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Carbohydrate and fat: considerations for energy and more

Hall, M. B., Eastridge, M. L.
ARS USDA Submissions 2014 v.30 pp. 140
acid detergent fiber, animal production, biofuels, byproducts, carbohydrate composition, cow feeding, dietary fat, digestibility, energy, fatty acids, feed composition, feed conversion, fructans, grains, lipid content, livestock and meat industry, metabolism, metabolizable energy, methane production, microbial growth, milk, neutral detergent fiber, nutrients, pectins, prices, risk, rumen, rumen fermentation, rumen microorganisms, ruminal acidosis, rumination, starch, sugars
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 (NFC). Fiber now encompasses acid detergent fiber (ADF), neutral detergent fiber (NDF), physically effective fiber, and fiber digestibility to describe the effect on diet composition, rumination, rumen fill, potential fermentability, and nutrient contribution. The NFC is now parsed into sugars and fructans (both in water-soluble carbohydrates), starch, pectins, and others, all of which may 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- vs. 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 (FA) to alter the metabolic function of specific tissues or to alter the FA 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 the use of traditional diets which tended to be rich in grains. Fat and carbohydrate feeding recommendations may need to change with diets high in low-starch byproducts. We need to learn how diets with substantially more byproduct 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.