Friday, June 9, 2023

A Lasting Effort

Layer On The Residuals For Effective Season-Long Weed Management 

⋅ BY AMANDA HUBER & PRATAP DEVKOTA, UNIVERSITY OF FLORIDA EXTENSION WEED SPECIALIST ⋅

Editor’s Note: The 2023 Panhandle Row Crop Short Course at the Jackson County Extension office in Marianna, Florida, in March garnered a record-breaking attendance of more than 160 people, the most in at least a decade. University of Florida Extension weed specialist Pratap Devkota presented the peanut and cotton weed management section to the growers in attendance. The information on residual weed management from Dr. Devkota is presented here:

Successful weed control involves good management practices in all phases of peanut production. Weeds compete with peanuts for moisture, nutrients and light, with the greatest competition usually occurring during the first six weeks after planting. So, controlling weeds from before planting and early in the season is critical in peanut production. Moreover, layered residual herbicides with postemergence applications are needed for effective season-long control of troublesome weeds.

Residual or Soil-Applied Herbicides • Help to reduce selection pressure on POST herbicides — delay development of herbicide resistant weeds. • Weed control for about six to eight weeks. • Can be applied: – With burndown herbicides. – Preplant incorporation (PPI). – Preemergence (PRE) or at planting application (before crop emerges). – In crop season with POST herbicides. Factors Limiting Residual Herbicide Efficacy • Uniform soil surface coverage is essential. • Weather factor is critical. – Rainfall/irrigation needed to incorporate and activate herbicide into soil. – Rainfall stimulates weed germination. – Cool and wet weather can also result in crop injury. • Dry period after herbicide application is often a concern.

But First, Crop Rotation

One of the foundational cultural practices in any row-crop system is crop rotation. It is as important a part of a good peanut weed control program as it is a benefit to reducing insects, diseases and nematodes. Certain broadleaf weeds, which are not easily controlled in peanuts, may be controlled by herbicides that can be used in a preceding crop such as corn. 

Proper weed identification is also a critical component before deciding on a management program, especially with the use of herbicides. 

Effective weed control in peanuts is generally obtained by using herbicide programs that consist of a preplant incorporated or preemergence treatment, followed by a cracking/early postemergence treatment and a postemergence treatment. The cracking/early postemergence treatment, if properly timed, is generally the most critical application in a peanut weed control program. Maximum effectiveness will be achieved if the application is timed to the emergence of the weeds or made to weeds less than 3 to 4 inches tall. 

Follow all label instructions and precautions carefully to avoid crop injury or poor weed control. Peanuts under stress from cold and wet weather, thrips injury, etc., may be subject to injury from early season herbicide applications.

Off To A Good Start

Residual or soil-applied herbicides provide weed control for the first six to eight weeks, allowing the emerging peanuts to get a vigorous start. It also helps reduce selection pressure on postemergence herbicides later in the season. 

Residual herbicides provide weed control for the first six to eight weeks, allowing the emerging peanuts to get a vigorous start, and helps reduce selection pressure on postemergence herbicides later in the season.

Residual herbicides can be applied with burndown herbicides, incorporated in the soil before planting, applied at planting and before emergence and even during the season with postemergence sprays.  

Recommendations for effective residual herbicides are offered for the following groups of weeds. For small-seeded grasses such as goosegrass, crowfoot grass, large crabgrass and barnyardgrass, yellow herbicides such as Prowl, Sonalan or Treflan, along with Dual, Outlook, Strongarm, Valor or Brake are effective. 

Small-seeded broadleaf weeds would need similar herbicide treatments.

For large-seeded grasses, like Texas panicum or broadleaf signalgrass, Strongarm plus Valor would be an effective option.

Likewise, Strongarm plus Valor can be applied with other products for management of large-seeded broadleaf weeds such as Coffeeweed, tropic croton, spiderwort and morningglories.

Various factors can limit the efficacy of residual herbicides, which include coverage and water needed for activation. Uniform soil coverage is essential for residual herbicides so that it serves as a uniform herbicide layer. Rainfall or irrigation is also very important to incorporate and activate herbicides. Rainfall also helps stimulate weed germination and the uptake of the herbicide. Cool, wet weather may cause more herbicide injury on peanut. A dry period after the herbicide application can reduce efficacy. 

Dry Weather Considerations

If there is no rain in the forecast, consider applying a rate on the higher end of the recommended range. For completely dryland fields, incorporate residual herbicides into the soil at 1 to 2 inches. 

If there is no rain for 10 days or more after application, expect some loss of activity. If weeds emerge before a rainfall and herbicide activation and are still small and more sporadic, shallow cultivation of the row middles may be warranted.

For weeds that are dense and 3 inches tall, apply a cracking treatment or early postemergence herbicide sooner than previously planned. 

Tank-mixing residual herbicides with cracking and early postemergence application is important to increase the likelihood of adequate activation for later season weed control. 

Activation Precipitation Amounts

How much rainfall is needed for activation and when is it needed? 

Optimum Rainfall and Activation Timing. Herbicide and their respective Rainfall consideration. Prowl, Sonalan: 0.5 to 1 inch within two days after application. Valor: At least 0.25 inch. Strongarm: 0.5 inch or more is required for initial activation. Dual Magnum: 0.5 inches on coarse soils, 1 inch on fine textured within two days after application. Warrant: 0.25 to 0.75 inches within 7 days after application. Zidua: 0.5 inches before weed emergence. Brake: >0.5 inches after application

Prowl and Sonalan need 0.5 to 1 inch within two days after application. Valor herbicide needs at least 0.25 inch. For initial activation, Strongarm needs 0.5 inch or more. 

Dual Magnum depends on the soil texture: On coarse soils, 0.5 inch is needed, but 1 inch is needed on fine-textured soils within two days after application.

Warrant requires 0.25 inches to 0.75 inches of rainfall or irrigation within seven days after application, and Zidua needs 0.5 inches of rainfall or irrigation before weed emergence for effective activation.

Research has shown that Brake, a new herbicide that is still pending registration in many states, needs more than 0.5 inches after activation.

In a perfect world, every field would get 0.5 to 1 inch of rainfall or irrigation within two to three days after application to provide activation and optimal herbicide performance. However, we rarely live in this perfect world.  

Generally, 0.5 to 1 inch of precipitation within the first week is good, and about 2 inches or more of rain or irrigation spread over the next two to four weeks is optimal.

Scout Before An At-Crack Application 

If weed management for the season has gotten off to a good start, producers may not need a cracking treatment. So, scouting fields thoroughly before deciding to apply an at-crack treatment is warranted. If preemergence herbicides got good activation and the field looks clean, there may not be a need for a treatment. 

If the field is already looking pretty weedy, then an at-cracking herbicide application is needed to get weed pressure back under control. PG

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