Effect of starch content and processing method on in situ ruminal and in vitro intestinal digestion of barley grain in beef heifers

Rate of barley starch digestion in the rumen is critical issue in high-grain fed beef cattle because it is associated with growth performance and health. The digestion rate of starch varies with variety, grain processing or starch content. The objectives of this study were to evaluate the effect of starch content and processing method of barley grain on in situ ruminal dry matter (DM) and starch digestion kinetics, and to develop a model to predict the rate of DM digestion of barley grain. The study was a 2×2 factorial arrangement of treatments: starch content (low vs. high) and processing method (PM; grinding vs. dry rolling). Ten barley samples with 5 low (571±17g/kg) and 5 high (660±7g/kg DM) starch were either ground 2mm or dry-rolled with processing index (PI) of 0.75 (PI=bulk density processed/bulk density whole). Three beef heifers (650kg body weight) fitted with rumen cannulas and fed diet consisting (g/kg DM) of 700 barley silage and 300 barley grain were used for in situ incubation. In situ degradation kinetics of DM, starch, neutral detergent fibre (NDF) and crude protein (CP) were estimated after 0, 2, 4, 8, 12, 24, and 48h of ruminal incubation. Data from DM, starch, NDF and CP degradation at different times of incubation were fitted to a model y=a+b(1−e−c(t−L)). In vitro intestinal disappearances of DM, starch, NDF and CP of ruminal residue after 12h of incubation were determined using a modified three-step procedure. Interactions between starch content and PM on the degradation kinetic parameters of DM, starch, NDF and CP were seldom observed. High starch barley had greater (P<0.01) a and effective degradability (ED) of DM and starch, but it had less (P=0.03) b of starch versus low starch barley. Ground barley had greater (P<0.01) a, c and ED of DM and starch, and lower (P<0.01) b compared with dry-rolled barley. Intestinal disappearance DM and starch of rumen residues were affected by both starch content and PM. Starch content of barley grain and manipulating processing method could effectively alter rumen digestion of barley grain.