INVITED REVIEW: Carbohydrate and fat: Considerations for energy and more

Historically, carbohydrates and fats were valued on their caloric contributions to diets. Feeding recommendations for these feed fractions now address inclusion levels as well as consideration of the positive and negative effects of specific types of these nutrients. Feed carbohydrate characterization has expanded beyond fiber and nonfiber carbohydrates. Fiber now encompasses ADF, NDF, physically effective fiber, and fiber digestibility to describe effects on diet composition, rumination, rumen fill, potential fermentability, and nutrient contribution. The nonfiber carbohydrates are now parsed into sugars and fructans (both in water-soluble carbohydrates), starch, pectins, and others, all of which can differ in their effects on rumen pH or support of microbial growth. Dietary fat has the advantage of providing energy without increasing the risk of ruminal acidosis. However, there are specific considerations for amounts and types fed in high-versus low-forage diets. Fats can affect ruminal fermentation, having the potential to depress fiber digestion or affect ruminal methane production. Considerable research in recent years has focused on providing specific dietary fatty acids to alter the metabolic function of specific tissues or to alter the fatty acid content of milk for nutraceutical purposes. Rising grain prices and diversion of fats for biofuel are driving livestock industries to seek alternative nutrient sources. Most of the nutritional research on which current recommendations are based involved use of traditional diets that tended to be rich in grains. Fat and carbohydrate feeding recommendations might need to change with diets high in low-starch by-products. We need to learn how diets with substantially more by-product feedstuffs ferment and pass from the rumen and how they affect nutrient supply and feed efficiency. We can then better predict digestion and the effects on metabolism and thus target supplementation to have the greatest positive effect on food-animal production.