Timed artificial insemination according to diameter of the preovulatory follicle improves fertility in lactating beef cows

The objective of this study was to evaluate whether timed artificial insemination (TAI) according to the diameter of the dominant preovulatory follicle (POF) would improve pregnancy rates in beef cows. In this study, lactating Nelore cows (Bos indicus; n=413), with 430-640 kg body weight, which were submitted to an estradiol/progesterone-based TAI protocol. On Day 0, cows given a progesterone device (1.9g Progesterone; CIDR®, Sao Paulo, Brazil) plus 2 mg of oestradiol benzoate (Bioestrogen®, Curitiba, Brasil) im, to synchronize follicular waves. On Day 10 (07:00 a.m.), the diameter of the POF was assessed by ultrasonography, and cows were randomized into Control (n=209) and Block (n=204) Groups. Control-Group cows were inseminated 48 h after CIDR removal, soon after ultrasound examination (08:00 a.m. on Day 10), and Block-group cows were inseminated at four time points according to the diameter of the POF as described by Pfeifer et al. (Reproduction, Fertility and Development, abstract, p 97, 2015): B0 (POF ≥ 15 mm, TAI at 08:00 a.m. on Day 10, n=51), B1 (POF 13 to 14.9 mm, TAI at 02:00 p.m. on Day 10, n=70), B2 (POF 10.1 to 12.9 mm, TAI at 08:00 a.m. on Day 11, n=54), and B3 (POF ≤10 mm, TAI at 02:00 p.m. on Day 11, n=28). Pregnancy status was assessed 30 days post-AI by ultrasonography. The diameter of the pre-ovulatory follicles on Day 10 was analyzed by analysis of variance – Factor ANOVA and the means between groups were compared by Tukey’s post hoc test. Pregnancy rate was evaluated by Chi-square analysis. There was no difference in the diameter of the POF 48h after CIDR removal (P=0.4) between Groups. The pregnancy rate was greater in the Block-Group (63.2%, 129/204) than in the Control Group (48.8%, 102/209; P=0.002). In conclusion, our results demonstrate that TAI results can be improved when performed according to the diameter of the preovulatory follicle. This technique may represent an effective tool to improve fertility of lactating beef cows receiving TAI protocols utilizing estradiol as ovulation inducer.