Efeito da aplicação de ocitocina durante a ordenha sobre a eficiência reprodutiva de vacas mestiças submetidas à inseminação artificial em tempo fixo

Abstract

In cows, the "critical time" of the estrous cycle determined by the physiological state that must be defined to maintain the corpus luteum (CL) and pregnancy or luteolysis and a new estrus occurs 15 to 19 days after ovulation. The mechanisms involved in premature luteolysis and regression of the CL are not yet understood complements. It is probable that the early CL regression is related to PGF2a release mediated by the endometrium lower concentrations of P4 receptors, higher concentrations of OT receptors, or both, in the endometrium. Thus, uterine receptors for P4 and OT may influence the secretion of PGF2a from the 10th day of the cycle, during which the CL is more sensitive to the luteolytic action of PGF2a. This study aimed to evaluate the effect of exogenous OT, postpartum crossbred cows on reproductive efficiency, with emphasis on the period of maternal recognition of pregnancy. The experiment was conducted in Bom Jesus do Itabapoana, Northwest Region Fluminense between September 2013 and January 2014. Twenty- four cows were entered into a TAI program (D0 - progestin releasing intravaginal device + estradiol benzoate (EB - 2 mg, im); D8 - intravaginal device removal and application of PGF2a (0.526 mg, im); D9 - BE (1 mg, im); D10 - IA). After ovulation, cows were divided randomly into two groups: GO (n=12): oxytocin (25 IU, iv, milking morning and afternoon), GC (n=12): without OT. Sonographic evaluations were performed on days 10, 15, 18, 28 and 42. The diameter of CL was calculated as the average of the two major axes and volumes (vol) using the formula for volume of a sphere (V = 4/3p × (D / 2) 3) where D is the average diameter (DM) of CL. On day 28 of pregnancy diagnosis, confirmed at 45 and 60 days was carried out. We observed an increase in GC DM CL between the 10th and 18th. In GO DM CL decreased among 10th and 15th and most cows expressed estrus before day 18 (9/12 - 75%), six (50%) with return to estrus between the 15th and 18th. The vol. CL was higher in cows GC at all times with a significant difference (p<0.05) compared to GO on the 15th. Cows in the CG had vol. CL higher at all times and had a higher pregnancy rate (41.7%) compared to GO (25 %). The rates of fertilization and maintenance of pregnancy until day 42 demonstrates the low reproductive efficiency in GO compared to the CG (p=0.0022). With successive inseminations at 90 days one seven cows were empty in CG (8.33%) and GO (58.3%), respectively. The pattern of development and regression of the CL confirms the involvement of OT in the reproductive activity of lactating cows. The low reproductive efficiency in GO is probably due to the action of OT, confirming the hypothesis that exogenous OT can inhibit the maternal recognition of pregnancy.