Effect of ruminal exposure and subsequent microbial contamination on dry matter and protein degradability of various feedstuffs

Abstract

The object of the experiment was to estimate the magnitude of error and factors affecting degradability estimated in sacco caused by bacterial contamination. Dry matter (DM) and crude protein (CP) degradability of five feeds (alfalfa hay, wheat straw, maize, sunflower meal and fish meal) with different physical characteristics, CP, neutral detergent fibre and crude fibre content was determined in sacco using fistulated dairy cows. Washed feed residues after 2, 4, 8, 16, 24 and 48 h (plus 72 h for forages) incubation were analysed for DM, CP and diaminopimelic acid (DAPA) as a bacterial marker. Bacterial DM as percent of total residue DM and bacterial CP as percentage of total CP, increased linearly within incubation time. Bacterial colonisation took place mainly during the first 8 h. Colonisation of wheat straw was slower, compared to the other feeds and continued up to 72 h. Bacterial DM in percentage of total DM of residues in bags for various time of incubation was, respectively, 1.0–6.2%, 2.8–6.7%, 5.6–13.6%, 3.8–16.7% and 4.3–28.6% for maize, fish meal, alfalfa hay, sunflower meal and wheat straw. The effect of bacterial contamination of feed residues on protein degradability was very high for wheat straw (46–162% units) and alfalfa hay (9–13%), moderate for maize and sunflower meal (3–6% units) and low and not significant for fish meal (2–3% units). Bacterial contamination is an important source of error, especially for low protein feeds, when degradability is being estimated to calculate protein value of feeds according to new protein systems. The results support the need for a microbial correction of in sacco residues.