New to LibertyLink Soybean? - Here's your Primer

LibertyLink soybeans are finally starting to get the use in Ohio that they deserve.  Use of the LibertyLink system accomplishes several things - resolving current herbicide-resistant weed problems and reducing the emphasis on glyphosate use that continues to drive the development of resistant weed populations.  Our primary glyphosate weed problems in the state are still marestail, giant ragweed, and common ragweed.  Waterhemp problems are slowly increasing and Palmer amaranth is showing up here and there.  Glyphosate resistance in these species often occurs in conjunction with site 2 resistance (ALS inhibitors), which has greatly increased reliance on site 14 herbicides (PPO inhibitors – fomesafen, Cobra).  Not surprisingly, we have common ragweed populations now resistant to site 14 herbicides.  POST applications of glufosinate can be very effective for control of ragweeds and marestail when used in the appropriate herbicide system.   Glufosinate applications should be part of an herbicide program that includes a comprehensive preplant burndown application or tillage for a weedfree start, and broad-spectrum residual herbicides.  Some other things to consider to get the most out of glufosinate: 

- The number of glufosinate products is on the increase.  The products that are listed in this year’s weed control guide, Liberty, Cheetah, and Interline, have similar loading, rates, and labels, and have performed similarly in our research.  There are undoubtedly differences in adjuvants and other ingredients among glufosinate formulations that can affect foaming and other qualities, but we don’t evaluate this.  Level of support in the event of problems probably varies among manufacturers also.

- Any number of residual premix products can fit into the LibertyLink system.  Glufosinate can be weak on lambsquarters, pigweed species, and certain grasses - yellow foxtail and barnyardgrass.  The residual herbicide should cover these broadleaf weeds for sure, which is not a problem for most premix products.  At least some residual activity on ragweeds and marestail is also desirable. 

- avoiding problems with POST grass control with glufosinate can be resolved 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 will also control volunteer corn, which may not be adequately controlled by glufosinate alone.

- Use of comprehensive residual herbicides usually creates a situation where one POST application of glufosinate is sufficient.  The exception is usually giant ragweed, which is best managed with two POST applications – the first when ragweed is 4 to 8 inches tall, and the second about three weeks later. 

- 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.  Glufosinate activity can be reduced in cool, cloudy conditions, and many applicators try to apply primarily during periods of relatively warm and sunny weather to avoid performance problems.

- Glufosinate has generally been underutilized in the state, and must of us have not thought too much about the potential for overuse and the possible development of glufosinate-resistant weed populations.  Glufosinate has been intensively used for several years now for the management of Palmer amaranth in the southern US, and glufosinate resistance has apparently developed in some fields.  At this point, we assume that overuse of any herbicide site of action on the five major problem weeds in the Midwest – giant and common ragweed, marestail, waterhemp, and Palmer amaranth – can result in resistance.  Glufosinate is no exception to this, so as its burndown and POST use continues to ramp up consider: 1) how often it’s being used in the rotation; 2) how often it’s being used for control of the same weed species.  For example, there may be better strategies than use of glufosinate in both the burndown and POST applications for control of marestail or giant ragweed.  Deciding where the use of glufosinate is most beneficial, and then integrating it with other herbicide sites of action and applications, can result in a better shot at long term utility.

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.