Modeling changes in rumination time and milk yield to detect the onset of heat stress in lactating dairy cows of different physiological traits

Determining the temperature-humidity index (THI) threshold for the onset of heat stress (HS) can serve as a decision-making tool in HS mitigation management in dairy cows. While classic thresholds are based on production losses, other physiological and behavioral changes may appear at lower THI. The objective of this observational study was to model the changes of daily rumination time (RT) and milk yield (MY) of lactating dairy cows to identify potential HS thresholds of different indicators. Daily RT and MY were collected from Chinese Holstein cows of 1,007 cow-lactations 532 ± 72.8 min/d RT; 38.0 ± 8.92 kg/d MY (mean ± SD). Daily mean THI in the barn ranged from 25.6 to 84.8, and HS duration (i.e., h/d when THI was above 68) ranged from 0.0 to 24.0 h/d. Four data sets were used to evaluate differences in parity (primiparous or multiparous), lactation stage for each parity (early, peak, mid, or late lactation at 1-40, 41-60, 61-160, or 161- 305 DIM, respectively), and production level of multiparous cows (low or high averaging 37.1 or 52.1 kg/d MY, respectively). The marginal effect of mean, minimum, and maximum THI and HS duration on RT or MY were analyzed using generalized additive mixed model framework. Pointwise slopes of the fitted means were calculated and were used to determine the HS threshold (based on mean THI or HS duration) and, subsequently, the average rate of decrease in RT or MY upon reaching the threshold. The study demonstrates that, for most of the cow groups, a significant decrease in RT was detected at a lower mean THI (around 57.2 to 62.2) than losses in MY (mean THI 64.9 or higher). The same trend was observed when the effect of HS duration was evaluated, wherein it generally took a shorter HS duration to detect the onset of the decrease in RT than in MY. Upon reaching the threshold, cows decreased their RT by 1.98 to 4.67 min/d/unit mean THI or 0.64 to 3.64 min/d per hour more of HS duration, and their MY by 0.041 to 0.171 kg/d/unit mean THI or 0.036 to 0.112 kg/d per hour more of HS duration. The onset and the severity of HS responses differed according to cow parity, lactation stage, and production level with high-producing, multiparous cows in mid-lactation being the most susceptible to HS. Therefore, our results suggest that management strategies to mitigate HS should be applied based on the animal's heat tolerance which should be before mean THI exceeds 57. More studies may be needed to incorporate intensity, duration, and possibly timing of heat exposure to better understand the daily heat load that the different cow types can tolerate.