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Tips For Weed Control in the Southern Plains

Wednesday, February 1, 2012
filed under: Weeds

By Phil Stahlman*

1. Prevent weed seed production in the preceding crop and fallow period. Individual plants of most weed species can produce hundreds to thousands of seeds. Preventing weed seed production in the preceding crop and/or fallow period prior to planting sunflower reduces the number of weeds to be controlled in the sunflower crop. The higher the weed density, the greater the possibility some weeds may escape control to compete with the crop and/or interfere with harvesting.

There are few options to control escapes or late-emerging weeds in sunflower. No herbicide is registered for use over-the-top (postemergence) to control emerged broadleaves in conventional sunflower; only in herbicide-resistant hybrids is this possible. Then, only Beyond® can be applied in Clearfield™ hybrids and only Express® in ExpressSun™ hybrids. (See Tip #5 for additional information on Clearfield and ExpressSun sunflower technologies.)

2. Knowledge of history is the key to success. Knowledge of field history, including weed presence, crop rotation and herbicide use, will help anticipate which weed species are most likely to occur — a necessity in selecting an appropriate herbicide program. Some herbicides, such as Prowl® H2O and generic products containing pendimethalin or trifluralin, control grass weeds more effectively than broadleaf weeds; while Spartan® controls several common broadleaf weeds but very few grass weeds. Dual Magnum® controls several annual grasses and some small-seeded broadleaf weeds. In most instances, however, mixtures of complimentary herbicides, such as Spartan plus Prowl H2O or Dual Magnum, are needed for satisfactory broadspectrum weed control.

If the previous crop was grain or forage sorghum or Roundup Ready® corn, soybean or possibly cotton, then volunteer plants of the previous crop will be hard-to-control weeds in sunflower. Whereas glyphosate can be used to control volunteer grain or forage sorghum plants prior to planting sunflower, glyphosate will not control volunteer plants of Roundup Ready crops.

Grass herbicides such as Select®, Select® Max, Assure II®, Poast® or various generic products containing clethodim or quizalofop will control volunteer corn and grain sorghum plants — but not volunteer plants of cotton or soybeans. These herbicides will also control many late-emerging annual grasses.

In-crop control of emerged susceptible weeds can be achieved by applying glyphosate using specialized spray hoods between rows; but there is risk of crop injury, and few growers are set up to make this type of application.

3. Start clean and stand tall and evenly spaced. Whether using conventional tillage, reduced or minimum tillage, or a no-tillage production system, it is essential to plant sunflower into a weed-free seedbed for maximum production potential. Early spring weed growth in a no-tillage system often requires at least two burndown applications ahead of sunflower planting.

Because of low crop population and wide row spacing, sunflower is most vulnerable to weed interference early in the growing season before plants are large enough for leaves to shade weeds growing between plants within rows and between crop rows. Lack of uniformity in plant spacing and skips within planted rows allow sunlight penetration through the canopy to the benefit of weeds and detriment of the sunflower. Weeds flourish in skips, and species such as cocklebur and devil’s claw can interfere with harvesting.

4. Know the critical periods of weed interference. There are two critical periods of weed interference, and both are influenced by weed density and environmental conditions. The first critical period is the length of time weed control must be maintained to prevent yield loss. The second critical period is the length of time weeds emerging simultaneously with the crop can compete before reducing crop yield.

Numerous studies have shown that two to four weeks of weed control in sunflower is needed to prevent yield loss. At Hays, Kan., longspine sandbur plants emerging two weeks later than sunflower developed about 80% fewer tillers and 50 to 80% fewer burs, compared to plants emerging simultaneously with sunflower. Sandbur plants emerging four weeks later than sunflower developed few tillers and produced fewer than six burs per plant.

Recent studies in Nebraska and Europe showed the length of time weeds can remain in the crop grown without use of a preemergence herbicide also was two to four weeks after emergence. By then, sunflower has three to four leaves. Use of a pre-emergence herbicide extended the allowable time of weed presence without reducing yield by about two weeks.

5. Not all sunflower types are the same. Most herbicides registered for use in sunflower can be used on both confection and oilseed types. Herbicide-resistant sunflower hybrids possess a trait that confers resistance to specific herbicides. Currently, there are only two such sunflower technologies: Clearfield sunflower with resistance to Beyond herbicide and ExpressSun sunflower with resistance to Express herbicide. Though both Beyond and Express have the same acetolactate synthase (ALS)-inhibiting mode of action, misapplying Express on Clearfield sunflower or Beyond on ExpressSun sunflower — or applying any ALS-inhibiting herbicides to any sunflower hybrid — will result in severe injury or plant death.

Sunflower hybrids of either of these herbicide-resistant technologies currently are not as common in the central/southern Great Plains as are traditional sunflower hybrids. Growers planting either of the herbicide-resistant sunflower types must be vigilant against misapplication of Beyond or Express to traditional sunflower — and must take precaution to avoid spray tank contamination when spraying traditional sunflower following herbicide application to herbicide-resistant sunflower.

6. Use the right rate at the right time. Length of residual weed control from soil-applied herbicides is affected by many factors. The temptation is to use the lowest possible rate to minimize cost without considering soil type differences within fields, time of application or variable environmental conditions.

Though low rates of most herbicides can be quite effective under favorable conditions, consistency of performance and length of residual weed control frequently are improved by using higher recommended rates for specific soil type and organic matter content.

Herbicides applied a few days up to three weeks preplant generally will not provide effective weed control as long into the growing season as will applications made after planting (assuming adequate moisture for activation both times). So higher rates usually are recommended for early preplant applications compared to pre-emergence applications. The probability of timely activating rainfall is increased with early preplant application, which may omit the need for and offset the cost of an added burndown treatment at planting.

7. Tip 6, Second Verse: Kochia and Palmer amaranth control. Kochia has long been a common weed in sunflower, and populations of Palmer amaranth are on the increase. Kochia mostly germinates from March through mid-June when conditions are favorable, whereas Palmer amaranth can emerge as late as August and still produce seed. Lower rates of many soil-active herbicides have dissipated by then.

Spartan effectively controls several common broadleaf species, including Russian thistle, Palmer amaranth and kochia (including ALS- and glyphosate-resistant biotypes); but higher rates are needed on fine-textured soil with high organic matter content. At Hays, Kan., Spartan has generally provided better broadleaf weed control than either Prowl H20 or Dual Magnum. Preplant Spartan treatments 14 days or more before planting generally have provided better weed control (especially kochia) and crop tolerance than pre-emergence applications at planting. Spartan Charge® (sulfentrazone plus carfentrazone) and Spartan Advance® (sulfentrazone plus glyphosate) have enhanced burndown characteristics compared to Spartan. However, Spartan Charge will not control emerged grass weeds, so glyphosate should be added if grasses are present.

Dual Magnum is effective on Palmer amaranth, but once again higher rates are needed for season-long control. Tank mixtures of Spartan and Dual Magnum provide more-complete weed control than what is provided by either herbicide alone.

8. Use appropriate adjuvants at the right rates. Labels of most postemergence herbicides specify the use of one or more adjuvants to aid performance. The primary function of non-ionic surfactants is to increase spray coverage on plant surfaces, while crop oil concentrates, methylated seed oils and nitrogen fertilizer solutions (to a lesser extent) primarily facilitate penetration and uptake of herbicides through leaf surfaces.

Surfactant use rates typically range from 0.25 to 0.5% volume to volume (1-2 qt/100 gal), while the recommended use rate for crop oil concentrates and methylated seed oils is 1% volume to volume (4 qt/100 gal) or not less than 1.25 pt/A when spray volume is less than 10 gallons per acre.

Another category of adjuvants includes water conditioners and pH modifiers. Ammonium sulfate (AMS) is both a nitrogen source and a conditioner of hard water due to high cation concentration. The addition of AMS to spray solutions containing glyphosate is recommended at rates of 8.5 to 17 lbs dry ammonium sulfate per 100 gal of spray solution (or 2.5 to 5 gal of liquid AMS per 100 gal). AMS in solution disassociates and the sulfate binds with cations in the spray solution, thus preventing the development of herbicide (especially glyphosate)-cation complexes that tend to have lower absorption into plant leaves.

Most herbicide solutions have slightly acidic to neutral pH. Adjuvants marketed as pH modifiers lower the spray solution to pH 3.0 or less, so that the herbicide molecules are less prone to binding with positively charged salts that inhibit herbicide activity. Research on such products’ effectiveness has resulted in mixed findings.

* Phil Stahlman is research weed scientist with Kansas State University, Hays.
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