Influence of nitrogen rates on the growth and grain yield of two dry bean genotypes under redroot pigweed (Amaranthus retroflexus) competition

Nitrogen (N) fertilizer that used to raise seed yield can affect weed competition. In order to study the effect of N arte and growth habits on dry bean competitiveness, a two-year field experiment as complete block design were conducted with factorial arrangement and three replications at a field experiment of Tarbiat Modares University in 2009 and 2010. First factor included determinate erect (cv. D81083) and indeterminate semi-erect (cv. Gholi) dry bean growth habits. Second factor was N application rates at 0, 25, 50 and 100% of recommended N rate on the basis of the grain yield goal for each genotype. Third factor was redroot pigweed densities of 0, 2 and 14 plants m-1. N application increased leaf chlorophyll and leaf area index for both bean genotypes when they grown under no and low weed density. Under high weed density, the maximum N application compared with no N application increased leaf chlorophyll for Gholi cultivar; however leaf chlorophyll for D81083 line increased at 50 and 100 % of maximum N requirement. Under high weed pressure, N application had no significant effect on leaf area index for D81083 line and decreased the leaf area index for the Gholi cultivar. The increase of weed density decreased leaf area index in both bean genotypes. An increase in N fertilizer increased seed number and see yield for both bean genotypes when they grown under no and low weed density. The maximum seed yield loss associate with redroot competition obtained at high N rate and high weed pressure.