With the hot climatic conditions in Australian summers, heat stress can have varied effects on the milk production of lactating dairy cows.
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.
‘‘Lactating cows show the optimum milk production performance within the thermoneutrality zone with an upper limit of temperature around 26°C,’’ Dr Brendan Cullen said.
‘‘With high temperature conditions in summer, the heat dissipation ability and milk production of lactating cows are reduced.’’
The study was conducted at the Dookie Campus robotic dairy farm and used 109 Holstein cows between February 1 and February 18, 2017 consisting 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 were grouped into three calving groups — early stage of lactation (January 2017 calving), mid stage of lactation (August to September 2016 calving) and late stage of lactation (April to May 2016 calving).
Daily milk production was recorded by the 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 about seven days after the heat stress event.
For the early lactation stage group, milk production increase rate dropped from 2.2kg/day in baseline period to 0.17kg/day during heat stress event, and then only recovered to 0.79kg/day after 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 the dairy industry should avoid calving in summer period, unless there is an efficient heat stress mitigation strategy in place on farm.’’
Future study will be conducted with the robotic milking system to understand the impact of consecutive heat stress events on cows with different stages of lactation and diverse genetic backgrounds.