Herbage contributes much dietary protein to ruminant diets. High-producing ruminants need part of the dietary protein to be resistant to ruminal degradation. Estimates of herbage protein degradability have not considered agronomic factors in herbage production. This research examined how herbage species, maturity stage, plant part, and nitrogen (N) fertilization modify the N composition of herbage;Smooth bromegrass (Bromus inermis Leyss.), timothy (Phleum pratense L.), red clover (Trifolium pratense L.), and alfalfa (Medicago sativa L.) were harvested at four, two-week intervals during spring of 1984 and 1985. Total herbage, leaves, and stems were analysed for herbage quality and various N fractions. Less than 13% of the total N in herbage was available fiber N (AFN) (neutral detergent fiber N (NDFN) minus acid detergent fiber N (ADFN)). Available fiber N increased with herbage maturity. Legumes had twice the N in ADF as did grasses. Concentrations of NDFN and ADFN in leaves and stems reflected the different amounts and proportions of tissues in each organ. Stems had more ADFN, which reflects the greater amount of lignified structural tissue. Leaves had more NDFN, which mirrors the greater amount of mesophyll tissue. Immature herbage was most digestible, least in NDF, and highest in N concentration;Smooth bromegrass (SBG) was grown in mixture with alfalfa or fertilized with three levels of N in combination with three levels of nitrapyrin, a nitrification inhibitor, and harvested at four, two-week intervals during spring of 1985 and 1986. Total herbage of SBG was analyzed for various N fractions. Nitrapyrin had no effect on chemical composition of herbage. Nitrogen fertilizer increased both dry matter yield and N concentration in SBG, but did not alter available fiber N or herbage quality;It is concluded that herbage should be managed to optimize the yield of digestible energy to best use the large proportion of degradable N in herbage.