Heat stress and grazing

By Jeff Lehmkuhler, University of Kentucky, Department of Animal & Food Sciences

Forage availability is a key driver of stocker calf performance followed by forage quality. As we move through the spring months and begin to see temperatures increase, forage growth slows. Previous research demonstrates that the photosynthesis of plants is negatively impacted by increasing temperatures. Photosynthetic rates of tall fescue can be reduced when temperatures reach 86/77 degrees Fahrenheit, day/night. These warmer temperatures slow forage growth of our perennial cool-season forages.

More importantly, research has demonstrated that soil surface temperatures can have a larger effect on photosynthesis than air temperature. Close grazing or mowing exposes more soil to direct sunlight increasing soil surface temperature. Chris Teutsch’s research with tall fescue at the Princeton Extension and Research Center demonstrated that clipping forage weekly to 1-inch versus 4.5 inches in height weekly increased plant crown sensor daily maximum temperature by 10 degrees Fahrenheit. Close clipping led to an increase in warm-season annual forages such as crabgrass due to the temperature stress on the cool-season forage. Reducing stocking density or implementing a managed grazing system to better manage forage residual heights may help cool-season forages be more persistent.

Be mindful of feeder calves that are not shedding winter hair coats. Studies show that lower hair coat scores, better shedding, improve daily gains during the grazing season. Several factors may be involved with shedding of winter hair including fescue alkaloids, genetics, plane of nutrition, and others. Recently, researchers from the southeast reported breed differences in hair coat scores when grazing tall fescue with Charolais-sired calves having less hair than Hereford-sired calves. Calves that don’t shed will be more susceptible to heat stress. Ensure stocker calves always have access to clean water. As temperatures increase water intake will increase creating more demand on your water system. Ensure the floats and valves are in working order, that tanks are clean and not fouled with fecal contamination. Spring- and pond-fed tanks may accumulate sediment and should be cleaned out routinely.

Shade should be available during periods of heat stress. University of Missouri research demonstrated that stocker steers grazing Kentucky 31 tall fescue gain 0.3 pounds per day more when they had access to shade compared to those that did not have shade. Other studies have shown added performance when cattle have shade access during periods of high temperatures. Shade can either be natural from trees or man-made using structures with shade cloth. Shade cloth should have a minimum of 50% of light exclusion.

As forage quality and availability declines in mid-summer combined with increased temperatures, cattle performance may dip. To combat this, supplementation can increase the plane of nutrition of stockers sustaining higher performance. Strategies will be dependent on feed prices, target levels of gain, marketing windows, and other factors. A higher protein supplement, 20% to 28% crude protein, targeted a low rate of supplementation near 0.5% of body weight can increase protein intake to combat declining protein in the forage. If there is a need to increase supplementation rates to achieve either higher rates of gain or stretch forage, a low starch, highly digestible fiber coproduct feedstuff that is 14% to 16% crude protein can be utilized. Using commodity blends containing corn at 50% or less with soyhulls, distillers grains, corn gluten feed and other quality coproducts can be offered to boost energy and protein intakes of grazing cattle. Research would suggest at higher feeding rates of supplements that have minimal impacts on ruminal microbes every pound of supplement fed will lower forage intake by about ½ pound. Point is that at 0.5% to 1.5% of body weight supplementation levels, forage substitution won’t be 1:1 with the supplement fed.

Consider these management factors for maintaining stocker gains during the summer. Take a few minutes to evaluate your current management and see if there are opportunities to adjust management to maintain or increase gains during the heat of the summer. Be sure to maintain animal health, keep internal parasites in check, utilize implants if your market allows, consider feeding an ionophore to combat coccidiosis and improve energy utilization from forages. Best of luck this summer and the markets appear to have some optimism looking at the futures prices. Consult with your veterinarian, feed dealer and county Extension agent for additional information.

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