Weed Management Topics of Interest--September 2020
With the now-completed or soon-to-be-completed soybean harvest in the Midsouth, it is time to consider management of problematic weeds that plague the crop. The below articles are good sources of information when addressing this issue at this time.
In an article titled “Glyphosate-resistant Italian rygrass Management: Now is the Time!“ (La. Crops Newsletter, Vol. 10, Issue 8, Sept. 2020, p. 13-14), LSU AgCenter Specialists Drs. Daniel Stephenson and Josh Copes provide guidance on measures that can be used to control this problem weed. Major points in their article follow.
• A control program is divided into fall, winter, and spring treatments.
• The fall program is the best strategy for controlling glyphosate-resistant (GR) Italian ryegrass. It begins with residual herbicides that are applied mid-October to mid-November. Numerous residual herbicide choices are presented by the authors, and the choice of which one(s) to use is dictated by which crop is to be planted the following spring. Those choices are presented along with residual herbicide choices that can be applied if a fall cover crop is to be planted.
• Winter programs (mid-January to mid-February) are limited to only one herbicide (clethodim). There is a maximum ryegrass size restriction for that application and a preplant application interval if corn or rice will be the following crop.
• A spring control program is extremely limited (paraquat only) and is the least preferred option. A tank-mix partner with the paraquat is dictated by the following crop to be planted.
A Take Action Partner Update (Sept. 2020) from USB’s TakeAction Pesticide-Resistance Management program has two new resources of value to soybean producers.
The first is “Weed Out Resistance–Know Your Weeds–The 11 Biggest Resistance Threats” (technical editing led by Dr. Bob Hartzler of Iowa State Univ.) that is a poster with pictures of 11 weed species that are the most problematic herbicide-resistant (HR) weeds. Of particular importance to Midsouth growers are barnyardgrass, Palmer amaranth, Italian ryegrass, horseweed (marestail), and johnsongrass. With the picture of each weed is a bar showing the herbicide groups that weed is resistant to.
The second is Palmer Amaranth Management in Soybeans Fact Sheet, with technical editing led by Dr. Christy Sprague of Michigan State Univ. Important points from that article follow.
• Palmer amaranth is the most competitive and aggressive pigweed species. It emerges later than many other summer annual weeds, and continues to emerge throughout the growing season.
• Its rapid growth rate makes control with postemergence (PE) herbicides difficult because there is only a very short window for effective control.
• It has prolific seed production, which is a significant contributor to its perpetuation and spread. The high number of seeds produced per plant severely limits its eradication once established.
• In the last 3+ decades, Palmer amaranth has evolved resistance to 8 different sites of action.
• Control of this weed in soybeans is enhanced by earlier planting, narrow rows, and optimum soybean plant populations that are uniform–i.e., no skips that provide unshaded soil.
• Start clean by removing all Palmer amaranth plants with tillage or a burndown herbicide before planting.
• Plant soybean varieties with different HR traits–e.g., Liberty Link (LL), LL GT27, Enlist E3, or RR2 Xtend.
• Apply effective residual herbicides prior to or soon after soybean planting (see the Fact Sheet for suggested residual herbicides and herbicide mixes).
• Apply PE herbicides in a timely manner–i.e., before emerged Palmer amaranth plants exceed 3 in. in height (see the Fact Sheet for suggested herbicides and herbicide mixes for application to GR, LL, LL GT27, Enlist E3, and RR2X varieties).
• Apply effective residual herbicides with PE applications (see the Fact Sheet for suggested residual herbicides and premixes to apply at this time).
• It is important to reduce or eliminate seed production from Palmer amaranth. Do this by hand-removing escaped weeds and harvesting fields with escaped weeds last.
Following the above strategies will substantially reduce HR Palmer amaranth in soybean fields, and more importantly, will delay or minimize the future development of HR weeds of this species. Again, the above-linked USB TakeAction Fact Sheet provides the most up-to-date information for managing this problematic weed in soybeans.
Composed by Larry G. Heatherly, Sept. 2020, larryheatherly@bellsouth.net