Herbicide-resistant weeds don't look like monsters. They don't have thorns or giant leaves. They simply survive chemical applications that kill their susceptible cousins.

Because it takes a greenhouse test to tell them apart it rnay be difficult to distinguish between weed escapes and a resistant outbreak.

"Every weed control failure you may have in the future isn't going to be due to resistant weeds," says Leon Wrage, South Dakota State University extension weed specialist.

According to a list prepared by Du Pont, poor herbicide performance sull

can be caused by many factors, including:

Poor application coverage.

Improper timing. Weeds may be too big when sprayed to be controlled.

Lack of rain. It takes moisture to activate some herbicides.

Plant stress. When under stress, weeds may not absorb herbicide.

Failure to use a needed additive.

Some herbicides give top performance only when tank mixed with surfactants or additives.

Herbicide resistance may be involved if the following conditions exist:

The same herbicide has been used on the same field for several years.

Past weed control has been very good.

Other reasons for poor performance have been eliminated.

Control is poor on only one or two weed species, but good on othe~

For example, let's say lambs-quarters, mustard and kochia had been controlled very well in the past, but now only lambsquarters and mustard are being controlled. Herbicide resistance may be involved.

Weed experts say the following four steps can help you head off development of herbicide-resistant weeds:

1) Rotate crops. Crop rotation usually means using a diverse herbicide program, making it difficult for resistant weeds to increase.

2) Rotate and tank mix herbicides. Using herbicides with different chemistry and modes of action keeps resistant weeds in check - with little opportunity to grow and go to seed.

3) Use short-residual rather than long-residual herbicides. Herbicides that last for a long time in the soil increase the selective pressure on resistant weeds. The longer susceptible types are suppressed and resistant ones allowed to grow, the more likely resistant weeds will increase enough to dominate the species.

4) Where practical, use tillage in conjunction with herbicides. The best weed management program uses a balanced variety of control methods,. including herbicides with different modes of action and tillage.

Through the years we have let plant populations creep up to dangerous levels, as can be seen by the effects of this year's drought. We need to reevaluate our strategy on the basis or the total tillage program utilized on a farm.

I will make production decisions late, depending on the rainfall this fall. We need a big charge of precipitation then to bring moisture balance back to normal next year. There's a lot of herbicide and fertilizer on top of the soil this year. The top layer needs to be cleansed.

We will be doing plenty of soil testing next year to locate fertility and dryness problems.

Despite the bad press no-till has received this year, the practice has worked well for us on corn following beans in sandy soil, perhaps because we had a very good weed-control program going into planting season. The bean stubble protected plants in the mids of dry, blowing dust tbat eroded conventionally tilled soil. We plan to use no-till more extensively next year. by. Bob Andersan

Norwest Bank

Des Moines, Iowa

MINNESOTA SCIENCE