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A comparison was made of the effect of season and maturity on the in situ ruminal breakdown and intestinal protein digestion in dairy cows between intensively N-fertilized grass (whole sward) and moderately N-ferti!ized ryegrass and clover from a mixed sward. From May lo September 1990. eight consecutive cuts were made, representing alternate harvests of late and early swards. Limited effects of fertilizer-N on chemical and ruminal degradation characteristics and intestinal digestion of grasses were observed. Clover had higher ash, crude protein (CP) and lignin and lower hemicellulose. cellulose and sugar contents than grass. Moreover, soluble fractions of organic matter (OM) and CP were higher, and nimen-undegradable OM and CP fractions were lower. Soluble fractions of grasses and clover decreased and undegradable fractions increased during the season. More mature swards showed increased undegradable fractions. Degradation rates of OM and CP in clover were, respectively, higher than or similar to those of grass. Seasonal effects on degradation rates differed between grass and clover. Effectively rumendegradable (g kg"' DM) carbohydrates and CP. escaped protein and intestinal digestion of escaped protein were higher in clover than in grasses. Inclusion of clover in swards may result in higher Correspondence: Dr A. Sieg, Instiltjie for Animal Science and Hcallh (TD-DLO). PO Box 160, 8200 AD. l^ely.^iuid. the Nelhcrlands post-rumen protein supply. Supplementation of clover-based diets is suggested to compensate for an increased loss of N in the rumen.

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