The objective of the study was to evaluate the interval from onset of oestrus to time of artificial insemination (AI) to obtain the optimum pregnancy rate with sex‐sorted semen in Holstein heifers. Heifers in oestrus were detected and inseminated only by using heat–rumination neck collar comprised electronic identification tag at the age of 13–14 months. Heifers (n = 283) were randomly assigned to one of three groups according to the timing of insemination at 12–16 hr (G1, n = 97), at 16.1–20 hr (G2, n = 94) and at 20.1–24 hr (G3, n = 92) after reaching the activity threshold. The mean duration of oestrus was 18.6 ± 0.1 hr, and mean peak activity was found at 7.5 ± 0.1 hr after activity threshold. The mean interval from activity threshold to ovulation was 29.4 ± 0.4 hr. The overall pregnancy per AI (P/AI) was 53.0% at 29–35 days and 50.9% at 60–66 days after AI. There was a significant reduction between G1 (13.8 ± 1.4 hr) and G3 (7.9 ± 1.4 hr) related to the intervals from AI to ovulation time. Sex‐sorted semen resulted in significantly higher P/AI at 29–35 days when heifers inseminated in G3 (60.9%) after oestrus than those inseminated in G1 (49.5%) and G2 (48.9%). In terms of fertility, when the temperature–humidity index (THI) was below the threshold value (THI ≤65) at the time of AI, there was a tendency (≤65; 57.2% vs. > 65; 47.1%) for high pregnancy rate. There was no effect of sire on P/AI. In addition, the interaction of the technician with the time of AI was found significant, and three‐way interaction of technician, sire and time of AI was tended to be significant on pregnancy rate. Thus, in addition to delaying the time of insemination (between 20.1 and 24 hr) after oestrous detection, THI and experienced technician were also found to be critical factors in increasing fertility with the use of sex‐sorted semen in Holstein heifers.

中文翻译：

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延迟分性别精液授精时间对荷斯坦小母牛妊娠率的影响。

该研究的目的是评估从发情开始到人工授精 (AI) 时间的间隔，以获得荷斯坦小母牛性别分选精液的最佳妊娠率。发情小母牛在 13-14 月龄时仅通过使用包含电子识别标签的热反刍颈圈进行检测和授精。 根据 12-16 小时（G1，n  = 97）、16.1-20 小时（G2，n  = 94）和20.1-24 小时的授精时间，将小母牛（n = 283）随机分配到三组之一小时 (G3, n = 92) 达到活动阈值后。发情的平均持续时间为 18.6 ± 0.1 小时，平均峰值活动出现在活动阈值后 7.5 ± 0.1 小时。从活动阈值到排卵的平均间隔为 29.4 ± 0.4 小时。每次 AI 的总体妊娠率 (P/AI) 在 AI 后 29-35 天为 53.0%，在 AI 后 60-66 天为 50.9%。G1 (13.8 ± 1.4 小时) 和 G3 (7.9 ± 1.4 小时) 之间的显着减少与从 AI 到排卵时间的间隔有关。与在 G1 (49.5%) 和 G2 (48.9%) 中受精的小母牛相比，在发情后 G3 (60.9%) 受精的小母牛在 29-35 天时，性别分选的精液显着更高的 P/AI。在生育方面，当温湿度指数（THI）低于阈值（THI≤65）时，AI时有偏高的趋势（≤65；57.2% vs.>65；47.1%）。怀孕率。父系对 P/AI 没有影响。此外，技术人员与人工智能时间的交互作用显着，技术人员、父亲和人工智能时间的三向交互对怀孕率有显着影响。因此，除了在发情检测后延迟授精时间（在 20.1 和 24 小时之间）之外，还发现 THI 和经验丰富的技术人员是在荷斯坦小母牛中使用按性别分选的精液提高生育能力的关键因素。