Serum hormone profiles, pregnancy rates, and offspring performance of Rambouillet ewes treated with recombinant bovine somatotropin before breeding

An experiment was conducted to examine effects of bovine ST (bST) on serum hormone concentrations, pregnancy rates, and offspring performance. Before initiation of a fall breeding period, 75 Rambouillet ewes (68.8 +/- 1.5 kg) received an intravaginal insert containing 0.3 g of progesterone (P4) to synchronize onset of estrus. After 12 d, inserts were removed (d 0), and ewes (stratified by BW and age) received either 0 (control, n = 37) or 250 (n = 38) mg of recombinant bST (Posilac, Monsanto, St. Louis, MO, subcutaneously). Ewes were joined with fertile rams 24 h after insert removal. Blood samples were collected from 12 ewes in each treatment group daily from d 0 to 20 after insert removal. Serum IGF-I concentrations were 315 and 437 (+/- 58) ng/mL in control and bST-treated ewes 2 d after receiving bST (P = 0.02) and remained increased (P < 0.03) in bST-treated ewes throughout the 13-d period (P < 0.05). Serum prolactin (P > 0.10) and estradiol (P = 0.65) were similar between treatments. Serum triiodothyronine (T3) and thyroxine (T4) concentrations were similar (P > 0.20) between treatments from d 0 through 8. Controls had greater (P < 0.04) serum T3 and T4 concentrations than treated ewes did until d 18. Serum P4 was similar (P > 0.10) in control and bST-treated ewes from d 0 through 3 but was increased (P < 0.05) from d 4 to 8 in control ewes. Serum P4 was again similar (P > 0.10) between treatments from d 9 to 20. Serum insulin concentrations were 0.44 and 1.74 (+/- 0.19) ng/mL in control and bST-treated ewes, respectively, 1 d after receiving bST (P < 0.001) and remained increased (P < 0.03) in bST-treated ewes through d 9 (P < 0.03). Serum glucose was increased (P = 0.003) from d 0 to 10 in bST-treated ewes compared with controls. Thirty-three of 37 (89%) control ewes were pregnant, whereas 27 of 38 (71%) bST-treated ewes were pregnant (P = 0.05). As a percentage of ewes lambing, 61% and 39% of control ewes produced single and twin lambs, respectively, compared with 41% and 59% of bST-treated ewes (P = 0.12). Lamb 60-d adjusted weaning weights were 23.0 and 21.2 (+/- 0.65) kg for offspring produced by control and bST-treated dams, respectively (P = 0.04). In conclusion, serum IGF-I, insulin, and glucose were greater whereas serum T3, T4, and P4 were less in bST-treated ewes than in controls. Pregnancy rates and offspring adjusted weaning weights were decreased by bST treatment immediately before breeding.