The use of fixed shelling percentage biases genotype selection in hybrid maize multi-environment yield trials

Context or problem: Phenotyping is an integral part of plant breeding operations. In many cases the trait measured is not identical to the target trait for reasons of speed and or cost. This is a form of indirect selection, where correlation between the trait measured and the target phenotype influences the rate of genetic gain. Low correlations lead to slow rates of genetic gain. In sub-Saharan African maize breeding programs, maize grain yield in breeding experimental plots is measured as a field weight (FW), which includes the grain and cob. The weight of grain from each plot is estimated as a standard proportion of grain to total ear weight using a shelling percentage of 80 %. This approach assumes that there is no genetic, environment or genetic by environment interaction in shelling percentage which, if present, would contribute to slower rates of genetic gain for grain yield. Objective or research question: This study investigated the magnitude of genetic and environmental variation in shelling percentage and its impact on selection in six hybrid maize multi -environment yield trials in Ethiopia over two seasons. Methods: The data of shelled grain weight (SW) and cob weight (CW) from the trials were analyzed using a bivariate linear mixed model. Results: Genetic variances for both traits varied across the six testing sites ranging from 0.199 to 2.975 for SW and from 0.029 to 0.245 for CW. The genetic correlations between pairs of sites for SW and CW also varied, indicating the existence of genotype by environment interaction for these traits. Additionally, the bivariate regressions between FW and SW indicated there was substantial genetic deviation around the 80 % shelling response, and this relationship was impacted by environmental influences. Conclusion: The use of a constant relationship of 80 % shelling biases grain yield prediction in multi -environment hybrid maize yield trials and thus reduces the rate of genetic gain in maize breeding programs. Implications or significance: Taking into account the variations in the shelling percentage of the genotypes across sites in predicting grain yield from field weight improves the accuracy of genotype selection and the rate of genetic gain in maize breeding programs.