Louisiana: Don’t Ignore Weeds Between Now And December

Italian ryegrass. Image from Mississippi State University

Italian ryegrass. Image from Mississippi State University

It’s not hard to notice how green corn fields are following harvest, and we all know what that color means: weeds. Managing weeds post-harvest is not popular with farmers; however, weed seed production and subsequent seed deposition on the soil should be a great concern, especially if glyphosate-resistant Palmer amaranth and/or waterhemp are present.

We know that it usually takes only one month for a newly emerged pigweed to produce viable seed this time of the year, so action needs to be taken monthly to stop seed production until the first killing frost.

Even if glyphosate-resistant pigweeds are not present, preventing seed production of grass weeds like barnyardgrass or browntop millet will pay dividends next year.

Mowing, tilling and spraying herbicide are options to minimize or prevent weed seed production. I suggest mowing no sooner than two weeks after harvest, perform tillage that will destroy all plant material three to four weeks after mowing, then apply at least 0.75 lb ai/A of paraquat plus a residual herbicide three to four weeks after tillage.

Residual herbicide options include Goal, Valor, LeadOff and Dual Magnum. If corn is harvested in mid-August, mowing would occur in late August and tillage in late September, followed by the herbicide application in late October. September tillage could be when producers are preparing fields for next year.

Also, applying a residual herbicide in mid to late October will provide residual control of many winter annual weeds as well. If mowing and tillage are not options, then paraquat at 0.75 lb ai/A plus Valor is a good option to minimize pigweed seed production. Glyphosate is a good option for control of grass weeds.

Glyphosate-resistant Italian ryegrass has become a major issue in the past couple of years. Farmers in Mississippi have been dealing with this pest for almost 10 years. Mississippi State University weed scientists developed a program for glyphosate-resistant Italian ryegrass control. The program begins with management strategies implemented in the fall, such as a residual herbicide application like S-metolachlor, Zidua, Boundary and others.

Aggressive Approaches

Double disking is also an option. Application of clethodim in the winter (January) is their second line of defense.

Finally, if the producer waits until spring to target Italian ryegrass, the third line of defense is sequential applications of paraquat spaced two weeks apart with atrazine, metribuzin or diuron tank-mixed with the first paraquat application.

It is best to target Italian ryegrass in the fall. Unfortunately, Louisiana producers are tank-mixing clethodim with their burndown in the spring for Italian ryegrass. This strategy is proving to be ineffective for numerous reasons. Currently, the LSU AgCenter is investigating possible clethodim-resistant Italian ryegrass in Louisiana. Clethodim resistance has been documented in Mississippi.

Therefore, Louisiana producers should use fall-applied residual herbicides and spring clethodim or paraquat applications to manage glyphosate-resistant Italian ryegrass and avoid clethodim-resistance.

Horseweed, which is also known as mare’s tail, was a tremendous problem for Louisiana corn, cotton and soybean producers this year. Horseweed predominately germinates in the fall, but little above-ground plant growth occurs until the spring. Spring emergence of horseweed has also been observed; however, most of the horseweed populations Louisiana producers face emerge in the fall.

The best course of action is to scout for horseweed in the fall and apply 2,4-D or dicamba when found. If horseweed populations are targeted with spring burndown applications, 2,4-D, dicamba and Sharpen tank-mixed with glyphosate or Liberty are good options. Ensuring complete kill prior to planting the crop is crucial.