D. Alan Peters, Jason A. Bond, James L. Griffin and Jeffrey M. Ellis
The best time to plant corn in Louisiana is from mid to late March. During that time lower soil temperatures can inhibit germination of weed seeds and delay growth of emerged seedlings, which helps with weed management. The critical time to remove weeds from corn for maximum yield ranges from four to six weeks after planting.
In some cases, depending on weed spectrum, use of soil-applied herbicides alone can be sufficient for season-long control, especially when corn growth is rapid and canopy closure occurs early in the season. Producers in the mid-South have traditionally relied on soil-applied residual herbicides in corn weed control programs; however, the development of herbicide-tolerant corn may mean a change in strategy. Use of effective postemergence herbicides could eliminate the need for a soil-applied treatment.
The herbicide-tolerant technologies developed for postemergence control of weeds in corn include Roundup Ready, Liberty Link and Clearfield. Roundup Ready corn is tolerant of glyphosate, the active ingredient in Roundup formulations and Touchdown. Liberty Link is tolerant of glufosinate, the active ingredient in Liberty. Clearfield corn is tolerant of a combination of imazapyr and imazethapyr, the active ingredients in Lightning. Each offers alternative weed management options for corn producers in the South.
Roundup Ready, Liberty Link
Experiments were conducted over two years (1998 and 1999) to evaluate weed control programs and corn response using the corn hybrids Dekalb 580 RR (Roundup Ready) and Cargill 7750 LL (Liberty Link) planted in late March. Treatments included:
Bicep (Dual plus atrazine) as a stand-alone preemergence program
Bicep, Dual, Prowl or atrazine preemergence followed by Roundup Ultra at 1.5 pints per acre or Liberty at 20 ounces per acre early postemergence
Atrazine plus Roundup Ultra at 1.5 pints per acre or Liberty at 20 ounces per acre early postemergence
Roundup Ultra or Liberty at the same rates early postemergence
Roundup Ultra at 2 pints per acre or Liberty at 28 ounces per acre late postemergence
Roundup Ultra at 1.5 pints per acre or Liberty at 20 ounces per acre both early and late postemergence
Accent plus Buctril early postemergence, a standard program used in conventional corn, was included for comparison. Early postemergence applications were made in late April to early May when weeds were 0.5 inch to 5 inches tall and late applications to 1- to 14-inch weeds in late May.
In 1999, 0.6 inch of rainfall was received within seven days of application. But in 1998, rainfall of only 0.16 inch was received within 21 days of application, and no rain fell from week 7 through 10. Even though total rainfall was greater during the growing season the second year (18.2 inches as compared to 7.4 inches), rainfall was limited during grain fill.
For the two years, 28 days after application of early postemergence treatments, results were as follows:
Broadleaf signalgrass was controlled 74 percent to 93 percent with Bicep alone preemergence and, when followed by Roundup Ultra or Liberty, 96 percent to 100 percent. For comparison, Roundup Ultra or Liberty applied early controlled broadleaf signalgrass 73 percent to 100 percent and, when applied late, 91 percent to 100 percent. Broadleaf signalgrass was controlled 70 percent to 96 percent with Accent plus Buctril.
Pitted morningglory and prickly sida were controlled most consistently with Bicep or atrazine followed by Roundup Ultra or Liberty, or atrazine plus Roundup Ultra or Liberty (91 percent to 100 percent). This compares with 78 percent to 100 percent control of pitted morningglory and 83 percent to 100 percent control of prickly sida with Roundup Ultra or Liberty applied alone early. Accent plus Buctril controlled these weeds 68 percent to 100 percent.
Because of inadequate rainfall, the yield potential of the hybrids was not realized, and differences observed in weed control were not reflected in yield.
Roundup Ultra, Liberty
In another study, Roundup Ultra at 2, 4 and 6 pints per acre and Liberty at 28, 56 and 84 ounces per acre (1x, 2x and 3x rates) were applied to the respective hybrids at 4-, 6- and 8-leaf. Corn yield for each hybrid was equivalent to the nontreated weed-free control in 1998, when only 7 inches of rainfall was received during the growing season and in 1999, when corn was irrigated as needed. Yield the second year was more than 200 bushels per acre and averaged 1.3 times higher than in 1998. The consistency in response of the hybrids to the herbicides over the two diverse years indicated a high level of tolerance.
Experiments were conducted over the same two years to evaluate crop response and weed control with Lightning in Clearfield (Pioneer 3395 IR) corn. Weed control programs included standard preemergence and post-emergence herbicides applied alone and in combination with Lightning postemergence. The hybrid was highly tolerant to Lightning applied from 5- to 9-leaf at twice the labeled rate of 1.28 ounces per acre.
Bicep or Prowl plus atrazine preemergence controlled broadleaf signalgrass 50 percent to 86 percent and itchgrass no more than 69 percent. Lightning preemergence controlled these weeds no more than 29 percent. Broadleaf signalgrass control was increased to 89 percent to 98 percent when Lightning followed Prowl, atrazine or Bicep preemergence. These same treatments controlled itchgrass 76 percent to 83 percent. When only Lightning was applied postemergence, broadleaf signalgrass was controlled 64 percent and 88 percent and itchgrass 71 percent.
In the first year, corn yield was highest when Lightning alone was applied postemergence, but in the second year, yield for this treatment was no higher than when Bicep was applied. Conditions both years were not conducive to the maximum yield potential of the hybrid.
New Systems Can Be Effective
Results indicate that depending on weed spectrum and timeliness of application, Roundup Ultra, Liberty and Lightning can provide effective postemergence control when used as stand-alone programs. Hybrids were tolerant to the respective herbicides. Consistency in season-long weed control was enhanced in many cases when the herbicides followed a preemergence treatment. Preemergence herbicides can be beneficial in delaying emergence and slowing growth rate of weeds to allow flexibility in timing a postemergence application. When only postemergence herbicide programs are used, application timing is critical to reduce weed competition and to secure maximum yield potential of the hybrid. The decision to use a soil-applied herbicide in corn also should be based on negative environmental effects associated with movement of the herbicide from fields when significant rainfall occurs soon after application and the potential residual effect on other crops should replanting be required. In none of these studies was johnsongrass a problem weed; however, past research has shown that Roundup Ultra, Liberty and Lightning all control johnsongrass and are, therefore, viable options.
Since Roundup Ultra, Liberty and Lightning do not have significant soil residual activity, multiple applications may be required to obtain season-long control. When conditions are conducive to rapid early season corn growth, coverage of weeds underneath the crop canopy with spray solution may be hindered, requiring use of drop nozzles.Selection of herbicide programs in corn should be based on grower preference, yield potential of the hybrid and economics. At present, the LSU AgCenter does not recommend herbicide-tolerant hybrids that allow for use of glyphosate herbicides (Roundup Ready), Liberty (Liberty Link) or Lightning (Clearfield). However, when higher yielding hybrids are developed for the mid-South climate, this may change.