Effect of pelletizing on the feeding value of rice straw in steam-flaked corn growing-finishing diets for feedlot cattle

Trial 1. Three cannulated crossbred steers were used to compare the effects of ground versus pelletized rice straw on characteristics of digestion. Treatments consisted of a steam-flaked corn-based diet containing 120 g/kg sudangrass (SG, positive control), ground rice straw (GRS) or pelletized rice straw (PRS). Ruminal digestion of organic matter was similar across treatments (P<0.10). Ruminal digestion of neutral detergent fiber was low, averaging 140 g/kg, and not affected (P>0.10) by forage source. Ruminal starch digestion was lower (5%; P<0.05) for sudangrass than for rice straw diets, and greater (8%, P<0.01) for pelletized versus ground straw. Ruminal microbial efficiency was greater (9.7%; P<0.05) for ground than for pelletized rice straw. Total tract digestion of dry matter was greater (3%, P<0.10) for sudangrass than for rice straw treatments and NDF digestion was lower (26%; P<0.05) for rice straw than for sudangrass diets. Total tract starch digestion was greater (2%, P<0.01) for pelletized than for ground rice straw diets, due largely to greater ruminal starch digestion. Treatment effects were numerically similar for ground sudangrass and ground rice straw diets. Pelletized rice straw increased (P<0.05) ruminal total volatile fatty acid (VFA) and propionate concentrations by 24 and 47% respectively, and decreased ruminal pH (9%, P<0.05), ruminal acetate:propionate molar ratio (27%, P<0.05), and estimated methane production (28%, P<0.05). Trial 2. Ninety crossbred yearling steers were used in a 112 day finishing trial to evaluate the influence of pelletizing on the feeding value of rice straw. Treatments were the same as Trial 1. Substituting ground rice straw for sudangrass had no detrimental effects (P>0.10) on energy intake or average daily gain (ADG). However, DMI and ADG were greater (11%, P<0.05, and 12%, P<0.10, respectively) for ground than for pelletized rice straw supplemented diets. There were no treatment effects (P>0.10) on dressing percentage, which had a carcass to live weight ratio averaging 0.642. Steers fed the sudangrass diet had a lower yield grade (16%; P<0.01), marbling score (13%; P<0.05), and fat thickness (15%; P<0.10) than those fed the rice straw diets. Feeding pelletized straw reduced marbling score (10%, P<0.10), and fat thickness (20%; P<0.10) when compared with ground rice straw. As expected, NEm and NEg were greater (5%; P<0.01) for sudangrass than for rice straw diets. Pelletizing rice straw did not decrease (P=0.56) the dietary net energy (NE) value of the rice straw supplemented diets. Thus, the difference in ADG due to feeding ground versus pelletized rice straw is attributable solely to treatment effects on DMI. We conclude that at lower levels of rice straw inclusion (<150 g/kg) fed to steers in growing-finishing diets, pelletizing the straw may reduce intake and gain through a satiety response mechanism, possibly associated with increased propionate production.