Technical Abstract: Cover crops can provide many benefits to cropping systems including provision of significant quantities of fixed N (legumes) that is readily mineralized. In the humid, temperate mid-Atlantic area of the U.S.A., winter annual cover crops such as hairy vetch produce abundant biomass and N before summer crops are planted in spring. Although N mineralized from a legume cover crop can contribute to meeting the N requirement of crops such as corn, it also may not be used efficiently by crops and can be as prone to loss from the environment as fertilizer N. This research was conducted to determine whether hairy vetch or a hairy vetch-rye mixture that were allowed to grow high levels of biomass with high N content (200 to 250 kg/ha) could meet the N requirements of sweet corn and to determine their Nitrogen Use Efficiency relative to that of fertilizer N. Marketable yield of sweet corn was increased by cover crops compared to an unfertilized, no-cover crop control but cover crops reduced yield compared to the no-cover crop control with fertilizer N. Reduced plant population that reduced the number of ears per ha accounted for yield reduction by cover crops compared to the no-cover crop plus fertilizer N control. The Nitrogen Use Efficiency of sweet corn was higher for fertilizer N than for hairy vetch N and least for combinations of fertilizer N and cover crop N. Results suggest that growing sweet corn without tillage in high biomass levels of cover crops can interfere with crop establishment, reduce the efficiency of crop production, and allow for potentially high N losses into the environment. Legume cover crops may be used more efficiently in no-tillage cropping systems by killing earlier than crop planting, developing reliable planting equipment for residue management, developing improved guidelines for supplemental applications of fertilizer N, and following harvest with a cover crop adapted for capturing excess soil N.