ASAS/ADSA Physiology: Nutritional Regulation of Ovarian Function/Ovarian Biology

A number of reproductive parameters are altered in lactating dairy cows including: twinning rate, expression of estrus, conception rate, pregnancy loss, and incidence of anovulation. We hypothesized that the high feed consumption associated with lactation has fundamentally altered reproductive physiology due to increased metabolism of steroids. We evaluated liver blood flow and steroid metabolism under different physiological conditions using continuous infusion of bromosulfothalein ( 90% metabolized by one pass through liver) and/or estradiol and progesterone. In lactating or non-lactating cows, acute feeding produced an increase in liver blood flow of 20-30% that was maximal by 2 h after feeding. There was a corresponding decrease of 30% in circulating estradiol and progesterone concentrations following acute feeding of cows continuously infused with steroids. The baseline liver blood flow was much greater in lactating ( 1600 l/h) as compared to non-lactating ( 800 l/h) cows. Similarly, estradiol and progesterone metabolism was much greater in lactating as compared to non-lactating cows. Analysis of normal estrous cycles showed striking differences in lactating cows vs. heifers that are consistent with high steroid metabolism. Peak serum estradiol near estrus was lower in single-ovulating lactating cows (7.1.5 pg/ml) than heifers (9.4.6) in spite of larger preovulatory follicle diameter in cows (17.4.5 mm) than heifers (14.8.2). Similarly, circulating progesterone concentrations were lower for cows than heifers from d 5 to 14 of the estrous cycle (on day 14: 4.1.3 vs. 6.1.4 ng/ml); whereas, luteal volume was greater for cows than heifers (on day 14: 8272932 vs. 5415276 mm3). There were also differences in double ovulation rate (cows-25.0%; heifers-1.8%) and embryo quality on day 5 after normal ovulation (n = 27-28 per group each flushed 3 different times). The percentage of degenerated embryos (Grade 5) was greater (p<0.05) in lactating cows (61.9%) vs. heifers (6.3%) during summer or comparing lactating (41.7%) vs. non-lactating cows (17.6%) during winter. Thus, changes in reproductive physiology may be a consequence of the high feed consumption, high liver blood flow and resulting increase in steroid metabolism associated with high milk production.