Fall-applied herbicides
By Lyndon Brush

Applying a residual herbicide during the fall can control certain weeds into the spring planting season. But there are drawbacks. If you're thinking about applying a residual herbicide this fall, read this.

Fall-applied herbicides can be an excellent way to control some weeds in some crops. However, this does not mean that this works for all cropping practices. Anytime you consider using herbicides to control weeds you have to look at the weed spectrum, the crop you are raising and the future crops to be grown in the field. Then you should select the best weed management option.

Recently, chemical manufacturers, chemical dealers and farmers have been talking about, and in some cases promoting, the use of fall-applied herbicides on no-till corn and soybean fields. I will take a look at some situations where fall-applied herbicides may or may not work.

First, let us consider using a fall-applied herbicide to control a weed such as musk thistle in established cool-season grass pasture or hay fields. Musk thistle is a biennial weed that requires two years to complete its life cycle. It will germinate in the fall, go dormant in the winter and then resume growth in the spring.

During its first year, the plant will stay in a small rosette form. During the second spring, the plant will bolt upright and produce a flower that contains the seed head. Fields infected with musk thistles will have various stages of plants growing throughout the field.

Fall applying Tordon 22K or Grazon P+D is an excellent way to not only control the new germinating plants but to also work on the plants that have grown a year. Herbicide applications can also work in the spring prior to the musk thistle bolting upright and putting out its reproductive seed head. Once the musk thistle has switched to its reproductive stage, control with herbicides generally declines.

Another type of fall herbicide application that can work successfully is in lawn and turf areas when controlling broadleaf weeds such as dandelions, which are a perennial type of weed, and other true winter annual weeds such as chickweed, henbit, deadnettle, shepherdspurse, etc. However, if the lawn problem is summer annual weeds, then the ideal application of a herbicide would be in the spring of the year when those types of weeds are starting their growth cycle.

Now let's discuss fall applying herbicides for no-till crops such as corn and soybeans. Using fall-applied, residual herbicides to get no-till fields off to a clean start in the spring by controlling the winter annual weeds is being promoted by some.

This practice leads to earlier planting because the bare ground may warm up quicker, it may help eliminate a burndown application prior to planting, it can spread out workload, etc. The idea of putting a residual herbicide down in the fall of the year in the state of Missouri is to control winter weeds plus provide residual weed control. When the crop is planted, it is, in my opinion, a questionable agronomic practice.

We all know that when planting a crop we need to start with a clean field whether it is in a no-till or a conventional till field. However, with the products on the market today, a successful burndown treatment applied in a timely fashion with the correct herbicide and the correct herbicide rate will create a clean field close to planting.

Applying a residual herbicide at planting may not even provide season-long weed control, so how do you expect applying a residual herbicide five to seven months prior to planting corn or soybeans to hold up? Time and time again the best weed-control programs in corn and soybeans are the two-pass programs. Applying a herbicide at planting or close to planting followed by a planned, timely, post-emergence treatment based on weeds present will always provide good, consistent weed control.

Herbicides break down in the soil primarily either through microbial degradation or chemical hydrolysis. In MFA's trade territory, winter weather patterns can be very different from one year to the next. If a residual herbicide is applied in the fall and weather conditions are cold enough to stop microbial degradation of the product, the herbicide will lie there until spring.

If a warm winter occurs, keeping soil temperatures above 45 degrees F, microbes will stay active and can break down the herbicide into an inactive material for weed control. For products that primarily break down through chemical hydrolysis, moisture is needed to drive this reaction. If soil temperatures are warm and plenty of moisture is present, chemical hydrolysis can occur and break down the herbicide into an inactive material for weed control. The weather dictates when the weeds will germinate and what happens to a herbicide.

Also consider springtime. Will you be able to plant at the optimum time, will you plant really early or will your planting be delayed due to weather conditions? When planting is delayed, more time is given for the summer weeds to start to germinate. Remember the dry, early spring of 2000? Normally, we do not see the widespread planting of corn and soybeans this early.

Also consider the environmental impact of the product that is being used. Does it move in the soil or will it stay there throughout the fall and winter? A herbicide's purpose is to control weeds so they will not compete with the growing crop. How important is applying a residual type of herbicide to control weeds when a crop is not growing and will not be growing for five to seven months?

There are some excellent herbicide products on the market. Some of these herbicide products will control some winter annual weeds. If you feel it is in your best interest to apply fall-applied herbicides, that is fine. But remember these points: you still may have to apply a burndown product; you still may have to provide a residual type of herbicide if you are not going to post-apply any other herbicide; your soil temperature may or may not be warmer than a non-treated area; you still may not be able to plant earlier; and you still have to address your other weed issues.

I do not see any corn or soybean fields where the only weed issue in the life cycle of the crop is winter annual weeds. Finally, what happens if you decide to switch crops? It's pretty hard to do if you already have fall applied your herbicide.

All sorts of claims and guarantees are sometimes made on products. If a chemical company feels it has a product that can be fall applied that will provide you, the grower, cost-effective benefits and is safe for the environment, then show us the research data. I am sick and tired of hearing "trust me, it works."

The plots I saw this year still needed a burndown application applied prior to crop planting. Putting down a residual product, just because of a guarantee that says you can control your winter annual weeds this fall, may not be all that wise of an investment.

Agronomically speaking, we use herbicides to control weeds in a timely fashion for the crop we are growing. I still believe optimum corn and soybean planting is done in the spring of the year. Let's keep applying the herbicides when the growing crop needs the herbicide to control the weeds present.

Lyndon Brush is a staff agronomist for MFA Incorporated.