Genetic trend in milk fat percent is highly responsive to the relative economic value of milk fat and milk protein in the New Zealand dairy sector

Abstract

Milk components are important traits within Breeding Worth (BWSI), the national selection index of the New Zealand dairy industry. Breeding Worth is an economic index, and trait weightings are calculated based on the economic value (EV) that each trait contributes to a dairy farm business. The EVs are updated annually, but key parameters like the absolute and relative values of milk solids, and the relative value of milk fat and milk protein are included as a 5-year rolling average to mitigate against volatility in animal rankings. In 2015, the global price for milk fat began to increase, and between 2015 and 2022 the value of milk fat relative to milk protein rose rapidly year on year. The increase in the dollar value of milk fat relative to milk protein was gradually mirrored in BWSI weightings, and although the response in milk fat yield was modest, a clear inflection in the genetic trend for milk fat percent was observed from 2018 onwards. We sought to understand the drivers for this change in the rate of genetic gain, especially given the multi-breed composition of the NZ dairy herd. For a trait like milk fat percent, which differs between the Jersey and Holstein-Friesian breeds, NZ farmers can use both breed substitution and within-breed selection to alter the performance of their herd. We report that both breed substitution and within-breed selection appear to have contributed to the genetic trend in milk fat percent. The use of Holstein-Friesian sires declined between 2015 and 2020 in favor of Holstein-Friesian cross Jersey sires. Similarly, a 15 year decline in the use of Jersey sires was reversed in 2015, with a small increase recorded between 2015 and 2020.  The within-breed response to the change in relative weightings on milk fat and milk protein is also notable, demonstrating the power of coordinated selection towards a breeding objective. Our findings highlight the importance of carefully considering the approach used for determining weighting factors within a selection index, especially for traits as responsive to selection as milk fat.