The Challenge of Controlling Weeds

Patrick Kirchhofer - Peoria County Farm Bureau

Winter is an important time for a family farm operation, as all of the meetings attended and knowledge attained will soon be put into practice in the field. For grain farmers, the months of December, January and February are the time to secure seed, fertilizer, herbicide and other crop inputs. Many decisions have to be made—what seed to plant in each field, how much fertilizer to apply, which weed control practice to use—in addition to preparing machinery, record keeping and taxes. Livestock farmers do double-duty during these months, as they need to tend to their animals as well as make plans for the upcoming growing season.

One of the challenges facing farmers in recent years involves weeds that are resistant to traditional crop protectants such as Roundup. To tackle this ongoing challenge, farmers are diversifying their herbicides. What are these nuisance weeds? Waterhemp, marestail, Palmer amaranth, giant ragweed… and the list continues to grow.

Waterhemp seems to be the most invasive weed species in Illinois. In the past, it's commonly been referred to as pigweed. There are several variations of the species, including redroot pigweed, smooth pigweed and tall waterhemp, but common waterhemp is the real troublemaker. Over the past decade, it has built up a resistance to Roundup, which had been the "go-to" herbicide for farmers until the last two to three years, with the onset of the resistance buildup.

It can quickly get complicated to explain, but there are a variety of herbicides that can be used to control weeds—and different "modes of action" for each. Some are contact herbicides, which inhibit photosynthesis in the part of the plant it touches. Other herbicides are systemic, meaning the plant will absorb the herbicide through its leaves and translocate it to all parts of the plant, including the roots. Roundup works this way, which is why it has been so successful.

Waterhemp is a challenge for farmers in a number of ways. First, it is a prolific seed producer; one plant can produce well over 100,000 seeds. Second, it will germinate throughout the growing season—even in the middle of summer when corn or soybeans are already growing in the field, making weed control much more difficult. A third challenge is its fast growth rate—more than an inch a day. This is especially a problem in soybean fields, as soybeans do not grow nearly as fast. A farmer’s soybean field might be weed-free on July 1st, but covered with waterhemp weeds by the end of July.

In addition, waterhemp grows taller than its competition. If a soybean plant is three feet tall, waterhemp will grow four feet. If a corn plant is eight feet tall, waterhemp will grow nine feet. The species has grown up to 12 feet tall!

So what's the problem if there are a few weeds in a field of soybeans or corn? For one thing, weeds compete with the crop for moisture, sunlight and nutrients. As a result, there will be a reduction in yield, which could be significant if weed pressure is high. Second, weeds must be run through the combine at harvest, reducing efficiency as the machine requires extra power, fuel and time to harvest the field. They may even wrap around the reel at the front of the combine, or cause the cylinder to plug up.

Weed seeds that run through the combine will either be placed in the grain tank with the soybeans, or come out the back of the combine with the rest of the plant material (stems, leaves and pods) and be spread on the field, ready to grow with next year's crop. If the weed seeds go into the grain tank with the soybean seeds, they will reduce the quality and pureness of the soybeans. Ground-up soybeans, called soy meal, are fed to livestock (mainly chickens and pigs), whole soybeans are exported to other countries (primarily China), or soybeans can be used to make a renewable biofuel (CityLink buses, for example, operate on a 20-percent blend of soy diesel).

Weeds are yet another challenge for farmers. You can till them out with machinery, spray them with herbicides or manually pull them out—and there are probably not too many enthusiastic for that option. iBi

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