Beam S W, Butler W R
Dept of Animal Science, University of California, Davis 95616, USA.
J Reprod Fertil Suppl. 1999;54:411-24.
As milk production has increased during the past four decades, conception rates in lactating cows have declined. Although reduced reproductive performance has been associated with high milk yields, measures of postpartum ovarian activity have been more closely related to energy balance. The relationship between daily energy balance and postpartum reproductive activity is confirmed by longer intervals to first ovulation in cows with greater body condition loss. Patterns in daily energy balance, such as improvement from nadir, have been correlated with enhanced follicular function and a shorter interval to first ovulation. Such observations are consistent with increased LH pulse frequency following the energy balance nadir in lactating dairy cows. Evidence indicates a primarily hypothalamic locus for the modulation of LH secretion during negative energy balance. Formation of follicular waves after parturition begins synchronously in response to increased FSH in the first week postpartum, and is typically not a limiting factor in reproductive recrudescence. Altered follicular responsiveness to gonadotrophic support through changes in metabolic hormones such as insulin-like growth factor I (IGF-I) and insulin may contribute to impaired function of dominant follicles early postpartum. Positive relationships between changes in energy balance, peripheral IGF-I and function of dominant follicles support the identification of IGF-I and the day of the energy balance nadir as metabolic modulators of postpartum ovarian activity in dairy cows.
在过去的四十年间，随着牛奶产量的增加，泌乳奶牛的受孕率却有所下降。尽管繁殖性能下降与高产奶量有关，但产后卵巢活动的指标与能量平衡的关系更为密切。体况损失更大的奶牛首次排卵间隔时间更长，这证实了每日能量平衡与产后繁殖活动之间的关系。每日能量平衡的模式，如从最低点开始改善，与卵泡功能增强和首次排卵间隔时间缩短相关。这些观察结果与泌乳奶牛能量平衡最低点后促黄体生成素（LH）脉冲频率增加一致。有证据表明，在负能量平衡期间，LH分泌的调节主要发生在下丘脑。产后第一周，随着促卵泡生成素（FSH）增加，产后卵泡波的形成开始同步，通常不是生殖恢复的限制因素。通过代谢激素如胰岛素样生长因子I（IGF-I）和胰岛素的变化，卵泡对促性腺激素支持的反应性改变，可能导致产后早期优势卵泡功能受损。能量平衡变化、外周IGF-I与优势卵泡功能之间的正相关关系，支持将IGF-I和能量平衡最低点的日期确定为奶牛产后卵巢活动的代谢调节因子。
