Heat stress modifies cow behavior
Dairy cows experiencing even mild heat stress spend more time standing compared to cows not experiencing heat stress. This change in cow behavior is most likely related to the cow’s attempt to increase the amount of surface area needed to dissipate heat and decrease her core body temperature. Although there has not been a research trial looking at the direct impact of heat stress on incidence of lameness, we do know: (1) as cows spend less time lying down and more time standing, the incidence of lameness increases and (2) heat-stressed cows spend more time standing. Thus, one could assume that heat-stressed cows would have a higher incidence of lameness, and any practices that reduce heat stress and standing times of dairy cows would likely decrease the proportion of dairy cows becoming lame.
Modifying diets for heat-stressed dairy cows
- Maintain effective fiber intake: Adequate effective fiber is necessary for maintaining rumination, buffering the rumen contents, and efficiently digesting forages and grain components of the diet. Heat stress increases the rate of respiration and panting, decreases rumination time, and results in a decrease in the amount of saliva and bicarbonate in the blood. These changes result in a decreased buffering of the rumen and blood. Thus, decreasing the fiber content and increasing the amount of starch in a diet is the last change you want to make in an attempt to increase the energy of the diet because ruminal acidosis could result. However, feeding excessive amounts of neutral detergent fiber (NDF) to dairy cows under heat stress is detrimental. High NDF forages are generally lower in forage quality and result in more heat of fermentation when digested in the rumen, and thus the dairy cow needs to dissipate more heat compared to consuming diets with adequate amounts of fiber.
- Feed highly digestible forages: Feeding higher-quality forages increases the energy content of the diet, helps maintain adequate rumination, and decreases the heat of fermentation associated with feeding lower-quality forages. Brown midrib forages (i.e., corn silage or forage sorghum) may be more beneficial in diets of heat-stressed dairy cows to improve digestibility of the fiber and, therefore, the amount of energy derived from the consumed diet.
- Adding fat to the diet: Adding fat to the diet is expected to decrease heat produced during the digestion of feeds while increasing the amount of energy available. Studies where fats have been fed to heat-stressed cows have shown inconsistent responses in improving milk production; some have improved milk production, and others have shown no response.
- Adding yeast cultures to diets: Yeast culture has been shown to improve fiber digestion and stabilize the rumen environment. In heat-stressed dairy cows supplemented with yeast, lower rectal temperatures and respiration rates were observed in several but not all studies. Several studies, but not all, have shown an increase in milk production of heat-stressed cows supplemented with yeast. In 1994, Huber and others summarized 14 lactation comparisons with 823 heat-stressed cows where yeast was or was not added to the diet. Overall, these comparisons showed a 2.2 pound/day increase in milk production with yeast supplementation with six comparisons showing significantly higher milk production with supplementation, three slightly higher, and the remaining five comparisons with no or slightly lower milk production. Two recent studies have shown no improvements in milk production with yeast supplementation, but one indicated improved feed efficiency. Early-lactation cows fed a higher proportion of concentrate may respond more favorably to yeast supplementation than mid- to late-lactation cows.
- Modifying mineral content of the diet: Heat-stressed dairy cows sweat, and their sweat contains high amounts of potassium and sodium, thus increasing their need for these minerals in summer rations. To achieve these increased concentrations of potassium and sodium and maintain adequate dietary cation-anion difference (DCAD), additional amounts of sodium bicarbonate, potassium carbonate, or both may need to be added to the diet. In addition, higher amounts of potassium reduce the absorption of magnesium, thus increasing the requirements for magnesium.