Effects of a combining feed grade urea and a slow-release product on performance, dietary energetics and carcass characteristics of steers fed finishing diets

Recent findings have shown that microbial nitrogen flow and digestible energy of diet are increased when urea (U) is combined with a slow-release urea product (SRU) in diets with a starch:acid detergent fibre (S:ADF) ratio of 4.5, while feed grade U has shown greater effects on growth performance or dietary energy utilization when the diet contains a S:ADF ratio of greater than 5.0. These results can be partially explained by the better synchronization of ruminal degradation rates between those U sources with the carbohydrates of the diets. Therefore, 60 crossbreed steers (372.4 +/- 15 kg) were used to evaluate the effects of combining U and SRU in a diet with a S:ADF ratio of 4.5 vs. U that was supplemented in diets with different S:ADF ratios (3.5, 4.5 and 5.5) on growth performance, dietary energetics and carcass characteristics. U combination did not affect average daily gain (ADG), but reduced dry matter intake [DMI, as % of body weight (BW)] enough to increase feed efficiency (G:F) and dietary net energy (NE). As the S:ADF ratio increased, the DMI, ADG, G:F and NE of diet increased linearly. Irrespective of the S:ADF ratio, U diets did not modify neither the observed-to-expected NE ratio nor the apparent retention per unit DMI, while U combination increased by 7.2% and 8.4%, respectively, the observed-to-expected dietary ratio and the apparent retention per unit DMI. U combination had no effect on carcass characteristics. As the S:ADF ratio increased, carcass weight and LM area were increased linearly. Combining feed grade U and SRU in diets with a 4.5 starch:fibre ratio resulted in positive effects on the efficiency of utilization of dietary energetics.