Is the field sprayer ready for late season duty?

Pre and post-harvest weed control can do a number on most problem weeds.

Posted: July 25, 2005

Don't put that field sprayer away yet. There are still good opportunities ahead to control some of the most prevalent perennial weeds through a pre-harvest herbicide application, while a post-harvest treatment can be effective at controlling winter annuals.

"Producers have to know their weeds, their crop, and the herbicide," advises Keith Topinka, Integrated Crop Management Specialist with Alberta Agriculture in Edmonton. "But a pre-harvest herbicide application can be very effective at controlling several perennial weeds."

While glyphosate-based herbicides today produced by a range of manufacturers are one of the most effective tools for pre-harvest weed control, there are some limitations, says Topinka. "You don't want to use glyphosate, for example, on crops intended for seed because germination can be affected," he says. "It's a matter of checking product registration labels for the details. Malt barley producers should also check their buyer's requirements before preharvest applications."

What's the objective

Producers also need to consider their expectations. If the pre-harvest treatment is intended to desiccate the crop, producers need to be patient. "They can't expect to apply glyphosate and then combine the crop within three or four days," he points out. "It may take anywhere from 10 to 21 days for the material to dry down."

If the prime objective is to desiccate the crop, using a different product such as Reglone may be the best option. "With glyphosate a producer can get good perennial weed control and a slow dry down," says Topinka. "While with Reglone you get speedy desiccation with much less impact on perennial weeds."

Perennial weed populations have been on the increase in Alberta in recent years. Factors such as reduced tillage practices, continuous cropping, weather and diversified crop rotations have contributed to a greater abundance of weeds such as Canada thistle, dandelions, field horsetail, quackgrass, perennial sow thistle, foxtail barley and yellow toadflax.

The most recent weed surveys show that the frequency of Canada thistle has increased from a 12th place ranking in 1980s to one of the top five weeds by 2003.

Good timing

Late summer is the best time to tackle these perennials, says Topinka. "The perennials are actively growing and transporting nutrients to the root system. So if you're able to apply a systemic herbicide such as glyphosate and penetrate the crop canopy to make contact with the weeds, it will be quickly carried into the plant's system."

According to a recently updated direct seeding fact sheet produced by Alberta Agriculture, this translocation results in the death of "hard to kill" perennial weeds. Pre-harvest glyphosate can be applied to barley, dry beans, canola, field pea, flax (including solin), lentil, oat, soybean, wheat and forage crops. The treatment will provide good to excellent control of several of the most common perennial weeds.

If weather and temperature co-operate, late fall is a great time for a post-harvest treatment to control several weeds ranging from perennial grasses to broadleaf weeds. Again, glyphosate can be effective on quackgrass and dandelion, while 2,4-D is effective on broadleaf winter annuals such as stinkweed, flixweed and downy brome.

For a more detailed look at perennial weed control under direct seeding systems check out the fact sheet on the Alberta Agriculture Ropin the Web site at: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex784?opendocument or request a copy of the fact sheet Agdex 519-6.

Calibrate sprayer

With opportunities for late season weed control it is important to ensure the field sprayer is operating at proper efficiency, says Brian Storozynsky, field sprayer technology specialist with Alberta's AgTech Centre in Lethbridge. If it wasn't done earlier in the year for pre-seeding and in-crop weed control, it's worth the effort to ensure the sprayer is properly calibrated he says.

The procedure takes time, but it can be worthwhile to reduce pesticide waste, ensure proper pesticide rates and reduce downtime, says Storozynsky. "There's nothing worse than coming up a few acres short after filling-up for an 80 acre field," he says. "Having to reload for those few acres is as time consuming as filling for the whole 80 acres and can be frustrating when adverse weather conditions are on the horizon."