Invited review: milk production and reproductive performance: modern interdisciplinary insights into an enduring axiom.

A general belief across the dairy community, both scientific and commercial, is that of an antagonistic association between milk production and reproductive performance of dairy cows. In this article, we critically review the evidence supporting this belief and discuss some of its limitations. Based on the fundamental principles of experimental design and inference, we consider relevant issues that, although critical to the very foundation of the perceived production-reproduction antagonism, seem to have been previously misrepresented or overlooked. In particular, we focus on issues of confounding, randomization, nature of inference, single- versus multiple-trait modeling, cow- versus herd-level modeling, and scope of inference, all within the context of dairy production systems. Taken together, these issues indicate that the production-reproduction antagonism may not be as pervasive as previously believed, suggesting the need for more rigorous methods of scientific investigation on this matter. We revisit the association between milk production and reproductive performance using a novel interdisciplinary approach based on cutting-edge statistical methods that accommodate some of the unique and previously ignored features of this problem. In fact, recent work supports a highly heterogeneous association between milk production and reproductive performance, whereby heterogeneity is partitioned across several scales and driven by many contributing factors, both physiological and managerial. We conclude that the relationship between milk production and reproductive performance is not necessarily that of a universal homogeneous antagonism and suggest better ways to study and even manage this association. A more comprehensive assessment that draws expertise from multiple scientific disciplines will be required to elicit management recommendations targeted to effectively optimize overall performance of dairy cows and commercial herds.