Higher Estrous Associated Temperatures (HEAT): Relationship with Different Estrous Active Behaviors

Ian E. Batey¹, Pablo Fioravanti¹, Kennedy Hill¹, Rebecca R. Payton¹, Hao Gan², Yang Zhao¹, F. Neal Schrick¹, Michael A. O’Neil³, and J. Lannett Edwards¹

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

2. Department of Biosystems Engineering and Soils Science, The University of Tennessee Institute of Agriculture and AgResearch, Knoxville, TN, USA, 37996

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

Abstract Text:

Estrous active cattle exhibit varying levels of higher estrous associated temperatures (HEAT) that are impactful on preovulatory follicle components of fertility importance. Although factors including follicle size, hormone levels, ambient conditions, and other animal variables provide some explanation, 52 to 95% of HEAT variation remains unexplained. Increased walking elevates body temperature; thus, it was hypothesized that estrous active behaviors like increased walking, mounting, and others are contributive. Although intuitive, objective was to identify active behaviors associated with HEAT when occurring after onset, up through maximum temperature in beef heifers and cows. Estrus was induced using a seven-day CIDR-PGF_2α protocol and defined as first time a female stood to be mounted by another. Onset of HEAT was when vaginal temperature (VTp; iButton attached to blank CIDR) increased 0.1°C above baseline. Monitored ‘continuously’ by a team of individuals for 5 days after PGF_2α, estrous active behaviors were recorded and summarized every 15 min. Females experiencing or performing estrous active behaviors (i.e., standing to be mounted versus mounting others) were distinguished. Number in a sexually active grouping when a female was also sexually active was recorded (PopSAG). A collar-based Axy5 triaxial accelerometer provided an additional estimate of estrous activity. Temperature-humidity index was recorded hourly (THI). Hierarchical linear regression models were performed using backwards manual selection to derive the best-fit models for VTp and the change in VTp from baseline (∆VTp). An instance of low stress handling occurred when five heifers exhibited HEAT and was accounted for in model. Eighteen of 24 Angus heifers displayed estrus (75%). Average time from HEAT start to maximum was 5.6 hours (range: 2.2 to 12.1). Variables significantly explaining VTp variation from HEAT onset to maximum (R² = 0.38) included actively mounting others, entropy (accelerometer estimate), baseline temperature and duration, low stress handling, and THI. Per each additional time a heifer mounted another in a 15-minute window, VTp increased by 0.05°C (P = 0.0002). Low stress handling, increased VTp by 0.33°C (P < 0.0001). The same variables, except baseline temperature, explained ∆VTp variation (R² = 0.35). Fourteen of 15 Angus/Angus cross cows displayed estrus (93.3%). Average time from HEAT start to maximum was 10.3 hours (range: 2.8 to 19.1). Variables significantly explaining VTp variation (R² = 0.37) included the act of actively mounting others, standing to be mounted, headbutting, PopSAG, movement variation (accelerometer estimate), and THI. For instance, each time a cow mounted another in a 15-minute window, VTp increased by 0.03°C (P = 0.01). For example, each time a cow stood to be mounted by another in a 15-minute window, VTp increased by 0.02°C (P = 0.04). For each unit increase in movement variation, VTp increased by 0.25 (P < 0.0001). For each additional participant in a sexually active group, VTp increased by 0.02°C in a 15-minute window (P = 0.01). Interestingly for each unit increase in THI, VTp decreased by 0.07°C for that same time period (P = 0.02). These same variables, with the addition of baseline temperature duration and entropy (accelerometer estimate), explained ∆VTp (R² = 0.48). In summary, different estrous active behaviors along with the size of sexually active group positively contributes to HEAT variability. 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.