Heat stress is also detrimental to the follicle that encloses the oocyte cell and can lead to more small and medium sized follicles and reduced dominance. There is a close correlation between these follicle alterations and changes in the endocrine system resulting in reduced concentrations of estradiol—a powerful and natural ovarian and placental estrogen that works to prepare the uterus for implantation of the fertilized ovum. Principal effects of lower estradiol levels include impaired estrus duration and intensity, and increased incidence of anestrus (Gwazdauskas et al., 1981; Wolfenson et al., 1988). High air temperatures 10 days before estrus are shown to be associated with low fertility (Al-Katanani et al. 1999) but even after pregnancy is established, there is a significant pregnancy loss in dairy cattle: about 10% of pregnancies at Day 40-50 resulting in fetal loss (Garcia-Ispierto et al., 2006).
When viewed directly with the THI, the correlation between temperature/humidity and pregnancy loss is very clear (Figure 5) and collectively all these changes and events reduce the probability of conception rate and fetal survival.