Developing replacement heifers is one of the most expensive and complex management decisions for a cow-calf producer, which also has long-term implications for profitability. The objective of this study was to determine the impact of converting spring-born heifers into a summer-calving herd on growth, reproductive performance, longevity, and productivity compared with early or late-born May heifers. Over a 3 yr period, Red Angus/Simmental crossbred heifers (n = 273) were utilized to determine the impact of converting March-born heifers (Convert; n = 90) to a May-calving herd compared with May-born heifers on reproductive performance, body weight (BW), and calf performance from a yearling to 5 yr of age. May-born heifers were retrospectively grouped into 2 different groups: heifers born in the first 21 d of calving as a heifer calf (Early; n = 123) or heifers born after the 21 d of calving as a heifer calf (Late; n = 60). Heifers were exposed to bulls for a 45-d breeding season with a bull-to-heifer ratio of 1:20. Heifers were synchronized with a single injection of prostaglandin F2α (5-mL i.m.; Lutalyse, Zoetis, Parsippany, NJ) 5 d after bulls were introduced in the pasture for breeding. Data were analyzed as a randomized design using the MIXED procedure (SAS Inst. Inc., Cary, NC, USA). All binomial data were analyzed using PROC GLIMMIX. Heifer served as the experimental unit with treatment and year developed set as fixed effects. Heifer BW at December, May, July, and pregnancy check (Oct) were greater (P < 0.01) for Convert heifers with no difference between Early and Late born heifers. Heifer BCS at pregnancy check was not different (P = 0.13) among heifer treatment groups. However, pregnancy rate tended (P = 0.08) to increase for Convert heifers with no difference between Early and Late born May heifers. After 5 yr of age, retention rate was increased (P = 0.04) in Convert cows compared with Early and Late May-born cows with no difference (P = 0.84) between the two May-born heifer groups. Total kilograms of calf weaned was greater (P = 0.04) in Convert cows than their counterparts with no differences (P = 0.94) between Early and Late May-born cows. This study implies that selecting replacement heifers from an earlier calving season for replacements in a later calving season increases the longevity and long-term productivity of the cowherd than selecting heifers within the same calving season. This may be even more important in resource-poor breeding environments that create reproductive challenges for heifers and rebreeding young cows.

中文翻译：

---

182 将春季出生的小母牛转化为夏季产犊的牛群可以提高奶牛的寿命和生产力


对于奶牛犊牛生产商来说，开发后备母牛是最昂贵和最复杂的管理决策之一，这也对盈利能力产生长期影响。本研究的目的是确定与早产或晚产的 May 小母牛相比，将春季出生的小母牛转化为夏季产犊牛群对生长、繁殖性能、寿命和生产力的影响。在 3 年的时间里，使用红安格斯/西门塔尔杂交小母牛 （n = 273） 来确定将 3 月出生的小母牛 （Convert;n = 90） 转化为 5 月产犊牛群与 5 月出生的小母牛相比对繁殖性能、体重 （BW） 和一岁至 5 岁犊牛性能的影响。5 月出生的小母牛被回顾性地分为 2 个不同的组：产犊前 21 天出生的小母牛犊（早期;n = 123）或产犊 21 天后出生的小母牛（晚期;n = 60）。小母牛在 45 天的繁殖季节与公牛接触，公牛与小母牛的比例为 1：20。小母牛与单次注射前列腺素 F2α （5 mL im;Lutalyse， Zoetis， Parsippany， NJ） 公牛被引入牧场进行繁殖后 5 天。使用 MIXED 程序 （SAS Inst. Inc.， Cary， NC， USA） 作为随机设计分析数据。使用 PROC GLIMMIX 分析所有二项式数据。小母牛作为实验单位，处理和年发展设置为固定效应。转换小母牛在 12 月、5 月、7 月和怀孕检查（10 月）的小母牛体重较高 （P < 0.01），早产小母牛和晚产小母牛之间没有差异。妊娠检查时的小母牛 BCS 在小母牛治疗组之间没有差异 （P = 0.13）。然而，怀孕率趋向 （P = 0.08） 增加 转换小母牛，早产和晚产小母牛之间没有差异。5 岁后，与 5 月初和晚产奶牛相比，转化率奶牛的保留率增加 （P = 0.04），两组 5 月出生的小母牛之间没有差异 （P = 0.84）。转化率奶牛断奶的总犊牛公斤数 （P = 0.04） 高于同类奶牛，5 月初和末期出生的奶牛之间没有差异 （P = 0.94）。这项研究表明，与在同一产犊季节选择小母牛相比，从较早的产犊季节选择后备母牛作为后备母牛，可以提高牛郎的寿命和长期生产力。这在资源匮乏的繁殖环境中可能更为重要，因为这些环境会给小母牛和再繁殖幼奶牛带来繁殖挑战。