Global warming is resulting in an overall increase in temperatures and in the frequency of extreme weather events. In dairy cattle, thresholds within the temperature-humidity index (THI) have been used to indicate points at which cattle will likely experience thermal stress (e.g., a THI threshold of 75 predicts thermal stress). However, high-yielding dairy cows that reside in temperate maritime climates may experience some degree of thermal discomfort below this threshold particularly when they are housed. Housing often results in high levels of humidity. The use of technology such as activity monitors and automated intake measures allow us to monitor responses. The aim of this study was to use technological solutions to assess behavioural changes in response to moderate increases in THI levels. Data from dairy cattle on an experimental unit were used. Data on daily lying times, lying bout frequency, step count, feed and water intake and milk yield were extracted for 8 pairs of warmer (THI<65) and 8 matching cooler (THI=43 to 60) periods. Warm and cool periods were no more than 5 weeks apart to ensure that the data from the same animals were being compared. The first three days from each period were analysed. Results showed that total daily lying time was shorter during warmer periods than cooler periods (P<0.05; means and SEMs (h): warm: 11.3±0.06h; cool: 11.8±0.06h). However, there was no effect of THI level on the no. of steps taken by cows (P>0.05 (counts) warm: 868±8 steps; cool: 878±9 steps). Water intake was higher during warm periods (P<0.05: (l) = warm: 81.2±0.7l; cool: 72.1±0.6), but there was no difference in feed intake (warm: 57.6±0.5kg; cool: 57.1±0.3kg). Milk yield was lower during warm periods than cool periods (P<0.05; (l): warm: 31.8±0.3; cool: 32.7±0.2). This suggests that behaviour and milk yield are mildly adversely affected even in conditions that are not traditionally regarded as exceeding cows’ ability to cope with thermal challenge
