The effect of recombinant bovine somatotropin on the pregnancy rate of resynchronized nellore cows submitted to fixed-time insemination

The objective of the study was to evaluate the effects of recombinant bovine somatotropin (rbST) during resynchronization on the pregnancy rates of Nellore cows submitted to fixed-time artificial insemination. A group comprised of 100 multiparae Nellore cows was submitted to a resynchronizing protocol: on day 0 the subjects received the Bovine Intravaginal Device with 1,0g of progesterone (P4), associated with intramuscular administration of 2,0mg of estradiol benzoate (IM); on day 8, the Bovine Intravaginal Device (DIB®) was removed, and 1,0mg of estradiol cypionate, 0,15mg of prostaglandin F2α and 300 UI of equine chorionic gonadotropin were administered; on day 10, fixed-time artificial insemination was conducted (FTAI ). The subjects were then randomized into Control Group (n=50) and Treated Group (n=50), of which the latter received 500mg of rbST on the eighth day of the protocol. Sixty days after the fixed-time artificial insemination, pregnancy diagnoses were conducted via rectal palpation. Blood samples were taken to measure plasma concentrations of glucose, triglycerides and cholesterol. Pregnancy rates were statistically evaluated through Generalized Linear Models Theory, and their significance was tested through Analysis of Deviance. No difference was found between groups, pregnancy rates were 40% and 48% for control and treated groups, respectively. Cholesterol plasma concentrations were also not affected by the treatment with rbST. There were significant increases in serum glucose and triglycerides between groups. The administration of rbST to multiparae Nellore cows, in a single dose, did not statistically interfere in the pregnancy rates. The resynchronization of estrus and re-insemination had a positive effect in cumulative pregnancy rates.