In the United States, cattle are commonly fed diets containing cereal grains. The presence of starch and sugars reduces fiber digestion, which may in turn depress intake. In this paper, chemical constraints that may be responsible for the decrease in fiber digestion are explored. A major factor appears to be rumen pH. Moderate depression in pH, to approximately 6.0, results in a small decrease in fiber digestion, but numbers of fibrolytic organisms are usually not affected. Further decreases to 5.5 or 5.0 result in depressed growth rates and decreased fibrolytic microbes, and fiber digestion may be completely inhibited. Proliferation of organisms on readily fermentable carbohydrates may increase the need for total nitrogen as both ammonia and amino acids. The value of amino acids to cellulolytic organisms appears to be primarily as sources of isobutyric, isovaleric, and 2-methylbutyric acids. This reinforces the need to establish dietary requirements for nonprotein nitrogen, degradable protein, and isoacids. Other factors affecting fiber digestion, such as inhibition of cellulytic enzymes and plant concentrations of lignins and phenyl propanoids, are also discussed.

Establishing conditions under which rumen fermentation will be optimized requires an understanding of the nutrient requirements of the mixed microbial population. The major nutrients required by rumen microbes are carbohydrates and proteins, but the most suitable sources and quantities needed to support maximum growth have not been determined. Digestion of proteins results in the production of peptides, which can accumulate in the rumen. Peptides are further hydrolyzed to amino acids, some of which are deaminated, producing ammonia. Although peptides, amino acids, and ammonia all may individually serve as sources of N for various microbes, the total population achieves the highest growth rate on mixtures of all three sources. In a somewhat analogous manner, carbohydrates are digested by exoenzymes to oligosaccharides that are available for crossfeeding by the mixed microbial population. Based on data from both in vitro and in vivo studies, there is general agreement that rate of digestion of carbohydrates is the major factor controlling the energy available for microbial growth; in addition, rate of digestion of total carbohydrate is directly related to proportion of starches, pectins, and sugars. Proteins affect both total fermentation and production of microbial DM per unit of carbohydrate fermented. It appears that the quantity of ruminally available protein needed to optimize microbial growth may, under some conditions, be as high as 14 to 15% of diet DM.

A method was developed to fractionate the neutral detergent‐soluble carbohydrates (NDSC) in feedstuffs. Differential solubilities of carbohydrates in 80:20 (v/v) ethanol/water were used to partition NDSC into organic acids (OA) and mono‐ and oligosaccharides soluble in ethanol/water from starch and neutral detergent‐soluble fibre (NDSF) which are insoluble. Mono‐ and oligosaccharides (total ethanol/water‐soluble carbohydrate) were measured on the ethanol/water extract, and starch was measured on the ethanol/water‐insoluble residue. The OA and NDSF, the two most compositionally diverse NDSC fractions, were estimated by difference. The method allows partitioning of the NDSC on a nutritionally relevant basis into (1) organic acids, (2) total ethanol/water‐soluble carbohydrate, (3) starch and (4) neutral detergent‐soluble fibre. The methods involved in this fractionation are relatively simple or are commonly used. © 1999 Society of Chemical Industry

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A study was conducted to determine the effect of various forms of N on the growth of ruminal microbes in a continuous culture system with solids and liquid dilution rates comparable to those of a high-producing dairy cow. Nitrogen forms were isolated soy protein, soy peptides, individual amino acids (AA) blended to profile soy protein, and urea, which were fed alone and in combinations so that the total N provided was 1.6% of the diet DM. The 100% soy protein treatment resulted in reduced digestion of N and nonstructural carbohydrate compared with other N forms, and outflow of bacterial N/24 h was less than when peptides were fed. This suggested that proteolysis rather than peptide uptake was the rate-limiting step in N utilization in this study. Non-urea N forms increased ADF digestion, total VFA production and the molar percentages of isobutyrate, isovalerate, and valerate compared to urea, which reflected the contribution of carbon skeletons of AA. When combinations of N forms were used, each form contributed an equal quantity of N, 50% of the total treatment, which was .8% of the diet DM. Combinations of N forms did not enhance, and in most cases reduced, ADF and NDF digestion when compared with individual N forms, and no combinations increased microbial growth over that of the individual forms. These results confirm that N forms other than ammonia are needed not only for maximum microbial growth, and they further demonstrate a need for non-protein N for the fiber digestion. In addition, results of this study suggest a requirement for a minimum level of peptide or AA N, which was met only when individual N forms were fed.