Effect of botanical characteristics, growth stage and method of conservation on factors related to the physical structure of forage – An attempt towards a better understanding of the effectiveness of fibre in ruminants

Forages differing in botanical characteristics, growth stage at harvest and conservation type were evaluated for their effects on variables assumed to be related to the physical characteristics of feed. Mixed swards harvested at young and mature growth stage were conserved either as silage, haylage or hay. Additionally, first and second cuts of pure swards of English ryegrass, Italian ryegrass, orchard grass, red clover and alfalfa were ensiled. Apart from chemical fibre analysis, particles size distribution was determined with a particle separator after drying. The energy spent for grinding through a 5-mm sieve was determined. Samples were incubated in sacco, measuring dry matter and fibre degradation kinetics, and in vitro, determining volatile fatty acids (VFA) and gas production. Although the proportion of particles >19 mm was larger in young mixed swards, the potentially degradable fractions and the intensity of in vitro fermentation were higher than in mature mixed swards. Silage making, compared to the preparation of haylage and hay, resulted in the highest proportion of particles >19 mm in length, but this did not adversely affect the extent of in vitro gas production which was lowest with hay. Orchard grass, with the highest fibre content, was less intensively fermented than the other grass and legume species. For these species, the potentially degradable fractions and the intensity of fermentation were higher in the first than in the second cut, whereas the differences between cuts remained small in orchard grass. Responses in grinding resistance were either lacking (mixed swards) or weak (individual plant species). Structure value and physically effective neutral detergent fibre described the physical structure of the forage similarly. Both concepts were suitable to determine differences in sward maturity, but not between conservation methods, and were closely correlated with dry matter and fibre degradation and the intensity of fermentation. Furthermore, the correlations showed that the contents of neutral detergent fibre are just as well suitable for estimating structure-related properties. However, the contents of acid detergent fibre and lignin gave more information about the expected profile of individual VFA.