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The effect of calf sex on dam milk yield in the New Zealand dairy cattle population
Studies in US, Canadian and French dairy cattle have ignited debate over whether the sex of calves influences milk yield of their dams. North American studies have shown milk yield is increased when either the calf born to initiate lactation or the calf in utero during lactation is female. A French study found a small effect of calf sex on milk yield in favor of males and concluded that this effect was too small to have any real effect on profit. The objectives of this study were to 1) investigate whether the sex of the calf born at the beginning of the current lactation or calf sex in neighboring lactations affects milk yield in 274,000 New Zealand Holstein Friesian and 85,000 Jersey cows, and 2) determine whether including an effect for calf sex in national evaluations is likely to affect selection response. Lactations 1-3 were analyzed separately. An animal model with fixed effects for herd-year and calf sex across the first three or four parities was fitted. This model was expanded to include a sex-specific slope for days-in-milk. Spearman rank correlations of breeding values were calculated separately for cows and bulls between the expanded model and an animal model with herd-year as a fixed effect and days-in-milk as a covariate. In the model ignoring days-in-milk, giving birth to a female rather than a male in the current lactation was associated with an increase of between 0.24±0.09 and 1.1±0.45% milk yield, depending on breed and lactation number. Calf sex in the preceding parity had an effect on milk yield for second lactation Holstein Friesians and third lactation Jerseys. There were significant interaction effects between calf sex in different parities in all lactations. After accounting for days-in-milk the main effect of calf sex was not significant except for second parity calf sex in the second lactation Holstein Friesians. The rank correlation between breeding values in the models that included or ignored calf sex was 1.00 for both bulls and cows in all lactations and both breeds. This indicates the calf-sex effect was primarily attributed to decreased lactation length in cows that gave birth to male calves, which can be attributed to their increased gestation length. Lactation ceases for the whole herd at once, so the delayed calving date will shorten lactation length. Including information on calf sex is unlikely to have any effect on selection response in New Zealand dairy cattle.
Keywords:
Calf sex, milk yield, Breeding values