Effects of administration of a growth promoting implant during the suckling phase or at weaning on growth, reproduction, and ovarian development in replacement heifers grazing native range.

Management strategies utilized during pre-breeding development of replacement heifers can impact fertility and the ovarian reserve. Angus-Hereford crossbred heifers (n = 233) were utilized over a 3-yr period to determine the effects of administration of a growth promoting implant at either branding or weaning on growth, reproduction, and ovarian development. Heifer calves were randomly assigned to one of three treatments: 1) nonimplanted controls (CON; n = 79), 2) implanted at approximately 2 mo of age (average calf age = 58 d) with Synovex-C (BIMP, n = 82), or 3) implanted at approximately 7 mo of age (average calf age = 210 d) with Synovex-C (WIMP; n = 72). In years 2 and 3, a subset of heifers (year 2 n = 16; year 3 n = 14) were unilaterally ovariectomized. Heifers implanted at 2 mo of age were heavier at weaning, yearling (mid-February; average calf age = 332 d), and at the beginning of the breeding season (P < 0.01) compared to CON and WIMP heifers. Average daily gain (ADG) was similar among treatments from weaning to yearling and weaning to the start of the breeding season (P ≥ 0.61); however, WIMP heifers had increased (P = 0.05) ADG from yearling to the start of the breeding season compared to BIMP heifers. Antral follicle count and reproductive tract scores were not influenced by implant treatment (P ≥ 0.18). Response to synchronization of estrus was increased (P = 0.02) in WIMP compared to CON heifers, with BIMP heifers similar to all other treatments. First service conception rates tended to be increased (P = 0.09) in CON heifers compared to WIMP heifers, with BIMP heifers similar to CON and WIMP. Final pregnancy rates were similar (P = 0.54) among treatments. A treatment × yr interaction was detected (P = 0.01) for the number of primordial follicles/section with increased primordial follicles in WIMP heifers in year 3 compared to BIMP and WIMP heifers in year 2 and CON heifers in year 3, as well as in BIMP compared to WIMP heifers in year 2. Utilization of growth promoting implants did not negatively impact postweaning reproductive development or compromise pregnancy rates in beef heifers. Based on these results, administration of a growth promoting Synovex-C implant at 2 mo of age may allow for increased body weight at weaning, without hindering reproductive performance.