Evaluation of agronomic traits

The agronomic characteristics evaluated were number of tillers, nodes on the main stem and plant height. Measurements were taken from ten plants selected at random from the middle of the two central rows of each plot. The number of tillers was determined 35 d after sowing. Nodes were counted during the spiking stage and plant height was measured just before physiological maturity from ground level to the last grain of the spike on the main stem.

Assessment of yield and grain physical quality

To determine grain yield, the two central rows of each plot was threshed. Once the gain was cleaned, it was weighed, recording moisture content and hectoliter and thousand-grain weight. Grain yield was expressed in t ha^-1 adjusted to 13 % moisture, the average of three samples. Moisture was determined with a digital moisture tester (BURROWS, model DMC750, Chicago, USA). Hectoliter weight (PHL) in kg hL^-1 was determined with the test tube method described in the Mexican norm NMX-FF-043-SCFI-2003 (^{Secretaría de Economía, 2003}). For the weight of one thousand grains (PMG), eight replications of 100 grains were counted and weighed. With these data, average, variance, standard deviation and coefficient of variation (CV) were calculated. The procedure was repeated until the CV was equal to or greater than 4. PMG was obtained by multiplying the arithmetic mean of the eight replications (^{ISTA, 2005}).

Statistical analysis

With the data, an ANOVA, comparison of means with the Tukey test (p≤0.05), and correlations were carried out using ^{SAS (2009)} software.

Agronomic characteristics

The number of tillers (NM) was determined (p≤0.05) by three factors and the interaction FSxC (Table 1). NM was higher with the genotypes were sown at the end of autumn and tillering coincided with cool temperatures in December and early January (Figure 1). In contrast, NM decreased when sowing was at the beginning of winter (Table 2) because of the gradual increase in temperature at the end of January and early February, which coincided with that development stage.

Table 1 F test statistical probability of the analysis of variance for agronomic characteristics, grain physical quality and grain yield of ten barley genotypes sown on five sowing dates of two agricultural cycles in El Bajío, Mexico. 

*Significant (p≤0.05); NS: not significant (p>0.05); FV: factor of variation; CV: coefficient of variation; GL: degrees of freedom; FS: sowing date; C: cycle; G: genotype; NM: number of tillers; NN: number of nodes; AP: plant height; PHL: hectoliter weight; PMG: thousand-grain weight; REN; grain yield.

Figure 1 High and low temperatures in Celaya, Guanajuato, Mexico, during the agricultural cycles 2012-2013 (year 1) and 2013-2014 (year 2). 

Table 2 Agronomic characteristics and grain physical quality of ten barley genotypes sown on five sowing dates in two agricultural cycles in El Bajío, Mexico. 

†Means in a column and variation factor with different letters, are statistically different (Tukey; p≤0.05). DMSH: least honest significant difference; FS: sowing date; C: cycle; NM: number of tillers; NN: number of nodes; AP: plant height; PHL: hectoliter weight; PMG: thousand-grain weight. FS represents the average for G in both C. C represents the average of FS and C in each C.

The number of tillers in the 2012-2013 agricultural cycle was 25 % higher than in the other cycle evaluated. Often, cool temperatures favor tillering (^{García and García, 1995}), but this is also in function of the efficient use of water, nutrient availability and solar radiation (^{Hussain et al., 2013}).

The number of tillers was higher in the genotypes developed for dryland cultivation: genotype M-177 had the highest number (Table 3). In the genotypes developed for irrigation, this characteristic was limited, manifesting the influence of genetic aptitude of each genotype (^{Tamm, 2003}). Other factors that influenced in the number of tillers per plant were sowing date and cultivar (^{García and García, 1995}).

Table 3 Agronomic characteristics and grain physical quality of ten genotypes sown on five sowing dates in two agricultural cycles in El Bajío, Mexico. 

†Means in a column and variation factor with different letters, are statistically different (Tukey; p≤0.05). DMSH: least honest significant difference; G: genotype; NM: number of tillers; NN: number of nodes; AP: plant height; PHL: hectoliter weight; PMG: thousand-grain weight. G represents the average of each G in FS and C.

The number of nodes (NN) was significant (p≤0.05) due to the factors FS and G (genotype), and the interaction FSxG (Table 1). Genotypes sown on FS3 had the highest NN, 8 % higher than that of genotypes sown on FS5, which had the lowest NN (Table 2). The genotypes Adabella and M-176 had the highest NN (Table 3). Generally, NN is in function of the cultivar. Each node is a meristematic zone from which leaves differentiate. In turn, leaves contribute to the accumulation of biomass and therefore to yield. Besides influence from genetic factors, NN also depends on agrometeorological factors (^{López, 1991}).

Plant height (AP) was significant (p≤0.05) by effect of the factors FS and G, and of the interaction FSxC (agricultural cycle) (Table 1). Plants sown at the beginning of winter had lower AP values (<80) (Table 2). The genotypes developed for dryland farming had higher AP than those developed for irrigated conditions, with the exception of Alina (Table 3). Genetically, low stature is desirable in genotypes generated for irrigated farming to prevent winds from lodging the plants during irrigation. AP is a morphological trait determined by the cultivar (^{Alam et al., 2007}) and is often affected by environmental conditions (^{Tamm, 2003}).

Grain yield and physical quality

Low temperatures during the 2012-2013 agricultural cycle affected yield and physical quality of the grain. Temperatures fell to 0.4, -0.5 and 0.8 °C on March 3, 4, and 5. The low temperatures 109 days after FS1 did not apparently harm grain development. However, 94 d after FS2, they affected grain physical quality (thin grains), and 79 d after FS3, because of the cold temperatures, seeds did not develop possibly because most of them were in an aqueous stage. With FS4, the low temperatures occurred 64 days when the crop was between the boot stage and spiking; there was damage to the foliage, and seeds were absent in the spikes of the earliest genotypes (M-173, Armida and Alina). With FS5, the low temperatures occurred 48 days when most of the genotypes were in the boot stage, and damage was observed only in the foliage.

Hectoliter weight (PHL) was determined (p≤0.05) by three factors and the interactions FSxC and FSxG (Table 1). With FS3, PHL was 7 % higher than that of FS5, which had the lowest PHL value (Table 2). This was likely the consequence of the stress caused by temperature increases in April and May (Figure 1), which coincided with grain filling and reduction of the biological cycle of the crop. The 2013-2014 agricultural cycle produced a higher PHL than the other cycle (Table 2) because of the low temperatures during sowing in the autumn of 2012, when PHL decreased. This showed the effect of climatic variations between the two years. Alina had the highest PHL, 5 % above the genotypes Adabella, Esperanza and M-173 (Table 3); the differences between genotypes were significant. Industrial quality and grain yield of barley is better with high PHL (^{López et al., 2005}). Generally, grain weight is determined by the duration of the grain filling stage (^{García del Moral et al., 2003}), but it decreases with high temperatures during this stage. (^{Alam et al., 2007}).

Thousand-grain weight (PMG) was also significant (p≤0.05) by effect of the three factors and the interactions FSxC, FSxG and FSxCxG (Table 1). Sowing in autumn obtained heavier grains; FS1 had the highest PMG, which was 9 and 19 % higher than FS4 and FS5, which had the lowest values. The PMG of Armida and Alina was 10 % higher than Adabella and M-173, which were the lowest. In the 2013-2014 agricultural cycle, PMG was 4 % higher than the other cycle evaluated (Table 2) because of the climatic variations between the two years studied. Generally, cultivar influences thousand-grain weight (^{Soleymani and Shahrajabian, 2012}). Grain physical quality is better when the crop develops in cool temperature environments because the time of dry matter accumulation is longer and there can be more soil moisture since evapotranspiration decreases (^{Copeland and McDonald, 1995}). In contrast, when the crop develops in high temperatures, PMG is usually lower since the weight of individual grains decreases (^{Hossain et al., 2012}). Other factors that influence grain weight are the quality of intercepted light and the position of the grain on the spike (^{Copeland and McDonald, 1995}).

Grain yield (REN) was determined (p≤0.05) by the three factors and the interactions (FSxC and FSxG (Table 1). In general, establishing the crop at the end of autumn allowed greater expression of REN. Nevertheless, REN of barley sown on FS3 in the 2012-2013 agricultural cycle decreased 35 %, relative to the value found in 2013-2014 (Table 4), due to the low temperatures in 2012-2013. REN of the crops sown at the beginning of winter decreased in both cycles evaluated. With the exception of FS5, in the 2013-2014 agricultural cycle, REN was higher than in the other evaluated cycle (Table 5). This is related to cold injury from low temperatures in March 2013. In both agricultural cycles the lowest REN was obtained with FS5 (Table 4) because the increase in temperature reduced the number of days to physiological maturity and, therefore, there was less time to accumulate reserves in the grain.

Table 4 Grain yield of barley genotypes sown on five sowing days in two agricultural cycles in El Bajío, Mexico. 

†Means in a column and variation factor with different letters, are statistically different (Tukey; p≤0.05). DMSH: least honest significant difference; FS: sowing date.

Table 5 Grain yield (t ha^-1) of ten barley genotypes, sown on five sowing dates in two agricultural cycles in El Bajío, Mexico. 

Means in a column and variation factor with different letters, are statistically different (Tukey; p≤0.05). DMSH: least honest significant difference; 𝑋 : mean; FS: sowing date.

The results of this study coincide with those of ^{Solís et al. (2004)}, who evaluated wheat cultivars in El Bajío. They found no differences in grain yield when sowing was between November 15 and December 15. However, when sowing was on January 15, grain yield was lower because the number of grains per spike decreased and the grains were smaller.

For small-grained cereals to have good expression of grain yield, the cultivars should be grown in relatively cool temperatures (^{García del Moral et al., 2003}). For this reason, when crops are established in periods that are not recommended, grain yield often decreases (^{Aslani and Mehrvar, 2012}).

^{Mendoza et al. (2011)} mention that grain yield is affected largely by sowing date, mainly because of the effect of temperature variation. Sowing wheat at the end of autumn in El Bajío permits higher grain yield, while sowing at the beginning of winter exposes the crop to high temperatures during the reproductive stage and thus shortens the period of grain formation leading to lower yields (Suaste- Franco et al., 2013).

The varieties Alina and Armida had the most outstanding yields (Table 5). These are varieties developed for the study region and recently released (^{Solano et al., 2009}; ^{Zamora et al., 2010}). There are cultivars recommended as suitable for the study area, but it is important to be familiar with the performance of genotypes developed for dryland farming since barley seed is produced in El Bajío.

In both agricultural cycles, the genotypes produced higher REN when sown on FS1 and FS2, and some genotypes, such as Alina and Armida also did so on FS3. However, when sown on FS4 and FS5, REN decreased considerably. This variation is generally due to the genotypes’ adaptations that are specific to given environmental conditions ^{Bolandi et al., 2012}). The genotypes selected for production with irrigation had higher yields than the genotypes selected for dryland and varied more in their behavior among sowing dates and growing cycles (Table 5). Similar results were reported by ^{Saad et al. (2013)} with barley cultivars in different environments.

The correlations showed that when NM, PHL and PMG increased, REN also increased (Table 6). ^{Hossain et al. (2012)} found similar results. Moreover, REN was higher when NN and AP increased; this is attributed to the association of REN with plant dry matter accumulation since there is higher availability of assimilates for grain formation (^{López-Castañeda, 2011}). In this study, expression of potential grain yield was influenced by environmental conditions. In both growing cycles evaluated the genotypes expressed higher NM, NN, AP, PHL, PMG and REN when sown on FS1, FS2 and FS3, but when sown on FS4 and FS5, these values decreased. Sowing on optimum or suitable sowing dates contributes to obtaining higher REN (^{Aslani and Mehrvar, 2012}). However, REN decreases principally when sowing occurs in periods that coincide with high temperatures during grain filling (^{Soleymani and Shahrajabian, 2012}).

Table 6 Mean squares of the correlation analysis for agronomic characteristics, yield and grain physical quality of ten barley genotypes sown on five sowing dates in two agricultural cycles in El Bajío, Mexico. 

*Significant (p≤0.05); NS: not significant (p>0.05); NM: number of tillers; NN: number of nodes; AP: plant height; PHL: hectoliter weight; PMG: thousand-grain weight; REN; grain yield.