The most common genes for semidwarf habit in modern wheat (Triticum aestivum L.) cultivars are found at the Rht‐B1 and Rht‐D1 loci on chromosomes 4B and 4D, respectively. An alternative gene for semidwarf habit, Rht8, has shown potential as a replacement for Rht‐B1b and Rht‐D1b in some environments. The objective of the present study was to assess the impact of the height‐reducing gene Rht8 relative to Rht‐B1b and Rht‐D1b on performance of spring wheat in Montana and Washington environments characterized by terminal drought stress. Evaluation of near‐isogenic lines developed in four genetic backgrounds showed that Rht‐B1b, Rht‐D1b, and Rht8 caused height reduction of 19, 20, and 6.5%, respectively, relative to wild‐type near‐isogenic lines over 12 environments. An increase in grain yield was associated with reduced height for lines containing Rht‐B1b and Rht‐D1b based on means over the four genetic backgrounds and 10 environments. Height reduction and yield increase associated with Rht‐B1b and Rht‐D1b were significant in most environments. Lines with Rht8 yielded less than wild‐type based on means over environments and in 3 of 10 individual environments. Reduced height lines with Rht‐B1b and Rht‐D1b tended to have a higher harvest index and more seed per spike than wild‐type lines and reduced height lines with Rht8. In sum, our results suggest that Rht‐B1b and Rht‐D1b are superior to Rht8 as a source for height reduction for spring wheat in the tested environments.

Crop seed size affects the competitive relationship between spring wheat (Triticum aestivum L.) and wild oat (Avena fatua L.). However, the mechanisms associated with the process are not known. The effect of wheat seed size on wild oat competition was assessed by a mechanistic approach involving yield and its determinants in these species. Wheat plants established from large and small seed were evaluated under different seeding rates and wild oat densities during 1999–2001 near Kalispell, MT. Linear structural model systems based on ontogenic diagrams were constructed for each seed size class. Spikes m−2 and panicles m−2 had the greatest positive effect on yield within each species. For wheat, the impact of the two later‐formed yield components on yield decreased in an ontogenic manner, whereas for wild oat, their relative contributions were similar in magnitude. Wheat plants derived from large seed had a noticeable negative effect on wild oat via a reduction in panicles m−2 and seed weight, whereas wheat established from small seed mainly affected wild oat panicles m−2 Wild oat competition reduced wheat spikes m−2 and kernels spike−1 in both seed size classes. However, these reductions were less for plants derived from large seed, which demonstrated enhanced compensatory ability. In summary, nongenetic variations in crop seed size affected the competitive dynamics between these species, where the major crop–weed interference mechanism involved wild oat seed weight.

Dryland maize (Zea mays L.) production in the U.S. western High Plains is hampered by variable yields because of substantial environmental variation in this region. This study was conducted to determine the degree to which the ranking of superior maize hybrids for dryland production in the western High Plains was predictable from performance of the same hybrids in highly productive, irrigated environments in the same region. Forty‐five maize hybrids were evaluated for grain yield performance under different water regimes in western Nebraska, eastern Wyoming, and northeastern Colorado in 1998 and 1999. The value of genotypic variance was by far larger in fully irrigated test environments (0.70) than in nonirrigated test environments (0.01–0.17). The genotypic mean repeatability in fully irrigated test environments (0.63) compared with that in nonirrigated test environments (0.18–0.69, respectively), and it showed correspondence with yield performance. The genetic correlation between fully and nonirrigated environments (0.72) was lower than that observed between all‐nonirrigated environments (0.78–1.02). Thus, the proportion of direct advance in the former case (0.63) was generally lower than in the latter (0.41–0.97). However, an environmental similarity ratio (ESR) derived from crossover interaction indicated that water‐contrasting environments were as similar (ESR = 0.53) as nonirrigated environments (ESR = 0.49) in ranking the maize hybrids. Selective identification of maize hybrids in irrigated environments for production under nonirrigated environments in the western High Plains might be a useful surrogate to direct selection in the latter environments.

Crossover interactions occur in evaluation trials when ranks of cultivars change across environments. Determining groups of environments within which crossover interactions are minimized may facilitate making cultivar recommendations. The goal of this research was to test a new approach for determining these environmental groups in which crossover interaction between a pair of cultivars was defined across all environments. The number of groups was based both on reduction in crossover interaction and repeatability of cultivar means within groups. The validity of this procedure was tested on three simulated data sets with known crossover interactions. For each data set, the approach divided the environments into the two groups that minimized crossover interactions. The approach also was applied to yield data from a maize (Zea mays L.) trial in which 59 environments previously had been clustered by a different measure of crossover interaction. Three groups of 12 environments and one group of 23 environments were defined. The previous clustering had identified six clusters. The amount of crossover interaction within the four environmental groups was reduced by 53% from the total crossover interaction in all 59 environments. The results of the clustering depended on whether all pairwise comparisons among cultivars or only the significant crossover interactions among the higher yielding cultivars were used. The latter method was deemed more appropriate when the goal is to recommend specific cultivars for specific groups of environments. Regardless of the approach used, clustering based on crossover interactions only has practical significance if these interactions are repeatable.

Maize (Zea mays L.) production in the western High Plains of the USA occurs under dryland and irrigated conditions. Thus, an ideal maize hybrid for this region would have stable and high average performance across environments that are highly variable in productivity. Stability of a hybrid in this research was defined as its across‐environmental variance. Nine commercial single‐cross hybrids and 36 related double crosses were evaluated for grain yield in four irrigated and eight dryland environments in Nebraska, Colorado, and Wyoming. Environmental means ranged from 2.10 to 12.01 Mg ha−1 The average superiority of the single crosses over the double crosses in mean grain yield was 11.5%. However, the double crosses were generally more stable than the single crosses. This was true regardless whether stability was measured across all 12 environments or only across the eight dryland environments. The greater stability of the double‐cross hybrids in the dryland environments was the result of fewer extremely low or high yields compared with the single crosses. Hybrids were ranked considering both stability and mean performance with a safety‐first index. Values of this index depended on the value chosen for acceptable minimal yield. Across all 12 environments, a double‐cross hybrid ranked the best when the minimal acceptable yield was <2.68 Mg ha−1, and across the eight dryland environments this happened when the minimal acceptable yield was <1.88 Mg ha−1 Possible use of other types of heterogeneous maize hybrids in the Western High Plains are discussed.