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Planting Good Ag Practices
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Palmer Amaranth Control
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Palmer Amaranth Control

Size is critically important when it comes to controlling this weed.

By David Jordan
Extension Agronomist,
North Carolina State University
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Control of Palmer amaranth has become a major issue in many areas of the peanut belt, especially with widespread resistance to ALS-inhibiting herbicides such as Cadre, Pursuit, Classic and Strongarm. In recent years, resistance to glyphosate has further increased the difficulty of controlling this weed in cotton and soybean.

Even though the challenge seems daunting, there are a number of things growers can do to help make postemergence herbicides perform better. Furthermore, some of these items do not directly impact peanut, but anything done in rotation crops that increases control of Palmer amaranth will no doubt pay dividends in peanut.

Key variables that affect herbicide performance are as follows:

Weed Size
Glyphosate, a glycine herbicide, and to a lesser degree the ALS-inhibiting herbicides, provided some flexibility in timing of application because these herbicides often controlled relatively large Palmer amaranth, especially in the case of glyphosate. However, it is critical that herbicides with modes of action different from the ALS inhibitors and glyphosate be applied timely.

Many times herbicides are applied well past the optimum timing recommended by university scientists, consultants, county agents and agribusiness representatives and based on product labels.

The HPPD-inhibiting herbicides (Laudis, Capreno, Impact, others) offer more flexibility than some postemergence herbicides, although they have not been studied extensively in North Carolina.

For paraquat (various formulations), glufosinate (Ignite 280), PPO inhibitors (Reflex, Cobra, Blazer, Resource, others), photosystem II inhibitors (atrazine, prometryn, others) and synthetic auxins (dicamba, 2,4-D), applications need to made when weeds are small.

While this can be a challenge for the producer, the reality is that these herbicides perform well only when Palmer amaranth is relatively small. Complicating this is the fact that Palmer amaranth grows rapidly and within just a few days can become too large for effective control.

The importance of timely application cannot be overemphasized when trying to manage this particular weed. The photograph and table accompanying this article shows the sizes of Palmer amaranth at which growers should make applications of commonly used postemergence herbicides.

Nozzle Selection
Many producers use air induction or other types of drift-reducing nozzles to apply glyphosate. Spray drift is directly related to droplet size, and drift-reducing nozzles achieve their purpose by producing larger droplets. However, as droplet size increases, the number of droplets per unit area of leaf surface, or spray coverage, is greatly reduced.

While air induction nozzles work well with glyphosate, the best results with contact herbicides will be achieved with flat-fan nozzles.

Expected response of Palmer amaranth to postemergence herbicides as influenced by weed size
Palmer amaranth size in inches and expectation
of complete control*
Herbicide Always Very
Often
Occas-
ionally
Almost
Never
Glyphosate 20 40 60 >60
ALS inhibitors 4 8 12 20
HPPD inhibitors 3 8 10 >10
Paraquat (peanut rates) 2 3 4 >6
Paraquat (burndown rate) 3 5 7 >10
Glufosinate 3 4 6 >10
PPO inhibitors 3 4 6 >6
Photosystem II inhibitors 3 4 5 >6
Auxins (not 2,4-DB) 2 3 5-Apr >6

 
*Control assumes good conditions at time of application, appropriate adjuvant, use of herbicides within each mode of action that are considered effective against Palmer amaranth and application to plants that have not developed resistance to that particular mode of action.

Information compiled by David Jordan and Alan York with assistance from Stanley Culpepper and Eric Prostko, University of Georgia, Larry Steckel, University of Tennessee. Additional information on weed management can be obtained through your local cooperative Extension service.

Spray Volume
Weed size is not the only variable that can influence weed control. In recent years, growers have understandably reduced spray volumes in order to reduce overall mixing time and the amount of time spent hauling water so that acreage can be covered as quickly as possible.

While this approach has worked for glyphosate, a systemic herbicide often performs more effectively at lower spray volumes than at higher spray volumes. Contact herbicides, including PPO inhibitors, paraquat and glufosinate, will perform more effectively at higher spray volumes.
Maximum coverage will improve weed control with these herbicides, and this will be especially true if herbicides are applied when weeds are large.

Adjuvant Use
Extensive use of glyphosate has minimized the need for growers to consider adjuvant selection because many of the glyphosate formulations contain a proprietary adjuvant. However, adjuvant selection becomes more critical when comparing across herbicides with other modes of action. Selecting a high quality adjuvant and applying it at the correct rate is important in maximizing weed control with postemergence herbicides.

Tankmix Compatibility
Applying two or more herbicides or pesticides can improve weed control in some but not all instances, and in some cases reduced weed control can be observed with tank mixtures. Knowledge on compatibility will be increasingly important as growers rely on greater diversity of modes of action to manage herbicide-susceptible and resistant weed biotypes.

Residual Herbicides
Total reliance on postemergence herbicides may not be sustainable for a number of reasons, especially with herbicide resistance becoming common in many fields. Residual herbicides have often been considered “insurance” simply because of the effectiveness of total postemergence systems with glyphosate.

Applying a preplant incorporated or preemergence residual herbicide has become increasingly important not only to control resistant biotypes but also to minimize early season weed interference, especially for aggressive weeds such as Palmer amaranth.

Residual herbicides give producers greater flexibility in scheduling postemergence applications, which can decrease the number of times growers are required to apply postemergence herbicides well past the optimum timing.

PG

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