The ideal dairy cow for pasture-based systems.

Key characteristics of the ideal grazing dairy cow include (1) excellent reproductive efficiency and longevity, (2) a propensity for high voluntary DMI of grazed pasture which is efficiently converted to output, (3) the ability to achieve high output of valuable products (i.e., milk and meat) per unit area from grazed pasture, (4) excellent health, (5) easy care, (6) low environmental footprint, (7) being robust to environmentally induced perturbations without compromising output, and (8) producing progeny that can readily be finished early off pasture (i.e., achieve the desired carcass weight and fat cover with minimal supplementation). Genotype-by-environment interaction is a phenomenon where the traits' performance difference between 2 genotypes (i.e., individual animals, strains, breeds) differs between environments (i.e., pasture versus confinement dairy production systems). This phenomenon may manifest either as a reranking of genotypes for suitability (i.e., reranking) or as a change in the magnitude of their differences, with the most suitable genotype always excelling irrespective of the environment (i.e., rescaling). Although marked genotype reranking for individual traits is generally limited across environments, rescaling of the trait variances, combined with environment-specific differences in (1) which traits matter and (2) the relative importance of these traits, can lead to important reranking on overall dairy cow total merit indexes. Substantial exploitable genetic variation exists across a range of cow-level traits that determine suitability for pasture-based production systems. However, limited routine access to sufficient phenotypic data for certain traits continues to constrain progress toward delivering the ideal cow for such systems.