Avoid summer calving: heat-stress study

By Simone Dunne

With the hot climatic conditions in the Australian summer, lactating dairy cows usually experience heat stress which could reduce their milk production.

Dookie Campus scientists from the Faculty of Veterinary and Agriculture Sciences at the University of Melbourne investigated the impact of heat stress on milk production performances of cows differing in stage of lactation in summer.

“For dairy farmers, it is very important that their lactating cows are in the best production condition as possible to produce milk,” Dr Paul Cheng said.

Dr Brendan Cullen said lactating cows showed the optimum milk production performance within the thermoneutrality zone with an upper limit of temperature around 26°C.

“With high temperature conditions in summer, the heat dissipation ability and milk production of lactating cows are reduced,” Dr Cullen said.

The study conducted at Dookie Campus Robotic Dairy Farm used 109 Holsten-Friesian cows between February 1 to 18, 2017. It consisted of a seven-day pre-heat stress period, during which the milk production records were used as a baseline data. It was then followed by a four-day heat stress period and a seven-day recovery period.

Cows grazed pasture and were grouped into three calving groups: “early stage of lactation” (January, 2017 calving), “mid stage of lactation” (August-September, 2016 calving) and “late stage of lactation” (April-May 2016, calving). Daily milk production was recorded by Robotic Milking System.

By the end of the four-day heat stress event, the late- and mid-lactation stage groups had decreased their milk production by 17 per cent and 15 per cent, respectively compared to their baseline, and the milk production recovered to their baseline level approximately seven days post the heat stress event. While for the early lactation stage group, their milk production increase rate dropped from 2.2 kg/day in baseline period to 0.17 kg/day during heat stress event, and then only recovered to 0.79 kg/day post the heat stress event.

“We can see from the results of this study that heat stress showed a strong negative impact on cow milk production performance,” Dr Cheng said.

“Dairy cows in different lactation stage had responded to heat stress differently.

“For the mid- and late- lactation cows, reduced milk production recovered soon after heat stress period. However, the early lactation cows could not recover and lost their production potential, which could imply that dairy industry should avoid calving in summer period, unless there is an efficient heat stress mitigation strategy in place on-farm.”

Dr Cullen said future study would be conducted with Robotic Milking System to understand the impact of consecutive heat stress events’ impact on cows with different stage of lactation and diverse genetic background.

The study was published in Australasian Dairy Science Symposium 2018 Proceedings. The work was a collaborative effort by Dr Paul (Long) Cheng, Dr Brendan Cullen, and Prof Frank Dunshea from the University of Melbourne, and Chole Charlot from AgroParisTech (France), and Dr DongWen Luo from AgResearch (New Zealand).