Nitrogen (N) assimilation by weed species may be partially attributed to the physiological differences between C₃ and C₄ plant anatomies.  However as weed population densities increase, competition for resources also increases.  Few studies have investigated the effects of weed densities and nitrogen rates on plant competition.  The objectives of our study were to quantify total nitrogen assimilation of common lambsquarters (Chenopodium album), a C₃ species, and redroot pigweed (Amaranthus retroflexus), a C₄ species, grown under three N supplies (0, 67, and 134 kg N ha^-1) and four plant densities (1, 2, 4, and 8 weeds kg^-1 soil) to examine differences in weed growth, biomass accumulation, and N use efficiency.  A greenhouse study was initiated in February 2011 using a split, split-plot randomized complete block design. Whole-plots consisted of weed species and subplots consisted of harvest date (21 or 35 days after emergence).  Nitrogen rate and plant density were randomized within five replications of treatments and two timings of the experiment.  Data will be analyzed using PROC MIXED in SAS to determine significant differences between factors using Fisher’s Protected LSD at the 95% confidence level.  Shoot and root biomass accumulation was greater for redroot pigweed compared to common lambsquarters.  Shoot and root biomass also increased with increasing plant density.  Nitrogen application rate did not influence shoot biomass 21 days after emergence; however, 35 days after emergence, shoot biomass increased with N application rate.  Root biomass decreased as N application rate increased.  Nitrogen assimilation results will be presented.