Development of a heat load index for grazing dairy cattle
J.R. Bryant, F.J. Huddart, Karin E. Schütz
Abstract
ABSTRACT Many indices have been developed to predict heat stress in dairy cattle, but no indices exist specifically for extensively grazed dairy cattle where wind and solar radiation modify heat stress responses. Using a large database of respiration rates and matching weather data, we developed a Grazing Heat Load Index (HLI) to predict respiration rates in extensively grazed dairy cattle. The Grazing HLI includes ambient temperature, solar radiation, and wind speed for the hour immediately preceding the respiration rate measure and was more accurate at predicting respiration rates than existing temperature humidity indices. On average, we observed an increase of 4.21 breaths per minute per 1°C increase in ambient temperature, an increase of 5.89 breaths per minute per 1 megajoule/m 2 increase in solar radiation and a reduction of 1.70 breaths per minute per 1 km/hr increase in wind speed. We observed marked changes in panting score and percentage of drooling animals at Grazing HLI of greater than 70. This maybe a threshold when dairy cattle welfare is seriously compromised due to heat, however, noting that respiration rate starts to increase more steeply before this threshold.