lyzes chemical bonds between fibrous components in the cell wall. Sodium hydroxide is more effective than ammonia or urea, but it is more expensive and has greater environmental consequences, so ammonia or urea are more widely used. Berger et al. (1994) concluded, from 21 studies on crop residues and 6 on grasses, that ammoniation improved dry matter intake by 22 and 14 percent, respectively. With regard to digestibility, 32 studies on ammoniated crop residues and 10 on grasses demonstrated a 15 and 16 percent improvement, respectively. Urea enhanced intake by 13 percent and digestibility by 23 percent. Oxidation is an alternative chemical procedure that has been used to upgrade roughages and microbial and enzymatic methods have been developed and tested as well. Steam treatment is an additional physical process that has been developed. However, none of these latter processes are widely used in North America at present. For details, the reader is referred to Berger et al. (1994).
Processing can significantly improve the nutritive value of cereal grains for beef cattle. The most common physical processes used are rolling or grinding the grain, with or without additional moisture; and this is done chiefly to rupture the pericarp and expose starch granules to aid digestion (Beauchemin et al., 1994). In a few cases (see below), processing of whole grain for beef cattle is not beneficial; but this is the exception rather than the rule. When processing is used, results are often variable and unpredictable. Furthermore, processing can affect nutrient requirements in a subtle fashion. To rationalize these effects, significant principles about grain processing will be discussed first.
Cattle are less able than other ruminants in the ability to masticate whole grain (Theurer, 1986). Sorghum presents the greatest difficulty followed by wheat, barley, corn, and oats. Morgan and Campling (1978) found that younger cattle can digest whole grain better than older cattle; however, Campling (1991) concluded that further studies on a possible relationship between cattle age or weight digestion of grain are necessary. The ability of rumen microbes to digest grain depends on particle size (Galyean et al., 1981; Beauchemin et al., 1994)—fine particles are digested more rapidly than coarse particles. Microbial digestion proceeds from the inside to the outside of the kernel, and the protein matrix, which surrounds starch granules in the endosperm, is a barrier to the effective digestion of starch (McAllister et al., 1990a). For this and related reasons, there are major differences between the rates at which grains are digested; for example, barley is digested more rapidly than corn (McAllister et al., 1990b). Rapid acid production from the fermentation of starch in the rumen is undesirable; thus, starch bypassing digestion in the rumen altogether can be beneficial, hence processes that inhibit digestion of grain protein will decrease starch digestion in the rumen (Fluharty and Loerch, 1989). Because heat has a major influence on protein digestion, any process using heat treatment is likely to influence grain nutritive value. Unfortunately, in the heat treatment of grain, the relationships of time, temperature, and moisture to protein digestibility are ill-defined; therefore, effects of heat treatments on grain nutritive value would be difficult to interpret. This is further complicated because heat gelatinizes starch, which facilitates microbial digestion (Theurer, 1986) and could therefore offset some or all of the effects of heat. Enhanced microbial protein synthesis and decreased grain protein degradability were associated with steam processing and rolling of sorghum to produce a lighter flake (Xiong et al., 1991). Zinn (1990a) found that the longer the corn was steamed, the faster nonammonia nitrogen was processed in the duodenum of cattle. Roughage source and amount influence dynamics of rumen liquid and particulate flow and may, therefore, influence grain digestion in the rumen (Goetsch et al., 1987).
Instrinsic characteristics of grains affect the rate or extent of starch digestion and can reduce benefits from processing. One factor is the form of starch and the other is the presence of tannins. Amylopectin is more digestible than amylose; hence, waxy grains are more digestible than other grains (Sherrod et al., 1969). Tannins present in bird-resistant grains, for example, sorghums, reduce digestibility (Maxson et al., 1973). Within varieties of the same grain, total digestible nutrients (TDN) varied as much as 7 percent (Parrot et al., 1969). Grain quality for beef cattle is positively associated with grain density or fiber content, as shown for barley by Mathison et al. (1991a) and Engstrom et al. (1992).
Grain that is fermented less rapidly and extensively in the rumen can escape microbial digestion and may be digested enzymatically in the small intestine. In a review of many trials, Owens et al. (1986) estimated that cattle are 42 percent more efficient in utilizing starch when it is digested in the abomasum and small intestine compared to the forestomach. Thus, processes that cause starch to escape rumen digestion could be beneficial, provided it is effectively digested in the intestine and not passed further to the caecum, where fermentation can resume and significant depletion of nitrogen from the animal may result (Owens et al., 1986). The concept of limited starch digestion in the small intestine does not seem plausible. Furthermore, digestion in the hindgut does not usually compensate for reduced digestion in the rumen (Goetsch et al., 1987).