A field experiment was conducted to determine the effect of nitrogen (N) supply on competition between canola cultivars and their natural weed populations. The experiment was carried out as a randomized complete block design with a factorial treatment arrangement and three replications. Four canola cultivars (Hyola420, Hayola308, RGS-003, and PF), three nitrogen rates (0, 100, 200 kg ha-1), and two weed management regimes (weed-free condition, weeded weekly and weedy condition, not weeded throughout the growing season) were considered for the experiment. Results showed that canola grain yield significantly increased when N rate increased from 0 to 200 kg N ha-1. The highest grain yields were recorded for Hayola 420 at 0 and 100 kg N ha-1 and for RGS003 at 200 kg N ha-1. Averaged over N rates and cultivars, weed interference significantly reduced the grain yield. Canola grain yield was positively correlated with canola biomass, height, plant N concentration, and plant N uptake, but negatively correlated with weed biomass, and not correlated with HI (p<0.01). Regardless of canola cultivar, weed biomass significantly reduced with increasing the N rate (p<0.01), suggesting that high levels of N favor the crop over the weeds. Both at 0 and 100 kg N ha-1 (averaged across weed management regimes), RGS003 and Hayola 308 had the highest and lowest nitrogen uptakes, respectively. On the other hand, at 200 kg N ha-1 and regardless of weed management regime, Hayola 308 and Hayola 420 had similar plant N uptake, which was significantly lower than that in RGS003. Plant nitrogen uptake was significantly reduced by weed competition (23%, 23%, and 42% at 200, 100, and 0 kg N ha-1, respectively). This experiment suggested that canola grain yield was affected more by weed interference at low N rate.

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