Heat increment of the dietary ingredients plays a role in both the efficiency with which nutrients are utilized and heat generation potential of the diet. There is a potential for formulation of diets that are particularly favorable for use during hot weather. High fiber diets result in production of large quantities of ruminal acetate, which is used with less efficiency than propionate. The efficiency of use for ME from high fiber diets is lower than for diets with high levels of rapidly fermentable carbohydrate, providing a scientific basis for the practice of feeding low fiber diets during hot weather. Low fiber diets must be implemented cautiously because of the necessity for sufficient fiber to promote good rumen function and to maintain animal health and well-being.
Dietary fats have a relatively low heat increment because of a high efficiency of use by the cow.
Fats may be of particular value during hot weather and may be undervalued by current energy equations because of the lower heat increment. Results from research with fat feeding during hot conditions have been variable. Rumen inert fat allows inclusion of relatively high levels of fat in the diet, and experiments to elucidate the effects of feeding these diets during hot weather should be conducted. Because high levels of dietary fat reduces the amount of fermentable energy in the rumen, the quantity of protein and ruminal escape value for that protein may be particularly critical when feeding high fat diets. Practical management considerations such as frequent feeding and ready water availability are logical.