Reaction norms used to quantify the responses of New Zealand dairy cattle of mixed breeds to nutritional environment

A reaction norm, where phenotype or genotype is expressed as a function of environment, can be used to measure environmental sensitivity. This study investigates the environmental sensitivity of dairy cattle genotypes (or breeds), in relation to the range of herd milksolid yields (a proxy for feeding level) in New Zealand. Genetic variation in environmental sensitivity was observed with overseas Holstein Friesian genotypes, originating largely from North America, exhibiting higher environmental sensitivity levels and environmental optimums than New Zealand Friesian and Jersey genotypes in a range of feeding level environments. Overseas Holstein Friesian genotypes were more likely to be considered specialists (i.e., perform to high levels in superior feeding level environments), whereas New Zealand Friesian and Jersey genotypes were largely generalists (i.e., tolerant to changes in environment), or specialists in a grassland‐type environment. This confirmed the theory that generalist genotypes evolve in heterogeneous environments, whereas specialist genotypes evolve in homogeneous environments. However, variations in environmental sensitivity within genotypes did exist. Generalist genotypes appear to be indirectly selected in New Zealand as environmental sensitivity levels are decreasing. Potential ways in which environmental sensitivity information can be used in dairy cattle systems and evaluation are suggested.