EFFECT OF HIGH TEMPERATURE-HUMIDITY CONDITIONS ON MILK-PRODUCTION OF DAIRY HERDS GRAZED ON FARMS IN A PASTURE-BASED FEED SYSTEM

The influence of stressful temperature-humidity conditions on output from dairy cows on farms near Wauchope (31°S Lat.), Australia, was examined by comparing weekly milk production and Temperature-Humidity Index (THI) values over a two-year period (1973 to 1975). Production during weeks with relatively high THI values (70 or over) was higher than production in weeks with lower temperature-humidity stress. In autumn and winter when THI values were low, the quantity and quality of pasture available was inadequate to sustain high levels of milk production. Short-term (weekly) responses of milk output to changes in temperature-humidity conditions above THI values of 70 were not consistent, either in terms of the direction or magnitude of production change. THI values may not have been high enough to adversely affect week-to-week production from predominantly Jersey herds, although some depression in lactation yields may have occurred. The reduction in digestibility and palatability of grasses during late summer appeared to be of greater importance in influencing production patterns than the direct effects of stressful temperature-humidity conditions on cows. © 1979 Swets & Zeitlinger B.V.