Studies compared ovarian function, circulating steroids, fertilization, and early embryonic development between nulliparous heifers (n = 29) and lactating Holsteins (n = 31; >40 kg milk/d) during summer, or between lactating (n = 27; >45 kg milk/d) and dry cows (n = 26) during winter. Daily blood sampling and ovarian ultrasonography were performed. During summer, cows had more multiple ovulations (17.9 vs. 1.9%) and larger ovulatory follicles (16.8 ± 0.5 vs. 14.9 ± 0.2 mm; mean ± SEM), but had lower maximal serum estradiol (7.9 ± 0.8 vs. 11.3 ± 0.6 pg/ml). Similarly, maximal serum progesterone during the estrous cycle was lower for cows (5.6 ± 0.5 vs. 7.3 ± 0.4 ng/ml), whereas maximal luteal tissue volume was greater (11120 ± 678 vs. 7303 ± 308 mm3). During winter, multiple ovulations were similar between lactating and dry cows (17.9 vs. 17.2%). Maximal serum estradiol was also similar (7.6 ± 0.5 vs. 8.5 ± 0.6 pg/ml) although lactating cows had larger ovulatory follicles (18.6 ± 0.4 vs. 16.2 ± 0.4 mm). Six d after ovulation, lactating cows had greater luteal tissue volume (7599 ± 475 vs. 5139 ± 460 mm3) but had similar serum progesterone (2.2 ± 0.2 vs. 1.9 ± 0.1 ng/ml) compared with dry cows. Six d after artificial insemination at estrus, embryos/ova were recovered and evaluated. During summer, fertilization was 55% for cows and 100% for heifers. Sperm were attached to 80% of unfertilized ova (17.8 ± 12.1 sperm/ova) implicating the oocyte as the cause of reduced fertilization. Embryos from lactating cows had inferior quality, but more accessory sperm (37.3 ± 5.8 vs. 22.4 ± 5.5 sperm/embryo). During winter, fertilization was 87.8% for lactating and 89.5% for dry cows. Embryo quality from lactating cows was inferior to dry cows but embryos had similar numbers of accessory sperm (42.0 ± 9.4 vs. 36.5 ± 6.3). Thus, fertilization rate was reduced in lactating cows only during summer suggesting that fertilization failure is not the underlying cause of reduced fertility in non-heat stressed lactating dairy cows. However, even in the absence of heat stress, embryos of lactating dairy cows were inferior in quality to embryos from nonlactating females as early as on d 6 after estrus with a high percentage of non-viable embryos. Thus, poor fertility in lactating cows is already detectable by d 6 and may be due to lower circulating steroid concentrations in spite of larger follicular and luteal sizes.
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机译：研究比较了夏季或哺乳期（n = 27；> 45）之间，未产小母牛（n = 29）和泌乳荷斯坦奶牛（n = 31；> 40 kg牛奶/天）之间的卵巢功能，循环类固醇，受精和早期胚胎发育。公斤/天）和干燥的母牛（n = 26）在冬季。每天进行血液采样和卵巢超声检查。在夏季，奶牛的排卵次数较多（17.9％vs. 1.9％），排卵卵泡较大（16.8±0.5 vs. 14.9±0.2 mm；平均值±SEM），但最大血清雌二醇含量较低（7.9±0.8 vs 11.3±0.6）。 pg / ml）。同样，发情周期内最大的血清孕酮水平较低（5.6±0.5 vs. 7.3±0.4 ng / ml），而最大的黄体组织体积较大（11120±678 vs 7303±308 mm 3 ）。在冬季，泌乳和干燥母牛的多次排卵相似（17.9％vs. 17.2％）。最高血清雌二醇也相似（7.6±0.5 vs. 8.5±0.6 pg / ml），但泌乳母牛的排卵卵泡较大（18.6±0.4 vs. 16.2±0.4 mm）。排卵六天后，泌乳母牛的黄体组织体积更大（7599±475 vs. 5139±460 mm 3 ），但血清黄体酮相似（2.2±0.2 vs. 1.9±0.1 ng / ml）与干牛。发情期人工授精六天后，回收并评估了胚胎/卵。在夏季，母牛的受精率为55％，母牛的受精率为100％。精子附着在80％的未受精卵（17.8±12.1精子/卵）上，提示卵母细胞是受精率降低的原因。泌乳母牛的胚胎质量较差，但辅助精子更多（37.3±5.8 vs. 22.4±5.5精子/胚胎）。在冬季，泌乳的受精率为87.8％，而奶牛的受精率为89.5％。泌乳母牛的胚胎质量不及干母牛，但胚胎的副精子数量相似（42.0±9.4对36.5±6.3）。因此，仅在夏季，泌乳母牛的受精率降低，这表明受精失败并不是非热应激泌乳奶牛生育力降低的根本原因。然而，即使在没有热应激的情况下，早在发情后第6天，泌乳奶牛的胚胎的质量也就低于未哺乳雌性的胚胎，其中有很高比例的无生命的胚胎。因此，在第6天已经可以检测到泌乳母牛的低生育力，这可能是由于卵泡和黄体较大而循环中的类固醇浓度较低所致。
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