| 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. |
