Effects of grain source and monensin level on growth performance, carcass traits and fatty acid profile in feedlot beef steers

A study was conducted to evaluate growth performance, carcass traits, and composition of fatty acid in carcass of feedlot steers fed diets varying in grain source and monensin levels. Two hundred crossbred steers (initial body weight [BW] 488 ± 36.9 kg) were blocked by BW, allotted to 20 pens, and then randomly assigned to 5 treatments (4 pens per treatment) with 2 × 2 + 1 factorial arrangement. Treatments were barley (100 g/kg barley silage, 900 g/kg barley-based concentrate, and 28 mg/kg monensin) which is a standard feedlot diet for western Canadian feedlots, and diets substituting hard or soft wheat for barley combining with 28 or 44 mg/kg monensin. Dry matter intake (DMI) was higher (P < 0.02) for wheat than for barley diets but it was not different between hard and soft wheat diets. Increasing monensin supplementation reduced (P < 0.01) DMI. Final BW, average daily gain and gain:feed were not different between treatments. Carcass traits were not affected by treatments except that dressing fraction was greater (P < 0.05) for steers fed barley than wheat diets. Additionally, steers fed soft wheat had less (P < 0.05) back fat and greater (P < 0.05) meat yield compared with hard wheat diet. Substitution of wheat for barley grain did not affect the total monounsaturated fatty acid and polyunsaturated fatty acid, but decreased (P < 0.05) vaccenic acid (t11-18:1; VA) and a-linolenic acid (18:3n-3; ALA) in the pars costalis diaphragmatic muscles of beef cattle. These results indicate that wheat can effectively replace barley grain in finishing ration without negatively influencing growth performance, carcass traits, and FA composition in beef. Supplementing monensin with higher level than currently practical level had no evident effect on growth rate, feed efficiency and carcass traits, although DMI was decreased.