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Tips For Weed Control in the Northern Plains

Wednesday, February 1, 2012
filed under: Weeds

By Richard Zollinger*

1. Use rotation crops to reduce weed pressure. Herbicide choices for weed control in sunflower are less than for many other crops. To successfully manage weeds in sunflower, use the preceding crops to reduce the weed seed bank — especially for difficult-to-control weeds like Canada thistle, common ragweed, wild buckwheat, nightshade and biennial wormwood. Herbicides registered in corn, wheat and most other grass crops will control problem broadleaf weeds. Glyphosate-resistant crops grown prior to sunflower can also reduce weed infestations from the popular and effective herbicide, glyphosate.

Weed surveys conducted in North Dakota show that many common weeds that were a problem 35 years ago are still present today, even though many new and effective herbicide chemistries have been invented. The surveys show the same weeds are still here, but their populations have significantly been reduced. Crop rotation, along with herbicide rotation, may contribute to this seed bank reduction.

2. Know your weed spectrum. Correctly identifying weeds in your field is the first principle of weed management. If you do not know the weeds or mistake weed identity, the chosen herbicide will miss the mark. Herbicides are an expensive part of crop production, and choosing the right herbicide and the right herbicide rate is critical to controlling your particular weed spectrum.

There are several examples of weeds that growers might mistake for other weeds. Biennial wormwood is often confused with common ragweed, as is eastern black nightshade for redroot pigweed. Volunteer glyphosate-

resistant canola appears in fields out of nowhere, and growers might assume it is wild mustard. Bromes species, like downy brome and Japanese brome, are increasing all over North Dakota due to a wet cycle we are currently in. Some may mistake bromes for foxtail or wild oat. Many believe annual bromes will emerge only in the fall because of their winter-annual life cycle, but observation has shown they can emerge in the spring as well.

Weeds resistant to different herbicides are continually appearing in fields. Waterhemp looks very much like redroot pigweed, but is much more difficult to control as it has become resistant to at least six different herbicide modes of action, including glyphosate. Your application of glyphosate may control most of the redroot pigweed but miss the waterhemp. Correctly identify your weeds and choose a weed management program that will control those weeds.

3. Use foundation treatments. It is likely that most sunflower production is in no-till or minimum-till conditions. Eliminating steel forces growers to rely more on chemical weed control.

Foundation weed control is comprised of burndown and pre-emergence residual herbicides. The burndown application will likely be glyphosate plus another herbicide approved for use prior to planting sunflower. The end-goal of a burndown treatment is a weed-free seedbed, and it may require higher rates, depending on the weeds present.

Residual pre-emergence herbicides can be applied after seeding but prior to sunflower emergence. Pre-emerge herbicides may not provide 100% weed control but will decrease a significant portion of the weed population. This will remove a lot of pressure from the postemergence herbicides to do all of the weed control work.

North Dakota pesticide use surveys show very low foundation herbicide use in glyphosate-resistant crops. The 2008 survey indicated that only 4% of the soybean and 8% of the corn acres received a pre-emergence herbicide treatment, while 37% of the dry bean and 66% of sunflower acres were treated with a pre-emergence herbicides. Spartan and Prowl are popular herbicides applied pre-emergence in sunflower. They control many “pivot” weeds in sunflower fields and provide season-long residual weed control.

No broadspectrum postemergence herbicides are registered in sunflower, but Express is registered in ExpressSun sunflower and Beyond in Clearfield sunflower. A foundation herbicide program can benefit all three of these production systems.

4. Know your sunflower type. Current seed technology may make it hard to remember where each crop was planted. Roundup Ready crops, Roundup Ready/STS soybean, Liberty Link crops, Clearfield crops and ExpressSun sunflower are some herbicide-resistant technologies available. Misapplication of the wrong herbicide on a herbicide-resistant crop will result in severe injury.

Clearfield sunflower and ExpressSun sunflower both allow ALS-inhibiting mode-of-action herbicides for weed control. Express is from the sulfonylurea (SU) chemistry and Beyond is an imidazolinone (IMI); but both are ALS- inhibiting mode-of-action products — which might make a grower think it doesn’t matter which herbicide is used on which type of sunflower. But only Beyond can be used on Clearfield sunflower, and only Express can be used on ExpressSun sunflower. These technologies are herbicide-specific, and using any other ALS herbicide, whether SU, IMI, TPS or SACT herbicide chemistries, will result in severe sunflower injury and plant death.

5. Spray small weeds. Herbicides are more effective on small weeds. Growers routinely wait until most weeds have emerged before spraying their postemergence herbicides. By waiting, early emerging weeds can be quite large at the time of application and may not be controlled. Spraying when weeds are small will allow herbicides to give more-consistent weed control and control of “marginally” controlled weeds. A foundation herbicide program will support earlier applications to small weeds.

The type of weed resistance has changed from the resistance we saw back in the 1990s. Weeds that developed resistance to ALS-inhibiting herbicides in past decades exhibited an “on/off” response like a light switch. Either they were highly susceptible or highly resistant. Resistance to glyphosate is different. Small weeds are much more susceptible to herbicide phytotoxicity, and resistance increases as plants get bigger. Spraying when weeds are small will result in better overall weed control.

6. Respect the one. “Respect the one” has reference to how resistance biotypes show up in fields. It usually starts as one or just a few plants in a patch in a field. When a plant naturally mutates and produces seed giving rise to resistance to a certain herbicide, or if weed seed from a resistant plant is transported into that field, a few plants will escape the respective herbicide. Some growers may see the small patch and think it is a sprayer skip or nonperformance from the herbicide due to a variety of reasons. Growers may not suspect the small patch is the beginning of weed resistance in the field; and if the plants are allowed to develop seed, that small patch can quickly grow to encompass most of the field in the following years.

If the plants were physically removed from the field (by hand weeding or rogueing), in essence the resistant genes would be eradicated and the problem stopped dead in its tracks. Many “older and wiser” growers I talk to confirm that pulling errant weeds was a normal practice many years ago, but has gone into hibernation due to the many effective herbicides presently on the market giving near-complete weed control.

7. Optimize herbicide activity with appropriate adjuvants. Most post-emergence herbicides require one or more adjuvants to optimize herbicide effectiveness. Adjuvants are broadly classified as surfactants, oils and fertilizer. Herbicide labels may allow a broad array of adjuvants, but rarely specify one particular adjuvant type.

Adjuvants are not regulated, thereby allowing an overabundance of adjuvants — even within adjuvant classes. Some adjuvants work and some don’t. Unbiased third-party adjuvant testing is rarely done, making it harder for growers to choose effective adjuvants. NDSU is one of the few universities that has an adjuvant and formulation testing program. Much of the information is printed in the annual North Dakota Weed Control Guide.

In general, methylated seed oil (MSO) adjuvants are more effective than crop oil concentrate (COC or petroleum oil) adjuvants, while COC are more effective than nonionic surfactants (NIS). Label direction must be followed with addition of adjuvants, as some may be too aggressive and compromise crop safety. Always add a fertilizer source — either 28% urea ammonium nitrate (UAN) or ammonium sulfate (AMS) — if the label allows. The ammonium increases penetration, absorption and translocation of most all systemic herbicides on the market.

8. Use adjuvants at the right rates. Many oil adjuvants are recommended on a % volume basis (e.g., 1% v/v). Growers who use a high spray volume of 15 to 25 gpa will have sufficient oil adjuvant to enhance herbicide activity. However, most growers in the U.S. use a lower spray volume because glyphosate is the principle herbicide used and glyphosate is more active in low spray volumes. Weed control will be reduced when growers apply low spray volumes of 5 to 10 gpa and use oil adjuvants on a % volume basis reduced because there will not be sufficient concentration of the adjuvant in the spray tank for the area covered.

Research at NDSU has shown that oil adjuvant applied on an area basis (e.g., pt/A) will significantly increase herbicide activity — regardless of spray volume used — and will result in better weed control. The general rule of thumb for oil adjuvants is to apply at 1% v/v (label directions from most postemergence herbicides) but not less than 1.25 pt/A.

* Richard Zollinger is extension weed specialist with North Dakota State University, Fargo.
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