By Alvaro Garcia South Dakata State University Extension

Hot weather conditions create challenges for grazing beef cattle. Animals need to meet their energy requirements while maintaining the balance between internal heat production and the environmental heat load. There are several physiological means for the body to dissipate heat, however those that involve body mechanisms result in increased energy maintenance requirements. During hot days, animals seek shade, wind, or water to wade in, to cool off and increase heat losses by convection and/or conduction. Heat is also produced inside the animal as part of their metabolism a significant part of which is feed fermentation in the rumen. When the physiological mechanisms (e.g. sweating, panting) and behavioral changes (e.g. searching for cooler spots) designed to cope with rising ambient temperatures reach a maximum, and the animal has exhausted these heat abating options, the only remaining alternative is to reduce feed intake. The challenge when beef cattle are supplemented roughage is that it generates significantly more heat of fermentation when compared to concentrate. This is particularly important for the animal (and the producer!) when high fiber feeds (bales) are offered during hot summer days since of all feeds, they generate the most heat of fermentation.

Working around cattle behavioral changes

The time of the day when most grazing activities occur is determined by ambient temperatures. During hot summer days, there is a shift in grazing patterns with more early morning, late afternoon, and night grazing. Night grazing is performed in the area close to where it ended in the evening as there is little directional grazing at night. Under grazing conditions, feed intake is regulated by a balance between gut fill and clearance of feed from the rumen (digestibility). There is general consensus that energy intake is regulated rather than intake of specific individual nutrients. Feed intake is determined by an integration of physiological, physical, and psychogenic mechanisms. When high-quality, high-energy forages are fed, energy is regulated by the physiological energy demand. In other terms, the animal will graze until the energy requirements are met provided there are no other constraints (e.g. adverse environmental conditions). With high-fiber, low-energy forages (e.g. corn stalks, straw, mature grass, etc.), physical fill limits intake; the animal eats until the rumen distension causes the cessation regardless of whether intake of energy is enough to fulfill requirements.

Choosing the right supplement

When supplementing grazing cattle, the objective is to balance otherwise deficient nutrients and do so in an efficient and profitable manner. From everything addressed above it is clearly important to take into consideration the feeds and the season. For example, feeds that generate excessive heat of fermentation are inappropriate as a supplement during hot summer days. Regrettably, when pasture is scarce there are little options beyond feeding bales. One has to make the economic effort though as to feed the best the budget can afford so the animals will thrive during the summer. On the other hand, supplementing with concentrates during hot weather, although they increase the nutrient density of the diet and produce less heat of fermentation, may result in cases of sub-clinical to clinical acidosis and may reduce the rate of fiber digestion.

Energy needs

Energy is probably one of the first limiting nutrients under summer grazing conditions. Energy is mostly derived from mature pastures and forages of lower digestibility that result in the intake of less total digestible nutrients. Energy deficiency can be primary (resulting from low forage fiber digestibility) or secondary to a suboptimal level of other nutrients such as protein. For the case of a secondary energy deficiency, supplementing with the nutrient which is deficient (e.g. protein) will result in increased digestible organic matter being utilized by the rumen microbes. In instances where sparse and/or overly mature pastures are grazed, more often than not energy/protein deficiency are combined. Minerals and vitamins are also important nutrients to take into consideration. Of the macro minerals (those present in greater concentration in the diet) phosphorus, sodium, and chlorine are the most likely to be deficient under range conditions.


Deficiencies should always be a concern when animals graze pastures that have been under stress (e.g. drought) over a prolonged period of time. Cobalt, copper, iodine, manganese, selenium, and zinc of the micro or trace minerals are more than likely the ones that will be deficient under range conditions. However, in certain areas selenium may be present in concentrations that are potentially toxic. This can occur via selenium accumulator plants or via traditional native or tame grasses. It is also possible that secondary deficiencies can be caused by mineral antagonisms. This is particularly true for copper. Iron, molybdenum, and sulfur are all potent antagonists against copper absorption. In pastures that are mature and dry, beta-carotenes, which are precursors of vitamin A, are often deficient. Plants pro-vitamin content can be assessed fairly accurately by looking at their color. Green indicates the presence of beta-carotenes, whereas a yellowish color indicates their depletion and/or oxidation. Under these circumstances it is also more than likely than vitamin E can also be deficient.

Environmental considerations

The environment plays an important role in the ability of animals to perform according to expectations. Energy is usually one of the first limiting nutrients; its deficiency results from an imbalance between intake and expenditure, and it is greatly influenced by environmental conditions. An energy deficiency can also be secondary to suboptimal levels of protein in the diet. Among the factors that play a big role in the energy balance are ambient temperatures and work performed by the animal. Choosing the right supplement is important not only from an economic perspective but also from an animal performance standpoint. Fibrous roughages of limited digestibility are best utilized during cold weather when heat of fermentation is useful in maintaining the animal’s body temperature.