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 three-year 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) non-implanted 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 7mo of age (average calf age = 210 d) with Synovex-C (WIMP; n = 72). In yr 2 and 3 a subset of heifers (yr 2 n = 16; yr 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 yr 3 compared to BIMP and WIMP heifers in yr 2 and CON heifers in yr 3, as well as in BIMP compared to WIMP heifers in yr 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.

中文翻译：

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在哺乳期或断奶时施用促生长植入物对放牧后备小母牛生长、繁殖和卵巢发育的影响

在后备母牛的育种前开发过程中使用的管理策略会影响生育力和卵巢储备。安格斯-赫里福德杂交小母牛 (n = 233) 在三年期间被用于确定在烙印或断奶时施用促生长植入物对生长、繁殖和卵巢发育的影响。小母牛被随机分配到三种治疗方法之一：1) 非植入对照（CON；n = 79），2) 在大约 2 月龄（平均小牛年龄 = 58 天）植入 Synovex-C（BIMP，n = 82)，或 3) 在大约 7 个月大时（平均小腿年龄 = 210 天）植入 Synovex-C (WIMP；n = 72)。在第 2 年和第 3 年，一部分小母牛（第 2 年 n = 16；第 3 年 n = 14）被单侧切除卵巢。在 2 个月大时植入的小母牛在断奶、一岁时（2 月中旬；平均小牛年龄 = 332 天），并且在繁殖季节开始时（P < 0.01）与 CON 和 WIMP 小母牛相比。从断奶到一岁和断奶到繁殖季节开始的处理之间的平均日增重 (ADG) 相似 (P ≥ 0.61)，但是，WIMP 小母牛从一岁到繁殖季节开始增加 (P = 0.05) ADG与 BIMP 小母牛相比。窦卵泡计数和生殖道评分不受种植体治疗的影响 (P ≥ 0.18)。与 CON 小母牛相比，WIMP 对同步发情的反应增加 (P = 0.02)，B​​IMP 小母牛与所有其他处理相似。与 WIMP 小母牛相比，CON 小母牛的首次配种受孕率趋于增加 (P = 0.09)，BIMP 小母牛与 CON 和 WIMP 相似。不同治疗组的最终妊娠率相似 (P = 0.54)。与第 2 年的 BIMP 和 WIMP 小母牛以及第 3 年的 CON 小母牛以及BIMP 与 WIMP 小母牛在第 2 年的比较。促生长植入物的使用不会对肉牛小母牛的断奶后生殖发育产生负面影响或降低妊娠率。基于这些结果，在 2 月龄时施用促生长的 Synovex-C 植入物可能会增加断奶时的体重，而不会影响繁殖性能。以及 BIMP 与 WIMP 小母牛在第 2 年的比较。促生长植入物的使用不会对肉牛小母牛的断奶后繁殖发育产生负面影响或降低妊娠率。基于这些结果，在 2 月龄时施用促生长的 Synovex-C 植入物可能会增加断奶时的体重，而不会影响繁殖性能。以及 BIMP 与 WIMP 小母牛在第 2 年的比较。促生长植入物的使用不会对肉牛小母牛的断奶后繁殖发育产生负面影响或降低妊娠率。基于这些结果，在 2 月龄时施用促生长的 Synovex-C 植入物可能会增加断奶时的体重，而不会影响繁殖性能。