Variation in spring emergence periodicity (both before and after crop seeding) of summer annual weeds is a potentially exploitable attribute that may be applied to weed management in canola. Tillage intensity, which is decreasing in the Great Plains of North America, may influence emergence periodicity of summer annual weeds. Emergence periodicity of common lambsquarters, field pennycress, green foxtail, redroot pigweed, wild buckwheat, wild mustard, and wild oat were monitored during the spring of 2000 in 17 producers' canola fields across southern Manitoba, Canada. The fields represented a region of approximately 2 million ha and included a broad range of soil types, agronomic practices, environmental conditions, and seedbank distributions. Fields were grouped into one of two broad tillage classifications (conventional or conservation). For most species, except redroot pigweed and wild mustard, conservation tillage promoted earlier emergence than conventional tillage in terms of both thermal and chronological time. The differences were significant even though there was only a limited range of tillage intensity for the two tillage classes within this region. Onset of canola crop emergence preceded that of all but one weed species in the conservation-tillage fields and five weed species in the conventional-tillage fields. This suggests that canola seeded in conservation- vs. conventional-tillage systems may have a competitive advantage by way of an earlier relative time of crop emergence. The influence of tillage system on weed emergence periodicity is likely due to the influence of tillage on the vertical origin of weed seedling recruitment because measurements of soil temperature and soil moisture did not help to fully explain the differences in emergence periodicity between tillage systems. The results from this study will facilitate weed control timing decisions in canola and provide validation data for weed emergence models.

Bullied, W. J., Van Acker, R. C., Marginet, A. M. and Kenkel, N. C. 2006. Agronomic and environmental factors influence weed composition and canola competitiveness in southern Manitoba. Can. J. Plant Sci. 86: 591-599. Canola yield in Manitoba has reached a plateau in recent years. The causes for this, as related to agronomic, environmental, weed interference and canola competitiveness factors, were identified using observational data from 31 canola fields in southern Manitoba in a 2-yr on-farm research study. Agronomic and environmental factors contributing to weed density and composition were determined with multivariate canonical correspondence analysis. Agronomic and environmental factors most influential on absolute and relative canola biomass were determined with multiple regression analysis. Most weeds were adaptable across a broad range of crop environments; however, some functional groups of weeds were either positively or negatively favored by specific environmental or agronomic conditions. Absolute canola biomass prior to bolting was greater as soil growing degree days (GDD) increased and canola was dense and seeded early. Lower weed density, increased soil GDD, and reduced surface soil moisture were significant factors contributing to higher relative canola biomass. Results from this study indicate that seeding canola early and at a rate sufficient to achieve a dense crop stand can increase canola competitiveness and reduce weed interference. 591-599. Depuis quelques années, le rendement du canola a atteint un plateau au Manitoba. Les auteurs ont cherché les raisons de ce problème au niveau de l'agronomie, de l'environnement, de la concurrence des adventices et de la compétitivité du canola en analysant les observations effectuées dans 31 champs de canola du sud du Manitoba lors d'un projet de deux ans poursuivi sur le terrain. Ils ont ainsi déterminé les facteurs agronomiques et environnementaux qui affectent la densité et la composition des peuplements d'adventices par une analyse factorielle de correspondance canonique à variables multiples. Les paramètres agronomiques et environnementaux qui exercent la plus grande influence sur la biomasse absolue et relative du canola ont été établis par analyse de régres-sion multiple. La plupart des adventices s'adaptent à une vaste gamme de milieux, dans les cultures, mais certaines conditions agricoles ou environnementales ont une incidence positive ou négative sur quelques groupes fonctionnels de mauvaises herbes. La biomasse absolue du canola avant la montaison augmente avec la hausse du nombre de degrés-jours de croissance et lorsqu'on sème la culture hâtivement à une grande densité. Une faible densité de peuplement des adventice, un nombre accru de degrés-jours de croissance et une teneur en eau plus faible dans le sol de surface sont des facteurs qui favorisent sensiblement une biomasse relative plus élevée du canola. Les résultats de l'étude laissent croire qu'on peut atténuer la concurrence des adventices et accroître la compétitiv-ité du canola en le...