In situ degradation of perennial rye grass from grazed pastures during theseason at two levels of nitrogen fertilization

In situ incubations were carried out to investigate the effect of rate of n itrogen fertilization and season on rumen degradation of ryegrass. Samples of Lolium perenne of grazed herbage, fertilized at two levels of nitrogen ( 250 and 550 kg N/ha/year), were collected in May, July-August, September an d October, and incubated in nylon bags in the rumen of three grazing dairy cows with a rumen cannula. Theoretical degradability in the rumen varied be tween 56 acid 64% for organic matter, between 60 and 71% for crude protein and between 59 and 74% for neutral-detergent fibre. Theoretical degradabili ty was lower in summer than in spring and especially in autumn (P < 0.001). Reducing N fertilizer application decreased the theoretical degradability in the rumen (P < 0.001). Differences between rates of N application were l ess pronounced in autumn. (C) Elsevier/Inra.