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2006 Weed Control Notebook

Saturday, April 15, 2006
filed under: Weeds

First 3 to 4 Weeks Key for Weed Control

Thus, maintaining a weed-free sunflower crop for the first 3 to 4 weeks is key for minimizing weed competition and maximizing yield. Because sunflowers are usually planted in low densities and grow slowly during the first several weeks, weeds which emerge and become established during this time can be very competitive and reduce sunflower yield potential. It is essential that sunflower be planted into a seedbed free of growing weeds.

An early glyphosate burndown provides the sunflower crop an equal footing with weed competition. Optimum would be a glyphosate burndown several weeks before planting to control the season’s first flush of weeds, including small wild buckwheat at the two to three-leaf stage, followed up with another glyphosate application at planting or before emergence to control additional weeds. If making only one glyphosate burndown, don’t let weeds get too large. When weeds get too big, it can be difficult to get all of the axillary buds along the stem killed, and you may get regrowth or an incomplete kill.

Timing of Preplants is Critical

Pendimax (Dow AgroSciences), Prowl and Prowl H20 (BASF) and Pendant (Agriliance) all have the same active ingredient (pendimethalin). Prowl H20, new in 2004, is marketed as having less odor, staining, and volatility compared to the conventional formulation. As well, observations indicate that it may be a bit more effective in high-residue situations such as no-till, because it doesn't bind as readily to stalks and other debris. Sonalan (ethalfluralin) and Trifluralin products must be incorporated. It’s generally recommended to incorporate pendimethalin herbicides as well. Without incorporation, moisture becomes the critical component to getting the herbicide into the soil and down to the root zone where weeds germinate. Thus, allowing ample time for preplant products to activate – at least three weeks before planting (some labels specify up to 30 days before planting no-till) – is extremely important. Because a liquid product is more likely to adhere to field residue, a granular product is recommended in minimum/no-till situations. Experts say granules work better than liquid at sifting through residue to get to the soil.

Top Weeds in Sunflower Fields in 2005

Listed in order of leading incidence, from the 2005 NSA Sunflower Survey:

North Dakota/Minnesota: Broadleaf weeds – kochia, Canada thistle, redroot pigweed, biennial wormwood, marshelder, wild buckwheat, cocklebur. Grassy weeds – green foxtail, volunteer grain, yellow foxtail.

South Dakota: Broadleaf weeds – redroot pigweed, kochia, Russian thistle, wild sunflower, cocklebur, wild buckwheat, lambsquarters. Grassy weeds – green foxtail, volunteer grain.

Kansas: Combined broadleaf and grassy weeds, in order of incidence – palmer amaranth, puncturevine, volunteer grain, kochia, green foxtail.

Colorado: Combined broadleaf and grassy weeds, in order of incidence – puncturevine, palmer amaranth, kochia, devils claw, cocklebur, green foxtail.

See more details of the 2005 U.S. Sunflower Survey online at - click on ‘USA Sunflower Survey.’

Moechnig new SDSU Extension Weed Specialist

Mike Moechnig is the new South Dakota State University extension weed specialist, taking over for distinguished professor emeritus Leon Wrage, who retired last year. Moechnig earned both his Ph.D. and his master’s degree in agronomy from the University of Wisconsin-Madison. Before coming to SDSU, Moechnig was a post-graduate researcher at the University of California-Davis. Contact him by phone at (605) 688-4591, e-mail

Web site:

Select Max Has Improved Uptake

Valent has introduced post-emerge herbicide Select Max, a different formulation compared to Select, with ‘inside technology’ optimized for improved uptake and translocation for control of volunteer Roundup Ready corn, problematic annual and perennial grasses and volunteer cereals. NDSU extension weed specialist Richard Zollinger notes that product absorption on wheat is about 50% better with the new post formulation.

Recognize the Strengths and Limitations of Beyond in Clearfield Sunflower

Beyond/Clearfield sunflower offers excellent control of marshelder, cocklebur, devil’s claw, and a number of grasses and broadleaf weeds. Performance is best at early postemergence from V2 to V8 (V2-V4 optimum). Apply before broadleaf weeds exceed a height or width of 3 inches, and before grass weeds exceed 4-5 inches. Beyond will not control ALS-resistant kochia or other ALS-resistant weeds, and the herbicide shouldn’t be viewed as a rescue treatment for large weeds that have gotten out of hand. A preplant treatment may still be needed, depending on the field. Pyrethroid insecticides (Warrior, Asana XL, Baythroid, Scout X-Tra) are safe to tank mix with Beyond. This might be considered for treating stem weevils, for example. Of course, Beyond can only be used on Clearfield sunflower hybrids.

Stewardship is key with Clearfield sunflower. To maintain continued effectiveness of this herbicide technology, always grow it in rotation with other non-Clearfield crops and non-ALS mode-of-action herbicides, limiting reliance on ALS herbicides to no more than two out of four years in the same field. To reduce the risk of Clearfield sunflower outcrossing with wild sunflower, be sure to control wild sunflower with non-ALS burndown herbicides in no-till/min-till fields or tillage with conventional-till. As well, control wild sunflower in adjacent areas to Clearfield sunflower fields (road ditches, field borders, fence rows) through the use of non-ALS herbicides and/or mowing prior to seed set.

Herbicide Drift in Sunflower

If you have sunflower plants growing abnormally within a field this season, stunted or distorted in appearance and you can’t figure out why, it’s possible it could be herbicide drift. With more broadleaf and glyphosate tolerant crops being grown, sunflower can be particularly vulnerable, since it is a taller, high profile crop. Considerable variation in symptomology can occur depending on the herbicide product and rate, sunflower stage, and environment. Symptoms may include height reduction, abnormal twisting and bending of stems and/or leaf petioles, malformed leaves, abnormal lumps or knots on the roots, branching that produces smaller multiple heads similar to wild sunflower, malformed heads or heads with sterility, or plant death.

See photos of herbicide damage in sunflower online at Factors that affect herbicide drift (and details of herbicide drift research on sunflower) are outlined in NDSU Extension Bulletin A-657 online at Proper documentation for managing suspected herbicide drift damage can be found NDSU Extension Bulletin WC-751 online at

Organic Matter, Spray Water Can Affect Herbicide Performance

Some herbicides are partially adsorbed and inactivated by soil organic matter, both high and low. Thus, test organic matter to help make herbicide product decisions. OM levels change very slowly, so testing once every 5 years should be adequate. Spray carrier water can also reduce the effectiveness of herbicides. Water high in sodium bicarbonate reduces the effectiveness of 2,4-D and MCPA amines (not esters), Poast, glyphosate, and dicamba. High salt levels in spray water can reduce weed control in nearly all situations. Calcium and, to a lesser degree, magnesium are antagonistic to 2,4-D and MCPA amine, dicamba, and glyphosate. Analysis of spray water sources will determine possible effects on herbicide efficacy. In North Dakota, water samples can be tested (for about $25) at the following laboratory: NDSU Soil and Water Environmental Laboratory, (701) 231-7864, Waldron 202, NDSU, Fargo, ND 58105-5575.

Tips for Controlling Weeds in Dry Weather

Weeds do not grow as quickly in dry conditions and metabolism is slowed. Many weeds are shallow rooted and stop growing in dry soil. Perennial weeds may handle drought better because underground roots can tap into moisture deeper in the soil profile.

This does not mean perennials will not be affected by drought. Look at leaf composition: plants grown in dry conditions have smaller leaves and develop a thicker cuticle which amounts into less droplets intercepting leaves and less herbicide absorbed. Good adjuvants can help overcome leaf barriers like thick cuticle. Most POST herbicides used are translocated, so movement through the cuticle and then through the plant to growing points are critical for adequate control.

Where product labels permit, addition of crop oil concentrate rather than nonionic surfactant usually results in greater herbicide activity. Some products also allow for N-based spray additives, which tend to improve efficacy of certain products during periods of slight stress.

Applying herbicides in the early morning (except glyphosate) may improve weed control in dry conditions. During the night, the plant has recovered some from the heat and stress of the previous day. The leaves may not be wilted as much, increasing the leaf’s surface area. The dew may help to keep spray droplets hydrated longer allowing better absorption. Outside of proper calibration, good working components, and quality nozzles, there is not much to improve performance from your sprayer in dry conditions.

Essentially all postemergence herbicides have a statement on the label regarding weed growth and environmental conditions. A typical example is “Do not apply to grasses or crops under stress such as stress due to lack of unsatisfactory control may result.” To attain adequate control, weeds must be actively growing and must not exceed the maximum growth stage.

For some weeds, delaying postemergence control efforts until moisture stress is relieved is not feasible. Many postemergence treatments are effective only on small weeds, and small weeds are always easier to control than larger ones. For weeds under drought stress, use the highest labeled rates for the herbicides and possibly alter adjuvants if label allows. – Bev Durgan, U of M extension weed specialist, Richard Zollinger, NDSU extension weed specialist

Weed Research Results at Minot, Carrington

The NDSU North Central Research Extension Center in Minot has an excellent online database on weed control research conducted there from 1998-2005 on over a dozen crops, at Results of 15 research projects related to sunflower are presented here, including volunteer canola control in sunflower, sunflower response to Valor herbicide, and weed control programs in Clearfield sunflower, all three of which were conducted last year.

A link to results of weed control research results on nine crops and a dozen sunflower research projects conducted at the Carrington NDSU Research Extension Center can be found at:

Weed Management Info Online

North Dakota State University Weed Science

with link to 2006 NDSU Weed Control Guide

South Dakota State University Extension

Kansas State University Weed Management Page

Nebraska 2006 Weed Control Guide

CDMS Ag Chem Information Services

Online ag pesticide labels

Weed Science Society of America (includes link to weed photos and I.D.)

2006 Weed Control Notebook – Tracy Sayler

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