497 Effects of post-weaning plane of nutrition and estrus synchronization on reproductive performance of Bos indicus-influenced beef heifers

Abstract

A 3-year study evaluated the effectiveness of estrus synchronization of beef heifers undergoing different post-weaning growth rates. On day 0 of each yr, Angus × Brahman heifers (60 heifers/year; 310 days of age) were stratified by initial BW and age, and randomly assigned, in a 3 × 2 factorial design, to achieve a low (LOW; 0.45 kg/d), medium (MED; 0.73 kg/d), or high (HIG; 1.00 kg/d) growth rate from d 0 to 168 (5 heifers/pasture; 4 pastures/diet/yr). Supplements were formulated to allow LOW, MED, and HIG heifers to achieve 55.0, 62.5, and 70.0%, respectively, of mature weight at initiation of breeding season (d 84). On d 65, all heifers within each pasture (2 pastures/diet) were enrolled (SYNC) or not (NOSYNC) in an estrus-synchronization protocol, which consisted of intravaginal controlled internal drug release (1.38 g P4) insertion on d 65 and removal on d 75, followed by 100-µg i.m. injection of GnRH on d 77, and 25-mg i.m. injection of PGF2α on d 84. Heifers were stratified by diet (1 pasture/diet) and exposed to Angus × Brahman bulls from d 84 to 168 (1 yearling bull/pasture). Pasture was the experimental unit, whereas year, heifer (pasture), and pasture (diet × synchronization) were included as random effects in all analyses. Growth performance, and puberty and calving distribution were analyzed using MIXED and GLIMMIX procedures of SAS, respectively. Interactions among growth rate, estrus synchronization, and time were not detected for any variable (P ≥ 0.26). Overall heifer ADG achieved only 51 to 56% of targeted ADG, and mature BW on d 84 did not differ between MED and HIG heifers (57.6 vs. 58.6 ± 2.40%; P = 0.30), but both were heavier than LOW heifers (53.7 ± 2.40%; P < 0.0001). Percentage of pubertal heifers on day 84 was greater for MED and HIG vs. LOW heifers (44.4 and 45.4 vs. 26.5 ± 6.77% P ≤ 0.05), whereas estrus synchronization increased attainment of puberty on d 84 (23.5 vs. 48.7 ± 4.64% for NOSYNC and SYNC heifers; P < 0.01). Pregnancy and calving rates were not affected by diet or estrus synchronization (P ≥ 0.24), however heifers calved 9 d earlier with the use of estrus synchronization (P = 0.02). Estrus synchronization resulted in earlier attainment of puberty and pregnancy in Bos indicus-influenced beef heifers; however, at the low rates of BW gain achieved in the current study, estrus synchronization did not impact overall reproductive success.