Cows pregnant with a daughter produce more milk
Using 2.39 million lactation records from 1.49 million dairy cows, the research team from Harvard University (Dr. Katie Hinde) and Kansas State University (Dr. Barry Bradford) showed that the sex of the fetus influences the capacity of the cow to synthesize milk during lactation.
Cows favor daughters, producing significantly more milk for daughters than for sons across lactation. Using a sub-sample of this dataset, they further demonstrated that the effects of fetal sex interact dynamically across parities, whereby the sex of the fetus being gestated can enhance or diminish the production of milk during an established lactation. The sex of the first calf is particularly important, and can influence how much milk their later siblings get. For example (Figure 1), a cow is pregnant with Calf 2 while she’s still producing milk for Calf 1: if Calf 1 is a son, the cow has persistently decreased milk production, especially if she has another son the second-time round. If she has a daughter next, that partly reverses the deficit, but not completely. By contrast, having a daughter the first time round ‘protects’ a cow from the negative effects of a second-round son. Specifically, gestation of a daughter on the first parity increases milk production by 445 kg over the first two lactations. That’s almost a 3% bump in milk production. These results showed a dramatic and sustained programming of mammary function by offspring in utero.
Figure 1. A) First-parity cows having a daughter produced significantly more milk than those having a son, but gestating a daughter in pregnancy 2 increased milk production in cows that had a son first. B) Second-parity milk production is greatest in cows that had a daughter in pregnancy 1. Additionally, cows with a son in pregnancy 1 showed increased milk production if they had a daughter in pregnancy 2. Data based on 113,750 cows (label: S = son, D =daughter).
Why do cows invest more in daughters? We don’t know the answer, but it’s possible that the extra milk could speed up a daughter’s development, allowing her to reproduce at a younger age and amass more babies over her lifetime. On the other hand, bulls start reproducing later than females, so they’ve got more time to make up for any shortfall in milk. In dairy cows, mothers may be able under-invest in a son with relatively less consequence without affecting the number of future offspring. Another possible explanation provided by Dr. Hinde is that daughters are better at manipulating their mothers into making more milk by making the right hormones. Female foetuses produce more oestrogens that could cross the placenta into the mother’s bloodstream, and influence the development of her breast glands.
The use of sex semen by dairy producers to ensure most of the calves they breed are females may be a cost-effective way to improve milk production.
