Effect of heat processing of protein sources on the disappearance of their selenium from mobile bags in the digestive tract of dairy cows

Abstract

It was proposed that the organic selenium (Se) in ruminant feedstuffs, a large proportion of which is present in the protein molecule, should become available for digestion to the same extent as the protein in a feedstuff when passing through the digestive tract of the ruminant. Factors such as heat processing of feedstuffs that decrease protein degradability in the rumen should also reduce the proportion of its Se metabolised during microbial fermentation in the rumen. This should increase the proportion of unchanged dietary Se reaching the lower digestive tract. Six plant protein sources were roasted in a laboratory scale electric drum roaster at different temperatures and time intervals, and four fish meal and four blood meal samples were obtained from different processing plants. The mobile bag technique was used to measure the disappearance of crude protein (CP) and Se from the feedstuffs when incubated in the rumen and passing through the lower digestive tract of dairy cows. For the plant protein sources, the disappearances of Se and CP in the rumen and lower digestive tract were correlated, with coefficients of determination ( r 2) of 0.718 and 0.717, respectively. However, for the animal protein sources these correlations were low. The ruminal disappearance of Se from all unprocessed plant proteins was higher ( P<0.05) than those from their corresponding heat-processed samples. Ruminal disappearances of CP in the same samples showed the same trend as Se, viz. that heat processing reduced ( P<0.05) the ruminal disappearance of CP from these feedstuffs. Only small proportions of Se in the unprocessed plant proteins disappeared from the bags in the lower digestive tract, but after heat treatment these proportions were significantly higher. Protein disappearances followed a similar pattern. It is evident from this study that the fate of Se in the organic form in feeds is affected by the extent of exposure of the feed to ruminal fermentation. Factors affecting ruminal protein degradation such as heat processing will determine also whether Se will be subjected to microbial reduction or incorporation into microbial proteins, or would escape rumen microbial fermentation. Evidence suggested that the overprocessing of some of the test feeds reduced the availability of their Se in the digestive tract. It is concluded that heat processing of feedstuffs can contribute to the factors affecting the bio-availability of Se in feeds.