Can you see the difference in weed growth between the left side of the above photo and the right side? The photo was taken May 25, 1999 as the field was about to be seeded. Prior to seeding, the entire field received an application of Roundup. In the original colour photo, you can see that the Roundup did its job as the weeds are a sickly yellow colour. But why the difference in weed populations between the 2 sides of the photo?
In 1998, the field had been seeded to foundation barley so a 12 foot buffer strip had been cultivated around the field throughout the growing season. While no plant counts were done in May 1999, you can see that there was a virtual mat of weeds on the cultivated side, while there were few weeds in the standing stubble. The predominant weeds in the field this spring were stink weed and shepherd's purse. When I relayed my observations to Dr. Doug Derksen Weed-Crop Ecologist with Agriculture and Agri-Food Canada in Brandon, he was not surprised. He indicated that while some winter annual weeds prefer no tillage, others thrive on soil disturbance. "Stink weed and shepherd's purse are two winter annuals that prefer conventional tillage over no tillage", he said. With that in mind then, working stubble in the fall to control these two weeds will not be very effective. Applying 2,4-D in late fall continues to be a more effective method for reducing their numbers.
Tillage can also stimulate weed growth if the field has been direct seeded for a few years. Dr. Derksen said, "If a field has had little tillage for a few years and then is suddenly tilled, you can create a huge weed mess! Tillage removes weed seeds from the surface and plants them."
If you're planning to work your stubble this fall to "dry it out" for next spring, consider the weed problems that may be created. If you go ahead and work the field anyway, be prepared to control the winter annuals early. While moisture conservation wasn't a big concern for much of Saskatchewan this past spring, there have been many springs when we've wanted to save every drop of moisture. Leaving the winter annuals until just before seeding may be costly in terms of lost moisture and nutrients. It's estimated that for every 10 days a winter annual is allowed to grow in the spring, it can remove one inch of moisture from the soil. If the crop to be seeded is a cereal, then an application of 2,4-D early in the spring will look after the winter annuals. An application of glyphosate is recommended if the crop to be seeded is a broad leaf.
No weed control system is perfect. Herbicides don't control all the weeds all the time but then, neither does tillage. Before pulling out the cultivator this fall, look at the photo one more time. Can you see the difference in the amount of weeds between the cultivated and uncultivated areas? Which side do you prefer?