Environmental sensitivity in New Zealand dairy cattle

Area of Science:

• Animal Breeding and Genetics
• Dairy Science
• Environmental Adaptation

Background:

• Understanding genetic by environment interactions is crucial for effective animal breeding programs.
• Sire reranking, where the genetic merit of sires changes across different environments, can complicate breeding decisions.
• New Zealand's diverse agricultural environments necessitate an evaluation of their impact on dairy sire performance.

Purpose of the Study:

• To quantify within-breed sire reranking for milk production traits in New Zealand's varied environments.
• To assess the impact of environmental factors like milk solids yield, heat load index, herd size, and altitude on sire rankings.
• To determine if separate breeding schemes are required for different environmental conditions in New Zealand.

Main Methods:

• Characterizing herd environments based on milk solids (MS) yield, heat load index (HLI), herd size, and altitude.
• Applying single-trait and bivariate sire models to estimate genetic parameters and correlations.
• Analyzing genetic correlations between sires in extreme environmental character states.

Main Results:

• Sire reranking for milk production traits was minimal, with genetic correlations around 0.9 between contrasting environments.
• Significant differences in MS yield (227 vs. 376 kg/cow) and HLI (61.4 vs. 69.6) showed low sire reranking.
• Estimated breeding value correlations in extreme environments were generally low but consistent with prediction reliability.

Conclusions:

• New Zealand's environments are not sufficiently diverse to necessitate separate breeding schemes for dairy sires.
• Current breeding strategies can effectively rank sires across the range of encountered environmental conditions.
• The study supports a unified approach to dairy cattle breeding in New Zealand, optimizing genetic gain for milk production traits.