Canterbury's hot summer weather is not an impediment to dairy cow productivity, says a Lincoln University researcher who will present his findings to dairy scientists at an Australasian symposium at Lincoln University next week.

Associate Professor Graham Barrell, of Lincoln University's Faculty of Agriculture and Life Sciences, is one of over 100 scientists who will present their research at the 4th Australasian Dairy Science Symposium over 31 August - 2 September. The theme of the symposium is "Meeting the challenges of pasture-based dairying".

His study suggests that reconsideration might be appropriate for the local New Zealand application of the commonly accepted international figure for predicting the onset of heat stress in dairy cattle - a Temperature Humidity Index (THI) value of 72.

Associate Professor Barrell says that while accepting the international evidence that THI values in excess of 72 may cause heat stress in dairy cows, the questions are really how much above 72 and for what duration?

"For New Zealand it may be more accurate to base predictions on a measure of accumulated heat load such as THI x time, or on a three-day average of peak values (as some researchers have done), or on a heat-load index that takes solar radiation into account," he says. "Many of the published studies on heat load in dairy cows have been carried out in the hotter and more humid regions of the world, where the daily minimum THI value can remain elevated for many days at a time. "It is therefore possible that the threshold peak THI on a single day that causes heat load effects on production in New Zealand dairy cows could be set higher than 72."

A three-part research project led by Associate Professor Barrell and conducted on 13 dairy farms in the Selwyn District of Canterbury, including the Lincoln University Dairy Farm, and involving an analysis of Canterbury meteorological data for a recent five-year period, revealed that - Many sites in the region experienced at least three days per month in summer when the peak THI exceeded 72. While regular changes in body temperature occurred throughout the day in dairy cattle, peaking in the afternoons, there was no evidence of cause and effect between THI and body temperature and until the former exceeded 72. Analysis of data from the Selwyn farms for two months of summer 2008-2009, when 19 days out of 90 had peak THI values exceeding 72, showed no evidence of a negative connection between the THI values and the amount of milk solids produced. "Our findings indicate that the episodes of hot weather which occur in Canterbury do not produce large enough heat loads in dairy cows to impact negatively on their milk production," says Associate Professor Barrell. "The hot days in Canterbury during summer were not associated with a loss of milk production in dairy cows, even on occasions when the THI reached values in the high 70s. The fact that the cows on the farms in our study were able to maintain productivity on these occasions suggests that the heat load was never excessive.

"This research provides a positive signal for the future of the industry here as it indicates that Canterbury does not have a summer climate that could be considered too severe for dairying."