Increasing Heifer Calves Without Using Sexed-Semen

Key Findings

 

Data from this experiment indicated that older cows tend to produce more heifer calves than younger cows. Thus, finding ways to reduce the culling rate, such as improving footing, reducing mastitis, and keeping cows healthy, should lower the cull rate resulting in an older herd and an increase in the chances of heifers being born.

 

The earlier days in milk or the age a cow conceives will increase the odds of the dairy producer getting a male calf. These data do not imply that farmers should wait to breed their cows, but it does suggest that reducing the culling rate may increase the chances of having more heifer calves born.

About the Co-Author

Peter Erickson, Professor of Agriculture, Nutrition, and Food Systems

Contact information: Peter.Erickson@unh.edu
603-862-1341, Peter Erickson Lab website

Having adequate numbers of heifer calves to replace cows that are culled is essential for the continual success of any dairy farm. There has been much animal research examining the likelihood of female or male offspring. Sexed-semen has resulted in many heifer calves on dairy farms. Is there a way in which dairy producers can increase the number of heifers born without using it?

Darwin suggested that some animal species can exhibit significant shifts in the proportions of male and female offspring that are born, though environmental conditions and mechanisms that cause these changes are unclear (Rosenfeld and Roberts, 2004). Trivers and Willard (1973) noted that in polygamous species a small proportion of males, typically the larger and more aggressive ones, share most of the lifetime reproductive success. Lower- ranking males often sire no offspring at all. However, a majority of females, regardless of social ranking or body condition, will be impregnated by that select group of males.

The hypothesis suggested by Travers and Willard (1973) states that females in the best body condition would tend to produce offspring of the gender that favors the sex of greater variance, namely males. The male offspring would benefit from greater parental investment and most likely as adults join the elite group of breeding males. As a consequence, the females are likely to pass on their genes to more of their offspring's progeny. Conversely, females lower in the social structure or poorer body condition would invest more in female progeny because their daughters rather than their sons are likely to have greater lifetime reproductive success (Rosenfeld and Roberts, 2004).

Data from the University of Illinois and the University of New Hampshire dairy herds were collected and summarized for calf sex, the number of services to achieve conception, and the lactation number of the lactating dairy cow when she conceived. Logistic regression procedures were used to analyze the data.

The data set was edited to delete those cow observations when the number of services to achieve conception (n = 6) or lactation number (n = 2) were greater than 9. These cows would typically be classified as “do not breed” and sold for harvest. The final data set contained 2,987 calvings, which consisted of 1,406 females and 1,581 males (47.1 and 52.9% for females and males, respectively).

The frequency distribution of the number of services to achieve conception was highest for the first service and progressively declined with increasing services (Table 1). Logistic stepwise regression indicated that the number of services to achieve conception was statistically significant (P < 0.02) in predicting the ratio of female to male calves. Calculation of odds ratios indicated that as the lactation number increased the likelihood of getting a bull calf decreased.