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

ABSTRACT

A study was conducted to quantify the influence of behavior, physical characteristics, and seminal traits of bulls on reproductive performance in a multisire herd. Braunvieh (n = 6) and Bonsmara (n = 6) bulls (18 to 24 mo) were evaluated for BCS, social dominance rank (SDR), serving capacity, sperm motility and morphology, and fertility-associated antigen classification of an ejaculate. Bulls (n = 12) were mated with multiparous cows (n = 305) for 90 d. Paternity was verified by DNA typing. All bulls were fertility-associated antigen-positive. Bull breed, sperm motility and morphology, and SDR were associated with number of calves sired per bull (P

Key words: bull, fertility, semen, mating behavior

INTRODUCTION

A breeding soundness exam can be performed to assess physical and structural soundness, spermatozoal motility, spermatozoal morphology, and scrotal circumference with the objective to identify bulls of satisfactory breeding potential. Additional factors such as serving capacity (SC) and social dominance rank (SDR) are known to influence reproductive performance (Blockey, L978, 1979). The presence of a specific protein, fertility-associated antigen (FAA) on the spermatozoal plasma membrane is positively associated with number of calves sired by bulls used in natural service mating programs (Bellin et al., 1998). Traditional beef cattle production systems frequently use multisire breeding herds with an approximate ratio of 1 bull per 25 to 30 females (Fox, 1974). Reproductive performance is variable among bulls in multisire breeding herds. Identification of factors associated with reproductive success would enhance accuracy of selection of bulls for use in multisire herds. Tropically adapted (Bonsmara) and temperately adapted (Braunvieh) breeds of bulls were utilized in semiarid range environment of the current study. The objectives of this study were to quantify the effects of seminal (sperm motility, sperm morphology, FAA classification, and scrotal circumference), physical (BW, BCS, and breed) and behavioral (SC and SDR) traits of young bulls in multisire herds on 1) number of calves sired and 2) number of calves born early in the calving season.

MATERIALS AND METHODS

An experiment was conducted at the Texas AgriLife Research and Extension Center, in Uvalde, TX, during the spring breeding season. All experimental procedures were approved by the Texas A&M University Animal Care and Use Committee.

Braunvieh (n = 6) and Bonsmara (n = 6) bulls (18 to 24 mo of age) were evaluated for BCS, SDR, SC, sperm motility and morphology, and FAA status (positive or negative). Body condition score was assessed by 2 evaluators on a scale from 1 to 9, with 1 being emaciated and 9 being obese (Whitman, 1975). The score from the 2 evaluators was averaged for body condition and used in analysis of these traits.

Social dominance rank and SC were determined by methods described by Carpenter (1990). Lutalyse (25 mg, i.m., Pharmacia-Upjohn, Kalamazoo, MI) was administered on d -14 and -3 to synchronize estrus in crossbred cows (n = 14). Cows were observed for estrus between 0700 and 0800 h on d 0 to identify females for the SC test. Serving capacity was determined by quantifying the number of mounts, intromissions, and ejaculations performed by each bull when exposed for 20 min to a group of cows in estrus. Groups of 6 bulls were evaluated with 5 estrous cows, with all bulls being individually identified. If a female became nonreceptive (i.e., exhibited evasive action or no longer accepted mounts), then she was replaced with a new estrous cow before initiating assessment of the next bull group. For the purposes of analysis, bulls that completed 2 or more services during the 20-min test were classified as high SC, and bulls that completed one or no services were classified as low SC (Carpenter et al., 1992).

The SDR test was performed by recording each encounter of dominance as bulls competed for access to a limited feed source for 20 min. Within each breed, bulls were managed as a common group from weaning to the start of the breeding season. Thus, we assessed SDR and SC among bull groups in which dominance order was expected to be more (within breed) rather than less stable (across breeds). Blockey (1979) reported that social dominance order was significantly more stable among mixed-age bulls that had grazed together for at least 4 mo than for 2-yr-old bulls that had grazed together for 4 mo or less. The ratio of encounters initiated to those won, lost, or tied was calculated. Winning was defined as the spatial displacement of the other animal (Blockey and Lade, 1974). A tie was determined when no displacement was apparent. Behavioral displaces used to evaluate encounters included displacement by visual contact, by threatening gesture (vocalization or head throw), by approach, by head butt to the body, and by head-to-head contact. The results of each encounter among pairs of bulls were recorded and used to determine a composite dominance ranking. Ratios used to determine dominance ranking were defined as total number of agonistic behaviors won per total number of agonistic behaviors (Ologun et al., 1981). The most dominant bull was assigned a rank of 1 and the most subordinate bull a rank of 6 within each breed group. For the purposes of analysis, bulls which were ranked 1, 2, or 3 in their breed group for social dominance were designated "dominant", and bulls ranked 4, 5, or 6 were designated "submissive".