Abstract
The aim of this study was to evaluate the effect of administration of rbST at different days during a timed AI protocol on fertility during the summer and winter seasons. A total of 1961 breedings from 1027 lactating Holstein were enrolled. Bovine somatotropin was administrated after the voluntary waiting period every 14 days until dry-off. Animals were split into two groups dependent on the timing of the bST treatment and the beginning of the timed AI protocol based on estradiol and progesterone. Group 1 (GP1) received rbST at d-17, d-3 and d11 and Group 2 (GP2) at d-10, d4 and d18, relative to timed AI. Body temperature (BT) was measured at the moment of AI and temperature ≥ 39.1° C was considered as high body temperature. Data were analyzed using GLIMMIX procedure of SAS. Animals that were bred in the summer in GP1 had higher pregnancy/AI when compared with animals in GP2 (36.1% vs. 27.1%; P < 0.01); no differences were found in winter. Animals that had high BT at the moment of AI and were in GP1 had higher pregnancy/AI than animals that had high BT and were in GP2, this was true for both the summer and winter seasons (Summer - 37.2% vs 22.4%; Winter – 38.9% vs 28.3%, P = 0.05). In conclusion, animals that were bred in the summer and had high body temperature at the moment of AI benefited from being in GP1 by having higher pregnancy/AI when compared with animals in GP2. The administration of rbST at certain days within a timed AI protocol may improve follicle and embryo quality, thus resulting in higher pregnancy outcomes.