Ma, B. L., Biswas, D. K., Zhou, Q. P. and Ren, C. Z. 2012. Comparisons among cultivars of wheat, hulled and hulless oats: Effects of N fertilization on growth and yield. Can. J. Plant Sci. 92: 1213-1222. There is limited comparative research on the responses of crop growth and yield to N fertilization in hulless (Avena nuda) and hulled (Avena sativa) oats. A field study was conducted for 3 yr at Ottawa, ON, to assess the growth and grain yield of current wheat (Triticum aestivum), hulled and hulless oat cultivars with three N fertility levels. Averaged across the 3 yr, both the mid and high (75 and 150 kg N ha^-1) N treatments demonstrated statistically similar, but higher, leaf area index (LAI) and total dry matter (TDM) than the 0 N treatment, regardless of crop types. At final harvest, with increasing N supply, both hulled oat Gosling and wheat cultivar AC Brio had similar and higher TDM than the hulless oat VAO-2 in 2006, whereas both oat cultivars produced much lower (P<0.01) grain yield than wheat in 2008, primarily due to early lodging of the oat cultivars. Overall, the wheat cultivar AC Brio produced the greatest grain yield with the highest harvest index (HI), followed by hulled Gosling; the hulless oat had the lowest grain yield and the lowest HI. Compared with the wheat cultivar, the lower grain yield of both hulled and hulless oats was attributed to a lower (P<0.05) HI and more severe crop lodging (especially when lodging occurred early), coupled with a higher LAI at flowering. Our results suggest that enhancing HI and resistance to crop lodging are the primary targets for both the hulled and hulless oat cultivar improvement in eastern Canada.