Ninomiya, S., Goto, Y., Huricha et al. 2023. Lying posture as a behavioural indicator of heat stress in dairy cows. Applied Animal Behaviour Science 265, 105981.

Dairy cows tend to stand as a heat stress response, but the increased tendency to stand adversely affects their productivity and welfare. Thus, taking measures against heat stress before cows spend more time standing is important. Therefore, this study focused on the lying postures of cows (lying with the head turned back (HB) and lying with the head up and still (HS)) as behavioural indicators of countermeasures. It is predicted that if cows adopt a posture that exposes a large body surface area to the outside air as an adaptive response to heat stress when they are in a lying position, the expression time of HB will decrease. Six Holstein dairy cows which had calved two or three times were observed using video filming, and their behaviour was recorded for 12 days throughout the year. The temperature and humidity in the cow barn were measured and used to calculate the temperature-humidity index (THI). Standing, lying down, rumination, and HB were recorded continuously, and HS was instantaneously sampled every 30 s. Lying, standing, and rumination durations, and the time ratios of HB and HS while lying down were calculated. The data obtained from 19:30 h to 4:30 h and from 10:00 h to 15:00 h were used as nighttime and daytime data, respectively; however, data obtained from the dry milk period were excluded. Data on each behaviour during the daytime or nighttime were applied to four linear mixed models (linear, quadratic, and cubic regressions, and broken stick), including THI and milk yield as explanatory variables, and the model with the lowest value of Akaike’s information criterion was selected. A broken-stick model was selected for the analysis of lying and THI in the nighttime, and the result showed that lying time decreased with a THI value of 75 as a boundary. A linear model was selected for the analysis of HB and THI in the nighttime, and the result showed that the time ratio of HB while lying down decreased with increasing THI. These results suggest HB may be a behavioural indicator of heat stress before cows spend more time standing as a heat stress response.