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

Plasma concentrations of LH, prolactin, progesterone and oestradiol-17 beta were measured during post-partum anoestrus 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 three suckling cows exhibited oestrus within 2 days after PRID removal (Day 73) whereas none of the untreated cows showed oestrus before Day 93. Five of six non-suckling cows showed oestrus 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 three of these characteristics were increased for prolactin. The number of prolactin spikes and the length of anoestrus in suckling cows were correlated (r = +0.87, P less than 0.05) as were mean basal LH level and length of anoestrus (r = -0.89, P less than 0.05). A decrease in plasma LH occurred in suckling and non-suckling cows during the period (Days 60-72) they were treated with gonadal hormones.