Farm advice: How heat impacts cows

By: Jacqueline McGowan, DairyNZ senior solutions and development specialist

DairyNZ explains the effects of heat stress on cows


We’ve all heard people say that some days are too hot for cows, but have you ever considered how much the heat is impacting them?

Historically, understanding heat stress has been based on noticeably reduced milk production, but now we understand that cows experience discomfort before production drops.

Cows’ stoic behaviour means their discomfort is not as obvious to us. Things such as increased standing, aggression around the trough, and increased breathing rates are easy to miss when we do not spend our whole day in the paddock with them.

Increased milk production means cows have more metabolic heat to manage and, therefore, require more support to get through summer in comfort.

To understand the effect of heat stress on your farm, I would like you to do two easy things:

  • Count how many breaths your best cow for production takes in 10 seconds
  • Research how many hours of risk your cows might face

Watch your best cow breathing

You might still feel sceptical about the impact of heat on your herd, and you could be right. The best way to find out is to get out and look at your cows, between 1pm and 3pm on a summer day.

Respiration is the earliest and easiest indicator we have on how a cow is coping, and it only costs a stroll through the paddock. High production increases susceptibility, so use your best cow rather than your smallest heifer.

Once you have selected a cow, count how many breaths she takes in 10 seconds. The normal rate of breathing is 26 and 40 breaths per minute or four to seven breaths in 10 seconds. If she takes more than 10 breaths, she’s experiencing enough heat stress to reduce her feed intake and affect milk production.

How many hours of risk?

The hours of heat that will affect milk production differ regionally, ranging from two hours per day around Gore to 11 hours per day around Whangārei.

However, the hours that affect their comfort levels are an additional four to six hours per day, in every region. If cows are exposed to all that heat, the estimated impact on milk production ranges from 1.5kgMS to 7kgMS per cow between December and February.

Many farms already have systems that reduce the risk for at least some of that time, but it’s good to assess what you’re already doing in each of those risk hours and whether you can apply some of the tips below.

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