Weed Control Considerations for Early Seeded Crops

Many fields in the southwest were too wet to be cultivated or treated with herbicide in 1999 and this allowed annual and perennial weeds to produce large numbers of seeds. While this may have increased soil seed reserves, it is still difficult to predict the distribution and density of weeds in 2000. For example, the lack of snow cover during the winter may have resulted in higher than normal winter kill. Alternatively, fields that were cultivated last fall may contain many buried seeds, which are more likely to survive alternating temperatures and wetting/drying cycles compared with seeds on the soil surface. The most important factor determining weed distribution and density however, will be the environmental conditions this spring. As in recent years, it is likely that weeds favoured by cool and/or moist springs, e.g. wild oats, wild mustard, wild buckwheat and Canada thistle, will be favoured over warm season species such as foxtail and kochia.

Seeding crops early may enable them to get established and so give them the jump on weeds (this may contribute to the increased yield of earlier sown crops according to Manitoba Crop Insurance data). However, the germination and growth of weeds is also regulated by temperature and soil moisture so that weed competition may also be earlier than usual this spring. The yield potential of the crop is determined early in the season and most research indicates that weeds emerging after the 3 leaf stage of a healthy, vigorous crop have little impact on crop yield. Alternatively, the crop should be kept weed free up to the 3 leaf stage in order to maximize yields.