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Weed Management Techniques

By Dr. Harold Willis

Weeds are generally the first battle of the growing season. They seem to be especially bad in row crops such as corn. There are more kinds of noxious weeds today than there were 50 years ago, and thanks to herbicides, we now have many weed varieties resistant to the most powerful poisons we can throw at them, and even populations of soil bacteria in some areas of the corn belt that gobble up certain herbicides before they can kill the weeds!


The simple truth is that nearly all weeds grow best in poor soil—unbalanced, tight, depleted, “dead” soil. And that is just what our modern agricultural methods have created over the last few decades. In loose, fertile soil with the right balance of nutrients and beneficial organisms, weeds just don’t grow well, believe it or not. And crops do grow well, and can out-compete the weeds.

So how can you keep weeds under control? There are several ways. First of all, tillage methods that prepare a good seedbed in the row but leave a rough, poor seedbed between the rows (plus the use of a starter fertilizer) will allow the corn to sprout first and get ahead of the weeds. If weeds get a head start, many species release chemical secretions (phytotoxins) that inhibit crop growth, cutting yields (the process of one plant inhibiting another is called allelopathy). Many crops can do the same to weeds if they sprout first. Some weeds also reduce yield by competing for light, soil mois­ture, and nutrients. In Minnesota, grain yields have been reduced from 16 to 93% (51% average) by weeds, according to studies by Iowa State University. Weeds grow faster than corn in cool weather, so early-planted corn is at a disadvantage.

A second weapon in the fight against weeds is timely culti­vation, with shovel, rotary, or disk cultivators, rotary hoes, spike tooth and spring tooth harrows, etc. Some weeds are easiest to control when small, when their roots are shallow and the corn roots are already deep. Rotary hoes and rotary cultivators are most effective when the corn is small, but after it is over six inches tall, shovel cultivators are most effective. Be sure to only cultivate deep enough to kill weeds and not to prune the corn roots. As the season progresses, stay toward the middle of the row. Cultivation also has the benefit of breaking a crust and aerating the soil—which can greatly stimulate crop growth and health. If your soil has a crust, it is wise to cultivate even if there are few weeds to kill. A “dust mulch” formed by breaking a crust is also a good weed inhibitor. Another excellent tool to use is a disk hiller (pairs of disks, one on each side of the row with cultivator sweeps between rows), which will throw soil into a ridge against the stalks of older corn plants. This process will help control weeds, stimulate corn prop root growth, and form a furrow between the rows that will trap rainfall (hilling and row direction are therefore best at right angles to the slope of a hill). Other unusual weed cultivation methods are burning them to death with propane-fueled flame cultivators (good for small in-the-row weeds) and electrocuting them with a 15,000–20,000 volt electrostatic rig (which works on weeds taller than the crop, but the equipment is expensive).

A cornfield aerial view with weeds. Knowing what weeds are growing in your fields, and why, will help you manage your soil health.

One seldom-used method of cropping corn can reduce weeds; that is, intercropping, planting a different crop between the corn rows, such as soybeans or other legumes, forages, or sod. Gener­ally, corn yields are not reduced (although wider rows must be used), but the other crop will suffer from shading (but if a forage or grass will be in rotation the next year, then it will already be established). Experiments with planting alternating narrow strips (6 or 12 rows) of corn and soybeans have shown that corn yields can be increased 15 to 50 bushels per acre because of increased sunlight available to the outside rows of corn. The soybean yield is reduced 10 to 20%, however.

The use of herbicides to control weeds is at best a crutch, which only creates more problems than it solves. Besides killing plants, herbicides kill or at least upset beneficial soil organisms, leading to reduced humus production and tight, anaerobic, toxic soil—an ideal soil for weeds. Some herbicides actually stimulate germination of weeds other than the ones they kill. Also, herbicides or their break­ down products can pollute ground water and get into crops—the food we and our animals eat—with often-unknown consequences. If they absolutely have to be used to save a crop, herbicides can sometimes be reduced in dosage, especially if used with a surfactant (wetting agent). But with good soil, they may not be needed at all.

So that brings us to our last method of combating weeds and the ultimate solution in the long run—healthy soil. By having a loose, well-aerated soil (plenty of humus is the most effective way), and the right balance of nutrients (high calcium; more avail­able phosphorus than potassium), most weeds just won’t want to grow. They can actually become sick and die! I have seen it hap­pen, in less than one year of soil improvement.

Source: How to Grow Top Quality Corn