Dairy farmers are being advised to ensure their herds are cool in summer months to help keep up milk production.
DairyNZ animal welfare developer Dr Adele Arnold says cows stop eating when they get too hot, and if they can't cool down they'll produce less milk.
Signs of heat stress include breathing with an open mouth, and a respiration rate of more than 60 breaths per minute.
Dr Arnold says any activity - even walking to the milking shed - can increase the risk of heat stress, while digesting food also produces heat and on hot days this can overload the animal's system.
Cows can be grazed close to the dairy so they don't have to walk as far, she says, and sprinklers can be used to cool them down while they're waiting to be milked.