CASE STUDY: Influence of Seminal, Physical, and Mating Behavior Traits of Bulls on Number of Calves Sired per Bull in a Multisire Herd

Professional Animal Scientist, Apr 2008 by Whitworth, W A, Forrest, D W, Sprott, L R, Holloway, J W, Warrington, B G

RESULTS AND DISCUSSION

Pregnancy rate was 91.5% (270 of 295 total cows), and weaning rate was 90.2% (266 calves weaned from 295 cows). The percentage of male calves weaned (53.4%) was similar to the sex ratio of calves sired by natural mating of suckled beef cows and beef heifers in previous studies (Lamb et al., 2006; Larson et al., 2006). Paternity was determined for 94.4% of the calves submitted for DNA typing (251 of 266 calves weaned). The percentages of calves for which paternity was determined were similar between male (95.6%) and female (92.7%) calves (P > 0.25; χ^sup 2^ = 1.13).

The Braunvieh bulls sired more calves than the Bonsmara bulls (174 calves vs. 77 calves, P

Upon sequential deletion of independent variables in an ANOVA model, breed of bull, spermatozoal normality and motility, and SDR were the variables which explained most of the differences (R^sup 2^ = 0.748; P = 0.029; Table 3) in the number of calves sired per bull. Because all bulls were FAA-positive, FAA classification was not included in the regression model. Social dominance has been previously been shown to positively affect number of calves sired per bull (Carpenter et al., 1990).

Bulls with ≤ 20% abnormal sperm had a higher proportion (P 20% abnormal sperm (Table 4). The proportion of calves born early in the calving season that were sired by bulls either with 21 to 30% abnormal sperm or with > 30% abnormal sperm did not differ (P > 0.10, χ^sup 2^ = 2.48, data not shown). Previous studies have enumerated the deleterious effects of increased numbers of morphologically abnormal sperm on conception rates (Nöthling and Arndt, 1995; Saacke, et al., 1998). Nöthling and Arndt (1995) reported a direct correlation between first service conception rate and the percentage of normal sperm in an ejaculate. A high percentage of abnormal sperm are filtered by barriers in the female reproductive tract (Saacke et al., 2000). However, most sperm which are filtered out of the ejaculate are those which exhibit midpiece and tail defects. Sperm which have slightly misshapen heads, including abnormal acrosome shape and chromatin condensation, have the ability to gain access to the ovum. Although acrosome shape and chromatin condensation were not assessed in the current study, sperm with these types of abnormalities have been associated with increased embryo mortality and decreased fertilization rates (Saacke et al., 2000).

IMPLICATIONS

This study evaluated the relative influence of physical, seminal, and behavioral traits of bulls on the number of calves sired in a multisire herd. Semen quality and social dominance were favorably associated with number of calves sired. More of the calves born during the first 40 d of the calving season were sired by bulls with ≥ 80% than by bulls with

ACKNOWLEDGMENTS

Special thanks to M. E. Bellin, J. N. Oyarzo, and R. L. Ax (Department of Animal Sciences, University of Arizona, Tucson 85721) for analysis of spermatozoal samples for FAA classification.

LITERATURE CITED

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Bellin, M. E., H. E. Hawkins, and R. L. Ax. 1994. Fertility of range beef bulls grouped according to presence or absence of heparinbinding proteins in sperm membranes and seminal fluid. J. Anim. Sci. 72:2441.

Bellin, M. E., J. N. Oyarzo, H. E. Hawkins, H. Zhang, R. G. Smith, D. W. Forrest, L. R. Sprott, and R. L. Ax. 1998. Fertility-associated antigen on bull sperm indicates fertility potential. J. Anim. Sci. 76:2032.

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