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6 steps to good weed control in LibertyLink soybean system

TAGS: Weeds
giant ragweed
JUDGE WEED SIZE CAREFULLY: What size are these giant ragweed plants? Can you tell by looking? Purdue's Bill Johnson says it’s critical to know weed size and plan applications of glufosinate accordingly.
Applying glufosinate requires a different approach than applying glyphosate.

Both glyphosate and glufosinate are broad-spectrum herbicides that can offer effective weed control in soybeans tolerant to the chemistry. However, that’s where the similarity ends. Each has different characteristics that influence how it should be applied. If you try to apply glufosinate in the same way you apply glyphosate, you could set yourself up for weed control failure.

“The LibertyLink system and glufosinate herbicide can be effective tools to help break the cycle of continuous glyphosate use,” Bill Johnson says. “But it must be applied properly.”

Johnson, an Extension weed control specialist at Purdue University; his cohort Travis Legleiter; and Mark Loux, an Extension weed control specialist at Ohio State University, compiled a simple guide containing tips to help achieve successful weed control in LibertyLink soybeans.

Here are six tips from their guide.

1. Set sprayer settings correctly. “The minimum total spray volume for applying glufosinate is 15 gallons per acre,” Johnson says. If weed pressure is intense with lots of weed canopy, 20 gallons per acre is better, he says. Glufosinate is primarily a contact herbicide, so coverage on weeds is key. By contrast, glyphosate is translocated into the plant.

2. Select spray nozzles that produce medium to coarse droplets. This right size of droplets offers the best chance for success, Johnson says. Stay away from nozzles that produce coarse to ultra-coarse droplets to get better coverage. However, if droplets are too fine, spray drift becomes an issue.

3. Put the correct adjuvant in the tank. Apply Liberty with ammonium sulfate, Johnson says. Add AMS at a rate of 3 pounds per acre. You can also add antifoam and drift-control agents as needed.

4. Take time of day into account. There can be a time-of-day effect if you’re spraying glufosinate, Johnson says. The weed specialists suggest spraying glufosinate from two hours after dawn to two hours before sunset. Spraying too early or too late in the day with this chemical can produce reduced weed control. Avoid foggy, misty conditions or when weeds are under stress from environmental conditions. These include drought, cool temperatures or extended cloudy conditions. Expect best results when spraying in warm, sunny weather.

5. Pay attention to weed size restrictions on the label. Glufosinate works best on many weeds when they’re small, Johnson says. For most weeds, you can apply glufosinate on tolerant soybeans when weeds are 3 to 6 inches tall at a rate of 29 fluid ounces per acre. However, if you’re after Palmer amaranth or waterhemp, apply before weeds are 4 inches tall.

If weeds are taller than you like, say 6 to 10 inches tall, you can go up to 36 ounces per acre, although a second application cannot exceed 29 ounces. The total allowed rate per year is 65 ounces per acre.

6. Adjust approach for intense giant ragweed pressure. If the field has a dense population of giant ragweed, the specialists recommend a planned two-pass postemergence program. The first application can be made when plants are 6 to 10 inches tall, and the second three weeks later. Note that application of residual herbicides is suggested for giant ragweed and other tough weeds.

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