The period of the follicular phase during which the uterus of mares shows estrus-like echotexture influences the subsequent pregnancy rate

Likelihood of pregnancy after embryo transfer is reduced in recipient mares with a short preceding oestrus

“…Furthermore, the ITO was also positively correlated with the likelihood of pregnancy, which agrees with previous observations in Thoroughbred mares [7,8]. Furthermore, the ITO was also positively correlated with the likelihood of pregnancy, which agrees with previous observations in Thoroughbred mares [7,8].…”

“…These results parallel previous studies in Polo, Standardbred and Thoroughbred mares [4,7,8], in which there was a clear association between the ITO and the likelihood of pregnancy, with the lowest fertility in mares that ovulated soon after PGF. Following PGFinduced luteolysis, mares had significant differences in the duration of oestrus, and consequently the likelihood of pregnancy, with a marked difference between mares that ovulated 6 or fewer days after treatment with PGF compared to mares that ovulated >6 days after PGF.…”

“…The development of endometrial oedema is a phenomenon observed during oestrus, and involves engorgement of the endometrial folds with fluid in response to follicular oestrogens in the absence of progesterone [5,6]. As for oestrous duration, there is significant variability in the duration of the period of endometrial oedema during oestrus, with individual mares showing endometrial oedema for none, one, a few, or several days prior to ovulation [1,7]. The primary factor regulating both the development and subsequent dissipation of endometrial oedema is the balance between circulating concentrations of oestradiol and progesterone [6].…”

“…It has been suggested that the threshold circulating progesterone concentration necessary to inhibit endometrial oedema ranges from 1 to 2 ng/mL [6]. However, the progesterone concentration does not appear to be the only factor regulating the presence of endometrial oedema since a small percentage of mares [1,7] show little or no oedema during the follicular phase despite basal concentrations of progesterone (1 ng/mL) during a so called 'silent oestrus' [1].…”

“…Several studies have shown a positive correlation between the length of the follicular phase and likelihood of establishing pregnancy in mares following PGF-induced [4,7,8] or spontaneous luteolysis [7]. Several studies have shown a positive correlation between the length of the follicular phase and likelihood of establishing pregnancy in mares following PGF-induced [4,7,8] or spontaneous luteolysis [7].…”