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Why is it difficult to improve both milk production and fertility in dairy cows at the same time?

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Red Poll cows — Animal breeding Red Poll cow and calf.

Improving both milk production and fertility in dairy cows simultaneously is challenging due to a negative genetic correlation between the two traits.

When traits are negatively correlated, selecting for one often hinders progress in the other. In the case of dairy cattle, this difficulty arises primarily from the metabolic demands of high milk production. As breeders select for increased milk yield, the cows prioritize the partitioning of nutrients toward production, particularly during early lactation. This large metabolic investment makes it physiologically more difficult for high-producing cows to conceive, resulting in lower fertility rates.1

Furthermore, these traits differ in their heritability. Milk production is considered to have intermediate heritability, making it relatively responsive to selective breeding. In contrast, fertility traits generally have low heritability, meaning they are more strongly influenced by environmental factors and are harder to improve through genetic selection alone. Consequently, focusing on one trait often necessitates a trade-off, where gains in production may lead to a reduction in fertility or vice versa.1