farmgate: What Is Stealing Your Nitrogen And Starving Your Corn?
Your decisions for use of Roundup Ready or Liberty Link corn have already been made. It is either in your field, or in your machine shed awaiting a brief ride in a planter box. But your decisions for a post emergent weed control system in corn may have brought some unexpected consequences. You don’t like surprises, so let’s take a look at what to expect this year in those corn fields.
The formula is simple. Plant a seed corn that has the genetics to resist glyphosate or glufosinate. Apply your side dress nitrogen. Spray the field with the proper herbicide for the corn. Pat your self on the back for your management decisions. Pocket a bit less money next fall.
Everything is in your plan except the last step and that is one that is becoming an issue for many farmers using a Roundup or Liberty Link weed control system. Not that the seed and chemical are more expensive, but the issue with the nitrogen is growing larger. Several Cornbelt agronomists are pointing out the problem, and Mark Loux at Ohio State is the latest in the C.O.R.N. newsletter reiterates the challenge of annual grasses in corn soaking up the nitrogen before the crop gets a chance to utilize it.
Loux and Purdue colleagues all expect expansion of Roundup and Liberty Link systems, which means most farmers are moving away from soil applied herbicides such as atrazine to eliminate the grass before it begins interfering with the corn crop. The pre-plant herbicides left the fields clean for an early start for the corn, and when the anhydrous ammonia applicator worked through the fields a couple weeks later, the rows were still fairly clean. That will not be the case where the glyphosate and glufosinate systems are implemented. Fields will have a broad spectrum of weeds using the same nutrients that corn will need, including the expensive nitrogen that will be applied. Researchers are uncertain just how much nitrogen will be siphoned off by a health crop of weeds and grasses, and work is being done to quantify the exact loss and determine the impact on corn yield.
The research that Loux and colleagues describes focuses just on the impact of grass, since the researchers eliminated the broadleaf weeds and allowed the grass to grow through the time that side-dress nitrogen was applied. The results indicated the grass accumulated a substantial amount of nitrogen. When the grass was 12 inches tall, it had consumed 50 to 63 lbs of nitrogen per acre in a 1999 study and 16 to 32 lbs in a 2000 study. The researchers found that when the grass was removed at a 9 inch height or more, the corn yield was reduced and the corn had accumulated less nitrogen compared to corn in fields free of any grass or weed competition.
The researchers had several recommendations for farmers who will be using a Roundup or Liberty Link weed control system:
1) When grass emerges with corn, and if the density reaches 30 plants per square foot, the grass should be controlled before it reaches the 6 inch height.
2) The best opportunity for utilizing side-dress nitrogen to recover yield due to early-season weed interference probably involves injection of the N into the soil after postemergence herbicides are applied.
3) To minimize the influence of grass weeds on N accumulation and corn yield, use residual herbicides either by themselves or in a tank mix with glyphosate or glufosinate.
4) Expect a potential 6% corn yield reduction if weeds and grasses are not controlled before reaching a 6 inch height.
5) Consider a late season nitrogen application to replace the nitrogen consumed by any weeds that were not controlled early.
Summary:
Just because nitrogen is applied to boost your corn yield does not mean that other weeds and grasses present in the field are not allowed to use it. If it is there, the non-corn plants will use it and that reduces the amount of nitrogen available to the corn. The problems created by the shift from a soil applied herbicide to a post emergent herbicide in the Roundup and Liberty Link programs will mean weeds and grasses will be present when side-dress nitrogen is applied in several weeks. Yield loss in corn may be significant unless the grasses are controlled in a timely manner.
Posted by Stu Ellis on May 7, 2008 12:27 AM to farmgate