Nutritional anestrus in beef cows: effects of body condition and ovariectomy on serum luteinizing hormone and insulin-like growth factor I.

The objectives of this experiment were to determine if a postcastration increase in concentrations of LH occurs in nutritionally anestrous beef cows and to examine the relationships between body energy reserves and secretion of LH and insulin-like growth factor-I (IGF-I). Nonpregnant, nonlactating, Hereford cows were fed to maintain (M) body weight (BW), body condition score (BCS), and normal estrous cycles (n = 5) or were fed a restricted (R) diet for 26 wk to lose BW and BCS and to become anestrus (n = 10). At 5-7 wk after the initiation of anestrus, R cows were randomly allotted to be ovariectomized (OVX) via flank incision (n 5) or to remain intact (INT, n = 5). OVX was performed when R cows became anestrous. All M cows were OVX. Serum was collected frequently the day before and during the first 10 days after OVX, and concentrations of progesterone, estradiol, LH and IGF-I were quantified. On Day 10 after OVX, 1 mg of estradiol was injected into 3 cows from each group and serum was collected for 30 h. After OVX, there was a treatment-by-day effect for mean serum LH and IGF-I concentrations. Concentrations of LH increased (p < 0.01) and concentrations of IGF-I decreased (p < 0.05) in M-OVX cows when compared with R-OVX and R-INT cows. Concentrations of LH and IGF-I were similar for R-OVX and R-INT cows. The number of LH pulses was similar for M and R cows. Estradiol treatment increased LH secretion in all M cows but not in all R cows. Concentrations of IGF-I in serum increased (p < 0.05) in M-OVX cows after treatment with estradiol, but IGF-I was not altered by estradiol treatment of R cows. We conclude that secretion of LH and IGF-I is greater, before and during the first 10 days after OVX, in cows fed to maintain BW than in nutritionally restricted cows that become anestrous. One mechanism by which reduced body energy reserves decrease secretion of LH may be independent of ovarian control and may involve decreased IGF-I secretions. Estradiol can acutely enhance concentrations of IGF-I only in beef cows with adequate body energy reserves.