Effects of conventional or low bull to female ratio and utilization of reproductive tract scores in extensively-managed, natural mating breeding groups

The current study involved two experiments which were conducted at the Texas A&M Agricultural Research and Extension Center in Uvalde (semi-arid environment) from 2002 to 2004. In experiment one, Bonsmara bulls (n = 19; 20-24 mo of age) were joined with multiparous, crossbred females (n =586) for 90 d in 2003 and 2004. Bulls were allotted by selected physical traits, seminal traits, social rank, and serving capacity to one of two bull to female ratio (BFR) treatments: Conventional (1:21-1:29; n = 6 pastures) or Low (1:47-1:52; n = 2 pastures) BFR. Pregnancy rate (P = 0.33), calving rate (P = 0.26), and calving date (P = 0.22) did not differ between Conventional and Low BFR treatments. Post-breeding evaluation of bulls in 2002 (n = 16) indicated that social rank, but not seminal traits, was significantly correlated with pre-breeding values (P < 0.05). The current study demonstrates that Low BFR can be utilized in single- and multisire, 90-d breeding pastures of up to 2,090 ha without adversely affecting reproductive performance. In experiment two, yearling, one-half or three-quarter Bonsmara heifers (n = 106; 11-14 mo of age) were palpated per rectum and assigned a reproductive tract score (RTS) immediately prior to the beginning of the breeding season. Reproductive performance was measured in their two subsequent breeding years in order to estimate the value of the RTS system in extensively-managed, natural mating, 90-d breeding season programs. RTS was positively correlated (p < 0.01) with frame score (r = 0.25), age (r = 0.31), weaning weight (r = 0.47), and the weight of the heifer on the day of RTS exam (r = 0.56). The RTS means by dam parity also differed (P < 0.03). A lower (P < 0.01) percentage of females conceived during each of their first two breeding seasons for heifers of RTS 1 and 2 (65.2%) than for heifers of RTS 3, 4, and 5 (91.2%). Females with a RTS of 1 had a lower pregnancy rate over each of their first two breeding seasons, conceived later during their first breeding season, weaned lighter first calves, and remained lighter each year for fall body weight and body condition score than did heifers with RTS of 2 to 5 (P < 0.05). Collectively, the results of the current study indicate that heifers with a RTS of 1 immediately prior to a 90-d breeding season should be culled. Consideration should also be given to eliminating RTS 2 heifers, but further studies will be needed to confirm the potential economic advantage of this practice.