, L. L. 2012. Nitrogen cycling, profit margins and sweet corn yield under fall cover crop systems. Can. J. Soil Sci. 92: 353Á365. In order to improve N best management practices in southwestern Ontario vegetable farming, the effect of cover crops on N dynamics in the fall and spring prior to sweet corn planting and during sweet corn season was assessed. The experiment was a split plot design in a fresh green pea Á cover crop Á sweet corn rotation that took place over 2 site-years at Bothwell and Ridgetown in 2006Á2007 and 2007Á2008, respectively. The main plot factor was fall cover crop type with five treatments including oat (Avena sativa L.), cereal rye (Secale cereale L.), oilseed radish (OSR; Raphanus sativus L. var. oleoferus Metzg Stokes), mixture OSR plus cereal rye (OSR&rye) and a no cover crop control. Compared with no cover crop, sweet corn profit margins were higher by $450 ha (1 for oat at Bothwell and $1300 and $760 ha (1 for OSR and OSR&rye, respectively, at Ridgetown. By comparing plant available N over the cover crop season, the cover crops tested were more effective at preventing N loss at Bothwell than at Ridgetown likely due to higher precipitation and sandier soil at Bothwell. Despite differences in site characteristics, cover crops did not result in increased plant available N compared with no-cover during the sweet corn season at either site, indicating that these cover crops will not provide an N credit to the following crop and growers should not modify N fertilizer applications based on cover crops. n te´moin, soit l'absence de culture-abri. Comparativement au traitement te´moin, la marge de profit du maı¨s sucre´a augmente´de 450 $ par hectare avec l'avoine, a`Bothwell, ainsi que de 1 300 $ et de 760 $ par hectare avec le radis et le me´lange radis/seigle, respectivement, a`Ridgetown. Quand on compare la quantite´de N a`la disposition des plantes durant la croissance de la culture-abri, on constate que les cultures teste´es pre´viennent mieux les pertes de N a`Bothwell qu'aR idgetown, sans doute a`cause de pre´cipitations plus abondantes et d'un sol plus sablonneux. En de´pit des diffe´rences observe´es au niveau des particularite´s du site, la culture-abri n'a accru la quantite´de N disponible pour les plantes a`aucun des deux endroits pendant la saison de croissance du maı¨s sucre´, comparativement au traitement te´moin, signe que les cultures teste´es n'ajoutent pas du N au sol pour la culture subse´quente et que les cultivateurs ne devraient pas modifier leurs applications d'engrais N en fonction de la culture-abri.Mots clé s: É conomique, culture de´robe´e, perte de nitrates, avoine, seigle, radis, bilan azoteẂ ith over 30 000 ha of land in production, sweet corn is the most widespread field-grown vegetable crop in Canada. With approximately 50% of the national acreage, Ontario is the largest sweet corn producing province with a farm value of $25.5 million in 2007 (Mailvaganam 2008). With such a large acreage, it is important that sweet corn production not only optimizes returns b...