I was in Indianapolis a couple weeks ago for the annual Dairy Science meetings and went to some interesting talks. My favorite was one on a study where they looked at the cows milk production with respect to the sex of the calf she had given birth to.
They looked at data from over 2.5 million lactations and found that cows that gave birth to heifer (female) calves produced significantly more milk over the course of the lactation. To start with only the first calf and subsequent lactation were analyzed then second calvings and lactations were added to the mix. What was suprising was that the sex of the first calf impacted the milk production of the second lactation. Cows that had a heifer as their first calf produced more milk in their second lactation regardless of the sex of their second calf. The cows that had heifers both times produced the most milk in their second lactation.
So why would this matter to a farmer; isn't calf sex basically just chance?
Nope not necessarily.
While with traditional practices it is basically a coin flip (48% female, 52% male) processes have been developed to sort sperm by sex chromosome. There is commercially available semen for dairy cows that is 90% X (female) bearing sperm.
So theoretically an enterprising farmer could get his cows to produce more milk in the future by using sexed semen, especially to attain the first pregnancy.
Calf sex influences whole-lactation milk and component production in Holstein cows. A. J. Carpenter, K. Hinde, J. S. Clay, and B. J. Bradford