Canola growers with excessive moisture in 2010 face many unknowns and issues for 2011. The following tips will help growers prepare these fields for 2011:
Get a soil analysis. Nutrient reserves will be difficult to predict, especially in low areas flooded for long periods. Lack of oxygen limits microbial breakdown of organic matter. Therefore nutrients made available through mineralization will be lower than expected in these low-lying areas. Leaching of nitrogen and sulphur and denitrification of nitrogen will be higher in those areas as well. Test saturated areas separately. If results are different from the rest of the field, adjust fertilizer rates for these areas to improve your dollar return and fertilizer efficiency.
Tillage can have unwanted side effects. Tilling to dry out wet soil can increase compaction of the topsoil and subsoil layers. Compaction reduces root growth, uptake of nutrients, and future tolerance of both flooded and dry conditions. Tillage in the spring can also create a rough seedbed, making consistent seed depth and good seed-to-soil contact hard to achieve.
Tillage can bring long-buried weed seeds to the surface. These seeds may germinate in large numbers when conditions are favour-able. In addition, tillage will have redistributed seeds that were on the surface throughout the tillage layer. This could extend the emergence window, making herbicide staging more difficult. Finally, tillage may stimulate the emergence of weeds that had declined in number under direct seeding management systems.
Unseeded fields can be considered a “break” from canola – as long as canola volunteers were controlled. Volunteer canola contributes to disease inoculum build up in the soil.
Residual herbicides may still be active. Oxygen is required for microbial breakdown. Saturated soil lacks oxygen. So, ironically, residual herbicide breakdown in wet soils can be as slow as breakdown in a drought year. Check herbicide records before seeding canola.
Weed pressure will depend on 2011 conditions, not 2010 conditions. Wet conditions in 2010 will not result in a significant shift in weed populations toward water-loving species. Unless there has been a recent introduction of a new weed from outside the field, historical populations should be a good guide to what species of weeds will emerge in 2011.
As for the weed seedbank, wet conditions may have caused many dormant seeds to germinate and may have resulted in heavier seed mortality from decay organisms. If weeds that emerged were effectively controlled, weed densities in 2011 may decline. But if weeds that emerged were not controlled and produced seed, the soil seed bank could have been replenished or even increased slightly for 2011.
Higher weed densities will require better herbicide spray coverage to maintain effective weed control, especially those with contact activity. Keep water-carrier volume at the high end of the recommended range to ensure that all weeds receive an adequate dose of herbicide.