Production: Whack Weeds Early

CS Oct 10 CoverFor early season weed control,
timing is the ‘biggest management concern.’

To set the stage for achieving the highest yield potential, Southern corn farmers have determined that tiny corn seedlings have got to emerge into a weed-free environment. In comparison to several other crops, corn has the luxury of having many effective herbicide choices. However, if they are not applied in a timely manner, they are not able to perform as well as they should.

Arkansas weed specialist Ken Smith agrees that farmers are fortunate to have an array of effective herbicides to control weeds in corn.

“Of course, in the South, atrazine is still the foundation herbicide,” Smith notes. “There is an old adage about new corn herbicides: ‘If you mix it with atrazine, it looks really good.’ Our low organic matter soils and low population of microorganisms that break down the herbicide make it more effective in the South than in the Midwest. With Roundup Ready and LibertyLink technologies combined with several available preemergence and postemergence herbicides, our biggest management concern is timeliness.”

Smith notes that the Arkansas Corn and Grain Sorghum Research and Promotion Board asked university personnel to develop a “one-shot” program that would provide season-long weed control with only one herbicide application.

“Our best weed control was from an early postemergence application of metolachlor, mesotrione and atrazine,” he says. “This worked very well in light-to-moderate weed pressure, but was not adequate for late- season morningglory control. Atrazine applications are restricted to corn 12 inches or less, and we still have sufficient time for morningglories to germinate prior to canopy closure after the atrazine breaks. Although the one-shot program provided good weed control, it was not always the most economical nor did it always provide the highest yield.

“Lately, we have started doing more work evaluating herbicide application timing,” Smith adds. “We have found that the field must be clean when the corn emerges. Weed removal with glyphosate or glufosinate at various timings after the corn has emerged shows that yield is lost if weeds are present when corn reaches the two-leaf tip stage of growth. This is usually three to five days after emergence. Based on this work, we feel it is important to use a good preemergence herbicide and follow it with a postemergence atrazine combination prior to the 12-inch stage of growth. For late-season morningglories, our best program has been mesotrione combined with glyphosate when corn is about 20 inches tall.”

Early Season Weed Management In Louisiana
Bill Williams, LSU AgCenter Extension/Research weed specialist, says most people think of preemergence or early postemergence applications when they think early season weed management in most crops. While preemergence and early postemergence applications are important, preplant weed management is the most important weed management component for Louisiana corn.

“Most of the corn in Louisiana is planted well ahead of the emergence of many summer weeds,” Williams says. “Most farmers are successfully controlling these summer weeds with glyphosate plus atrazine programs in mid-April to early May. Many producers also use metolachlor, Callisto and/or Resolve for improved residual weed control. It should be noted that there is a need for our growers to consider using more products that contain nicosulfuron (Accent) or Ignite in LibertyLink corn for resistance management. This is especially true for Johnsongrass.”

The Louisiana weed specialist points out that cool-season weeds present the biggest threat to successful corn production in Louisiana. There are over 20 commercially important cool-season weeds that must be removed six to eight weeks before planting, he adds. “Glyphosate plus 2,4-D is the most common program used for preplant weed management in corn. Note that in Louisiana, 2,4-D should not be used within two weeks of planting. Glyphosate plus 2,4-D controls the vast majority of cool-season weeds. However, ryegrass and henbit are becoming increasingly difficult to control.”

Glyphosate resistance in ryegrass has been confirmed in Louisiana and is believed to be widespread. Unfortunately, there are not any easy or “cheap” solutions to glyphosate-resistant ryegrass.


Ryegrass In Louisiana Corn

• Glyphosate-resistant ryegrass has been confirmed in the state.

• Reduced rates of clethodim will quickly
lead to clethodim-resistant ryegrass.

• After ryegrass develops seedheads,
atrazine is effective.

Source: Bill Williams, LSU AgCenter

Several fall and spring programs have been evaluated for managing ryegrass before planting row crops, Williams says. In fields where glyphosate resistance has not become a problem, farmers may want to consider using Resolve in their weed-eradication program. Resolve at 1.25 oz/A has been the only herbicide evaluated that improves ryegrass control when co-applied with glypho-sate compared to glyphosate alone.

“In spring, the herbicide clethodim – Select or Arrow – is rapidly becoming the standard for managing ryegrass,” Williams says. “Satisfactory control has been observed with from six to eight oz/A of clethodim. However, 12 to 14 oz/A of clethodim are generally required for ryegrass control.

“Clethodim is an ACCase herbicide with inhibitors that affect cell membrane production in grass plants and is very susceptible to weed resistance, so producers are cautioned to use at least 14 oz/A,” he explains. “Re-duced rates will quickly lead to clethodim-resistant ryegrass. Assure, Fusilade and Poast have not been as effective as clethodim, but additional rates and timings need to be considered. After ryegrass develops seedheads, atrazine on corn is effective.”

According to Williams, once a crop is planted, options for managing ryegrass are limited. In corn, programs that are based on the herbicide nicosulfuron (Accent or Steadfast Q) have provided the most consistent ryegrass control. However, there are fields where ryegrass resistant to ALS herbicides is suspected but has not yet been confirmed. Problems with suspected ALS-resistant ryegrass generally occur along railroad and highway rights-of-way. Nicosulfuron has not controlled ALS-resistant ryegrass. The only other option for managing glyphosate-resistant ryegrass in corn is 32 oz/A of Ignite in LibertyLink corn. Ignite will likely require multiple applications.

Poor henbit control is not a statewide problem yet, but it has increased from a few isolated cases in Tensas Parish in 2008 to widespread problems in Concordia, Tensas, Madison and East Carroll parishes in 2009 and in Catahoula Parish in 2010. The reason for increasing problems with henbit is still unknown. It’s suspected that henbit populations are becoming more tolerant to glyphosate plus 2,4-D. At the Northeast Research Station, henbit control from glyphosate plus 2,4-D averaged 80 percent five years ago but less than 30 percent – and often zero – in 2010.

In fields where henbit control has not been satisfactory, fall or early winter herbicide programs work best, Williams says. Pro-grams based on glyphosate co-applied with FirstShot and a residual herbicide in late December or January have performed well. The key to these programs is to make the applications while cutleaf evening primrose and henbit plants are small. 2,4-D can be used to clean up escaped primrose, if any, 30 days before planting corn. The herbicides Ignite (glufosinate) and paraquat are effective at cleaning up any henbit plants that escape before planting.

Early Postemergence Options
Erick Larson, Mississippi State University Extension/Research corn specialist says crop rotation to corn is perhaps the best option to control and/or help prevent glyphosate-resistant pigweed because many more effective herbicide options for pigweed are available for corn compared to cotton and soybeans.

“These herbicides have several different modes of action, which helps prevent resistance,” Larson says. “Several herbicides la-beled for use in corn, but not soybeans or cotton, have good residual and/or activity.”

In Larson’s “Grain Crops Update” news-letter, he discusses the early postemergence time period for corn weed control:

The transition to glyphosate-tolerant corn has enhanced the ability to control weeds in Mississippi corn production systems. How-ever, farmers can still benefit from supplemental residual herbicides and proper timing to achieve seasonal “one-pass” weed control, which they desire. Atrazine has long served as the standard for broadleaf weed control in corn and still plays a major role.

University Of Florida Extension Weighs In
Know your weeds and choose a herbicide that is effective for your specific weed problems. Generally, for preplant and preemergence applications, the weed problem must be anticipated since the weeds have not emerged at the time of application. This can best be done by observing the field in the fall and recording those weeds that are present and in what areas of the field they occur. These “weed maps” can be very useful the next spring in refreshing your memory and making decisions on which herbicides to purchase.

Herbicides are one of the most effective tools for successful weed control in corn. Preplant or preemergence applications combined with good management practices are important for ensuring that corn has the initial competitive advantage. The herbicides suggested in Table 1, Table 2 and Table 3 are those that have performed well in the Institute of Food and Agricultural Sciences (IFAS), University of Florida, research. When choosing a herbicide, consideration should be given to the crop that will follow in a rotation and whether the herbicide will carry over and injure the succeeding crop.

Production practices in no-till corn virtually eliminate effective cultivation, thus placing greater importance on effective chemical weed control. Chemical weed control in no-till corn is similar to that in conventional-planted corn with two basic differences: 1) Existing vegetative growth must be killed or suppressed with a herbicide at or before planting. These can be found under the Burndown Herbicides for No-Till Corn section (Table 1). 2) Herbicides requiring mechanical soil incorporation cannot be effectively used.

All other herbicides listed for preemergence or postemergence application can be used in no-till corn just as in conventional corn.

Visit to view the entire Weed Management In Corn section as well as all of the herbicide Tables to which this section refers.

Source: University of Florida IFAS Extension

However, the new bleachers or pigment- inhibiting herbicides, such as mesotrione (Callisto and other formulated mixtures, including Halex GT or Lexar) or tembotrione (Laudis) are proving to provide superior weed control, compared to the standard of atrazine/metolachlor (Bicep II Magnum) in many cases. We encourage our cooperators in the MSU Corn Verification Program to use Halex GT or Lexar tankmixed with glyphosate when weed troubles warrant. These herbicides can substantially improve weed control, particularly residual control of morningglory species.

Utilizing these mixtures in an early post-emergence time frame can also improve seasonal weed control, compared to preemergence application timing, particularly when planting in March. This is because most warm season annual weeds usually begin germinating in mid-April. Therefore, herbicides applied when soil temperatures are too cool for weeds to germinate may lose effectiveness before the corn canopy shades the soil. This early postemergence window (V3-V5 or 6- to 12-inch corn) also allows more days for application, compared to later timing, in the circumstance of wet weather. This is because corn develops height relatively slowly until the V6-growth stage, when rapid vegetative growth begins.

In all of the discussions about early season weed control and suggestions for achieving the best results when growing corn across the South, these university experts stressed herbicide application timing across all states.

So that Southern corn can achieve its highest yield potential, make sure from the beginning that the crop has a clean environment in which to grow.

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