**Abstract**

The main source of incomes in a dairy farm is milk sales, and any factor altering the production affects the farmers' income significantly. According to the Trivers-Willard hypothesis, if the cows' systems are generally good and offer competitive conditions, they produce more milk for bull calves. They also suggest that cows in a worse condition or of a genetically diverging strain invest more milk in heifer calves. The existence of a sex-bias in cows' milk production remains controversial even if it would open new insights on the economic impacts of using sex-sorted semen to enhance farm productivity. Sex-biased milk production in cows can vary, favoring one sex or the other and, sometimes, none. It seems to favor females in intensive production systems, while in other less intensive systems, this effect seems to disappear. This chapter intends to address available evidence on the sex-biased cows' milk production and discuss why further research forecasting this issue is needed, including other cattle populations and correlating the investment strategy with an animal welfare index. Besides, other factors, such as different housing and feedings, can impact the calf-sex milk production bias through pathways still to be understood.

**Keywords:** sex-biased milk production, secondary-sex effects, cattle, production system
