The Agronomy Brief with Darren Goebel

There’s more than one way to kill a weed… and you’ll need all of them.

By Darren Goebel

The biological world works through natural selection. Every organism is trying to survive, and those with the genes to survive a life-threatening challenge will pass on the advantageous trait to the next generation.

So, it goes with herbicide-resistant weeds. Several unwelcome plants have found a way to survive one chemical or even two, developing varieties that thrive when farmers only use one mode of action for weed control.

If you throw the same chemical at a weed over and over again, the species has enough genetic variability that eventually part of the population will live. Whenever you can interject other modes of action — both chemicals and cultural practices such as tillage and crop rotation — you change the game and make it more difficult for weeds to develop resistance.

A successful plan must be thought of as a weed-control system involving chemical and manual control. Whether you are fighting weeds or insects, you want to look for many different ways to control pests in order to thwart natural selection.

When you use tillage to control weeds, you are taking out already germinated weeds, literally cutting off the plant and its potential to steal sun, water and nutrients from your crop. You are also stopping the weed from creating seeds that might add more resistant traits in the weed seed bank. In approaching a resistant weed challenge and developing a strategy to fight it, a farmer should first define what weeds are a problem in the field and research how those plants emerge during the season.

For example, tall waterhemp, pigweed and the rest of the amaranth species are small-seeded summer annuals that continuously emerge throughout the growing season. If a grower has a major problem with weed resistance, one solution might be to turn the soil over with a plow, burying weed seeds deep and leaving them buried for at least eight years. However, this should be used as a last resort only when all else has failed.

Mare’s tail, on the other hand, emerges in the fall and the spring, so if you hit it with a pass of tillage at those times, you won’t have any mare’s tail in your field all season long. Use a disc or field cultivator to hit it in late fall or just prior to planting.

Tillage is just one weapon. You have to have a multi-pronged attack to control weeds successfully. Another pass of that miracle herbicide isn’t going to work as well as a complete weed-control system that may include a pre-emergence herbicide, a post-emergence herbicide (with several modes of action), a tillage pass to kill plants that do emerge, a cover crop to prevent weeds from emerging in winter and a deep-till pass when necessary.

The single-pronged approach (chemical only) was the cheapest way to control weeds a few years ago. But this approach allowed resistant genes to slip into the weed population. Today, that single-pronged approach is the most expensive one we can take as farmers, which should have us looking for newer and better answers.

Darren Goebel is an agronomist and the Director of Global Commercial Crop Care for AGCO. He has been a crop consultant and agronomist on virtually all crops and geographies, with time spent in Southeast Asia, China, India, Eastern Europe, Mexico, the US and Canada. Darren has spent extensive time consulting on precision agricultural and holds degrees in agronomy, agricultural economics and business. He grew up on a grain, livestock and specialty produce farm in Southwest Indiana.