Effects of extreme summer heat events on nutritive characteristics of dairy pastures in northern Victoria, Australia

Context The incidence of extreme heat events (prolonged temperatures above 35°C) is increasing in the northern Victorian dairying region in Australia. This increase is having a significant effect on the performance of perennial ryegrass, which has traditionally been the major perennial species in these pastures. Aim During seven extreme heat events, forage sampling was conducted on commercial farms to evaluate the effects of high temperatures on the nutritive characteristics of perennial ryegrass, tall fescue and lucerne. Our hypothesis was that reductions in nutritive characteristics with an increasing external temperature would vary among species. Methods Before, during and after heat events, replicated samples of the three forage species were collected and analysed for in vitro dry matter digestibility (IVDMD), neutral detergent fibre (NDF), acid detergent fibre (ADF), water soluble carbohydrates (WSC) and crude protein (CP). Several anti-nutritive compounds (alkaloids in perennial ryegrass, and saponins and coumestrols in lucerne) were also measured. Key results None of the temperate forage species evaluated maintained nutritive quality under high temperatures. Linear mixed model (LMM) analyses showed that for each degree rise between 23°C and 40°C, IVDMD and WSC concentrations decreased by 0.2% while NDF and ADF concentrations increased by 0.2% and 0.15% respectively. Concentrations of the alkaloid, ergovaline, in the base of the perennial ryegrass tiller were close to, or above, the recommended threshold level for animal health and welfare in three of the six samples taken during heat events. Conclusions These results suggest that extreme heat events have a detrimental effect on the nutritive characteristics of the evaluated forage species. Implications Heat-induced decreases in nutritive characteristics will potentially have serious implications for milk production and farm productivity. Further research, undertaken under controlled conditions of temperature, humidity and plant growth stage, is recommended to determine the full effect of these extreme summer conditions on forage nutritive characteristics.