486 Evaluation of beef heifers fed four different diets when selected for divergent digestible fiber intake

Abstract The objective of this study was to evaluate the factors which influence intake and fiber digestion efficiency of beef cattle previously selected for high or low digestible neutral detergent fiber (NDF) intake (DFI; g/kg BW0.75). Angus × Hereford cross heifers [n = 16; initial body weight (BW) = 584 ± 32.2 kg] previously classified as either high or low DFI when fed a high-forage diet (n = 8/treatment) were individually housed and fed one of 4 diets for ad libitum intake using a sequential offering of diets over four consecutive 28-d period. Diets included: 1) 100% grass hay; 2) 100% mature alfalfa hay; 3) 90% dry rolled barley + 10% alfalfa hay; and 4) 90% dry rolled barley + 10% grass hay (DM basis). Individual animal feed intake was recorded daily in addition to weekly BW. Rumen digesta samples were collected prior to feeding and 6 h after for 2 d within each period and analyzed for SCFA and NH3-N concentrations, and total rumen protozoa counts and identification. High DFI heifers had similar (P > 0.11) DMI compared with low DFI heifers in periods 1, 3, and 4, but had a tendency for greater DMI in period 2 and NDF intake during period 2 and 3 (7.84 vs. 7.07 kg, P = 0.07; 4.71 vs. 4.27 kg, P = 0.08; 2.56 vs. 2.28 kg, P = 0.10). ADG was greater for high DFI during the first two periods (0.61 vs. 0.13, P = 0.04; 0.44 vs. -0.09, P = 0.02); moreover, high DFI cattle finished the study with a heavier BW (708 vs. 661 kg, P = 0.04). Total SCFA concentrations before and 6 h after feeding did not differ (P ≥ 0.36) between treatments. NH3-N concentrations were not different before feeding during period 1, 3, and 4, but were greater for high DFI heifers during period 2 (9.85 vs. 8.39, P = 0.04). Additionally, there were no differences in NH3-N concentrations 6 h after feeding during any period. No differences were observed (P > 0.25) between treatment groups for total rumen protozoa counts for periods 1 and 2; however, there was a tendency for a greater proportion of Isotrichia before feeding during period 1 (3.67 vs. 2.41%, P = 0.07). In conclusion, differences in intake and the small changes in ruminal fermentation parameters only partially explain the differences in ADG observed between high and low DFI heifers.