Effects of grain source and monensin level on site and extent of digestion in feedlot heifers

Abstract Text: Many feedlot producers include wheat grain into the rations due to increasing supplies of feed wheat in North America, whereas few studies have documented the effects of wheat source on animal response. A study was conducted to examine feed intake and site and extent of digestion by substituting wheat grain (soft or hard) for barley and to determine whether increasing monensin level would improve feed digestion in feedlot heifers. Five ruminally and duodenally cannulated beef heifers were used in a 5 × 5 Latin square design with 2 × 2 + 1 factorial. Treatments were barley (10% barley silage, 90% barley-based concentrate, and 28 mg/kg monensin), and diets substituting soft or hard wheat for barley combining with 28 (low) or 44 (high) mg/kg monensin. The barley diet is a standard feedlot diet used in western Canadian feedlots. Contrasts were generated to compare barley vs. wheat diets in low monensin; soft vs. hard wheat; and low vs. high menensin in wheat diets. Intake of DM was affected neither by grain source (8.3 kg/d) nor by wheat source (7.7 kg/d), whereas increased monensin level reduced (P < 0.02) DMI from 8.1 to 7.3 kg/d. Inclusion of wheat in place of barley grain did not affect the flows of OM to the duodenum (3.8 kg/d) and digestibility (% of intake) of OM in the rumen (64.2%) and in the total tract (80.5%). However, digestibility of NDF (barley vs. wheat; 60.6 vs. 48.3%; P < 0.01) and that of starch (barley vs. wheat; 95.5 vs. 97.6%; P < 0.03) in the total digestive tract were different. Feeding soft vs. hard wheat delivered lower (P < 0.03) OM (soft vs. hard; 3.4 vs. 3.6 kg/d) and non-ammonia N (soft vs. hard; 164 vs. 178 g/d) to the duodenum with no differences in ruminal and intestinal digestibility of OM. Increased monensin supplementation decreased (P < 0.05) duodenal flows of OM from 3.7 to 3.3 kg/d, total N from 182 to 168 g/d, and microbial N from 99 to 87 g/d without affecting the site and extent of feed digestibility. These results indicated that wheat exhibited similar feed value to barley; high level monensin may potentially alleviate ruminal acidosis by reducing DMI of finishing cattle.

Keywords: feedlot heifers, grain source, digestibility