Publication Date

1989

Description

The nylon bag technique has been used for many years to explain variations in intake and/or digestibility between forages (Van Keuren and Heinemann, 1962; Demarquilly and Chenost, 1969). More recently, this technique has appeared a relatively simple method to estimate the extent and rate of feeds (particularly protein) degradation in the rumen (Mehrez and Orskov, 1977; Madsen and Hvelplund, 1985). In all these studies, it is implicitely accepted that the nitrogen remaining in bags after a stay of variable duration in the rumen corresponds to non degraded feed nitrogen. However, in recent years, sev­eral authors (Mathers and Aitchison, 1981 with DAPA and Varvikko and Lindberg, 1985 with 15N) have shown that a vari­able fraction of residual nitrogen is, in fact, bacterial protein. This bacterial contamination leads to an underestimation of feed protein degradability in the rumen, the extent of which varies according to the type of feed (Varvikko and Lindberg, 1985). The variations in percentage bacterial protein in bag residues and, more recently, the correction of forage protein degradability estimates which are necessary, have not yet been well defined due to the small number of feeds studies. This paper presents measurements of the impact of microbial contamination on forage protein degradability estimated by the in sacco method. They make it possible to calculate from simple chemical criteria the underestimation of forage protein degra­dability which would occur if the contamination is not taken into consideration. To obtain a number of results sufficient to provide this estimation, it was necessary to determine the bac­terial contamination of bag residues for a wide range of forages. The technique of decontamination of bag contents was prefered to the longer and more costly direct measurement of bacterial protein with markers (DAP, 15N and s35).

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Estimation of the Extent of Bacterial Contamination in Bag Residues and its Influence on In Sacco Measurements of Forage Nitrogen Degradation in Rumen

The nylon bag technique has been used for many years to explain variations in intake and/or digestibility between forages (Van Keuren and Heinemann, 1962; Demarquilly and Chenost, 1969). More recently, this technique has appeared a relatively simple method to estimate the extent and rate of feeds (particularly protein) degradation in the rumen (Mehrez and Orskov, 1977; Madsen and Hvelplund, 1985). In all these studies, it is implicitely accepted that the nitrogen remaining in bags after a stay of variable duration in the rumen corresponds to non degraded feed nitrogen. However, in recent years, sev­eral authors (Mathers and Aitchison, 1981 with DAPA and Varvikko and Lindberg, 1985 with 15N) have shown that a vari­able fraction of residual nitrogen is, in fact, bacterial protein. This bacterial contamination leads to an underestimation of feed protein degradability in the rumen, the extent of which varies according to the type of feed (Varvikko and Lindberg, 1985). The variations in percentage bacterial protein in bag residues and, more recently, the correction of forage protein degradability estimates which are necessary, have not yet been well defined due to the small number of feeds studies. This paper presents measurements of the impact of microbial contamination on forage protein degradability estimated by the in sacco method. They make it possible to calculate from simple chemical criteria the underestimation of forage protein degra­dability which would occur if the contamination is not taken into consideration. To obtain a number of results sufficient to provide this estimation, it was necessary to determine the bac­terial contamination of bag residues for a wide range of forages. The technique of decontamination of bag contents was prefered to the longer and more costly direct measurement of bacterial protein with markers (DAP, 15N and s35).