Milk production of the mother is associated with the birth weight rather than the sex of the calf

The amount of milk that mammals produce can be different after giving birth to offspring of different sexes. Usually, if there is any sex-bias, giving birth to male offspring leads to a higher milk volume and/or milk with a higher energy content. However, a study on US dairy cows showed higher 305-day milk yield after giving birth to female calves. The aim of our study was to investigate the effect of the sex of the calf on the subsequent 305-day milk production in Dutch dairy cows. Milk production records of 1,615,765 lactations of 861,273 Holstein Friesian cows from 7,303 herds throughout the Netherlands were used for analysis. Plain averages suggest that giving birth to male calves results in higher 305-day milk production: differences up to 90 kg are found in lactation 2 and 4. This changes dramatically when including gestation length, lactation length, calving ease and birth weight as fixed effects, and sire of cow and herd-year-season as random effects in the analysis. The effect of the sex of the calf on the subsequent 305-day milk yield is significant only for lactation 1 and 3, with a small positive effect after giving birth to a female calf in the first lactation and a small positive effect after giving birth to a male calf in the third lactation. The effect of the birth weight of the calf is significant in all lactations and seems more substantial: a calf of 55 kg instead of 25 kg at birth is associated with a 305-day milk production that is about 800 kg higher. What at first glance appears to be an effect of the sex of the calf is actually a confounding effect of the birth weight of the calf, because male calves tend to be heavier than female calves. It is not clear whether previous reports of sexbiased milk production can also be attributed to differences in birth weight of the offspring of different sexes.