Influence of Energy Source and RDP on Intake and Digestion in Beef Steers Fed Grass Hay-Based Diets
T. A. Baumann1, G. P. Lardy1, J. S. Caton1, W. W. Dvorak1, and V. L. Anderson2
1NDSU Department of Animal and Range Science
A 5 x 5 Latin square was used to determine effects of supplemental energy source (ENG; corn vs. soyhulls) and rumen degradable protein (RDP) on intake and digestion in steers fed grass hay. Steers (686 ± 51 kg BW) were housed in individual pens during each 14-d adaptation period and individual stalls during each 7-d collection period. Treatments were arranged as a 2 x 2 factorial plus one and consisted of 1) control (CON; grass hay, 7% CP); 2) grass hay plus 0.4 percent BW soyhulls (SH; 13.5% CP); 3) grass hay plus 0.4 percent BW SH and 0.15 percent BW sunflower meal (35% CP); 4) grass hay plus 0.4 percent BW corn (9.5% CP); and 5) grass hay plus 0.4 percent BW corn and 0.2 percent BW sunflower meal. Diets supplemented with RDP were formulated to have a 0 RDP balance with the 1996 NRC model. Preplanned contrasts included main effects of ENG and RDP, ENG x RDP interaction, and CON vs. supplemented (SUP) treatments. Supplementation increased total DMI compared to CON (1.67 vs. 1.45% BW; P < 0.001), but forage DMI was greater (P < 0.001) for CON compared to SUP (1.45 vs. 1.25% BW). Addition of RDP to SH had no effect (P = 0.31) on forage DMI, while addition to corn decreased (P = 0.01) forage DMI. No time x treatment interaction was present for ruminal pH (P = 0.79). Ruminal pH was higher (P < 0.001) for CON vs. SUP (6.69 vs. 6.56). There was an ENG x RDP interaction (P < 0.001) for ruminal pH; pH tended to increase with RDP addition to SH (6.58 vs. 6.63), but decreased with RDP addition to corn (6.60 vs. 6.46). Supplementation increased ruminal ammonia compared with CON (P < 0.001). Likewise, addition of RDP increased ruminal ammonia (P < 0.001). Total tract DM digestibility (DMD) was higher (P = 0.01) for SUP compared to CON (55.4 vs. 50.7%). Addition of RDP to SH decreased DMD (P = 0.04; 57.9 vs. 55.1%), while RDP addition to corn increased DMD (P = 0.04; 52.6 vs. 56.1%). Rates of in situ DM disappearance were not different (P = 0.34). For moderate quality forages, intake and digestion appear to respond differently to RDP addition depending on supplemental energy source. Additional research is needed to determine optimal RDP level and responses in diets using SH as the supplemental energy source.