by: Stephen B. Blezinger
Ph.D., PAS

Part 3

Over the last couple of issues we have been discussing different types of stress and the effects it has on the cow herd. In this final part of the series we'll take a look at nutritional and health stress and how it can also affect the animal.

Remember that stress is an external event or condition which results in a strain on a physiological system. This strain can be measured on the farm by reduced productivity (breeding, growth, milk production) or animal health. The effect of stress in beef cattle is significant and largely unquantified. Each year countless dollars are lost by producers and the industry due to stress effects from a variety of sources. As a point of reference, heat stress alone is estimated to cost the dairy industry in the United States as much as $900 million annually. Much of this stress and subsequent losses can be managed through proper management and nutrition.

Even with the advances in current management systems cattle feel the effects of a variety of stressors on a daily basis. As we discussed in a previous part of this series, stress is created by environmental conditions (heat, cold, humidity, dust, mud), facilities, handling, nutrition and health effects. Even the exceptional production potential of many cows (especially true in dairy cattle) creates stress in the animal given the level of nutrient intake and subsequent milk production in cattle selected for high milk production. In many parts of the United States, heat stress is probably given the most attention and certainly has commanded a large amount of research. Researchers in Florida suggested that the body of a cow begins to respond to warm temperatures in the environment at slightly over 70°F. Additionally, the sun's rays can directly have an adverse effect on fertility. All black cattle or cows with black markings will absorb more heat from the sun's rays, further elevating body temperature. Blood flow may be diverted from internal circulation to peripheral circulation in an attempt to reduce body temperature. The reduction in blood flow to internal organs including the uterus, oviducts and ovaries may reduce available nutrients and increase biochemical waste products at the tissue level.

The interaction of stress in the animal and nutrition has been examined at length. In each case the presence of stress resulted in physiological changes that were, in general, detrimental to the health or productivity of the animal (gains, feed efficiency, milk production). Researchers have examined how stress effects have affected the cow nutritionally, how it may reduce the intake of the various nutrients, how it can possibly affect the absorption and metabolism of required nutrients and if it affects the animal's actual requirements for these nutrients.

Effects on Feed/Forage Intake

One prominent effect of various types of stress is reduction of feed intake. When we examine heat stress we see that heat can reduce dry matter intake by as much as 35 percent. Even with the advances in management and development of cooling systems that can help reduce heat effects, in many cases feed intake can be reduced by 10 to 15 percent. With depressed feed intake comes a reduction in nutrient intake critical to milk production, reproduction and health. This is extended to growth in feeding or finishing cattle. Depression in milk production levels of 10 to 15 percent is commonly observed even in operations with adequate shade or other cooling methods in place. This drop in production can increase up to 40 or 50 percent in unprotected animals. While feeding programs or rations are commonly adjusted to compensate for the depressed intake of nutrients, it is still common to observe inadequate intake of a variety of nutrients under these circumstances. Nutritionally, protein and energy intake are often of the greatest concern yet mineral, trace mineral and vitamin intake are likewise affected and can affect the animal in less obvious, yet highly significant ways.

One particular nutritional stress period we note is during times of drought. Obviously during these periods, forages, the staple of the grazing cow's diet becomes in short supply. The quality of the forage also diminishes further accentuating the problem. This reduces the cow's ability to consume the nutrients she needs which limits her ability to perform as normal. This is emphasized even more if she is nursing a calf. All of these factors go together to greatly increase the stress level affecting her body resulting in diminished reproductive performance (this is the first effect -- since reproduction is not critical to survival it is the first system to be shut down when nutrients are in short supply), milking ability and the functioning her immune system. Her calf is similarly affected. With reduced milk production the nutrient flow (predominantly protein and energy) is decreased. The first, most obvious loss is to growth potential. Secondly, we see a depressed immune system.

A vicious cycle then becomes initiated. With stress we see a reduction in the animal's ability to fight off diseases. We also know that the disease process itself is stressful which further increases the stress level on the animal which depresses immunity even more. It also reduces the energy levels necessary for the animal to go out and graze or attempt to consume the nutrients necessary to support its requirements. This problem is further compounded in that drought typically is coupled with heat. If the animal is consuming less than adequate energy or other nutrient levels, it is less able to dissipate the heat from its body, further compounding the stress effects. It becomes obvious that once this cycle is started it is difficult to stop.

Intake limitations, unless addressed properly, also reduce protein intake. Intake of proper levels of protein is critical to normal rumen function and fiber digestion (the mainstay of the cow's diet). Protein is important for normal growth, milk production, reproduction and immunity. Less than adequate protein can result in a wide variety of performance and health related issues.

Significant Effects on Trace Mineral Status

Stress can have significant negative effects on the metabolism of trace minerals in the cow. Stress created by incidence of either mastitis or ketosis has been shown to alter zinc metabolism in dairy cattle. Orr et al. (1990) reported an increase in urinary copper and zinc excretion in cattle inoculated with IBRV. The findings from this study indicate that when an animal is stressed the excretion level of certain trace minerals increases. Furthermore, Nockels et al. (1993) reported that copper and zinc retention was decreased in steers injected with ACTH (a stressor), in conjunction with feed and water restriction. These studies, in conjunction with others, indicate that stress in the form of an infection (IBRV), a metabolic disorder (ketosis), or deprivation of feed and (or) water can increase copper and zinc depletion from the animal. Work by Greene and co-workers (1995) indicated that in many cases, because of the aforementioned issues, subclinical deficiencies in certain nutrients, especially minerals, might develop or even increase during periods of stress. Subsequently, enhanced mineral supplementation and use of organic or complexed trace minerals might be beneficial in preventing or correcting deficiencies during stress and period of low feed intake.

Conversely, stress effects may be lessened by improving trace mineral status in the animal. Chirase et al. (1991) reported that calves stressed by the internasal injection of infectious bovine rhinotracheitis virus (IBRV) showed improvements in dry matter intake, reduced fever levels and more rapid return to base line body weights when supplemented with zinc. Another study (Blezinger, et al., 1992) saw similar responses in stressed cattle when treated animals were provided increased dietary levels of Zn and Cu. This and other work has shown that stress depresses immune response in the animal but is responsive to trace mineral supplementation. Feeding of supplemental Se has been shown to lower the frequency and shorten the duration of clinical mastitis in lactating dairy cows (Smith et al., 1984). The prevalence of intramammary infection decreased as mean activity of glutathione peroxidase (GSH-Px) increased. Selenium is an essential component of this enzyme with activity responding to an improvement in Se status in the animal. The response of GSH-Px activity in beef heifers was researched with the delivery of Se via injection of a Se inclusive trace mineral solution resulting in a significant (P<.05) increase in GSH-Px activity in treated cattle (Fisher, 2007).

So what are the answers?

These and other studies provide that health and nutrition have effects on stress and vice versa. The goal here, in all situations is to reduce stress as much as possible and to reduce the factors that create stress as much as possible. Certainly some factors cannot be eliminated such as environmental or temperature stress. Cattle can be provided protection as much as possible in terms of shading or wind breaks but to provide a constant thermoneutral (temperature at which no energy is required to cool or warm the body) is impossible. At some point in their lives cattle will encounter a variety of temperature or weather effects. Providing shade, wind breaks, shelter of some sort (this might be a wooded area) can all be useful to minimize weather effects. Availability of ponds for cooling purposes has also proven positive but alternate, clean water sources should also be available for drinking purposes.

Additionally, it is necessary, for proper management, to handle cattle in order to provide vaccinations or injections, for identification purposes, deworming, dehorning, etc. Given that a working pen and squeeze chute are not the cow's natural environment, her exposure to this equipment will cause stress at some level. To minimize stress handling be sure that facilities are well designed and provide for a natural “flow” or movement of the animal. Handle cattle quietly and calmly. Minimize loud noises and leave the dogs on the truck. Minimize the use of tools such as electric cattle prods and also use good, safe handling and management practices. This minimizes stress on the animal and the producer.

Providing a nutritionally sound diet as regularly as possible minimizes stress and offsets stress effects. This means balancing the base forage with supplements as necessary. A well managed forage program minimizes the need for additional supplementation which also helps reduce cost. Recognizing that during stress conditions (heat, cold, rain, snow) additional nutrients might be needed or supplements may need to be formulated differently in order to effectively deliver a proper nutrient balance. This means all nutrients – protein, energy, minerals and vitamins. Finally, planning should be in place in the event of drought or forage shortfalls so that this component of the animal's diet is accounted for and so that it doesn't cost extra in terms of lost performance or added unnecessary expense. Working with your nutritionist can help balance your overall program and put in place plans to offset environmental challenges such as drought.

Finally, a sound health program is critical. Reducing the incidence of disease (respiratory, digestive, reproductive) or other conditions such as foot rot, internal or external parasites, pinkeye, etc. all go to minimizing the effects that these health issues can have on increasing stress levels and actually amplifying the health issues faced by the animal. A proper vaccination and treatment program, designed with your veterinarian specifically for your herd can significantly reduce the health risks and subsequent stress effects.


Some stress is inevitable and expected. The goal for every producer is to recognize the areas of his operation where stress on the animal can be minimized and work to reduce this drain on performance and profits.

Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, TX. He can be reached at 667 County Road 4711, Sulphur Springs, TX 75482, by phone at (903) 352-3475 or by e-mail at


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