Feeding the rumen Fine-tune rumen function for top cattle performance by Dr. Jim White The major nutrients needed by rumen microbes are proteins and carbohydrates. Both nutrients are required to maximize microbial growth, but each serves a different role in the process. Proteins-nitrogen sources (ammonia, peptides, and amino acids) have a major impact on the efficiency of microbial growth. It is in attempting to maximize the quantity of carbohydrate fermented that we cause most of the metabolic problems associated with rumen function. In attempting to maximize total carbohydrate fermented, the common approach is to reduce the fibrous carbohydrates in the ration and increase sugars and starches. If overdone, rumen dysfunction, acidosis, decreased microbial growth, metabolic distress for the cow, and results to unhappy calls to your friendly nutritionist. So think about rumen function by the following components. Sugars The sugar content of most common feeds fed to lactating cows can range from less than 1 to over 20 percent dry matter. As a consequence of the low levels of sugars in the ingredients in a typical diet for lactating cows, the total sugar content will often be only 1.5 to 3 percent of dry matter. Benefits of the right amount of sugar in the diet can come from several factors. These include a rapid increase in microbial growth due to the higher level of available energy, more efficient utilization of the nitrogen in diets high in soluble nitrogen or non-protein nitrogen, or the effects of altered volatile fatty acid ratios on production. In addition to molasses and whey products as the main sources of sugar that can be added to cattle rations, there are a number of other by-products that contain relatively high levels of sugar (almond hulls, candy by-products, corn syrup solids). The concern with by-products is having a consistent level of sugar in the product used—overfeeding sugar can result is rapid and drastic consequences. With the exception of pentoses, which are common in cool season grasses, feeding sugar results in the production of lactic acid in the rumen and a subsequent drop in rumen pH. The bad effects of feeding sugar are associated with the drop in rumen pH. This is consistent with field experience where acidotic animals will have reduced fiber digestibility—a fair amount coming through in the fecals. A major aspect of the current interest in adding sugars to ruminant diets is related to the effects on nitrogen metabolism and microbial growth. A reduction in ruminal ammonia concentration has been noted in nearly all studies in which sugars have been added to the diet; the sugar is quickly fermented, which effectively sops up any ammonia-soluble nitrogen laying about in the rumen.