The Odd Couple: Round-Up Ready Seed Corn and Conventional Herbicides
by Greg Stewart, OMAFRA Corn Specialist

30 years ago we could have only dreamed of a broad spectrum, postemerge herbicide that essentially controlled everything and left herbicide tolerant corn untouched. Now, with an ever-expanding use of herbicide tolerant corn, one might question why would you ever consider using conventional herbicide technology? Here are 4 considerations.

1) Hybrid Yield
Although there is great convenience in purchasing herbicide tolerant corn seed, it is important to not over-rate this convenience. Say for example, your conventional hybrid is 4 index points higher in an OCC table compared to comparable herbicide tolerant hybrids. That yield difference could represent 8 bushels or approximately $30 per acre! The herbicide cost savings cannot compete with that sort of yield loss! Clearly not all herbicide tolerant hybrids are going to yield less than their conventional counterparts but be aware of the yield impact and don’t surrender maximum profitability for simplicity.

2) Weed Resistance to Herbicides
If you have already converted a large percentage of your soybean acreage to Roundup Ready soybeans you may feel somewhat concerned about the potential for promoting weed resistance to glyphosate. This apprehension may increase as you swing more of your corn acreage into a Roundup Ready program. Weed resistance stewardship promotes the rotation of crops, tillage practices and herbicides. You may also consider that adding some conventional herbicides to your Roundup Ready program either as sequential applications or as tank mixes fits the stewardship aims. The conventional chemistry component will help in reducing the pressure to select weeds that might otherwise get exposed only to glyphosate.

3) Can I Get to Every Field On Time
Considering the use of conventional, residual herbicides after you have already spent additional money for seed corn that is herbicide tolerant may cause your farming partner to question whether you have slipped on that last patch of ice coming out of the barn. The logic or value of applying pre-emerge herbicides to fields where you have planted herbicide tolerant corn will vary with each operation, and essentially it becomes a question of timing. If you can get to every corn field on time (i.e. 3-4 leaf stage) before weed pressure robs yield, then forget it; do your post-emerge application and drive on. If however, your acreage, distance, soil type, other farming activities, custom applicator schedule,
etc. means that on a fairly regular basis you miss the beginning of the critical weed-free period, then you might want to reconsider. This strategy then becomes one of laying down a very low cost preemerge program, that gives some early season weed control, and builds some
flexibility into your post-emerge spray schedule. Then even if you can’t get to that field with your first glyphosate application until the crop is at the 6-7 leaf stage yield loss has been negligible, and you clean it up with a single postemerge trip over the field. This will not be a low cost herbicide system, but eliminating the yield losses from poor timing may make it a profit optimizer. A potential side benefit is that on some
fields or in some years the “bare bones” pre-emerge program may do the job beautifully and you walk away with herbicides saving as well.

4) Saving a Trip Over Field
The other approach to combining residual chemistry with herbicide tolerant technology revolves more around saving a pass over the field, or perhaps some additional reduction in the urgency of post-emerge spray timings. This approach calls for tank mixes of residual herbicides with the first application of glyphosate. This first application will absolutely need to be on time (i.e. 3-4 leaf stage) or for the reasons already discussed you may have lost yield. However, in this application, if you tank mix a residual herbicide such as Marksman or Atrazine, you plan on eliminating the application costs associated with spraying a second time. In addition, you prevent the potential yield loss associated with a second flush of weeds that you don’t get sprayed in time, i.e. by the 6-8 leaf stage.

While 30 years has made a big difference in the way we treat weed control, there is still a place for conventional herbicide technology.

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