In a recent article published by Plos One (Hinde K, Carpenter AJ, Clay JS, Bradford BJ (2014) Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex. PLoS ONE 9(2): e86169. doi:10.1371/journal.pone.0086169), the authors report a large effect of the sex of the calf on milk production, the female calves positively affecting milk production of their dams. According to these authors, this effect is observed for calves born at the onset of the lactation and also for calves the gestation of which overlaps with the lactation.

In this comment, we present a replication of this study in the two most numerous French dairy cattle breeds. We analyzed lactations in parity 1 to 3 from 2000 to 2008. This time period was chosen because since 1999 the sex of the calf has been a fully reliable information as part of the generalized animal identification system. And before 2009, use of sexed semen was negligible in France. Indeed it is known that sexed semen is used to inseminate the best cows and this can generate a bias in the estimation of the sex of calf effect on production. Similarly, dystocia is known to negatively affect subsequent production and is more frequent for male calves. To avoid such a bias due to dystocia, all data with birth conditions scored over 3 were discarded. The analyzed data sets included 1,434,000 and 7,467,000 lactations in Montbéliarde and Holstein breeds, respectively.

In a first analysis, we studied the effect of the sex of the calf the birth of which initiates the following lactation. The analyzed traits were milk, fat, and protein yields, and fat and protein contents. The statistical model was derived from the model used in the French genetic evaluation system for cattle (Robert-Granié et al, 1999) by adding the effect of the sex of the calf. This model also included the fixed effects of herd-year, of age at calving, of month of calving, and of preceding days dry in parity 2 and 3. The three latter effects were defined within year and region. The model also included the random effect of the breeding value of the cow and her permanent environment. The model accounted for heterogeneous variances between herds and year.

Results are presented in Table 1. Results presented correspond to the [(female calves) minus (male calves)] contrast. In Hilde et al’s paper, this contrast is strongly positive.

In first lactation, in Holstein as in Montbéliarde, the estimated effects were very low. A small favorable effect was observed for males in subsequent lactations of their dams. This effect reaches about 40 kg milk (eg about 0.5% of the mean), 0.6 kg fat, 0.6 kg protein. A small difference was also noticed for fat and protein contents (from 0.01 to 0.02%) in parity 2 and 3.
 
Table 1. Difference in milk production (female calf – male calf contrast) in the first 3 lactations
Montbéliarde breed Holstein breed
L1 L2 L3 L1-L3 mean L1 L2 L3 L1-L3 mean
Milk yield (kg) -1.9 -47.2 -40.8 -28.0 1.4 -42.7 -33.5 -23.3
Fat yield (kg) 0.66 -0.77 -0.53 -0.16 0.73 -0.63 -0.30 -0.06
Protein yield (kg) 0.01 -0.75 -0.53 -0.42 -0.12 -0.75 -0.50 -0.49
Fat content (%) 0.10 0.15 0.15 0.13 0.07 0.12 0.12 0.10
Protein content (%) 0.01 0.11 0.11 0.07 -0.01 0.07 0.07 0.03

These results were quite similar across years. For instance, in Holstein breed, the estimated effect on milk yield ranged from -62 to +86 kg for the 27 estimates (9 years x 3 parities) which represent less than 1% of the total lactation yield.

In a second study, we also analyzed the combined effect of the sex of the calf born and the sex of the calf carried during the first lactation of its dam. Editing rules were similar. However, to obtain the information about the sex of the second calf, cows were required to have a second calving. Table 2 presents the results for milk yield in first lactation, in both breeds.

Table 2. Difference in milk yield (kg) in 1st lactation according to the sex of calf born x sex of calf in gestation
Calf born x Carried calf Montbéliarde Holstein
Male x Male -2 -8
Male - Female 18 9
Female – Male -15 -16
Female - Female 0 0

As previously, in both breeds, differences in milk yield were very small.

In conclusion, our study does not confirm the results reported by Hilde et al. The effect of the sex of calf hardly affects the lactation of its dam. A small effect is observed in parity 2 and 3 and is similar across breeds. It is in favour of males, and not of females as reported by Hilde et al, and markedly smaller than their estimate. Similarly, the estimated effect of the sex of the calf in gestation on the simultaneous lactation is very small.
A major difference between the analysis of Hilde et al and ours is the statistical model used. In our approach, the model included a contemporary group (ie a herd-year effect) which limits the risk of bias.

A Barbat, R Lefebvre, D Boichard
INRA, UMR1313 GABI, Jouy en Josas, France