Applying nitrogen (N) to increase alfalfa (Medicago sativa L.) yield is a general practice in China that has caused serious weed infestation problems. Our objective was to determine the effect of N fertilizer and seeding rate on yields of alfalfa and weeds. The effects of four N fertilizer gradients (0, 50, 100, and 150 kg N hm^-2) and five alfalfa seeding rates (0, 5, 7.5, 10, and 15 kg hm^-2) were examined on dry matter of alfalfa and weeds using a three-year in situ controlled trial. The results showed:
• There were no significant differences among N fertilizer (N) for dry matter of alfalfa (DMA) during three years. Dry matter of grassy weeds (DMGW) and broadleaf weeds (DMBW) were significantly increased with the N fertilizer gradient in the seeding year.
• DMA was significantly increased with the seeding rates gradient in both 2012 and 2013. DMGW and DMBW were significantly decreased with the seeding rates gradient in all three years.
• Interactions between N fertilizer and seeding rates on DMA and DMBW in 2012 and DMGW and DMT in 2013 are significant (p<0.05). Although the study demonstrated that increasing seeding rate can suppress weed invasions, N fertilizer facilitated them.