Relationship Between TDN and OM Digested in Beef Cattle Finishing Diets

Abstract Text: Organic matter digestibility (OMD) is related to TDN. However, the relationship is unestablished for diets containing byproducts.  Three digestion studies were used to evaluate the relationship between TDN and OM digested. Total tract collection and OM analysis of feed and feces determined OMD which was multiplied by dietary OM content to determine OM digested (% of diet DM). Gross energy of feed and feces was determined by bomb calorimetry. Dietary DE was calculated and was converted to TDN using 4.4 Mcal DE / kg TDN. Experiment 1 utilized 45% high moisture corn, 40% Sweet Bran, and 10% corn silage diets (DM basis); Experiment 2 used diets containing 40% modified distillers grains plus solubles (DGS; DM basis) and increasing amounts of a corn stover pellet replacing dry rolled corn (DRC); Experiment 3 compared 80% DRC-based diets with one of two supplemental fat sources to diets with 25.5% distillers solubles, or 56% wet DGS. Regression was used to relate digestible OM to TDN. The initial model included experiment, animal within experiment, and treatment within experiment. A significant treatment within experiment effect (P  < 0.01) resulted in independent regression models for each experiment. Experiment 1 and 2 showed no treatment effect and no interaction between treatment and OM digested. Therefore, a single slope with a linear relationship was used. In Experiment 3, there was a tendency for a treatment effect (P  >  0.14). Results from Experiment 1 indicate OM digested was 3.58 percentage units (PU) greater than TDN content. In Experiment 2, OM digested was 11.1 PU less than TDN content. In Experiment 3, the OM digested in the corn diet was 3.96 PU greater than TDN. For the tallow diet, OM digested was 0.34 PU less than TDN, while in the corn oil diet, OM digested was 0.37 PU greater than TDN. In the solubles and wet DGS diets OM digested were less than TDN, 5.88 PU and 9.96 PU, respectively. These results suggest OM digested is consistent relative to TDN content of traditional, corn based diets.  In finishing diets containing DGS, the additional DE supplied by DGS is not accounted for when evaluating only OM digested.  This is likely due to the protein and fat content of DGS, which supplies additional energy relative to its OM content.  Reporting both DE and OM content of diets is beneficial as there is not a simple conversion factor between DE and OM digested.

Keywords: Bomb Calorimetry, OM digested,  TDN