In lactating dairy cow rations, carbohydrates compose about two thirds of the dry matter. Fiber (NDF = neutral detergent fiber) is the most abundant carbohydrate coming from the forage and other sources such as by-product feeds. Digestion of fiber, however, is usually lower than starch and sugars and as a result the energy value is less. There are some corn silage varieties that have higher fiber digestibilities and thus greater energy values. In addition soybean hulls, corn gluten feed, brewers grains, and distillers grains are good sources of digestible fiber.
The non-fiber carbohydrates (NFC) can be calculated by the equation: 100 – ((NDF – NDF protein) + crude protein + fat + minerals). In other words it is not NDF (corrected for protein content), crude protein, fat, or mineral. Another way to express is NFC = starch + sugar + soluble fiber. This soluble fiber does not appear in the NDF analysis and is not usually reported on feed test reports unlike starch and sugar. Soluble fiber is composed of beta-glucans, galactans, and pectin, is readily digestible and is an energy source. Digestion in the rumen yields more acetic acid rather than the propionic or lactic acids typical of starch and sugar digestion and is considered less of a risk for acidosis. Two feeds having relatively high levels of soluble fiber are citrus pulp (34% soluble fiber) and beet pulp (21%) according to the Degussa AminoCow Feed Program. Other feeds having some soluble fiber are soybean meal (10%), soybean hulls (10%), corn silage (10%), barley silage (8%), and brewers grains (8%). When reducing starch because of high corn prices it may be feasible to increase the level of soluble fiber but it should usually be limited to 8% of ration dry matter. Starch in dairy rations will typically be 20% or above and sugars less than 5% of the total dry matter. A ration containing 25% starch, 4% sugar, and 8% soluble fiber would have an NFC of 37%. Most rations have less than 40% NFC and more than this might put cows at risk of rumen acidosis.
In a recent analysis by Dr. Pat French of The Old Mill Troy, Inc. in Pennsylvania (personal communication March 2011) using Mid-Atlantic feed prices, barley (starch), citrus pulp (soluble fiber and sugar), soybean hulls (soluble fiber and digestible insoluble fiber), and molasses (sugar) were economical sources of carbohydrates. Before using, however, they should be figured into your feeding program along with your home-grown feeds and actual savings calculated using current prices. That is the only way to know if a feed fits and can reduce ration cost while supplying a needed component of the diet. In conclusion, soluble fiber, digestible insoluble fiber, and sugars have the potential to replace some expensive starch (corn) in the ration but avoid excessive amounts of each as well as total NFC.