Effect of Duration on Feed and Energy Substrate on the Digestive physiology of Finishing Feedlot Cattle

Abstract Text: The objective of this study was to determine the effect of dietary energy substrate and days on feed (DOF) on apparent total tract digestibility, rumen fermentation, short-chain fatty acid (SCFA) absorption and the arterial glucose clearance rate. Eight ruminally cannulated, cross-bred growing heifers were randomly allocated to one of the two dietary treatments. The control (CON) diet consisted of 75.2% barley grain, 9.8% canola meal, 9% mineral -vitamin supplement, and 6% barley silage (DM basis). To evaluate the effect of energy source, a high-lipid byproduct pellet was included to replace 60% of the barley grain and canola meal (HLP). Diets were similar in NEg (5.15 MJ/kg) and CP (13.7% DM). The study consisted of 4-consecutive 40-d periods with collections occurring in the last 6 d of each period. Dry matter intake did not differ among periods but the HLP group tended to eat less (P = 0.09). The ADG was greater for CON (P<0.05) than HLP and ADG decreased with advancing DOF. The ADG of CON was greater than HLP during first and the last periods (Trt×Period; P = 0.024). Heifers fed HLP tended to have greater mean ruminal pH (6.10 vs. 5.96; P = 0.07) than CON, but pH was not affected by DOF. The CON heifers had a greater digestibility for DM, OM, CP and NDF (P < 0.05) and the digestibility for DM, OM, CP, NDF and starch increased with advancing DOF (P < 0.05).  Crude fat digestibility of CON increased with DOF while that of HLP decreased (Trt×Period; P<0.05). Total SCFA concentration in the rumen was greater (P = 0.006) for CON (141.5 vs 128.08 mM/dL) than HLP and it tended to decrease with DOF (P = 0.098). The molar proportion of acetate increased and butyrate decreased with increasing DOF (P< 0.05) but propionate was not affected.  The rate of SCFA absorption was not affected, but the passage rate of chromium over period was decreased (P < 0.026). The arterial clearance rate of glucose was not affected by treatment or DOF. These data suggest that replacing 60% of the barley grain and canola meal with high lipid byproduct pellets negatively affects total tract digestibility and performance. Moreover, regardless of diet, with advancing DOF digestibility increases and SCFA concentration decreases without corresponding changes in SCFA absorption and as such, these changes do not explain the reduction in G:F with advancing DOF.

Keywords: Beef, digestibility, feeding-duration