Artificial shade sources still not cool enough for summer

During periods of heat stress, cattle increase blood flow to the skin surface to allow body heat to escape.

Cattle grazing toxic tall fescue have more difficulty dissipating heat because ergovaline, a chemical produced by the fungus, causes blood vessel constriction that reduces blood flow to the skin surface. We often see these cattle spend more time in shade, standing in farm ponds and panting in an attempt to reduce body temperature.

Reducing heat stress can improve productivity and providing shade is one of the primary ways.

Many pastures have abundant natural shade trees, but in some situations, such as management-intensive grazing systems, natural shade trees are not available in every paddock.

Previous research conducted at University of Missouri, University of Kentucky and University of Florida indicate that stocker cattle and growing replacement heifers had improved weight gains ranging from 0.20 to 0.89 pounds a day when provided shade compared with no shade.

Additionally, cows provided shade gained 0.54 to 0.87 pounds per day more than cows with no shade in studies at universities of Missouri and Arkansas. Cattlemen are generally aware that grazing cattle want and need shade, but these studies demonstrate the large magnitude lack of shade can have on cattle performance.

Shade structures

If adequate natural shade trees are not available, permanent or portable shade structures can be constructed. Current recommendations for design of shade structures are to use shade cloth that blocks a minimum of 80 percent of sunlight.

The size of the shade structure should be such that stocker calves weighing 400 pounds have 15 to 20 square feet of shade per head; calves weighing 800 pounds have 20 to 25 square feet of shade per head; and cows have 30 to 40 square feet of shade per head.

Two recent studies at Louisiana State University and Missouri State University have compared the performance of growing heifers provided either artificial shade structures with 80 percent shade cloth or natural shade trees.

In the Louisiana State University study, 700-pound heifers were provided 37 square feet per heifer of artificial shade and 42 square feet per heifer of natural shade. Heifers provided natural shade tended to gain 0.20 pounds a day more than heifers provided artificial shade.

In the Missouri State University study, 550-pound heifers were provided 28 square feet per heifer of artificial shade or abundant natural shade. Heifers provided natural shade gained 0.22 pounds a day more than heifers provided artificial shade.

A University of Arkansas study found that beef cows provided natural shade gained 0.53 pounds a day more than those provided artificial shade. Based on these studies, current recommendations for design of artificial shade, although better than no shade, may not be sufficient to allow similar animal performance as natural shade.

Previous research has evaluated several aspects of designing shade structures such as roof material, percent of sunlight reflected and height of the shade. When given the choice, cattle prefer shade that blocks greater than 50 percent of sunlight.

Shade materials

Roof material that blocked 60 percent of sunlight reduced the heat load index compared with material that blocked 30 percent of sunlight. The color (black versus silver) of the 60 percent shade material did not impact the heat load index.

Roof material that blocked 100 percent of sunlight further reduced the heat load index, respiration rate of cattle during the hot part of the day and the amount of time with cattle exhibiting elevated respiration rate compared with material that blocked 60 percent of sunlight.

However, care must be taken when selecting roof material as some materials, such as galvanized steel, even though blocking 100 percent of sunlight, do not reduce the radiant heat load under the shade. But aluminum painted white on top and black underneath is very effective at reducing radiant heat.

Many artificial shades are constructed at 7 to 9 feet high, but this may limit air movement and increase radiant heat under the shade. Shades at heights of 10 to 12 feet reduce radiant heat load as cattle have greater exposure to cool sky. But shades higher than 12 feet create a fast-moving shadow where cattle have to move to hot ground frequently.

Little research data is available comparing the effects of shade structures and materials on performance of cattle. Most available data compares environmental variables such as air temperature, ground temperature and air velocity underneath different shade structures and materials.

Many of the criteria mentioned above should be considered when designing and constructing shades to provide the most comfort to the animal while keeping in mind the cost of construction.

Ultimately, shade of some type will provide the largest benefit to cattle compared with no shade, but optimal shade design could result in additional improvements in animal performance.

PHOTO: When cattle consume endophyte fescue, it can cause blood vessel constriction that reduces blood flow to the skin surface. This can lead to cattle going to shade or standing in farm ponds. Staff photo.