. 2010. Volunteer wheat (Triticum aestivum L.) competition in corn (Zea mays L.). Can. J. Plant Sci. 90: 919Á924. Volunteer winter wheat (Triticum aestivum L.) is a sporadic weed found in corn (Zea mays L.) fields across southern Ontario. Eight trials were conducted over a 2-yr period (2006 and 2007) at four locations to determine the competitiveness of volunteer winter wheat in corn. A soft red winter wheat cultivar (Pioneer 25R47) was seeded at each location at densities of 0 to 30 seeds m (2 late in the fall, prior to corn planting the following spring. Volunteer wheat competition in corn resulted in reduced emergence of corn leaf collars. Competition with volunteer wheat resulted in a 5% decrease in corn leaf collars present at 21 and 70 d after emergence with volunteer wheat densities of 3.0 and 5.2 plants m . D'autre part, comparativement aux re´sultats obtenus sur les parcelles te´moin, la concurrence des repousses spontane´es re´duit la surface foliaire, le poids sec des feuilles, le poids sec des pousses, la hauteur du plant et de l'e´pi, ainsi que le rendement du maı¨s de 5%, a`une densite´de 5,1 a`6,0 plants au m 2 . Le degre´de compe´titivite´de´pend de la densite´des repousses spontane´es.Mots clé s: Densite´, surface foliaire, collet, poids sec des pousses, rendement Corn (Zea mays L.) is widely grown throughout Ontario. In 2009, corn growers seeded 722 000 ha of corn and harvested 6 375 000 tonnes of grain (McGee 2009). Early weed control is critical to successful corn production. Weeds compete with corn resulting in yield losses ranging from 5 to 35%, depending on weed species composition, weed density and the relative time of crop and weed emergence (Knezevic et al. 1994;Bosnic and Swanton 1997).Volunteer crops, similar to plants traditionally classified as weeds, are known to be competitive with crops (De Corby et al. 2007;O'Donovan et al. 2008). They emerge from seed that shattered prior to harvest or was lost during harvest operations the previous year (De Corby et al. 2007;O'Donovan et al. 2008;Bond and Walker 2009). Volunteer crops cause yield losses through resource competition, increase the cost of weed control, interfere with harvest operations and contaminate harvested grain (Deen et al. 2006;Clewis et al. 2008).Previous research on canola (Brassica rapa L. and Brassica napus L.), flax (Linum usitatissimum L.) and barley (Hordeum vulgare L.) has shown that volunteer crops reduce yield from 1 to 78% when present at the time of crop emergence. Volunteer wheat and barley reduced flax yields by 49 to 63% (Friesen et al. 1990). Marshall et al. (1989) reported that volunteer wheat and barley at densities of 30 plants m (2 caused a 17 and 35% yield reduction in rapeseed (Brassica napus L.), respectively. Lutman and Dixon (2008) found that volunteer barley reduced oilseed rape yields from 5 to 78% and overall plant growth from 25 to 91%. Volunteer wheat densities of 1 plant m (2 were reported by O'Donovan et al. (1988) to reduce canola yield by 1%.Abbreviations: DAE, days after emergence; OM, ...