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Beat the heat this summer

It has long been known that heat stress adversely affects production and reproduction in dairy cows worldwide. In the United States alone, Normand St-Pierre and co-workers at The Ohio State University have estimated that annual economic losses to heat stress in the dairy industry exceed $900 million per year.

A 2002 report estimated that 48 percent of dairy cows, or 4.2 million, in the United States are subjected to thermal or heat stress on an annual basis, negatively affecting milk yield, reproduction and health. Heat stress is unique in that it not only raises net energy requirements for maintenance but simultaneously reduces intake, which exacerbates the milk yield decline attributed to it.

However, not all of the reduction in milk yield during heat stress is associated with reduced feed intake. Research by Baumgard and Rhoads at the University of Arizona demonstrated that more than half of the milk loss in heat-stressed dairy cows is unrelated to a reduction in dry matter intake. Furthermore, the dairy cow shifts its metabolic fuel consumption during heat stress, burning more glucose and fewer nonesterified fatty acids.

The cow’s “lack of ability” to utilize body fat as a fuel during heat stress seems to be a metabolic adaptation to, in essence, preserve itself. Considerable work remains to further define the most effective nutritional strategies to minimize the impacts of heat stress on production in lactating dairy cows and to fully understand the metabolic alterations that occur in heat-stressed dairy cows.

Help them lose heat

Basic strategies to reduce the impact of heat stress include altering the environment around the cow by reducing heat gain (providing shade) and elevating heat loss (fans, soakers and evaporative cooling). In addition, producers and their advisers employ a number of nutritional strategies to maximize energy intake during periods of thermal stress. These include altering feeding times to allow more feed intake at night when it is cooler, increasing the energy density of the ration to reduce gut fill, providing high-quality forages and using total mixed rations to reduce feed sorting by cattle.

In addition to production losses, disease incidence and the immune system are compromised in heatstressed cows. This is associated with the release and elevation of cortisol common at the initial exposure to heat. Dry matter intake is compromised, and the cow’s mechanisms to digest and utilize nutrients are altered. As the cow’s system works to get rid of heat, it shunts nutrients to its extremities to aid in heat dissipation.

In reducing the cow’s heat load, nutrients are “freed up” to be utilized for more productive tasks, such as milk production, reproduction and immune function. Utilizing strategies to help cows deal with heat dissipation physically, with well-defined management protocols and, metabolically, with proven nutrition programs, alleviates the severity of heat stress and supports a faster recovery of the immune system. With this reduction in heat stress that the cow perceives, cortisol levels will begin to return to normal. This may result in improved health and productivity after periods of heat stress.

As dairy producers make plans to prepare for heat stress season, it’s important to double check the operation of and clean all fans, and test and service sprinklers, soakers or evaporative cooling systems. Work with your nutritionists to plan for necessary ration changes. When considering new or novel nutritional strategies to support normal health and immune status during heat stress, ask for a review of supporting research before making a final decision.


J.P. Jarrett

R.J. Collier

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