Estrus: Association with productive parameters and implications to fertility

Comparison between the fertility of timed-AI protocols vs. AI based on spontaneous or induced estrus is often inadequate. Day post-partum at AI and the consequent grouping of animals with different cyclic status, BCS, and overall health status is a confounding factor caused by many experimental designs. Previous studies observing the effect of concentration of progesterone during diestrus, concentration of estradiol and length of proestrus and follicular dominance minimize or neglect the effect of the expression of estrus on parameters such as fertilization rate, embryo quality, and endometrium receptivity. In one study, the likelihood of ovulation was greater for high vs. low relative increase estrus, but a more detailed experiment also showed slight differences in the timing of ovulation. Expression of estrus near AI also modified the expression of genes related with the immune system, adhesion molecules and prostaglandin synthesis in the endometrium (MX1, MX2, MYL12A, MMP19, CXCL10, IGLL1, SLPI, OTR, and COX-2) and those related with apoptosis, P4 synthesis, and prostaglandin receptor (CYP11A, BAX, and FPr) in the CL. The expression of estrus was associated with increased P/AI for timed-AI (38.9 vs. 25.5%) and embryo transfer (46.2 vs. 32.7%) protocols. Moreover, there was a decrease in pregnancy loss in both programs. Data from other recent studies involving spontaneous and estradiol cypionate induced estrus have shown that greater relative increase and longer duration of estrus, captured by different activity monitors, have a significant impact on P/AI (over 12% points across different studies). Intensity and duration of estrus were correlated with BCS, parity, and secondary behavior signs as expected, but only weakly associated with milk production. Follicle diameter and concentration of estradiol at estrus were also weakly correlated with estrus expression. Collectively, ovulation could partly explain the observed reduction in fertility, but it is clear that the endometrium and the CL play an important role that is independent of parameters such as parity, BCS, and milk production. Quantitative information from estrus events could be used to improve estrus detection quality and develop decision-making strategies at the farm level. Further studies in this field should aim to 1) better understand ovarian, embryo and endometrium mechanisms associated with either the expression or intensity of estrus and, 2) refine the collection of phenotypes related t