– Dr. Jeff Lehmkulher, PhD, PAS, Extension Professor University of Kentucky
Temperature and humidity both contribute to heat stress.
As the summer weather has hit full stride, take some time to focus on factors that impact animal performance during these months. Stocker calf performance reflects changes in the environment, plane of nutrition, and overall health of calves. Be mindful of the how summer weather can impact these three overarching factors and consider what you might alter or maintain to minimize the impact of these elements.
Heat stress is the first environmental factor that will impact animal performance during the summer months. The effect of heat stress is exacerbated by the alkaloids produced by the wild endophyte in Kentucky 31 tall fescue. Animals compensate during heat stress with increased respiration rate, increased skin vaporization (sweating), increased peripheral blood flow, decreased appetite to reduce metabolic heat production, and more time seeking relief by standing in the shade, congregating in water or grouped up in areas where urine and feces create a wallow. Increased respiration rate leads to greater energy expended for contraction and relaxation of the diaphragm. This doesn’t seem like it would be a big loss but sit there and double your breaths per minute for five minutes and see how you feel. Now consider doubling your respiration for several hours a day and the impact this would have on energy expended. Previous research has shown that cattle at thermoneutral conditions had respiration rates of about 23 breaths per minute while under heat stress this increased to 54 breaths per minute. This increased respiration rate is a key response to heat stress as well as increasing blood flow to periphery.
Compensation of heat stress can also occur through increased sweating or evaporative heat loss as periphery blood flow increases. Skin evaporative energy loss was observed to be 50% greater under heat stress than thermoneutral. However, when exposed to wild-type endophyte, skin vaporization was not increased due to a lack of skin temperature increase which may be attributed to vasoconstriction. Accumulated heat load by animals can be dissipated later into the night when ambient temperatures decline. However, during periods of high humidity and lack of nighttime temperatures falling, animals do not have significant reductions in core body temperature before the next day begins. Successive days of heat stress and minimal dissipation of accumulated heat load leads to severe health concerns for cattle. Add into the mix, the alkaloids from the wild endophyte in tall fescue leading to vasoconstriction reducing blood flow to the skin surface during these night hours limiting heat dissipation from sweating. All these factors combine to increase animal maintenance requirements by 7-25%. If maintenance energy requirements represent 65% of normal daily intake, a 15% increase in maintenance requirements as a result of heat stress would reduce gains significantly.
Providing shade is the first management strategy to help mitigate heat stress during the summer months. Shade helps to reduce heat loading from solar radiation. Additionally, ground surface temperatures under shade have been shown to be greatly reduced compared to unshaded areas. Shade can be natural such as wooded areas or man-made. Cattle will stand more during heat stress to allow more convection heat loss as air moves around the body. Shade should ideally provide sufficient room for cattle to stand in the shade without being crowded.
Often the question is how much shade should be provided. Consider the length from tip of nose to tail and width across the ribs of a mature cow. These measurements may be near 7’ x 3’ or 21 square feet and these measurements will vary. Spacing between animals is important so the actual shade provided will be greater than the size of the animal. Actual allocated area under shade of 30-40 square feet per cow may be necessary. The University of Nebraska recommends 20-25 square feet per animal for voluntary shade use in feed yards and 25-30 square feet for high-risk feeders on arrival. For man-made structures, ensure there is sufficient distance between the back of the animal while standing and the bottom of the shade structure to facilitate air movement through the structure. When possible, having shade structures that are portable will minimize wallows which can lead to high humidity under the shade from excessive urine and feces deposition. Additional information on shade structures can be found at https://www2.ca.uky.edu/agcomm/pubs/aen/aen99/aen99.pdf.
Consider developing shade areas during periods of higher temperatures and humidity to maintain the performance of grazing cattle if wooded areas are not readily available. Temporary electric fencing can be helpful in allocating different areas of wooded areas to minimize soil disturbance under trees and preventing development of wallows. Shade placed on ridges that have greater wind speeds will aid in moving air through the structures and cooling cattle. Ensure cattle have access to fresh, clean water as losses from sweating and increased respiration rates increase water requirements. Consider utilizing CAIP funds for shade or tree plantings for development of natural shade areas. Contact your county Extension office for additional information.