Why do fertility traits in livestock usually have low heritability?

In the context of animal breeding, fertility traits—such as the number of offspring in a litter or the number of services required for conception—typically exhibit low heritability.1

This low heritability occurs because a significant portion of the variation in these traits is driven by environmental factors rather than additive genetic differences. In livestock, reproduction is often heavily influenced by external conditions, including nutrition, care, and climate. Furthermore, there is often a negative genetic correlation between fertility and production traits. For instance, in high-producing dairy cows, a large amount of metabolic energy is partitioned toward milk production, which can make it more difficult for the animal to conceive. Because these environmental and physiological demands exert such a strong influence on reproductive outcomes, the proportion of total phenotypic variation attributable to genetics remains relatively small.231