Estimation of utilisable crude protein at the duodenum of dried distillers’ grains with solubles using a modified gas test

Abstract

The objective of this study was to characterise the variation of utilisable crude protein at the duodenum (uCP) of dried distillers’ grains with solubles (DDGS) for ruminants using a modified gas test and to predict the uCP in DDGS based on chemical composition. Thirteen samples originating from wheat, maize, barley or blends of different substrates were studied. The in vitro uCP was estimated using the modified Hohenheim gas test (moHGT). Samples were incubated in rumen fluid for 8 h, 24 h and 48 h followed by ammonia distillation. The obtained values were compared to reference values of uCP (based on the contents of crude protein (CP), in situ undegraded CP and metabolisable energy). The reference and in vitro values of uCP were calculated according to passage rates of 2, 5 and 8%/h (i.e., uCP₂, uCP₅ and uCP₈, respectively). The in vitro uCP₈ ranged from 214 to 320 g/kg DM and reference values from 158 to 302 g/kg DM. The in vitro uCP₂ was on average lower (by 7 g/kg DM) and in vitro uCP₈ was higher (by 56 g/kg DM) than their respective reference values. The in vitro uCP₅ and uCP₈ were correlated with reference values and the correlations were improved with increasing passage rates. When the differences of uCP content between in vitro and reference values were related to CP fractions, they increased with increasing content of CP fraction A and decreasing content of CP fraction B3 for uCP₈. The prediction of uCP values from chemical composition was not reliable. It was concluded that uCP can be predicted on the basis of the moHGT method and CP fractions. The accuracy of prediction improved upon the inclusion of CP fractions and neutral-detergent insoluble nitrogen. The present study revealed a significant variation in the uCP content of DDGS, which should be considered when formulating rations for dairy cows.

Keywords:

• distillers’ grains
• gas production
• in vitro
• microbial proteins
• protein degradation
• ruminants

Acknowledgements

This study was supported by the “Ministerium für Wissenschaft, Forschung und Kunst Baden Württemberg” with a doctoral scholarship for Edwin Westreicher-Kristen, which is gratefully acknowledged. The authors would like to thank Dr Christine Potthast for the recollection of the DDGS samples. Furthermore, the authors would like to thank Prof Dr Andreas Susenbeth from the Christian-Albrechts-Universität Kiel for providing critical comments.