Effects of Roughage Removal, Rumen Modifiers Inclusion and Post Ruminal Amino Acid Supply on Growth Performance in Beef Steers.

The objective of this study was to determine the effects of roughage removal, rumen modifiers and diets balanced to meet predicted AA requirement on beef steer growth performance. We hypothesized that growth performance of steers fed different rumen modifiers (monensin, Rumensin 90, Elanco Animal Health vs. essential oils [EO], Xtract X60-7065, Pancosma) would not differ, and that diets balanced to meet AA requirements as well as roughage removal would improve G:F and ADG. Crossbred steers (n=150; 284 ± 23kg BW) were randomly assigned to one of five diets. Four diets were balanced to meet AA requirements with a common feedlot diet used as control (CON; DM basis; 17% CP, 22% NDF, 12% fescue hay, 58% corn, 27% wet distillers grains with solubles [WDGS]; 300mg·steer^-1·d^-1 monensin). Two diets contained roughage (R), with monensin (RM) or EO (RE) (DM basis; 17% CP, 17% NDF, 9.4% fescue hay, 70% corn, 9.4% AminoPlus, 9% WDGS, 0.05% AjiPro [Ajinomoto]; and 300mg·steer^-1·d^-1 monensin or 800mg·steer^-1·d^-1 Xtract). Two diets replaced roughage with corn; non-roughage (NR) with monensin (NRM) or EO (NRE). Data were analyzed as a two-factor factorial arrangement plus control, with roughage and rumen modifiers as main effects and steer as experimental unit. Main effects were only considered when interaction was not significant (P>0.05) and data reported as least square means ± SEM. Control was compared only to RM. During the first 21d, steers consuming R had greater (P<0.01) DMI (8.54 vs. 6.78 ± 0.22kg·d^-1), ADG (2.81 vs. 2.01 ± 0.10kg) and G:F (0.34 vs. 0.30 ± 0.01) than NR. Steers consuming RM did not differ on DMI (P=0.08) and had greater ADG than CON (2.89 vs. 2.55 ± 0.09kg; P<0.01). At 42d, steers consuming R had greater (P<0.01) DMI (9.51 vs. 6.98 ± 0.22kg·d^-1) and ADG (2.39 vs. 1.90 ± 0.07kg) than NR. When monensin was fed, G:F did not differ regardless of roughage inclusion, however, when EO were added, calves consuming NR were more efficient than R (NRM=0.261, NRE=0.282, RM=0.262, RE=0.245 ± 0.008; P=0.02). Steers consuming RM had greater (P<0.01) DMI (9.52 vs. 8.15 ± 0.22kg·d^-1) and ADG (2.47 vs. 2.13 ± 0.06kg) than CON. In conclusion, balancing diets to meet AA requirements improved ADG during early growth. Calves consuming R were more efficient during the first 21d independent of rumen modifier. However, at d42 calves consuming NRE were more efficient than RE, and calves consuming NRM and RM did not differ.