Quantifying Adaptive Mechanisms and Ovarian Dynamics in Developing Heat Stress Beef Heifers

Abstract

The objective of this study was to quantify the adaptive mechanisms and ovarian dynamics in developing beef heifers which influence reproduction. We hypothesized that heat stress conditions would have negative implications for reproduction in heifer. A total of 12 crossbred heifers were obtained from the University of Arizona (U of A) Experiment Station V-V Ranch near Camp Verde, Arizona. Inclusion criteria for the study included matured mix breed heifers with an ideal pubertal weight and showing signs of estrus cyclicity. Heifers were stratified by body weight (BW) and randomly assigned to one of two treatment groups: heat stress (HS) or thermoneutral (TN) for 40-days (40d). HS was defined as exposure to a temperature humidity index (THI) of 73–83 while the TN (control) group was defined as exposure to a THI of 68. Over the 40d period, vital parameters (respiratory rate [RR], rectal temperatures [RT]) and water intake (WI) were taken twice daily. Blood samples, body weight (BW) and body condition scores (BCS) were collected biweekly. Average concentrations of cortisol, progesterone, non-esterified fatty acids (NEFA) and beta-hydroxybutyrate (BHB) were obtained from plasma samples. At day 40, non-terminal bilateral ovariectomies were performed and ovarian metrics were measured and analyzed. For repeated measurements, outcome variables were analyzed using a mixed-effects regression model with the inclusion of day and treatment as fixed effects and the random effect of the individual. Pairwise comparisons of the estimated marginal means were performed, and statistical significance was set at a Tukey corrected p-value of <0.05. For ovarian measurements, an ANOVA was performed with a threshold for significance set at a p-value <0.05.Results showed that there was a significant difference between average RR, where HS heifers had higher breaths per minute in the morning and afternoon (P<0.05). Afternoon RTs were significantly increased in HS heifers while morning RT were not statistically different. This indicated that our heat stress model was effective in inducing heat stress conditions within our experimental group. Regarding body metrics, BW and gain to feed ratios were not significantly different between groups. Although statistical differences were observed for average BCS (P<0.05), with HS heifers having an average BCS of 5 versus TN which had an average BCS of 4.8, this was not considered biologically significant. These results suggest that HS was not a major factor contributing to weight gain or feed efficiency. Average NEFA concentrations were significantly higher in TN heifers (P < 0.04); however, BHB concentrations did not differ between groups. This indicates that the HS group were high mobilizers for fatty acids but low oxidizers. Average plasma cortisol and progesterone were not statistically different, suggesting heat adaptation within our cohort. Out of the ovarian tissue metrics analyzed, a significant difference was observed in combined total antrafollicle surface count of both ovaries, where TN heifers had a higher count (P=0.05). This suggests that heat stress conditions caused a reduction of antra follicles, which decreases ovarian reserve, and, consequently, overall fertility. In conclusion, for this cohort a higher threshold for heat stress conditions would be needed to see differences in all outcome variables. This suggests that our cohort had as adaptive advantage to heat tolerance given that the heifers were native to Arizona. This pilot study is the first to assess heat stress in Arizona beef heifers, providing preliminary data on their early-stage physiological adaptations. It lays the groundwork for further research aimed at developing a selection tool to help producers identify ideal replacement heifers. Selecting the right replacements is crucial for cattle producers, as it enhances long-term herd productivity, reduces costs, and ultimately lowers meat prices for consumers.