Physiologic stress index of lactating dairy cows based on diurnal pattern of rectal temperature

Abstract Diurnal rectal temperatures of lactating Holstein cows were utilized in a series of studies involving shade modifications with spray and fan in addition to herd data during winter and summer to derive a physiologic index. Physiologic stress degree hours was the product of hours of rectal temperature greater than 39°C. Rectal temperature of cows in shade averaged 39.5°C for 19.6 hours (774.5°C h) compared to 39.3°C for 12 hours (471.6°C h) for shade with spray. Nighttime rectal temperature of cows in both groups was 38.5°C, but milk production declined significantly for cows in shade only. In a follow‐up study, shade modification with spray and fan eliminated hours of rectal temperature above 39°C while rectal temperature averaged 39.4°C for 12 hours for cows in shade (471.8°C h). Rectal temperature of cows in different stages and intensities of milk production was similar during winter. On a record hot day (41.4°C), diurnal pattern of rectal temperature identified high producing cows in early lac...