EFFECTS OF SYNOVEX C(R) IMPLANTS ON GROWTH-RATE, PELVIC AREA, REPRODUCTION, AND CALVING PERFORMANCE OF REPLACEMENT HEIFERS

Two trials were conducted to evaluate effects of Synovex C-(R) implant s on replacement heifers, given at two different ages. Crossbred heife r calves (n = 370) were allotted to four treatments: 1) nonimplanted c ontrols, 2) implanted at 2 mo, 3) implanted at 6 mo, and 4) implanted at both 2 and 6 mo of age. Heifers implanted at 2 mo gained 7 kg more (P = .01) by 6 mo than those not implanted at 2 mo. No differences wer e found in 22-mo weights. All implanted heifers had larger (P = .01) y earling pelvic area than controls. All heifers implanted at 6 mo conti nued to have larger (P = .01) pelvic area at 22 mo. All implanted heif ers had higher (P = .05) occurrence of non-ovulatory estrus. No differ ences were found among treatments in percentage of heifers puberal bef ore breeding, in estrus first 21 d of breeding, or in first-service co nception rate. In only one trial, pregnancy first 21 d and total pregn ancy in 63-d breeding season were decreased (P = .05) by implanting at 6 mo. At subsequent calving, an interaction existed between the effec ts of the 2- and the 6-mo implant for calf birth weight and pelvic are a:birth weight ratio. A single implant at either 2 or 6 mo decreased(P = .01) calving difficulty score; and implanting at both 2 and 6 mo sh owed the greatest reduction in calving difficulty. Implants had no sig nificant long-term effects on reproduction or calf production of 2-yr- old cows. Results showed implanting at 2 mo of age increased early wei ght gain and decreased calving difficulty scores; implanting at 6 mo i ncreased precalving pelvic area and decreased calving difficulty score s; and implanting at both 2 and 6 mo increased early weight gain, decr eased calving difficulty scores, but tended to reduce early fertility.