When do dry cows get heat stressed? Correlations of rectal temperature, respiration rate, and performance

Previous studies have documented the negative effects of heat stress during the dry period on dairy cow performance during the next lactation, but an easy method to assess heat stress for dry cow management is lacking. In an effort to determine a threshold for heat stress based on the physiological response of respiration rate, an easily measured variable, an analysis including data collected from 6 different studies (n = 144 cows) was performed to summarize the correlations among rectal temperature (RT) and respiration rate (RR) during the dry period, milk production during the first 8 wk of lactation (MK), calf birth weight (CW), body weight at calving (BW), gestation length (GL), and dry matter intake (DMI) pre- and postpartum. Studies were conducted in Florida during the summer, and dry cows were assigned to 2 treatments: heat stress (only shade; HT, n = 75 cows) and cooling (shade, fans, and soakers; CL, n = 69 cows). Average RT (0.3°C) and RR [26 breaths per min (bpm)] of HT cows increased compared with CL. In addition, the CL group produced more milk (2.8 kg/d) during the first 8 wk of the subsequent lactation relative to HT. Correlation analysis indicated that RR of HT was negatively correlated with MK and GL. Within HT, RT was also negatively correlated with MK and GL and tended to be negatively correlated with CW. In addition to being negatively correlated with RT and RR, HT MK was positively correlated with CW, GL, and DMI postpartum. In CL, no correlations were observed for RR, but RT was positively correlated with BW, and MK was positively correlated with DMI postpartum and tended to be positively correlated with GL. Moreover, the overall RT was 39.1 ± 0.48°C and RR was 61 ± 19.5 bpm, which indicates that RR over 61 bpm is an indicator of heat stress in cows during the dry period and can be easily assessed at the cow level. In summary, the more heat load a cow carries in the dry period, the greater the negative effects are on subsequent milk production, which may suggest a threshold for the effects of heat stress. Also, variation among cows within the HT group indicates that cows with longer gestation length have heavier calves, eat more postpartum, and produce more milk, but not as much as CL cows.