Assessment of environmental descriptors for studying genotype by environment interaction

Access to individual performance records opens up possibilities for treatment of genotype by environment interaction in international genetic evaluations. The ability to distinguish similar from dissimilar environments is thus important. Reaction norms were used in this study to evaluate several variables for their suitability to classify production environments for studies on genotype by environment interaction. Reaction norms describe the phenotypic expression of a genotype as a function of the environment, and can be modelled by random regression on environmental descriptors. Fifteen variables that measure aspects of management, genetic composition and climate were computed on herd level. Suitability of each variable to detect genotype by environment interaction was assessed through likelihood ratio tests for significant linear or quadratic random regression terms. First lactation records from approximately 40 000 Guernsey cows in four countries (Australia, Canada, USA and Republic of South Africa) were used. Variables with significant effect were: herd size, within-herd standard deviation of lactation yield, peak milk yield, persistency, days to peak production, calving pattern, age at first calving, rate of maturity, and annual rainfall. Genetic correlation between extreme environments for milk peak yield, within-herd standard deviation and annual rainfall were lower than 0.91, indicating that re-ranking of animals may occur. (C) 2003 Elsevier B.V. All rights reserved.