Modulation of reproductive hormones by suckling and exogenous gonadal hormones in young beef cows post partum

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Abstract
Plasma concentrations of LH [lutropin], prolactin, progesterone and estradiol-17.beta. were measured during postpartum anestrus in 12 young beef cows (6 suckling, 6 not suckling). A progesterone-releasing intravaginal device (PRID) was inserted on day 60 post partum for 12 days in 3 cows in each group. Two of 3 suckling cows exhibited estrus within 2 days after PRID removal (day 73); none of the untreated cows showed estrus before day 93. Five of 6 non-suckling cows showed estrus before day 60. Intensive bleeding periods on days 45, 58, 70 and 72 provided plasma samples for LH and prolactin measurements. Suckling did not affect the LH baseline or the number of LH spikes but did decrease the mean value of the spikes; all 3 characteristics were increased for prolactin. The number of prolactin spikes and the length of anestrus in suckling cows were correlated (r = +0.87, P < 0.05) as were mean basal LH level and length of anestrus (r = -0.89, P < 0.05). A decrease in plasma LH occurred in suckling and nonsuckling cows during the period (days 60-72) they were treated with gonadal hormones.