Studies on the in situ degradability corrected for bacterial contamination of concentrate feeds in steers

Stable 15N was used to evaluate the influence of bacterial contamination on in situ DM and N degradabilities (Dg) of meat and bone meal (MBM), soybean meal (SBM), and wheat bran (WB) in two steers. Bacterial DM and N contamination ranged from 2.4 to 28.6% and 2.1 to 56.8% of residual DM and N, respectively. Effective N degradability increased when bacterial contamination was taken into account (P < .05). The difference was low for MBM (2.4%) and for SBM (3.4%) but high for WB (12.2%). Theoretically, using solid-associated bacteria should give the most accurate correction for bacterial contamination; however, results showed that Dg of N based on liquid-associated bacteria were identical for MBM and SBM (P > .05) and different for WB (P < .05). In a second experiment, five treatments were applied to incubated feeds to remove bacteria fixed to the residues and consequently to determine directly the Dg of DM and N corrected for the bacterial contamination without the need for a marker. These treatments involved chilling for 6 h at 4 degrees C in saline solution alone (T1) or with a commercial detergent (T2), or with sodium dodecyl sulfate (T3) or with methylcellulose (T4), followed by pummeling in a stomacher for 5 min. The last treatment was only machine washing twice (T5). The Dg of DM can be directly determined following the first four treatments, nevertheless their application to MBM and SBM led to higher Dg of N than that corrected for the bacterial contamination determined in the first experiment (P < .05). This overestimation was higher than the observed underestimation when the correction for bacterial contamination was omitted. However, two treatments (T1 and T4) were able to dislodge bacterial N fixed to incubated WB and so the Dg of N could be measured directly. In conclusion, bacterial contamination of concentrate feeds incubated in the rumen may be extensive. In this case Dg must be corrected for this contamination or treatments for decontamination can be applied to residues before determining the Dg of N.