Is nutritional anestrus precipitated by subfunctional corpora lutea in beef cows?

Thirty-four multiparous, lactating, cyclic beef cows which calved in moderate body condition were used to determine effects of restricted nutrition on corpus luteum (CL) development and endocrine status. At 78 d postpartum, six cows were assigned to a control (CON) diet (26.0 Mcal ME), fed to increase bodyweight (BW) and body condition score (BCS), and the remaining 28 cows were fed to lose BW and BCS on a restricted (RES) diet (14.0 Mcal ME). Following a 40-d adjustment period on respective diets, estrous cycles were synchronized and cows bled daily for determination of progesterone (P4), luteinizing hormone (LH) and insulin (INS) beginning at the synchronized estrus. Ultrasonography was used to determine the ovulatory follicle and CL development. Control cows were maintained for one estrous cycle and were ovariectomized on day 11 of their second cycle. Ten cows on restricted diet (RES-C) continued to form a functional CL (P4 > 1.5 ng/ml at day 10 of an estrous cycle) through as many as 5 cycles, after which observations were discontinued. Fourteen cows on restricted diet (RES-A) were ovariectomized on day 11 of a cycle when a CL was identified by ultrasonography, but was subfunctional (P4 < 1.5 ng/ml on day 10 of that cycle). Four additional RES-A cows which had subfunctional CL were not ovariectomized but were bled for an additional 25 d. At ovariectomy, CL and ovarian weights were collected. Luteal tissue was prepared for evaluation of P4 synthesis, LH responsiveness in vitro, and for determination of P4 content and total LH receptors. Bodyweight and BCS increased in CON cows; whereas, RES cows lost BW and BCS (P < .05). In the cycle prior to ovariectomy, serum P4 and LH were not different in 18 RES-A cows which developed subfunctional CL in comparison to CON cows. Four RES-A cows not ovariectomized but bled for an additional 25 d neither exhibited estrus, ovulated, nor had P4 concentrations greater than .3 ng/ml. Serum INS was lower in RES-A cows during the cycle prior to ovariectomy than in CON cows (P < .05). During the 11-d period prior to ovariectomy, mean serum P4 and INS were lower in RES-A cows than in CON cows (P < .05); however, serum LH was not different. Furthermore, CL and ovarian weights, P4 content of CL, secretion of P4 by luteal tissue in response to LH in vitro and LH receptor number were not different between CON and RES-A cows. In conclusion, nutritional anestrus may be preceded by the formation of a CL with lower steroidogenic output in vivo. However, luteal tissue, collected from RES-A cows, did not appear to be subfunctional during in vitro incubation when substrate availability and gonadotropin support were equal between diets.