Comparison between two estradiol-progesterone based protocols for timed artificial insemination in blocks in lactating Nelore cows

The aim of this study was to compare the use of artificial insemination in time blocks (Artificial Insemination Blocks, AIB) using an 8 and 9 d estradiol-progesterone based protocol. In this experiment, lactating Nelore cows (n = 253) were subjected to two estradiol-progesterone based TAI protocols. On the morning of Day 10 (8d group, n = 124) or Day 11 (9d group, n = 129), cows were examined by ultrasonography to evaluate the diameter of the preovulatory follicle and were inseminated once at one of the following time points, according to the diameter of the pre-ovulatory follicle (POF): Block 0 (POF ≥ 15 mm, TAI 0 h after conventional TAI), Block 1 (POF 13.0-14.9 mm, TAI 6 h later), Block 2 (POF 10.1-12.9 mm, TAI 24 h later), and Block 3 (POF ≤ 10.0 mm, TAI 30 h later). The pregnancy per AI (P/AI) did not differ between 8d and 9d groups (P > 0.05). Considering only multiparous cows, however, P/AI tended to be greater in the 8d (64.1%) than in the 9d group (49.3%; P = 0.08). Cows from the 9d group tended to have a larger POF than cows from the 8d group (P = 0.07). In conclusion, these results provide evidence that there is no difference between 8d or 9d protocols when using the AIB technique. Use of the 8d estradiol-progesterone based protocol, however, tended to increase pregnancy in multiparous cows.