Effect of Nutrition on Response to Exogenous FSH in Beef Cattle

Forty cows were assigned to one of two groups and fed either a high (H) or low (L) level of ration (130% and 70% of NRC requirements for TDN, respectively). Over a period of 92 days, the H cows gained .63 kg/day and the L cows lost .15 kg/day. On day 98 of the feeding regimen, after a synchronized estrus and follicle stimulating hormone (FSH) injected twice daily for 5 days, the two groups were subdivided into four groups. Ten cows in each group were ovariectomized 15 hr after the last FSH injection to provide data on follicular development before ovulation (H-Fol and L-Fol). The other 10 cows in each group were bred twice by artificial insemination, at first detection of estrus and again 12 hr later, to provide ovulation and ovum fertilization data (H-Cl and L-Cl). Three H-Fol and five L-Fol cows ovulated before laparotomy and three L-Fol cows had cystic ovaries. L-Fol cows had higher ovarian weights (P<.01) and follicular fluid weights (P<.05) and more large follicles (P<.01) than H-Fol cows. The number of large follicles plus corpora lutea (CL) was correlated with body condition estimated at the time of laparotomy (r = .77; P<.01). Neither ovulation rate nor fertilization rate differed between cows in the H-Cl and L-Cl groups. Concentrations of luteinizing hormone (LH) did not differ between the H and L groups in serum collected weekly throughout the feed period or during the synchronized estrus and did not differ between the H-Cl and L-Cl groups during the luteal phase of the estrous cycle after superovulation. Progesterone concentration tended to be higher in serum of cows on the high feed than in those on low feed (9.6 ± 1.0 and 7.1 ± .8 ng/ml, respectively; P<.07) during the synchronized estrus before ovulation. When serum progesterone values were corrected for CL number, differences due to feed treatments were not significant.