Ruminal dry matter degradation of fresh grass in situ and cell wall content during vegetation | Ruminaler Abbau der Trockenmasse in situ und Gehakt Gerüstsubstanzen von Wiesenfutter waehrend der Vegetation

During vegetation radical morphological and chemical changes in fresh grass occur. Therefore, vegetative stage is the crucial influencing factor on the feeding value of grass forage. It determines the proportion and composition of the cell wall components and the nature of the cell content. The stem proportion increases at the expense of the leaf fraction accompanied by a considerable decrease in digestibility due to continuous formation of cell wall constituents and their lignification. In a comprehensive research project the influence of the vegetative stage of fresh grass on digestibility, feed intake and milk production was investigated in the course of three complete growing seasons. In this paper the dry matter degradation in the rumen - determined with the nylon bag-technique in situ - is presented. A meadow with a homogeneous botanical composition was divided into three sections to examine the vegetative process in the three growths. Every growth was harvested over a period of 7 weeks from grazing stage up to overripe grass using the grass of different maturities for the feeding trials. The in situ investigations were based on the specifications of Orskov et al. (1980), Michalet-Doreau et al. (1987), Madsen & Hvelplund (1994) and Huntington & Givens (1995). The incubation data were analysed according to the model of Orskov & McDonald (1979) and McDonald (1981). The proportion of crude protein decreased considerably faster during vegetation of the 1st growth than in the 2nd, and in the 2nd faster than during the 3rd growth. There were also significant interactions between growth number and vegetative stage regarding the fractions of crude fibre and cell wall components. Their proportions increased strongly during vegetation of the 1st growth while they remained almost unaffected in the 3rd growth. The 2nd growth lay in between. The vegetative stage influenced the degradability of dry matter of the fresh grass significantly. However, growth number also had an impact on degradability. There were significant interactions between growth number and vegetative stage regarding many degradation parameters, i.e. influence of vegetative stage was different in the three growths. Accounting for all vegetative stages, potential degradability was higher for the 1st growth than for growths 2 and 3 (78.3, 75.3 and 76.4 %, respectively). Whereas e.g. potential degradability of vegetative stages 7 differed marginally (70.4, 69.2 and 71.7 %), degradability of vegetative stage 1 from the 1st growth was higher than of 2nd and 3rd growth (84.7, 82.2 and 75.9 %). On average of all growths, the potential degradability decreased from 79.9 to 70.5 % during vegetation. On average fraction a decreased during the 7 weeks of vegetation from 21.0 to 17.2 % and fraction b from 58.9 to 53.2 %. The most obvious influence of vegetative stage became evident in parameter c (degradation rate). It decreased from 7.9 % per hour in vegetative stage 1 to 5.4 % at vegetative stage 7. In growth numbers 1 and 2 this decline was comparatively linear. In the 3rd growth the degradation rate decreased only at the beginning of vegetation and subsequently remained almost constant at 5.0 %. The effective degradability ED is a function of parameters a, b and c and accounts for passage rate of feed in the rumen, which competes with the fermentation of forage by the microbes, i.e. being lower than the potential degradability. E.g. ED5 (medium feeding level/outflow rate) decreased from 57.0 to 43.9 % during vegetation, this decline being more pronounced in the 2nd and 3rd growth. Averaging all vegetative stages ED5 for the three growths was 51.1, 49.7 and 49.5 %, respectively. Based on the results of this study it can be concluded that during vegetation especially for the 1st growth there is a strong decrease in degradability of DM and consequently a significant interaction between growth number and vegetative stage. Degradability of the later growths is lower.