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 application.

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 leaves.

The post-emergence application must be timed well for the following reasons:

* 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 coverage;

* Yield protection — Timely applications are more effective at protecting yields from early-season competition than late applications; and

* 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 instructions.

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 treatments.

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.