1. Introduction
Barley is unique among crop plants and is of tremendous importance to agriculture; it is the fourth most important cereal crop in the world, after maize, rice, and wheat [
1], and in Europe, approximately 12 million ha are cultivated with barley [
2]. Barley grain, in the form of malt, is a perfect nutritional source of yeast, which is very important for the brewing industry. In Germany in 2020, approximately 75% of the harvested spring barley grain was delivered as malting barley [
3]. In addition to achieving high yields, specific quality criteria for malting barley must be met to optimise industrial processes; barley grains are considered suitable for the malting and brewing industry if they have a protein content between 9.5% and 11.5% of dry weight, and if more than 90% of the harvested grains (i.e., the grain retention fraction) are larger than 2.5 mm [
4]. The price paid to farmers is much lower for grains not meeting these requirements, which can be downgraded to feed barley [
3]. However, the relationship between grain yield, grain size, and protein content is generally negative [
5]. To meet the malt quality requirements, therefore, grain yield and quality parameters of malting barley must be balanced. Furthermore, there has been a significant amount of work done to evaluate the factors influencing the malt barley yield and quality including different varieties, environmental conditions and agronomic practices (e.g., [
6,
7,
8,
9,
10,
11,
12]). Molina-Cano et al. [
6], based on 11 studies, showed contradictory results regarding genotype × environment interaction, and concluded that generally, the environmental effects had more influence on total variation than their interaction with genotypes. Therefore, it is necessary to evaluate the genetic variation of yield and quality traits of regionally available varieties, as they will be influenced by environmental conditions and the relationships between yield and quality traits.
The yield of small grain cereals is a product of two components: grain number and grain weight. Previous studies have demonstrated that while grain number in barley varies widely with location and season and typically accounts for the majority of the variation in yield across environments [
10,
13,
14,
15,
16,
17,
18,
19], the mean grain weight and size tend to be less variable and poorly correlated with yield [
10,
12,
13,
14,
20]. Because large grains provide a greater malt extract potential for the beer industry, grain size is an important parameter for malting quality, and is closely associated with the mean grain weight [
16]. Although grain weight is often considered to be the most stable yield component of barley [
13], significant genotypic and environmental variation can occur [
7,
8,
9].
It is known that, during the pre-anthesis period, the number of grains per spike and the number of spikes per m
2 is determined [
21,
22]. In contrast, it has been generally accepted that grain weight and size are primarily determined during the post-anthesis period [
16,
23,
24]. Grain weight and grain size were positively associated with the amount of radiation intercepted per grain during the stage of grain filling in the post-anthesis period [
25,
26] but were reduced by drought and heat stress [
27,
28,
29]. Water stress during the grain filling period has a negative effect on barley grain weight and size, primarily due to a reduction in the grain filling duration [
30,
31,
32,
33], and low radiation likely reduces the grain filling rate [
16,
17,
26,
31,
34]. The climate in the investigated region is also characterized by variability in the incident radiation over different years during the summer months [
29,
35], and thus, the natural occurrence of such an effect on the grain size of malting barley during post-anthesis is plausible. In particular, climate change in the past years has led to more frequent heat spells and droughts during summer [
29,
35]. In order to ensure yield stability and quality under variable weather conditions, a better understanding of the control of grain number and size and their response to genetic variation between years will play a crucial role in determining the possible limits to yield and quality of malting barley. Since the weather in different years varies greatly, we considered the year effect as the environmental effect in this study.
The grain protein concentration in cereal crops is dependent on a multitude of factors, including N supply, N uptake before anthesis, remobilization to the grain during grain filling [
11,
36,
37,
38], environmental conditions such as temperature and water stress [
28,
39], and genotype × environmental interactions [
40,
41]. Many of the above factors make it difficult for malting barley growers to control or implement practical methods to produce grain fulfilling the correct end-use specification. For spring malting barley grown in southern Germany, the Bavarian State Research Center for Agriculture (LfL) has recommended 120–140 kg N ha
−1 for spring malting barley. However, little research has been conducted on how years varying in air temperature, radiation, and precipitation affect the genotypic variation in grain yield, quality character, and yield components of spring malting barley, or what the relationships between yield and yield components under the recommended N fertilisation are.
Therefore, the aim of this study was to characterise the genotypic variation in grain yield, quality properties, and yield components which contribute to variations in grain yield and quality from 23 spring malting barley varieties in a two-year experiment.
4. Discussion
The average grain yield of spring malting barley from 23 genotypes in the current study was about 6.4 t ha
−1 in 2014 and 6.6 t ha
−1 in 2015, respectively (
Table 3 and
Table 4), which was slightly higher than the average yield of spring barley in the same years across different regions in southern Germany. The Bavarian State Research Center for Agriculture has conducted several long-term experiments in different regions in southern Germany, and accordingly reported that the grain yield of spring barley in 2014 and 2015 reached more than 6 t ha
−1 across the Bavarian State, which was the highest level recorded, compared to the average yield of about 4.8 t ha
−1 between 2004 and 2014 [
43,
44].
Our study showed a strong correlation between grain yield and grain number m
−2 in both years (
Table 5 and
Table 6), supporting previous findings in the literature across a range of environments [
10,
13,
16,
18,
19,
45,
46]. Furthermore, in this study, grain yield was not significantly associated with grain weight (
Table 5 and
Table 6). Gallagher et al. [
13] and Bulman et al. [
20] reported a similar finding that the mean grain weight tended to be less variable and was poorly correlated with yield.
Grain number per m
2 is the product of the number of spikes that remain at crop maturity with grain-bearing spikes that depend on tillering at early growth stages and tiller abortion in the later growth stages, and the number of grains per spike. An increase in grains per m
2 of wheat varieties from breeding progress in past decades was primarily attributed to increases in spikes per m
2 with little variation in grains spike
−1 observed, while similar studies in warmer regions such as Argentina have reported significant variation in grains spike
−1 among genotypes [
25]. The results from the present study demonstrate that there was a strong relationship between grain number m
−2 and spikes m
−2, while a poor correlation was found between grains m
−2 and grain spike
−1 (
Table 5 and
Table 6), which was more similar to the behavior of wheat genotypic variation in a West European climate. Therefore, this finding suggests that, to further increase the grain yield of spring barley under weather conditions in southern Germany, a high spike number per m
2 should be achieved during the vegetative growth stages.
The number of spikes at harvest depends on many developmental factors, including plant establishment, tillering dynamics, and tiller abortion to anthesis. In 2014, for example, the weather was favorable for spring barley. After a very dry and mild winter (the third warmest since weather records began in 1881 [
43]), the plants began to grow very early that year. Similarly, in 2015, the warm and dry weather in March caused the soil to dry quickly, which enabled sowing of spring barley in good time and under good conditions. However, the grain number per m
2 was higher in 2015 than in 2014. This was due to a higher spike number per m
2 and more grains per spike in 2015. The lower number of spikes per m
2 in 2014 may have been due to drought periods in April, May, and June (
Figure 1). In April, for example, the precipitation was approximately 30 mm in 2014 but was about 60 mm in 2015. A dry period also occurred in the middle of May, 2014. Drought periods during vegetative growth stages may have inhibited tillering in 2014 compared to 2015. Although the 23 spring barley genotypes were registered in different time periods and regions (
Table 1), this study surprisingly showed no significant genotypic variation in grain yield (
Table 3 and
Table 4).
Grain protein content is one of the most important factors in marketing malting barley. The primary objective is to maintain grain protein content between 9.5% and 11.5%. Studies in the literature have shown that grain N in cereals mainly represents N supply and N uptake in the vegetative organs until anthesis and the translocation of N reserves to grains during the grain filling phase [
22,
47,
48]. Studies have revealed that approximately 90% of N reserves translocates to grains during the grain filling phase [
47,
48]. Compared to the contribution of N translocation to grain N at the final harvest, the N uptake before anthesis may play a more important role in wheat [
49], whereas Bulman and Smith [
49] found genotypic variation in N uptake and translocation to the grain during the process of grain filling in barley. In this study, the N fertilizer application rate for spring malting barley was based on the official recommendation for southern Germany. Although the N supply rate was even lower in 2015, 22 barley varieties among 23 obtained protein content between 9.5% and 11.5% in 2015. The protein content varied with genotypes, ranging from 9.3% to 11.4%. In contrast, the protein content of only two varieties, IPZ 2427 and Union, reached more than 9.5% in 2014, with the protein content of the 23 genotypes ranging from 8.3% to 9.8%. Since IPZ 2427 is a new variety that has not yet been registered, while Union was registered in 1950 (
Table 1), our results suggest that the protein content from all modern spring barley varieties in 2014 could not meet the quality requirements for the brewing industry.
Owing to the higher N supply compared to that in 2015, the low protein content in 2014 seemed primarily to be associated with the N uptake before anthesis. Our results revealed that N uptake before anthesis was about 82 kg N ha
−1 in 2014 and 99 kg N ha
−1 in 2015, meaning the N uptake was 20% lower in 2014 than in 2015. Drought is a major factor that inhibits both N uptake and translocation. Compared to the weather in 2015, drought spells appeared more often in 2014, especially in April and May, the period before anthesis (
Figure 1). After anthesis, a further drought period occurred in June 2014 (
Figure 1), indicating that a reduction in N translocation in 2014 could also not be excluded.
Overall, this study suggests that in order to ensure that the protein content meets the quality requirement, the N application rate at the same site should vary with year when weather is different between years, suggesting that the environmental effects had more influence on total variation than genotypic effects. Currently, sensing technology is available for in-season N fertilisation of field crops. Differences in N status can already be detected in early and late tillering stages [
50]. Barmeier et al. [
51] and Barmeier and Schmidhalter [
52] reported that spectral sensing techniques can be used to recommend a more targeted N application for spring barley, since this not only allows for the detection of actual growth and N status, but can also be used to estimate soil nitrogen mineralisation [
53], which could be used for a targeted second N application correcting for possible nitrogen deficiency and avoiding surplus nitrogen fertilisation.
Although grain size contributed less to grain yield in both years, grain size is one of the most important parameters for malting barley. To meet the quality requirements of the brewing industry, the proportion of the grain with size > 2.5 mm must be greater than 90%. In contrast to no genotypic variation in grain yield, a significant effect of genotype and year was clearly observed on grain size (
Table 2), which is in agreement with the results of a previous study on the effects of genotypic and environmental factors on grain size [
5]. More interestingly, however, all barley varieties in 2014 showed more than 90% of grains that were larger than 2.5 mm, while only about 60% of the varieties in 2015 showed 90% grains that were greater than 2.5 mm. Since the grain weight and size are primarily determined during the post-anthesis period [
16,
17,
23,
24], this is likely due to the inhibition of grain filling during post-anthesis. Water and heat stresses and low radiation during the grain-filling period have a negative effect on barley grain weight and size. Drought and heat stress mainly induce a reduction in grain filling duration [
20,
30,
31,
32,
33], whereas low radiation likely leads to a reduction in the grain filling rate [
16,
17,
19,
26,
31,
34], as up to 90% of the grain dry matter of barley is acquired by photosynthesis during grain filling [
54,
55]. From the beginning of anthesis to dough ripening in 2015, the radiation intensity was 25% lower compared to the same period in 2014. In addition, the higher spike number per m
2 in 2015 may result in higher shade between plants. Kennedy et al. [
26] reported that the shade between plants could cause a reduction in grain size during grain filling. In 2015, the plants received more rainfall in June than in 2014. These results may indicate that radiation may play an important role in grain size during grain filling of spring barley. From the long-term experiment, the study by the LfL concluded that barley usually shows higher yield and better quality under sunny and drier conditions than in constantly cool and humid weather with a good water supply [
43,
44]. Furthermore, although there was a lack of evidence to show that drought and heat stress impacted grain filling in 2015, the grain filling duration was shortened because anthesis in 2015 began one week later than in 2014.