Synergy between selection for production and longevity and the use of extended lactation: insights from a resource allocation model in a dairy goat herd.

Although most of the genetic progress in production efficiency is achieved through selection at a global scale, locally, farm managers can also influence the selection process to better match genotypes and their varying herd environment. This study focused on the influence of a particular management decision--the use of extended lactation (EL) in dairy goat production systems--as it affects the survival and reproduction rates at the herd level, which may then shape different long-term selection responses. The objective was to understand and quantify the influences of EL and variability in achieved intake level on the responses to selection for production, reproduction, and longevity. An animal model of resource allocation between life functions was applied to the dairy goat. It predicts the trajectory of change in the herd genetic composition as affected by the feeding level and the selection pressure applied by the manager. During 40 yr, goats were selected for milk yield, reproduction, and, with a different selection weight for age (WAGE), for longevity. Under varying achieved intake levels, increasing WAGE improved the survival rate but a nonlinear effect was observed for the average milk yield and BCS. When moderately increasing WAGE from 0, resources were reallocated from lactation towards body reserves and survival, which led to a trade-off at the herd level between improving survival and BCS and increasing milk yield. When further increasing WAGE, old females became systematically preferred regardless of their reproductive status and the proportion of EL in the herd increased. Females undergoing EL had reduced energetic costs of reproduction, which improved their probability of survival. Across generations, an increased herd incidence of EL led to a relaxation of the selection pressure on the resource allocation to body reserves, which is normally imposed by the manager's priority to achieve successful reproduction at each mating. As selection for longevity progressed, the incidence of high-producing females increased within the herd, driving a long-term trend in increased milk production. Thus, the use of EL as a management tool led to an alleviation of the trade-off between milk yield progress and survival improvement. Although the model simplifies the underlying physiology of nutrient allocation, it provides insights into how farm manager strategies can influence the development of genotype × environment interactions and promote herd robustness.