Nitrogen (N) is a difficult nutrient to manage in organic farming systems, and yield reductions related to N deficiency have been reported in organic systems. Legume-based cover crops offer opportunities for biologically fixed N; however, improved quantification of N contribution is needed for cost-effective N management. A 2-yr experiment was conducted near Corvallis, OR, USA, in 2007 and 2008 to (1) evaluate biomass production and N accumulation from selected cover crop treatments, (2) compare the effects of fall-planted cover crops on broccoli [(Brassica oleraceae L. (Italica group)] yield, (3) estimate the quantity of feather meal-N replaced by cover crops. Cover crop treatments included common vetch (Vicia sativa L.), phacelia (Phacelia tanacetifolia Benth), oats (Avena sativa L.) and the mixtures phacelia plus vetch, oats plus vetch and a no-cover crop (fallow) treatment as the control. Using feather meal as an N source, four rates of N fertilizer (0, 100, 200 and 300 kg N ha−1) were randomized within each cover crop treatment in a randomized, split-plot design. Cover crop biomass and N accumulation differed between the 2 yr of the study. In 2007, total biomass accumulation ranged from 5000 to 10,000 kg ha−1, whereas in 2008, cover crop accumulation was 1500 to 5000 kg ha−1. Biomass of both phacelia and vetch (in mixtures or as sole crops) was reduced by 80% from 2007 to 2008, whereas oat biomass and weed biomass in the fallow plots was reduced by only 40% between the 2 yr. The accumulation of N was also reduced in 2008, with vetch (either as a sole crop or in mixtures) contributing less than a third of total N produced in 2007. In 2007, vetch and vetch-based cover crop mixtures increased broccoli yield compared with the fallow, providing 100–135 kg fertilizer equivalent N ha−1. But due to decreased cover crop biomass and N accumulation in 2008, vetch and vetch-based mixtures failed to increase broccoli yield, providing <20 kg N ha−1 fertilizer equivalence. In 2007, oats grown as a sole cover crop reduced broccoli yield when no supplemental N was applied. In 2008, both phacelia and oats reduced broccoli yield at all N levels, with estimated N fertilizer equivalence values of −80 to −95 kg N ha−1. Although legume and legume mixtures increased broccoli yield in only 1 yr of the experiment, addition of vetch to the mixtures reduced yield loss in both years compared with oats and phacelia grown as sole crops.