How to deal with Glyphosate Resistance and Weed Issues in Corn

Increasing populations of glyphosate-resistant and other herbicide-resistant weed species demand we adapt our corn weed control systems to address specific issues and prevent new ones.  This may require you to implement strategies for a specific weed, plan on multiple herbicide applications, and improve your application timing. 

Established ryegrass is going to outcompete young corn and reduce yield substantially.

Mississippi’s increasing Italian ryegrass populations are a huge threat to corn production.  It is extremely competitive because corn is planted early in the spring.  So as soon as the corn seed germinates, it is directly competing with ryegrass that is already well established and rapidly growing (see https://www.mississippi-crops.com/2011/03/12/key-corn-verification-findings/ for more information). Furthermore, glyphosate-resistant ryegrass has been documented in 14 Mississippi counties (including recently in Noxubee and Grenada), along with ALS-resistant ryegrass in 14 counties. This drastically limits effective herbicide control options.  Therefore, MSU Weed Scientists and Agronomists developed and published new strategies to aggressively control ryegrass during the fall, winter and spring, prior to corn planting (http://msucares.com/crops/weeds/herb_shortlist_lolmu_08-20-10.pdf). You could enhance control of later-emerging or escaped ryegrass by applying some atrazine near corn planting, as it has good activity on ryegrass – so don’t overlook this option.  Although these control strategies require multiple applications and obviously significant expense, if you choose to “just get by” and don’t achieve complete control, not only will it reduce crop yield, but you will also reseed your field.  Thus, you will need to “do it again” next year for the same or even greater problem, so don’t neglect the long-term cost.

Similarly, glyphosate-resistant horseweed (marestail) should be addressed in your burndown application, but if plants linger, go ahead and apply a postemergence herbicide to control them, perhaps considerably earlier than for warm-season annual species.  Since pigweeds are warm-season annual species, optimal herbicide timing will likely be similar to other such species.  This optimal timing will depend a lot upon the relationship between corn planting date and soil temperature, as most warm-season annual weed species do not readily germinate until soil temperature exceeds 60 degrees F.  Thus, preemergence herbicide timing is much better suited for late-planted corn, rather than early plantings, because soil temperatures will likely be warm enough to encourage weed germination.  Conversely, residual herbicides applied to early planted corn, when soils are too cool for weeds to germinate, will likely not be as effective, compared to later postemergence timings.

V6 growth stage corn is beginning rapid growth and leaves little margin for error for postemergence herbicide application.

If you do rely on postemergence herbicide applications, plan to start before the corn kicks into rapid growth growth stages (at V6 growth stage or taller than 12”), particularly if you are farming large acreage.  Your application timing window will shrink quickly, particularly if it rains, and you are more likely to injure the crop.   An early postemergence timing (V3-V5 or 6-12” corn) is preferable because it will increase the opportunity for ground applications, while likely improving  residual weed control.  For more information regarding how to determine corn growth stages see https://www.mississippi-crops.com/2011/04/21/how-to-determine-growth-stages-of-young-corn-or-sorghum/

We have always recommended residual herbicides for corn production systems to enhance seasonal weed control of various troublesome species.   However, incorporation of new pigment inhibiting herbicides or bleaching herbicides with existing standards can certainly improve your weed control by enhancing postemergence and residual activity in many cases, particularly for morningglories and glyphosate-resistant pigweeds.  These new benchmarks, which we typically use in MSU Corn Verification fields, include either or Halex GT (+ atrazine) or Lexar tank-mixed with glyphosate. These herbicide mixtures contain three or four diverse modes of action to attack weeds.  The pigment inhibiting herbicides are critically important for controlling glyphosate-resistant pigweeds, horseweed and large morningglories, because they generally provide good postemergence control, while atrazine is mostly dependent upon glyphosate for the system to control emerged weeds.  For more information regarding glyphosate-resistant Palmer Amaranth contol in corn see http://msucares.com/crops/weeds/herbshort_amapa_cor_09-28-10.pdf. 

The pigment inhibiting herbicides are critically important for controlling glyphosate-resistant pigweeds, horseweed and large morningglories, because they offer good control, while atrazine is dependent upon glyphosate to control emerged weeds.