Fermentation patterns and nutrient contents of forb-containing silages and their effects on microbial fermentation in the artificial rumen system RUSITEC

Abstract

SUMMARYGrassland forbs are important constituents of swards under low-input management. The hypothesis tested was that herbage from unfertilized swards consisting of a mixture of perennial ryegrass (Lolium perenne), white clover (Trifolium repens), dandelion (Taraxacum officinale) and ribwort (Plantago lanceolata) (treatment GCF) would exert a significant effect on the composition and quality of silage for rumen microbes compared with herbage from nitrogen-fertilized (50 kg N/ha/cut) pure ryegrass swards (treatment GN) or unfertilized ryegrass white clover swards (treatment GC). GCF had average proportions of clover and forb of 0·21 and 0·58, respectively. The crude protein and fibre fractions declined and non-fibrous carbohydrates (NFC) increased from GN over GC to GCF. Irrespective of cutting date (spring and summer), GN showed extremely high ammonia (NH3) concentrations, whereas GCF resulted in the highest concentrations of lactic acid coinciding with the lowest contents of butyric acid and ethanol. The ratios between the soluble and less soluble N fractions decreased in the order GN>GC>GCF. A similar sequence was observed in RUSITEC for the apparent disappearance rates of crude protein and fibre fractions. The concentration of NH3-N in the fermenter fluid declined in the order GN>GC>GCF. GCF produced less short-chain fatty acids (SCFA) than GN and GC. Microbial nitrogen (MN) flow was not significantly different between treatments but tended to decline in the order GN>GC>GCF, whereas the proportion of MN originating from NH3-N increased inversely. The efficiency of microbial protein synthesis was unaffected by the different silages. It is concluded that forb-rich swards are suitable for the preparation of high quality silages. There is a need for further investigation into postruminal availability and suitable supplementation for such silages in order to fully exploit their potential for improving the efficiency of nitrogen utilization in ruminant nutrition.