Enough already

by Greg Stewart, OMAFRA Corn Specialist

Weed control timing in corn

ONTARIO PRODUCERS ARE fortunate to have excellent researchers, industry personnel, and extension staff working in the area of weed control.

Herbicide tolerance in corn has significantly changed the way we control weeds today and all industry players have contributed to the producer’s ability to manage this technology.

One of the key elements of post-emergent weed control in herbicide tolerant corn is to understand the impact of weed control timing.

THE CRITICAL WEED FREE PERIOD
Ah yes, the glorious concept of the critical weed free period. Industry stakeholders have preached this concept, drawn pictures and graphs of it, set up programs and catchy names around for it, made movies of it, promoted products because of it and gasped if someone hadn’t heard of it.

It is pretty interesting stuff.  How could leaving a few weeds to compete with the corn crop for a few weeks in June have such a dramatic impact on yields? It’s like Hulk Hogan having the flu for a couple of weeks when he is ten to grow up to be a 135 pound weakling.

At any rate, it happens. Yes, corn producers need to be particularly sensitive about the critical weed free period. It is important to be timely with applications of post-emergent herbicides in order to avoid irreversible yield loss in corn.

Enough already, surely we are all tired of this story. Let’s figure out what it takes and get it done.

FLEXIBILITY AND TIMING ARE KEY
Missing the beginning of the critical weed free period at the three to four leaf stage of corn may be frowned upon, but it happens fairly regularly. Whether it is acreage, distances, soil type, cow milking, custom applicator scheduling, rain, weddings, or golf tournaments a significant numbers of producers miss the beginning of the critical weed free period.

Fortunately, additional flexibility in herbicide programs might reduce the potential yield loss associated with the poor timing of the first glyphosate application.

One strategy to increase flexibility is applying a pre-emergent program to give early season weed control. With this type of strategy, even if the first glyphosate application isn’t sprayed until the crop is at the six to seven leaf stage, yield loss will be negligible and the field can be cleaned up with a single post-emergent spray.  This may not be a low cost herbicide system, but eliminating the yield losses from poor timing may optimize profits. 

Highlights to date include:

1) A single application of Roundup at the three to four leaf stage often produces yields equal to the weed-free control.  This supports the idea that if you can always be on-time, the most economical weed control approach will usually be that early application of glyphosate.

2) Sequential applications of Roundup (three to four leaf stage, followed by seven to eight leaf stage) may give improved later season weed control but does not consistently improve yields over the single application. In 2008, in Elora, under heavy weed pressure the second application of glyphosate, tended to improve yields.

3) Where Aatrex, Aatrex and Prowl or Primextra II Magnum were applied pre-emergence at either full or half-rates, followed by a sequential application of Roundup when the corn was at the seven to eight leaf stage, corn yields were generally superior to those treatments where the half-rate pre-emergent products were used alone or where the seven to eight leaf stage application of Roundup was used in isolation. 

In this research, any of the pre-emergent options (some costing very little) seemed to significantly buffer the timing for the first application and reduce the losses associated with not getting the first post-emergent application done on time.

Dr. Swanton is quick to point out that he is not a proponent of reduced rate herbicide programs when used alone, as they increase overall risk. However, if low rates are used in combination with a follow-up treatment of glyphoate the approach can be very efficient.  This work reinforces the concept that low cost pre-emergent programs may add significant flexibility to a Roundup Ready weed control program both in terms of modes of action and spray timing. 

MAKING THE RESEARCH WORK FOR YOU
How a producer views this information can vary widely.

Some producers don’t have any problem getting their post-emergent spraying done on time.  Their yield losses due to early season weed competition are non-existent.  They have already moved on.

Other producers are not keen on planting herbicide tolerant seed and still applying conventional soil applied herbicides, therefore they need to make the necessary adjustments to ensure they get post-emergent spraying done on time.

Still other producers see pre-emergent applications in herbicide tolerant corn as a way to take the pressure off June spray schedules, widen application windows and protect yield.

All three ideas are valid. The key message remains painfully the same: delayed weed control often stings corn yields significantly. Please stop.