**5. Calving difficulty**

Calving difficulty is higher with larger calves [79]. It is also known that there is a higher frequency of dystocia in male calves' birth [60, 74, 79]. Dystocia significantly reduces the whole lactation milk yield [40, 50, 54, 69, 75–77], besides increasing veterinary treatment costs [76], and reducing cow fertility. After dystocia there is a higher incidence of metritis [77], ketosis [80, 81], both associated with a decrease in milk production. Also, an easy calving presumably leads to a higher milk production because it is associated with reduced stress and pain during calving, consequently leading to a lower energy imbalance that can cause more metabolic disorders [41].

Male calves are typically larger than females, and pose a greater risk of dystocia [1, 79, 82]. However, Hinde et al. [1] reported that sex-biased milk synthesis remained when analysis was restricted to a subset of females without record of dystocia, and included information on individual cows across the first and second parity, favoring females.

A Danish study found different results. Farmer assisted calving were associated with a higher milk yield while cows with no farmer assistance or with veterinary assistance during the most recent calving produced less milk. This means that mildly to moderate calving difficulties improved milk yield, while no assistance or the need for veterinary assistance decreased subsequent milk production. In the same study the interaction between sex of offspring and difficulty of calving was found to be insignificant [33]. Still, it must be also considered that dystocia might go unnoticed, nevertheless affecting milk production, which could lead to misreading of the sex-bias towards higher production after female calving because

of unidentified or unrecorded dystocia [40]. The effect of the different degrees of dystocia in milk production or for how long they persist remain unclear [83–85].

In UK Holstein-Friesian cows, moderate calving difficulties resulted in higher milk production. It is possible that some births not needing help and human supervision may experience real difficulties that go unnoticed and are wrongly registered as an easy calving, when they might have had some difficulties without the farmer's notice. Furthermore, it is likely that cows with highly valuated genetic material may be offered calving assistance from the farmer more often [67].

A reduction in milk production was observed between days in milk 10 and 90 after veterinary-assisted calving compared with non-assisted calving, leading to the conclusion that non-assisted cows presented a flatter lactation curve after peak yield [69]. One of the reasons is a reduced dry matter intake in the months postpartum [86].

In Jerseys the effects of calf gender in mothers milk production were not as pronounced as in Holstein-Friesians [40], which can point to a genetic selection of calf-sex biased milk production.
