Heat is produced as a result of microbial fermentation in the rumen. Low quality, stemmy forages generate more heat of fermentation inside the animal, contributing to the animal’s total heat load. Heat is used to maintain body temperature when ambient temperatures are low (below 40 F). However, when ambient temperatures are high the cow needs to maintain its body temperature within biological limits compatible with life. The cow options are limited; she can drink more fresh water (water evaporation from the body dissipates heat). She can also look for cool spots like ponds, shade, and places with good air circulation, or she can decrease her dry matter intake.
High quality forages are digested faster and result in less heat being produced. So, whenever feed intake decreases due to heat stress, consider enriching the ration. Energy is a particular concern, as most of it is derived from the fermentation of forages within the rumen. Increasing the energy density might require greater amounts of concentrate and/or byproducts. Keep in mind that shifting the grain/forage ratio may increase the risk of not having enough rumen “scratch factor” (effective fiber) to optimize rumination. When the rumen mat formation is inadequate, saliva production decreases (less chewing activity). This decreases rumen pH and allows more grain to be fermented in the abomasum and intestines. Adequate particle size and the use of buffers such as sodium bicarbonate are critical in these “hot rations” to minimize the risk of digestive disorders such as acidosis and displaced abomasum. Sweating aids in heat dissipation, and sodium and potassium are secreted in sweat. This will be a problem particularly with high producing dairy cows, which secrete more of these minerals in milk. When environmental temperatures are above 68 F, make sure that diets contain at least .25 lb of white salt per cow per day. Plenty of fresh, high quality water should also be available. Test it; you don’t want water already high in sodium. Alleviating heat stress in dairy cattle is a challenge in every South Dakota summer. Maintaining optimum nutrient balances and providing highly palatable, digestible feeds and ample supplies of fresh, clean water, along with shade and ventilation, will go far toward keeping your cows comfortable and their milk production up.
In the iGrow Feeding Strategies to Improve Intake in Heat Stressed Cows article, there is a checklist of feeding strategies that help reduce excessive drops in intake.