Weed growth and N assimilation usually increase with N application rate.With the increasing price of N fertilizer, a better understanding Nassimilation by weeds is necessary to maximize economic return. Total plantyield is generally independent of population density, except when plants arevery small or at very low population density. If plant yield is independentof population density, weed N assimilation may also be independent ofpopulation density. However, the effect of weed population density on Nassimilation has not been thoroughly investigated. A 2011controlled-environment study was established in East Lansing, MI, toevaluate the effect of weed population density and N application rate ongrowth and N assimilation by common lambsquarters and redroot pigweed. Studyfactors included four weed densities (1, 2, 4, and 8 plantspot−1), three N application rates (0, 67, and 134 kg N ha−1), and two weed species (redroot pigweed and commonlambsquarters). Weeds were destructively harvested 3 wk after emergence, andshoot height, biomass, total N concentration, N use efficiency, and Nassimilation were measured. Redroot pigweed was taller, had greater shootbiomass, and a greater shoot N assimilation than did common lambsquarters.With similar environmental conditions, redroot pigweed is expected to bemore competitive than common lambsquarters. Shoot N assimilation increasedwith increasing weed population density, indicating that N assimilation wasnot independent of population density 3 wk after emergence because weedswere small or at low population density.