With post-emergence herbicide applications, timing is
everything. Every year, corn yields are reduced, because herbicides are sprayed
after corn has exceeded the critical growth stage.
Two methods — the leaf-collar and plant-height methods — are
most commonly used when determining if the plants are ready for herbicide
The leaf-collar method is more accurate because it requires
counting the leaf only after it has emerged from the whorl. The plant-height
method involves simply measuring a plant’s height without counting
The post-emergence application must be timed well for the
* Better herbicide performance — A timely application
provides more effective control due to treating weeds at a more susceptible
stage and at a time when the crop canopy does not interfere with
* Yield protection — Timely applications are more effective
at protecting yields from early-season competition than late applications;
* Crop safety — Many post-emergence herbicides have
restrictions on how late they can be applied. Good intentions to meet early
application guidelines can be easily delayed due to weather.
Critical reproductive development is occurring in corn
plants during the early vegetative growth stages. It is easy to reduce rows per
ear or kernels per row or to cause kernel abortion, even with glyphosate
products applied to resistant hybrids.
Be aware that crop oils, surfactants and other adjuvants can
increase the risk of injury, so read and carefully follow label
Assessing the corn growth stage in relevance to applying
post-emergence herbicides is another key factor in optimizing corn yield.
Controlling weeds early is the best way to ensure maximized corn grain yields.
The critical control of weed competition begins three to six
weeks after planting. By then, corn plants are generally six to eight inches
tall and in the V2 or V3 growth stage.
Lack of rainfall or excess rainfall can drastically reduce
both herbicide effectiveness and corn yield. If you have rainfall concerns over
the performance of soil-applied herbicides, scout cornfields within one to two
weeks after planting to determine if weeds are escaping soil-applied
Wet weather patterns create a risk when relying solely on a
post-emergence or glyphosate-only program. The growth rate of weeds will help
you decide which herbicide to use.
Consult your Pioneer sales professional and follow label
instructions for your weed-management program. The longer weeds are left
uncontrolled, the more weeds take crucial nutrients from corn plants.