Herbicides to control perennials
   may be applied at the wrong
    time, and
Many perennials have extensive
   underground root systems and
   vegetative structures, which
   allow them to produce new 
   growth even after their tops have
   been cut or burned off by 
   chemicals.

Often, several other techniques may be required for successful control, such as:

Using wipe-on applicators to 
   apply nonselective, translocated
   herbicides to weeds taller than 
   the crop.
Spot treating to control or
   suppress perennials prior to
   planting, in the crop, or before
   harvest.
Spraying perennial weeds in 
   fence rows, field borders, and 
   other non-crop areas.
Hand-removing simple perennial
   weeds (like pokeweed and curly
   dock) and any volunteer trees. A
   large portion of the root or crown
   must be removed.

Reduced rates
When it comes to herbicides, no-till is usually considered a high-input system because it often relies on more than one herbicide. The primary difference in herbicide use between no-till and conventional tillage is the use of burndown herbicides, which can amount to an additional $8-$12 per acre. (Burndowns don't tax the environment because they are designed to be active only in the weed and neutralize upon contact with the soil.)
While it is sometimes possible to eliminate a bumdown treatment, this is more the exception to the rule. Proper use of adjuvants and surfactants as well as timing is critical. It may take several tries to finally be successful.
More and more producers are experimenting with reduced rates (rates below those on the herbicide label). This strategy can be effective, if weed pressures are low at or near the time of planting. Although reducing rates can cut herbicide costs, there is substantial risk involved. Should you have a failure in weed

control, the chemical company will probably not support any complaints, since the labeled rates are based on extensive research trials.
Successful weed control with reduced rates is dependent on at least three factors:

Scouting - You must know the type of weed, size and growth stage, and overall weed pressure. Reduced rates work best in fields with low weed pressure.

New global positioning system (GPS) backpacks allow farmers to scout fields and mark locations of pests on a hand held computer. The information could then be fed into a computer guidance system on sprayers to spot-apply herbicides.

Crop competition - Narrow-row crop production helps shade out emerging weeds. Wide-row systems make success with reduced rates more difficult. With wide rows, you may need to rely on row crop cultivation to alleviate weed pressure in the form of escapes and uncontrolled weeds.

Herbicide efficacy - Many herbicides lose effectiveness on certain weeds at reduced rates, so know the product and its capabilities before trying reduced rates. It's usually a good idea to check with your chemical company representative before applying reduced rates.
You may have noticed that using reduced rates is contrary to the seven-point weed management plan outlined on page 16. Let's try to make this the only allowable exception.

Note: Crops bioengineered for herbicide tolerance can be valuable when managing difficult weed populations. They should not be used in fields every year, but rather in rotation with other weed control strategies.