Investigations on the effect of forage source, grinding, and urea supplementation on ruminal fermentation and microbial protein flow in a semi-continuous rumen simulation system

Abstract

The objective of the present study was to compare the effect of maize silage and grass silage on microbial fermentation and protein flow in a semi-continuous rumen simulation system (Rusitec) when milling screen size (MSS) during grinding was varied. Oven-dried silages were milled through screens of 1, 4 or 9 mm pore size and incubated for 48 h in a Rusitec system. Furthermore, the effect of N supplementation to maize silage (MSS: 4 mm) was investigated and single dose vs. continuous infusion of urea-N were compared. Degradation of organic matter (OM), crude protein (CP), fibre fractions and non-structural carbohydrates (NSC) as well as short-chain fatty acid production differed significantly between forage sources. Urea-N supplementation improved the degradation of NSC, but not that of fibre fractions in maize silage. The way of urea supply had only marginal effects on fermentation characteristics. An increase in MSS, and consequently in mean feed particle size, led to an improvement in the degradation of OM, CP and NSC, but efficiency of microbial net protein synthesis (EMPS; mg microbial N flow/g degraded OM) and the microbial amino acid profile were less affected. EMPS was higher in grass silage than in maize silage and was improved by urea-N supplementation in maize silage. This study indicates that fermentation of NSC as well as EMPS during incubation of maize silage was limited by availability of NH₃-N. Furthermore, an increase in MSS above 1 mm seems to improve fermentation of silages in the Rusitec system.

Keywords:

• artificial rumen
• fermentation
• microbial protein
• particle size
• silage
• urea

Acknowledgements

This study was funded by Deutsche Forschungsgemeinschaft (DFG, Grant no. BO 2960/1-1). The authors would thank their co-workers at the Animal Nutrition Group of the Martin-Luther-Universität Halle-Wittenberg for their support in the conduction and chemical analyses of Experiment 1. Furthermore, the analyses of fresh forage characteristics by the ‘Landesanstalt für Landwirtschaftliche Chemie’ in Stuttgart-Hohenheim is gratefully acknowledged.