Advanced Preovulatory Follicle Progression Associated with Higher Estrous-Associated Temperatures (HEAT) and Impacted by Estrous Active Behaviors

Pablo Fioravanti¹, Sarah E. Moorey¹, Ian E. Batey¹, Megan D. Mills¹, Abigayle B. Pollock¹, Kennedy Hill¹, Michael A. O’Neil², Rebecca R. Payton¹, Emma A. Hessock¹, F. Neal Schrick¹, and J. Lannett Edwards¹

1. Department of Animal Science, The University of Tennessee Institute of Agriculture and AgResearch, Knoxville, TN, USA, 37996

2. Office of Innovative Technologies-Research Computing Support, The University of Tennessee, Knoxville, TN, USA, 37996

Abstract Text:

Females in estrus exhibit varying levels of higher estrous-associated temperatures (HEAT). To evaluate the extent to which this hormonally-induced, sexually-related form of exercise, alone or in combination with elevated body temperature, affects intrafollicular components after first mount, preovulatory follicular fluid and serum hormones, estradiol (E₂) and progesterone (P₄), were compared in heifers that were comingled with others or restricted from engaging in estrous active behaviors by placing in an individual pen adjacent but in sight of others. Estrus was induced in Angus heifers (n = 38) using a seven-day controlled internal drug release (CIDR)-PGF_2α protocol. Vaginal temperature was recorded every 5 minutes using a Thermochron iButton affixed to a blank CIDR. Animals were continuously monitored. Estrus was defined as the first time a heifer stood to be mounted at which point treatments were alternately assigned (comingled, n = 13; restricted, n = 12). Blood samples were obtained at CIDR removal, first mount and at follicle aspiration. Preovulatory follicle fluid was aspirated ~11.9 h after first mount. Serum estradiol (E₂) and progesterone (P₄) as well as follicular fluid P<sub>4 levels were determined by radioimmunoassay. Follicular fluid (diluted 1:2500) E₂ was assayed via ELISA. Hierarchical linear regression models were performed using hormone (dependent variables) and total area under HEAT curve, treatment (estrus active or restricted) and respective interaction. Best fit models were determined using backwards manual selection after ensuring lack of collinearity among other heifer covariates and data normality. Intrafollicular and serum levels of E₂ at ~11.9 ± 0.1 hours after 1^st mount were negatively related to HEAT. Per each unit increase in HEAT, intrafollicular E₂ decreased by -1.40 ng/ml (P = 0.0003), whereas serum E₂ decreased by -0.04 pg/ml (P = 0.0058). Restricted heifers had higher intrafollicular E₂ levels compared to those allowed to comingle with others after 1^st mount (388 versus 233 + 19.8 ng/mL; P < 0.0001). Neither HEAT nor restricting estrous active behaviors affected preovulatory follicle size at aspiration, change in follicle size from 1^st mount to aspiration, intrafollicular P<sub>4, intrafollicular E₂:P₄ ratio, and serum P<sub>4 levels at aspiration. Decreases in intrafollicular and circulating levels of E₂ are expected within 6 hours after the luteinizing hormone (LH) surge and continue thereafter. In our study where the preovulatory follicle was aspirated ~12 h after 1^st mount or ~8 h after the LH surge, a decrease in E₂ would be expected. Although subtle but impactful effects on the timing of the LH surge cannot be ruled out, advanced preovulatory follicle progression in heifers comingled with others is more likely attributable to the physiological differences that go along with exhibiting estrus active behaviors. To our knowledge, this is the first study to document that preovulatory follicle progression in estrual heifers is more advanced in females exhibiting estrus active behaviors and experiencing greater levels of HEAT. Consequences related to advancing follicle progression likely have impacts that carry over to the cumulus-oocyte complex contained therein and other components that may be involved in the initial cascade of events important for setting stage for ovulation later. Study was supported by Agriculture and Food Research Initiative Competitive Grant no. 2022-67015-36374 from the USDA National Institute of Food and Agriculture, UT Institute of Agriculture, and Animal Science Department.