Heat Stress & Productivity

Milk production drop is usually the most noticeable effect of heat stress events.

Other effects are less obvious but will still result in significant productivity losses.


Heat loads can build when farm infrastructure doesn't provide cooler conditions for the whole herd. Decreased milk production is the clearest cost, but some effects are less obvious and result in significant productivity losses. These include:

  • reduction in fertility & calving rates
  • low milk components
  • body condition loss
  • susceptibility to infection


Cows are more likely to have silent heats or shortened heats in hotter seasons. This is a result of reduced activity due to heat as well as alterations in hormonal activity that reduces the expression of oestrus behaviour.

Heat stress has been shown to decrease oestradiol production, a major female sex hormone that regulates oestrous, leading to ovarian inactivity. Alongside this, the hormonal imbalances impair the oocyte development. This results in lower conception rates.

Heat stress will effect the endometrium in the uterus.  This can result in reduced ability to sustain a pregnancy and increased embryo mortality. Additionally, growth hormones essential to embryo development are affected by heat stress.

A heat stressed dairy herd may experience a decreased 6-week and 100 day in-calf rates or increased not-in-calf rates. Higher heat loads affect digestion and nutrient acquisition by lowering feed consumption rates, which in turn, can affect the calf birth weight and viability. Reduced access to nutrients essential to calf development will have a negative impact on calf weight and viability.

In summary, the risks to calving and fertility presented by heat stress include;

  • reduced intensity and length of oestrus
  • decreased conception rates
  • increased risk of embryo death
  • decreased 6-week/100 day in-calf rates or increased not-in-calf rates
  • decreased calf birth weight and viability reduces.

Read more about heat detection & mating management



The metabolic changes associated with dissipating heat loads is energy intensive and believed to be responsible for reduced lactation and milk production1 Milk from heat stressed cows can have altered milk components with variations in proteins and fat content2,

A direct effect on milk quality will be due to the reduction in efficiency of the immune system resulting in increased risk of mastitis.

Care should also be taken when implementing any cooling processes with sprinklers.  Wet udders when sitting in potentially contaminated areas and immediately prior to milking can result in increased contamination. 
More information at Count Down Down Under.

In summary, some of the risks to milk production presented by heat stress include;

  • milk production can drop by 10-25% during heat stress, 40% in extremes
  • milk composition is affected with high to severe heat stress, with a decline in total protein
  • increased risk of udder infection, which, if occurs, results in increased somatic cell counts and sediments in milk



Increases in environmental temperature will suppress a cow's appetite. A noticeable difference in cows experiencing heat stress is a reduction in dry matter intake. Dry matter intake drops by 10-20% short term or long term depending on the length and duration of heat stress. The effort involved with keeping cool can result in 20-30% more maintenance energy needed to compensate.

Rumination and cud chewing decreases, along with the cow's ability to digest and absorb nutrients in feed is also decreased. The cow's body will open up blood vessels closer to their skin's surface, so the heat load can dissipate. As a result, blood moves away from the uterus, the gut and other internal organs.



Hot and humid conditions created after a summer storm or sprinkler use presents two main challenges to managing cow health;

  • maintaining the pH of rumen to prevent ruminal acidosis and ketosis
  • suppressed immune function alongside exposure to sources of opportunistic infection.

Large downpours over summer can quickly push up humidity in hot conditions and reduce the effectiveness of sweating as a form of evaporative cooling for cows. As heat loads increase because of this, the cow will increase their breathing rate and begin drooling. Saliva loss reduces the rumen pH because the bicarbonates in cow saliva can't act as a buffer to the rumen's acidity. This can be further impacted by feeding strategies.

Heat stress also reduces the cows dry matter intake (DMI), grazing during the day and lowers cud chewing. The natural process of rumen buffering through rumination and saliva bicarbonate is impaired. This is a common cause of sub-acute ruminal acidosis (SARA).

Cows prefer to eat in blocks in the cooler times of the morning and evening each day in hot weather. This will often be when in the dairy being offered high starch bail feed.  Cows tend to select against low quality forage/fibre if it is offered to them. They are less likely to use this to aid rumen balance than they would good quality forage. Additionally, a reduction in DMI can push metabolic energy sources from carbohydrates toward fats due to increase body tissue breakdown (ketosis).  This will also contribute to metabolic acidosis.

SARA plays a key role in the complex causes and origins of laminitis and associated diseases, such as claw lesions, white line disease, ulcers and lameness. Cows will roll and wallow in mud to alleviate heat loads and this presents a risk of environmental mastitis and other infections.

In summary, some of the risks to cow health presented by heat stress include:

  • rumen buffering capacity decreased
  • decrease in rumen pH
  • risk of ruminal acidosis and ketosis increased
  • risk of laminitis increased
  • immune function suppressed increasing susceptibility to infectious diseases
  • risk of mastitis increased.