Effect of nitrogen fertilizer and seeding rate on yield of alfalfa and weeds

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.