Nursing Enhances the Negative Effect of Estrogen on LH Release in the Cow1

Abstract

Twenty-three crossbred beef cows between 4 and 5 yr of age were assigned at random to one of six treatments: (1) ovariectomized 4 d postpartum (OVX) with early weaning of calves 21 d postpartum (OVX-EW; n=4), (2) OVX-EW and 17β-estradiol implants (OVX-E₂-EW; n=4), (3) OVX and normal nursing by calves throughout the experiment (OVX-NN; n=3), (4) OVX-NN and 17β-estradiol implants (OVX-E₂-NN; n=4), (5) intact cows and early weaning of calves 21 d postpartum (EW), (6) intact cows and normal nursed (NN). Blood was collected at 15-min intervals over a 4-h period once weekly during the 12-wk postpartum period in the OVX cows. Early weaned intact cows exhibited estrus 23 d sooner (P<.05) than normally nursed cows. A hormone level for each cow at each week was determined from the mean of the 17 samples collected over the 4 h period each week. There were no significant changes due to E₂ treatment, for concentrations of LH, FSH or number of pulses during wk 1 through 3. However, during wk 4 through 12 the linear and quadratic contrasts of wk × estrogen × nursing were significant for serum LH, indicating there was no difference between the treatments for EW and NN without E₂ but there was a large difference in the presence of E₂. During nursing E₂ suppressed serum LH below that of nonestrogen-treated cows while after weaning E₂ stimulated LH release above that of nonestrogen-treated cows. The linear contrast of wk × nursing interaction approached significance in regards to number of LH peaks. There were no significant interactions in either serum FSH or number of FSH pulses over the 4 to 12 wk period. From these data the hypothesis is proposed that the suckling stimulus increases time to first estrus by increasing the sensitivity of the hypothalamus to the negative feedback of estrogen during the postpartum period resulting in decreased LH release.