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BackgroundGenotype by environment interactions are currently ignored in national genetic evaluations of dairy cattle. However, this is often questioned, especially when environment or herd management is wide-ranging. The aim of this study was to assess genotype by environment interactions for production traits (milk, protein, fat yields and fat and protein contents) in French dairy cattle using an original approach to characterize the environments.MethodsGenetic parameters of production traits were estimated for three breeds (Holstein, Normande and Montbéliarde) using multiple-trait and reaction norm models. Variables derived from Herd Test Day profiles obtained after a test day model evaluation were used to define herd environment.ResultsMultiple-trait and reaction norm models gave similar results. Genetic correlations were very close to unity for all traits, except between some extreme environments. However, a relatively wide range of heritabilities by trait and breed was found across environments. This was more the case for milk, protein and fat yields than for protein and fat contents.ConclusionsNo real reranking of animals was observed across environments. However, a significant scale effect exists: the more intensive the herd management for milk yield, the larger the heritability.

Genotype by environment interactions are ignored in national genetic evaluations of dairy cattle. However, some breeders consider that the genetic merit of top sires is not correctly estimated for use in their own particular environment or with their own herd management. With the objective of later investigating genotype by environment interactions at the national level, we studied the relationship between herd management (e.g., feeding system, herd size, production, workforce) and herd-test-day (HTD) profiles for milk yield and contents of fat and protein using a database of 934 herds. Herd-test-day profiles, estimated using a test-day model, are thought to reflect herd management because they represent the part of production due only to month-to-month variations in environmental conditions of production; that is, those related mainly to feeding and climate conditions. Herd clustering based on results from a factor analysis of descriptors of HTD profiles was performed. Each cluster of herds was then characterized in terms of herd management. Three herd clusters were identified. Cluster 1 was characterized by low HTD profiles for milk yield and protein and fat contents as well as a low level of intensification. Cluster 2 was defined by a high milk yield HTD profile and a high level of intensification. Cluster 3 consisted of herds with HTD profiles showing high protein and fat contents but it was difficult to link this cluster to any specific herd management. The use of clusters based on HTD profiles as descriptors of distinct herd management systems will be used in a genotype by environment interaction study in France.

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