Barley grain is one of the most common grains used in diets for dairy and beef cattle. Due to its high gross energy and high energy digestibility (80%), barley grain has a high metabolisable energy value for ruminants (about 12.4 MJ/kg DM; Sauvant et al., 2004). However, its protein value is low with a digestible protein content of about 10% DM in diets with an adequate N balance (INRA, 2007). Due to its high content of rapidly degradable starch (nearly 50% of the DM), barley grain should be included in the diet at levels compatible with dietary recommendations on degradable starch (less than 40% of the DM). Other factors that influence pH buffering (for example particle length, forage NDF, electrolyte balance) should also be taken into consideration.
Effects of processing
Because the pericarp surrounding the endosperm of the barley kernel is extremely resistant to microbial degradation in the rumen, even in high-forage diets favoring mastication (Mathison, 1996), dry barley grain needs to be processed to improve its utilisation by beef and dairy cattle (Dehghan-banadaky et al., 2007). In lambs, however, processing (pelleting) did not prove to be necessary since feeding whole barley grain resulted in similar digestibility, prevented rumenitis and resulted in better subcutaneous fat quality (Orskov et al., 1974a; Orskov et al., 1974b).
Individual animal variation is high when animals are fed whole barley (Mathison, 1996). When barley is processed, for a given level of barley grain in the diet, animal performance depends not only on grain quality, but on the processing method, the extent of processing and their interactions (Dehghan-banadaky et al., 2007; Mathison, 1996). Incidence of bloating have been shown to be higher in steers fed whole rather than rolled barley, which is surprising since processing should increase degradability and the risk of bloating, but the mechanisms involved in these results are still unclear (Mathison, 1996).
Individual animal variation is high when animals are fed whole barley (Mathison, 1996). When barley is processed, for a given level of barley grain in the diet, animal performance depends not only on grain quality, but on the processing method, the extent of processing and their interactions (Dehghan-banadaky et al., 2007; Mathison, 1996). Incidence of bloating have been shown to be higher in steers fed whole, rather than rolled, barley, which is surprising since processing should increase degradability and the risk of bloating, but the mechanisms involved are still unclear (Mathison, 1996).
Grinding with a hammer mill is not an adequate process. It induces considerable dust due to shattering grain kernels and is generally detrimental to animal performance (Dehghan-banadaky et al., 2007). Finely ground barley grain ferments more rapidly than cracked barley grain, and may reduce productivity of cattle. Reduced feed intake (a reduction of 5% of DM intake compared to rolled barley given to steers), growth rate (down 100 g/d), feed efficiency (4.47 vs. 7.54), and fat depots (less 0.15 cm) have been observed with ground barley (Mathison, 1996).
Dry rolling, achieved by passing kernels between rotating rollers, is a common processing method. Adequate dry rolling increases rumen digestibility of grain and animal productivity (Dehghan-banadaky et al., 2007). In cattle fed high-grain diets, digestibility increased by 16% when rolled barley was fed rather than whole barley, related to an increase in starch digestibility of 37% (Mathison, 1996).
Tempering is achieved by raising the moisture content of the barley to 20-25% by adding water, mixing, and storing for 12-24 h prior to rolling (Dehghan-banadaky et al., 2007). This method reduces dustiness and production of fine particles. Although information is scarce, tempering of barley may slightly improve feed conversion (increased by 6.8%) in growing and finishing cattle, but the mechanisms are not clear since intake, starch digestibility, daily gain, and carcass characteristics are not affected (Mathison, 1996).
Steam rolling is achieved by application of steam above the roller mill prior to rolling. Compared to dry rolling, it reduces production of fine particles during rolling, allowing a more uniform particle size distribution. Short term (70 seconds) steam rolling was of no benefit in improving feeding value compared to dry rolling, where longer times (20 min) increased digestibility. However, there was generally little response in either live-weight gain or efficiency (Mathison, 1996).
Steam flaking is achieved by applying steam at low or high pressure and allowing the grain to cool before rolling. The combination of moisture, heat and pressure gelatinizes the starch granules. This process does not improve feed efficiency (Owens et al., 1997), because barley starch, once exposed to microbial organisms in the rumen, is readily degradable even without being gelatinized (Dehghan-banadaky et al., 2007). Neither pelleting with a low moisture content and a temperature around 80°C, nor extrusion affect rumen degradation of starch in barley grain, due to the very high level of rumen degradable starch before processing (Svihus et al., 2005).
Some processes can be applied to barley grain to control its rate of degradation in the rumen. Roasting, aldehyde treatment, and ammonia can decrease starch and rumen protein degradability (Mathison, 1996, Dehghan-banadaky et al., 2007). Ammoniation can increase milk production in dairy cows, but does not affect daily gain and feed efficiency in lambs (Mathison, 1996). Treating grains with NaOH may increase its ruminal starch digestibility without increasing the ruminal rate of starch release (Dehghan-banadaky et al., 2007). Ammonia or fibrolytic enzymes can increase hull degradation (Dehghan-banadaky et al., 2007). The effect of expanding is unclear, and could depend on temperature, treatment duration and particle size. At high heat input and low moisture content (toasting), the protein matrix becomes resistant to proteolysis and the rumen degradation of barley starch is decreased, but this does not reduce its intestinal digestibility in situ or in vivo (Svihus et al., 2005).