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

Numerous studies have shown the negative effects of heat stress during the dry period on dairy cow performance during the subsequent lactation. Exposure of cows to heat stress during the dry period is associated with compromised mammary cell formation and decreases in milk yield in the subsequent lactation. Moreover, a greater incidence of postpartum disorders and lower reproductive performance are associated with exposure of cows to heat stress during the dry period. Dry period heat stress also decreases dry matter intake and body weight, gestation length, and calf weight. Lactating cows start to experience the effects of heat stress when the temperature humidity index (THI) is higher than 68. Although the THI is a useful tool to assess the degree of heat stress potential on the cow, her own physiological responses to heat should be a better indicator of her degree of heat stress. Currently, no information is available to show a physiological threshold to identify that the individual dry cow is on the verge of experiencing significant heat stress. To better understand when dry cow heat stress affects performance, in the study of  Dr I.M. Toledo and colleagues at Florida, correlation analysis of rectal temperature and respiration rate during the dry period, milk production during the first 8 weeks of lactation, calf birth weight, cow body weight at calving, gestation length, and dry matter intake pre- and post-calving were performed. The results were published in JDS Communications, Volume 1 of 2020, page 21 to 24, with the title: When do dry cows get heat stressed? Correlations of rectal temperature, respiration rate, and performance.  

The analysis included data collected from six different studies with 144 cows to calculate the correlations among rectal temperature (RT) and respiration rate (RR) during the dry period, milk production during the first 8 weeks of lactation (MP), calf birth weight (CW), body weight at calving (BW), gestation length (GL), and dry matter intake (DMI) pre- and post-calving. Studies were conducted during the summer, and the dry cows were assigned to two treatments: heat stress (only shade; HS, 75 cows) and cooling (shade, fans, and soakers; CL, 69 cows). 

The average RT (0.3°C) and RR [26 breaths per min (bpm)] of HS cows increased compared with CL. In addition, the CL group produced more milk (2.8 kg per day) during the first 8 weeks of the subsequent lactation compared to HS. Correlation analysis indicated that RR of HS was negatively correlated with MP and GL. Within HS, RT was also negatively correlated with MP and GL and tended to be negatively correlated with CW. In addition to being negatively correlated with RT and RR, HS MP was positively correlated with CW, GL, and DMI post-calving. In CL, no correlations were observed for RR, but RT was positively correlated with BW, and MP was positively correlated with DMI post-calving and tended to be positively correlated with GL. 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. This can be easily assessed in individual cows.

In conclusion, 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. There was, however, considerable variation among cows within the HS group showing that cows with longer gestation length have heavier calves, eat more post-calving, and produce more milk, but not as much as the CL cows.