Heat stress is a physiological drain on the dairy cow and an economic drain to the dairy industry that has long-term implications to cow health and productivity. Even after the warm season has subsided, impacts of heat stress still linger.

Milk production can suffer dramatically due to metabolic adaptations the cow initiates to prioritize nutrient utilization. Using a step-wise approach and specified goals before, during and after heat stress is critical to transitioning lactating cows through this metabolically challenging situation.

Hot weather preparation

Before heat stress starts, it is important to make sure cow cooling strategies meet the following three criteria:

  • In place: Make sure fans and soakers are installed prior to the onset of heat stress and positioned appropriately.

  • Clean: The efficiency of fans and soakers are drastically reduced when they are dirty or clogged. These maintenance steps allow the pens to be more comfortable and consistent throughout.

  • Working properly: Broken fans and soakers lead to inconsistent pen environments. This can lead to crowding and bunching in undesirable areas, which in turn will intensify heat stress.

The onset of heat stress varies from region to region and year to year. To alleviate a guessing game, make sure these tasks are scheduled and completed early in the calendar year.

As heat stress begins, taking care of the cow is vital. A heat-stressed cow is a unique animal and should be treated accordingly. Managing a heat-stressed cow is different than managing that same cow the rest of the year.


Maintaining dry matter intake during heat stress is important, but a reduction in dry matter intake during heat stress is not responsible for all of the lost milk during heat stress. Cows are “programmed” to utilize nutrients differently during heat stress.

During heat stress, focus your attention on these three key areas in order to capture otherwise lost milk:

1. Adjust the diet
Keeping nutrient intake on track becomes paramount during heat stress, and adjustments in dietary nutrient concentrations and an increase in energy density are effective strategies.

Reprioritization of nutrient use during heat stress – to dissipate heat and away from milk production – makes it even more important to keep every mouthful of the TMR balanced to contribute most effectively to nutrient requirements and energy balance.

Actual dry matter intake needs to be well understood during heat stress. Energy density, fiber content and macrominerals, most specifically electrolytes, must be formulated with actual dry matter intake in mind.

Additionally, by increasing the energy density of the diet, the heat of fermentation (created by the rumen) will be reduced. This allows the cow’s total heat load to be lower.

2. Pay special attention to forages
High-quality forages will encourage dry matter intake, so it’s important to focus on forage quality and particle length upon delivery. Forage particle length must be considered with great care to keep the cows from sorting.

Sorting can lead to more problems during heat stress; when a cow sorts, she is making her own diet. During heat stress, fiber intake is vitally important to make sure the rumen is adequately buffered and therefore functioning. Much importance needs to be placed on the cow consuming the diet we provide versus the diet the cow may prefer.

3. Keep cows healthy
A healthy herd prior to the onset of any stressful event allows a cow to better cope with challenges that arise. An adequately functioning immune system prior to the onset of heat stress is a key factor in aiding cows through the warm season. The activated immune system is a huge drain on glucose, a milk production precursor.

When a cow’s immune response is rapid, efficient and well equipped to deal with insults, total glucose utilization is reduced because the activation is short-lived.

Utilizing this “at the ready” efficiency in immune function going into the warm season should keep the immune system’s draw on glucose to a minimum. Because of this, glucose is more available for the primary goal of a heat-stressed cow: heat dissipation – and now the secondary goal: milk production.

Nutritional strategies to support normal immune function are vital to minimize health insults and maximize production during heat stress.

When cooler days arrive

Bringing the cow through a hot summer is one thing, but maintaining diligence to get the cow “back to normal” when temperatures subside must also be addressed with great care. It is important to track dry matter intake closely to ensure the ration is being readjusted using a step-wise approach to get back to the pre-heat stress diet.

Gradually returning to the original ration over a defined period of time will allow little to no change to cow performance.

The overarching goals for heat stress are dissipating heat effectively, adequate and deliberate nutrient delivery and intake, optimized health by supporting a strong immune system and transitioning the cow through these stages as smoothly as possible.

Meeting these objectives will allow energy balance to favor milk production and reproduction to keep cows healthy and profitable during heat stress.  PD

Jamie Jarrett