Calf-sex bias in Holstein dairy milk production under extensive management

The economic impact of sex-biased milk production in the dairy industry, with the possibility of using sexed semen, can be of great importance to the sector. Studies performed in different countries about sex-biased milk production in the dairy industry reached different conclusions. The objective of the present work was to determine how the sex of the calves affected milk production, in extensive non-seasonal dairy production, where the animals are mainly kept outdoors, grazing all day long, with a moderate milk production. The data analyzed consisted in 45,712 lactation records, from 25,018 animals, coming from 367 farms, retrieved from 2009 to 2017. The cows under study had 2.85 +/- 1.78 recorded lactations, with a milk production of 8,414 +/- 2,155 kg and a protein and fat content of 3.23 +/- 0.24% and 3.68 +/- 0.56%, respectively. Considering the recorded lactations, after calving a male or a female, cows roughly produced the same amount of milk and protein content. However, the content of fat was slightly higher when they calved females (P < 0.001). The results obtained here, in conjunction with the ones reported in previous studies, point to the conclusion that there can be a sex biased milk production in dairy cows, nevertheless, this can vary, favoring one sex or the other and, sometimes, none. This bias seems to favor females in intensive production systems, while in other systems, like the one observed in Azores, Portugal, this was not observed.