Influence of dietary energy restriction on postpartum LH and GH secretory patterns in beef cows.

Postpartum LH and GH secretory patterns were evaluated with mature Brangus cows group fed either 50%M (n = 8) or 100%M (n = 7) NRC maintenance energy requirements starting two months prepartum and continuing for three months postpartum. Feed level was adjusted at 14 day intervals. Weights of cows and (or) calves were taken every 14 days before and after calving. Sequential blood samples were taken every 15 min for 8 h at biweekly intervals starting 14 days postpartum and continuing until 100 days. Serum LH and GH were determined by a validated double antibody RIA. A peak-detection algorithm was used to summarize hormone data into 5 variables for each cow at each bleeding period. Data were analyzed by GLM and regression using SAS procedures. Cows in the 100%M group were heavier (P<0.01) at calving time and at the end of the trial. There were no significant differences for weight of calves at birth or at the end of the study for the two energy levels. Total release of LH during secretory episodes was 5 fold greater (27 vs 5 ng/ml x min P<0.05) in cows receiving 100%M vs 50%M energy. Basal concentration and peak height of LH increased linearly with time postpartum in 50% but not 100% energy groups. For GH, cows receiving 50%M energy had more secretory spikes (2.6 vs 2.1 per 8 h; P<0.05), and higher basal concentration (14 vs 8 ng/ml; P<0.05), and a shorter duration of secretory episodes (82 vs 95 min; P