**4. Pregnancy and lactation length**

In New Zealand, with a seasonal calving system, the calf gender was reported to influence milk yield possibly through the increased gestation length of male calves [40]. In that study, the milk production tests were performed on the same date for all cows, so those calving a male would have their tests performed, on average, 2 days earlier. However, when the lactation length (reported longer in male calves) was included in the model, no effects existed of the calf gender over the increased production of milk [40]. At least part of the reported difference in milk production due to calf gender, was really due to methodological issues. The interval-centering method used provide a 10.8 ± 4.0 L higher milk yield if herd tests are 2 days later in lactation. However, the observed calf sex variance is too large to be explained only by this difference in herd test dates. When lactation length shortens depending on calving date, as well as the herd tests occurring 2 days earlier, the difference in milk yield is 26.9 ± 6.2 L. This difference is similar to the observed effect of calf gender on milk yield, further supporting that this effect is, at least partially, due to the different lactation length when male calves are born 2 days later [40].

It is difficult to establish any association between the calf gender and a presumed sex-biased milk production or a sex-biased pregnancy length, because of various existing confounder factors that may permeate such interaction. Mean pregnancy in length male calves is longer than in females, the difference also being affect by breed and parity [40, 75]. Also, primiparous cows tend to present shorter pregnancies than multiparous cows, the calves born lighter [76], albeit the risk for dystocia is also higher for first calving cows.

Recently, Atashi and Asaadi [77], using 252,798 lactations on 108,077 Holstein dairy cows in Iran showed that multiparous cows with longer gestations performed better in lactation than primiparous cows. This study also showed that multiparous cows with short gestation length had a lower yield at the beginning of lactation and higher raising and declining slopes of the lactation curve compared with cows presenting longer or average length of pregnancy.

The production system may also interfere with milk production performance of dairy cows. In seasonal breeding systems, late calving cows usually have a shorter lactation since the entire herd ceases lactation on the same day [40]. The

**219**

*Calf-Sex Influence in Bovine Milk Production DOI: http://dx.doi.org/10.5772/intechopen.93966*

the lactation curves are different.

**5. Calving difficulty**

metabolic disorders [41].

parity, favoring females.

lactation length is usually longer in non-seasonal systems because the lactation can continue until the milk yield of an individual cow drops below a point when it is more economical to dry the cow. In these conditions a weaker negative correlation between gestation and lactation length is observed compared with seasonal systems where all the cows are dried of on a single day [40]. However, even in non-seasonal systems lactation length was observed to be approximately four days shorter following the birth of a male calf compared to a female calf across the first four lactations [41]. Chegini et al. [41] found that cows calving female offspring had more persistent lactations than those that calving male offspring, suggesting that

Still, there is some controversy regarding the best methodology to apply when modeling the milk production (whether in milk yield or composition) to adequately account the effects of the gender of the calf. This is not an easy task, because it establishes a complex interaction with other parameters (e.g., pregnancy length, dystocia, and some cow related factors) that may act as confounding factors. Lactation length is one important factor affecting milk yield per lactation, leading to the need to introduce correction factors for lactation length in the models for milk production in cows. Lactation length in itself has a negative relationship with the annualized production of milk and milk solids [78]. Also, the milk yield and milk production curves change according to the lactation number, the persistency of the peak and lactational length, the cow genetics and the number or milking frequency, among other factors. Such aspects should also be considered in the lactation modeling studies. Models construct evaluate the lactation curves should be used that take all possible confounders into account simultaneously. Therefore, further investigation is necessary to confirm whether the shape of lactation curves differ based on calf gender and identify potential biological explanations for any such difference.

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

Male calves are typically larger than females, and pose a greater risk of dystocia

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

[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

### *Calf-Sex Influence in Bovine Milk Production DOI: http://dx.doi.org/10.5772/intechopen.93966*

*Animal Reproduction in Veterinary Medicine*

in milk production [74].

higher for first calving cows.

presenting longer or average length of pregnancy.

**4. Pregnancy and lactation length**

Modeling complex biological features, such as milk production, is challenging due to the number of inherent and environmental aspects that can influence them. Also, the statistical model used for analysis may influence to a certain point the results and data interpretation. One explanation for the differences of the several studies on calf-sex biased milk production can be related to the models used. For example, in one of the studies, Holstein Friesian cows calving males in the first three parities had significantly lower first lactation milk yield than cows calving two males followed by a female in the first three parities, but this observation is biased if models do not include lactation length. Also, there are no reasonable biological reasons why to test the effect of the gender of the third calf on the first lactation yield. In fact, the observed effect of calf gender on milk yield is due to an association between calf gender and milk yield rather than calf gender triggering a difference in milk yield. The alleged effect of the third parity calf gender on the first lactation milk yield was not apparent when lactation length was included in the models [40]. In beef cows, studies with limited samples led to different sex-biased milk production, pointing to either favors a son [71], or a daughter [72], or not show any sex-biases [73]. A study in the red Chittagong cattle found no effect of the calf-sex

In New Zealand, with a seasonal calving system, the calf gender was reported to influence milk yield possibly through the increased gestation length of male calves [40]. In that study, the milk production tests were performed on the same date for all cows, so those calving a male would have their tests performed, on average, 2 days earlier. However, when the lactation length (reported longer in male calves) was included in the model, no effects existed of the calf gender over the increased production of milk [40]. At least part of the reported difference in milk production due to calf gender, was really due to methodological issues. The interval-centering method used provide a 10.8 ± 4.0 L higher milk yield if herd tests are 2 days later in lactation. However, the observed calf sex variance is too large to be explained only by this difference in herd test dates. When lactation length shortens depending on calving date, as well as the herd tests occurring 2 days earlier, the difference in milk yield is 26.9 ± 6.2 L. This difference is similar to the observed effect of calf gender on milk yield, further supporting that this effect is, at least partially, due to the

It is difficult to establish any association between the calf gender and a presumed sex-biased milk production or a sex-biased pregnancy length, because of various existing confounder factors that may permeate such interaction. Mean pregnancy in length male calves is longer than in females, the difference also being affect by breed and parity [40, 75]. Also, primiparous cows tend to present shorter pregnancies than multiparous cows, the calves born lighter [76], albeit the risk for dystocia is also

Recently, Atashi and Asaadi [77], using 252,798 lactations on 108,077 Holstein dairy cows in Iran showed that multiparous cows with longer gestations performed better in lactation than primiparous cows. This study also showed that multiparous cows with short gestation length had a lower yield at the beginning of lactation and higher raising and declining slopes of the lactation curve compared with cows

The production system may also interfere with milk production performance

of dairy cows. In seasonal breeding systems, late calving cows usually have a shorter lactation since the entire herd ceases lactation on the same day [40]. The

different lactation length when male calves are born 2 days later [40].

**218**

lactation length is usually longer in non-seasonal systems because the lactation can continue until the milk yield of an individual cow drops below a point when it is more economical to dry the cow. In these conditions a weaker negative correlation between gestation and lactation length is observed compared with seasonal systems where all the cows are dried of on a single day [40]. However, even in non-seasonal systems lactation length was observed to be approximately four days shorter following the birth of a male calf compared to a female calf across the first four lactations [41]. Chegini et al. [41] found that cows calving female offspring had more persistent lactations than those that calving male offspring, suggesting that the lactation curves are different.

Still, there is some controversy regarding the best methodology to apply when modeling the milk production (whether in milk yield or composition) to adequately account the effects of the gender of the calf. This is not an easy task, because it establishes a complex interaction with other parameters (e.g., pregnancy length, dystocia, and some cow related factors) that may act as confounding factors. Lactation length is one important factor affecting milk yield per lactation, leading to the need to introduce correction factors for lactation length in the models for milk production in cows. Lactation length in itself has a negative relationship with the annualized production of milk and milk solids [78]. Also, the milk yield and milk production curves change according to the lactation number, the persistency of the peak and lactational length, the cow genetics and the number or milking frequency, among other factors. Such aspects should also be considered in the lactation modeling studies. Models construct evaluate the lactation curves should be used that take all possible confounders into account simultaneously. Therefore, further investigation is necessary to confirm whether the shape of lactation curves differ based on calf gender and identify potential biological explanations for any such difference.
