Big weeds: Get them now

May 11, 2016 - Issue 8

Seeding is the priority for many growers this week, but fields that still have lots of big winter annual weeds should get a pre-seed burnoff with a high rate of glyphosate before seeding. The herbicide needs only a day or two to get down into the growing point of these large winter annuals, then seeding can resume. The economic return will be higher than if the grower seeded over these large weeds and then took a chance on getting them sprayed before crop emergence.

Big weeds. Credit: Autumn Barnes

Big weeds. Credit: Autumn Barnes

Pre-seed rates of 1 litre per acre of 360 g/l low concentrate glyphosate formulations (0.67 l/ac. for 540 g/l high concentrate) will be more effective than 0.5 l/ac. (0.33 l/ac. high concentrate). The 1.5 l/ac. (or 1 l/ac. high concentrate) rate may be required. Tank mixing is recommended.

To improve glyphosate performance, apply…

—When temperatures are greater than 10°C, with the expectation that daytime highs will increase to greater than 15°C
—When nighttime lows are greater than 5°C (the warmer the better up to 10°C)
—Under bright sunshine (thin or sporadic clouds are also OK). Applications made in darkness or heavy cloud are less effective.
—Using water with low levels of hard water ions (Ca and Mg), iron, or suspended clay or organic matter.
—At water volumes of 5-10 gallons/acre. (Lower water volumes may help if only poor spray quality water is available.)
—When weeds are smaller, ideally at the 1-3 leaf stage.
—At speeds that minimize dusty conditions. Dust on leaves reduces herbicide uptake. If dust cannot be avoided, spray from downwind to upwind to minimize the dust deposition on unsprayed leaves. Or wait for rain to wash it off before spraying.

Further reading:

Tank mix glyphosate for pre-seed burnoff

Early weed control in cool or frost conditions

Canola Watch