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
2NDSU
Abstract
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.