Rumen degradability and microbial contamination of fish meal and meat meal measured by the in situ technique

Rumen degradation characteristics and effective degradability of six fish meals (FM) and five meat meals (MM) were measured in two trials by using nylon bag and rumen outflow rate techniques in rumen cannulated sheep. In trial 1, apparent degradation of crude protein (CP) of five FM and four MM samples from different processing plants were studied with three animals. In trial 2, microbial contamination in the incubated residues of one additional FM and MM sample was determined by continuous 15N intraruminal infusion and using isolated solid associated bacteria as reference sample, with four animals. These values were used to correct the estimates of apparent degradation of dry matter (DM) and CP. During both trials, animals were fed at the same intake level (40 g DM Kg−1 LW−0.75) with 2:1 hay to concentrate diets. Crude protein content (on DM) and buffer CP solubility of the samples ranged from 61.5% to 68.2% and 5.4% to 14.3% for FM and from 52.3% to 66.7% and 9.9% to 22.7% for MM, respectively. Apparent CP degradability ranged from 18.2% to 34.9% and 41.1% to 60.4% in FM and MM, respectively. Degradability values of CP in FM were conditioned by a lag phase (range 3.5–20.7 h) which was inversely correlated with the in situ (r=−0.76) and buffer (r=−0.52) CP solubilities. Buffer CP solubility explained 81% of the CP degradability variation and can be used as a predictor of it. Changes of microbial contamination with incubation time of residues, expressed as percentages of DM and N, fitted well to a simple mono-exponential model. Contamination levels were always very low, with higher values for MM than for FM. The fractional rate of microbial accumulation was markedly higher in the MM than in the FM sample. No correction for microbial contamination decreases CP degradability estimates (P<0.001), but this error (0.82% and 1.11% for FM and MM) can be considered of low nutritional significance in both meals.