The influence of forage particle size on rumen metabolic responses and nutrient utilization

Three trials were conducted to assess the influence of the barley silage particles as a source of effective fiber in total mixed rations for ruminants. Trial one determined the effective fiber characteristics of barley silage at different theoretical cut length; short barley silage (SBS) = 4.68 mm and long barley silage (LBS) = 18.75 mm in a replicated switch back design using six steers fitted with rumen fistulae to measure rumen fermentation patterns, kinetics, digestibility of cell wall constituents. In the second trial, four steers fitted with rumen fistulae in a 4 x 4 Latin square design were used to study the effect of barley silage particle size and concentrate level on the above variables. The third trial used eight multiparous lactating dairy cows in a replicated 4 x 4 Latin squire design to determined the effect of barley silage particle size and concentrate level on eating behavior, milk production and composition. In the first trial, feeding SBS increased DMI and total tract digestibility of DM,NDF and ADF compared with LBS. Mean ruminal pH was lower for steers fed SBS than those fed LBS but pH range was higher (P < 0.05) for SBS compared to LBS (0.36 vs 0.13 units respectively. Saliva production was 28.2% higher for LBS diets. Feeding SBS reduced rumen mean retention time but increased particulate passage rate. Interaction between particle size and time showed total VFA were higher for SBS diets 4 h post feeding while molar proportion of acetate and acetate: propionate ratio were reduced at the same time. It is concluded that reduced barley silage particle size can influence the quantity and nature of digestive end products by altering chemical and physical conditions in the rumen and rate of onset rumen fermentation. In the second study, feeding SBS at high concentrate rather than LBS diets reduced the molar proportion of acetate but increased the molar proportion of propionate. The daily means of rumen pH and ammonia concentration were lower for diets with 80% concentrate but not for the 50% concentrate diets. The passage rates for particulate matter and the retention time were decreased when SBS diets were fed. SBS diets reduced total digestibility of DM, CP, and NDF (65.5 vs 68.5, 66.5 vs 73.4 and 39.4 vs 45.9; P < 0.05) respectively. These results suggest that long particle size of the silage can promote higher rumen pH, reduced particulate passage thereby modifying rumen environment to favor optimum nutrient utilization when high concentrate diets. Results of the third trial showed that increasing silage particle size of the forage did not affect DMI. The 3.5 FCM and fat yields trended higher for increased particle size. Percent milk protein was higher for short particle size. Increasing concentrate levels in the diets increased proportions of milk protein, lactose but not milk fat. Cows fed SBS spent 90 minutes less per day chewing and ruminating as compared to those on long silage. Total chewing activity per kg forage intake was higher for cows on long silage as compared to those on short silage diets. It is concluded that longer barley silage particle size will modify rumen environment to favor optimum nutrient utilization when high energy density diets are being used