Degradable intake protein supplementation through the inclusion of urea in finishing diets containing distillers grains: effects on feedlot cattle performance, ruminal fermentation, and feed digestibility

Abstract

Degradable intake protein (DIP) represents the proportion of protein that is potentially fermented in the rumen. Ruminal DIP balance is calculated by the difference between DIP supply and requirements. The former is a function of dry matter intake and dietary DIP, and represents nitrogen (N) available for synthesis of microbial crude protein (MCP), which is used as a measurement of microbial growth or production of new microbial cells. Synthesis of MCP basically requires ammonia-N (NH3-N), carbon skeletons, and energy. While the last two are mainly derived from fermentation of dietary carbohydrates, dietary N represents the main NH3-N source. Consequently, DIP requirements represent rumen-degradable N needs for MCP synthesis, and are a function of available fermentable carbohydrates. High dietary inclusion of grain as well as more extensive grain processing methods can result in increased ruminal availability of rapidly-fermentable carbohydrates, which in turn may result in increased DIP requirements. In addition, corn-based diets may not supply adequate amounts of DIP because corn protein is considered to be approximately 60% undegradable. Despite great protein content and because of great undegradable protein concentration, small to moderate dietary inclusion of corn distillers grains (DG) may also result in DIP deficit. Experiments 1 and 2 described in Chapter 2 evaluated the effect of adding urea, a highly rumen-degradable N source, to a high-concentrate, moderate-DG-containing diet on feedlot cattle performance, ruminal fermentation, and feed digestibility. Results from both experiments indicate that due to a DIP deficit generated by the un-supplemented diet, the addition of urea resulted in enhanced ruminal fermentation and feed digestibility, and consequently improved animal performance. Because rates of degradation of carbohydrates and conventional urea do not match, beneficial effects may arise from the use of slow-release urea (SRU) sources over conventional urea when added to DIP-deficient diets. Therefore, experiments 1 and 2 described in Chapter 3 evaluated the effect of increasing DIP concentration through the inclusion of one of two SRU sources in comparison with the inclusion of conventional urea in DG-containing feedlot diets on ruminal fermentation and feed digestibility. Likely due to lack of DIP deficit with the un-supplemented diet, results from these experiments do not demonstrate potential beneficial effects of SRU sources over conventional urea. Several confluent factors are discussed that may explain lack of need of urea supplementation in Chapter 3 experiments. Because previous studies have demonstrated improved ruminal fermentation, feed digestibility, and animal performance when supplementing conventional urea to rapidly-fermentable, moderate-DG-containing diets, more research is warranted to evaluate the use of SRU in diets for which a DIP deficit is expressed.