather also influences other indicators is confirmed by several scientific papers. Hussain et al. [
32] found that, although there was a poor correlation between the FAO group and the percentage of borer damage, a stronger positive correlation was found between the percentage of damage and the average air temperature in June, ranging from 20 °C to 24.5 °C, and was negatively correlated with relative humidity, ranging from 50% to 80%.
Maize is a sensitive crop to drought and heat stress, especially if it occurs unevenly during the reproductive stages of development [
33]. The importance of the interaction of weather patterns during the growing season and the characteristics of the growing site has been emphasised by, e.g., Commission Regulation [
20]. Although at first glance the growing conditions for maize were very similar at both L1 and L2, there were differences between them that affected both the results of chemical analyses and nutritional quality indicators. The only factor that did not differ between the two sites was BMP. In the plot in Prague (L1), the DM, starch, NDF and ash contents were statistically lower than in Troubsko (L2). Conversely, the aNDFD, NEL and MPP contents were statistically higher (
Table 6). The differences in climatic conditions for each year are shown in
Table 2 and
Table 3. In L1, temperatures were lower and precipitation was higher than in L2 (and also in the normal long-term weather range) in all years during the growing season. According to Bažok et al. [
33], the co-occurrence of drought and heat is more severe for maize growth than a single stress.
3.3. The Influence of the Hybrid
The effects of the type of maize hybrids and their maturity stage on their yield and, for cattle, their nutritional characteristics have been presented by many authors [
34,
35,
36,
37,
38,
39,
40].
Table 7 shows the main characteristics of the maize hybrids we tested. The hybrids are ranked according to the FAO number that characterises the expected maturation period, with the first two hybrids (H1 and H2) belonging to the ‘early’ hybrids and the remainder belonging to the ‘early–medium’ hybrids. The small difference in the earliness of the hybrids (maximum of 40 FAO degrees) was chosen so that they could be harvested at a uniform date, which is what happened in both locations in each year. The plan for the harvest date was to follow the recommendation that the optimum grade for silage corn was a two-thirds milk line with some flexibility between a quarter and two-thirds [
20]. Harvest maturity at the two-thirds milk line of grain is also recommended by Marchesini et al. [
1]. Their research showed that at the same harvest date, the earlier hybrids measured higher DM and were predicted to have better fermentation results when used for silage. When earlier hybrids are harvested at the same date as those with higher FAO numbers, it can be assumed that a lower yield will be obtained [
41]. Their conclusions were obtained from 822 samples of fresh maize plants from hybrids of early and late classes harvested at three maturity stages (early, middle and late), for three consecutive years and in three locations with different soil fertility.
The largest difference in DM at harvest, 15 g.kg
−1, was between H3 and H4. This difference was between hybrids with different senescence patterns, with H3 being the only hybrid tested to have standard senescence (NSG). The other hybrids tended to be ‘stay-green’ (SG), which is a term used to describe genotypes that have delayed leaf senescence compared to reference genotypes [
42]. This was also reflected in the highest aNDF content and aNDFD digestibility (
Table 7). In addition, it is worth noting that SG hybrids are advantageous to grow because they have a ‘wider harvest window’, i.e., they can be harvested over a longer period without significant changes in the DM content and chemical composition of whole maize plants.
For the same type of hybrid (i.e., stay-green), the stage of grain maturity and the whole plant DM is closely related. When comparing the different types of hybrids (e.g., stay-green and dry down), this relationship may be different. Differences in DM content, nutrients and digestibility are also given by proportion grain (ear) to the other parts of the plant [
43].
Hybrids for testing were chosen so that the differences between them were not only in earliness. The difference between hybrids was also in grain type (H2 and H3 semi-flint, H1 and H4 turning to flint [
44]). However, this parameter probably did not have a significant effect on the results. The mode of ripening (senescence) may have had a greater effect; only H3 ripened uniformly, with the other hybrids having slight SG traits.
According to chemical composition, hybrid H1 had significantly lower CP and higher starch and ash content; hybrid H2 had significantly lower starch and ash content and higher aNDF and CP; and hybrid H3 had significantly lower starch and CP content and higher DM, aNDF and ash content. By maturity stage, the medium–late hybrid H3 had significantly lower starch and CP content and higher DM, aNDF and ash content.
The digestibility of aNDFD was lowest for H1, which was also reflected in NEL and MPP. This agrees with the statement of Jimenez et al. [
45] that due to higher NDF digestibility, maize tends to have higher NEL and a higher estimate of potential milk production per hectare and per ton DM (MPP), determined using the MILK 2006 program according to Shaver et al. [
8]. These indices (aNDFD, NEL and MPP) were higher in the later hybrids (H3 and H4). Hybrids H2 and H4 are intended not only for silage but also for use in biogas plants. However, this is not evident from the results; on the contrary, hybrid H4 had the lowest gas production. Agricultural practice requires more detailed and accurate research data and information. For farmers, choosing the right maize hybrid is crucial. Sometimes, however, under practical conditions, some characteristics of hybrids as declared by their sellers are not always apparent. In particular, the biogas maize ideotype is very difficult to find among current hybrids [
46].
3.4. Correlation between Nutrient Characteristics of Hybrids and Factors That May Have Influenced the Evaluation of Hybrids
Table 8 shows the correlations by year, location and hybrid characteristics. Relationships were analysed using the Pearson’s correlation coefficient (r, 1 or −1 depending on whether the variables were positively or negatively related [
47]). The r coefficient values for correlation were interpreted according to Prior and Haerling [
48]: very strong correlation (±0.91 to ±1.00); strong correlation (±0.68 to ±0.90); moderate correlation (±0.36 to ±0.67); weak correlation (±0.21 to ±0.35); and negligible correlation (0 to ±0.20). The colder and wetter the year, the higher the starch content (positively, moderately related) and the lower the aNDF and CP content, respectively, and the lower the BMP (negatively, strongly related). The cooler and wetter the site, the lower the DM and the higher the ash content but the lower the NEL and BMP. The BMP was not affected by location. The later the hybrid, the higher its energy value and MPP. While the correlations by years, locations or hybrids were significant mainly at the level of significance
p < 0.001, the correlations by hybrid character were significant at
p < 0.01. The starch content only slightly positively increased with the trend of being endosperm flint while being NSG. The SG hybrids had lower aNDF but higher aNDFD. The aNDFD, NEL and MPP values only weakly positively increased along with the trend of being endosperm flint and being NSG. This again supports the conclusions made by Herrmann and Rath [
46] that higher aNDFD tends to result in higher NEL and MPP in maize.