NATURAL-SELECTION EFFECTS IN WHEAT POPULATIONS GROWN UNDER CONTRASTING TILLAGE SYSTEMS

Use of no-till, a conservation-tillage management system, is increasing in the northwestern USA, to abate the serious soil erosion problem. This study examined the feasibility of improving the adaptation of winter wheat (Triticum aestivum L.) for no-till by natural selection. Five populations with genetic diversity for several traits, some of which presumably affected fitness of wheat under no-till, were studied. Two subpopulations were developed for each population by growing them during 1981 to 1985 in plots that had been rotary tilled (till) or directly sown (no-till) into standing barley (Hordeum vulgare L.) stubble at Pullman, WA. Trait means of the till treatment were always higher than their no-till counterparts, with the exception of one trait in a single population. Results did not indicate strong or consistent pressure for differential natural selection between the till and no-till treatments for several agronomic traits in most populations. The population that was putatively the most genetically diverse (Semidwarf Common Wheat USDA Blend) showed significant response to differential natural selection after pooling data across generations; both subpopulations had increased (P less-than-or-equal-to 0.05) biomass and grain yield means in the environments from which they had been derived. Plant height diverged in another population, with tall and short genotypes favored in the till and no-till environments, respectively. Most populations probably lacked sufficient genetic diversity to respond to differential natural selection for most of the traits; a high proportion of their parentage was derived from locally adapted genotypes that had been selected under conventional tillage. Using no-till selection pressure on genetically conservative populations that typify most wheat breeding programs probably is unwarranted. However, with populations that have been intentionally developed to achieve a broad genetic base, this approach may be justified.