Quantifying the effect of thermal environment on production traits in three breeds of dairy cattle in New Zealand

In this study, the effects of hot and cold conditions throughout New Zealand were assessed from 1990 to 2002 using daily milk yield, fat and protein concentration records from 19 201 spring calving first lactation cows comprising Holstein Friesian (HF), New Zealand Jersey (NZJ) and crossbred (1/2 HF and 1/2 NZJ; HF x NZJ). The effect of hot and cold conditions on test day records were assessed using a 3-day average of temperature-humidity index (THI) and a 3-day average cold stress index (CSI), which includes the effects of temperature, rain and wind. Hot conditions were associated with reductions in milk and milksolids yields, and fat and protein concentrations in all breeds. Reductions greater than 10 g of milksolids day(-1) per unit increase in 3-day average THI, started to occur at 3-day average THI of 68 in HF, 69 in HF x NZJ, and 75 in NZJ cattle. Temperature-humidity indices of 68 and 75 are approximately equivalent to temperatures of 21 and 25.5 degrees C, respectively at 75% humidity. Fat and protein concentrations were also reduced at high THI. There was also some evidence that HF cows of high genetic merit, as determined by their estimated breeding value for milk, exhibited greater milk yield reductions due to hot conditions than their low genetic merit counterparts. Cold conditions significantly reduced milksolids yields in HF and HF x NZJ cattle. However, the frequency of cold conditions where performance is likely to be compromised is low (1-3% of days). Feed supply and feed quality data for each herd test day would have improved the estimation of 3-day average THI and CSI thresholds when performance was reduced.