USA
August 30, 2016
Source: Ohio State University, Agronomic Crops Network
By Mark Loux
Continued problems with marestail and ragweeds this year have a number of growers considering the switch to LibertyLink soybeans for 2017. The LibertyLink system can certainly be a good choice for management of glyphosate-resistant populations of these weeds, along with waterhemp and Palmer amaranth. It’s essential to use the appropriate approach to LibertyLink soybeans to get the most out of it and avoid potential problems. Some things to consider as you make seed-buying decisions and think about your herbicide costs for next year (note – this is a recap of a March 7 C.O.R.N. article that has more detail):
- The active ingredient in Liberty – glufosinate – is available in a number of products now. The glufosinate products that are listed in this year’s weed control guide, Liberty, Cheetah, and Interline, have similar loading, rates, and labels. There is also a premix of glufosinate and fomesafen, Cheetah Max, that could be helpful for bigger ragweeds or waterhemp.
- POST glufosinate applications in LibertyLink soybeans should be part of an overall comprehensive herbicide program that includes a preplant herbicide treatment containing effective burndown herbicides and broad-spectrum residual herbicides. Do not shortcut the burndown program on marestail and other weeds and expect the POST glufosinate treatments to clean up a less than weed-free start. The POST glufosinate should be targeted for weeds that eventually emerge through the residual herbicides. Also – avoid use of glufosinate in the burndown treatment, since this can limit how much glufosinate can ultimately be applied POST.
- Omitting residual herbicides from LiberyLink soybeans is generally a good way to ensure problems. Any number of residual premix products can fit into the LibertyLink system. Glufosinate can be weak on lambsquarters, larger pigweed species (including waterhemp and Palmer), and certain grasses - yellow foxtail and barnyardgrass. The residual herbicides should cover these broadleaf weeds for sure, which is not a problem for most premix products. Residual activity on ragweeds and marestail is also desirable. With regard to marestail, our current recommendations for Roundup Ready soybeans are to use several non-ALS herbicides and higher rates, with the goal of not having to treat marestail POST. It’s probably not necessary to go to extremes on the residual program in LibertyLink, since the POST glufosinate can control later-emerging marestail.
- Avoiding problems with POST grass control in LibertyLink soybeans can be achieved in one of two ways: 1) use a residual herbicide premix with substantial activity on annual grasses; or 2) include a grass herbicides such as clethodim or Fusion in the POST glufosinate application. The latter approach may be most cost-effective and will also control volunteer corn, which often is not adequately controlled by glufosinate alone. This has become a routine practice for some growers and dealers to ensure effective grass control.
- Use of comprehensive residual herbicides usually creates a situation where one POST application of glufosinate is sufficient on many weeds. One exception is giant ragweed, which is best managed with two POST applications – the first when ragweed is 4 to 10 inches tall, and the second about three weeks later. Unless the ragweed population is very low, trying to get by with one glufosinate POST application will mean having to treat large plants that glufosinate can fail to control, which then regrow into late-season. Our research with giant ragweed clearly shows that the best approach for reducing populations is to bite the bullet and make two POST applications for several years (in rotation with an effective corn program), after which it may be possible to make one application work.
- POST application timing on waterhemp and Palmer amaranth is also an issue. For these weeds, the preplant residual herbicides should have substantial activity, and the POST glufosinate should be applied to small plants – less than 4 inches tall. A residual herbicide such as Zidua, metolachor, or Warrant can be included with the POST to minimize the need for a second POST application.
- Glufosinate is a contact herbicide, and it’s essential to optimize the application parameters to ensure maximum activity. Labels specify a minimum application volume of 15 gpa, and some applicators have found 20 gpa to be more effective. Nozzle and adjuvant selection should be geared toward production of primarily medium-sized droplets, avoiding a nozzle droplet distribution that is biased too much toward fine or large droplets. Applying in volumes too low is another good way to have problems with the LibertyLink system.
- Glufosinate activity can be reduced in cool, cloudy conditions, and there can be a need to base application decisions on weather. Many applicators try to apply primarily during periods of relatively warm and sunny weather to avoid performance problems. Using enough residual herbicide should result in enough flexibility in the POST application window to manage weather issues.
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