Rice Farming

Weed Control 101

Resistance management efforts can lead to higher yields


By Steve Gibbs

There is an old joke that gun control means hitting what you aim at. Weed control is a little like that, except that in the rice field, weeds, and especially grasses, are a moving target. Like all other living organisms, weeds are constantly changing. Genetic mutations are occurring within the plants’ chromosomes that allow them to better adapt and survive.

This includes developing resistance to the herbicides that growers use to keep their rice fields weed free. In other words, the products that work to control weeds today may not work as well in the future thanks to gene mutation.

Resistance management is a real challenge for farmers and researchers alike as they attempt to stay ahead of herbicide-resistant weeds. A great deal of research has been done to help the rice industry keep yields high, foreign matter low and chemical costs manageable. In order to be weed-free, America’s rice farmers must stay informed and be flexible enough to implement proven agronomic practices and sound resistance management techniques. It all starts with accurate information.

‘Combination punches’ help reduce chance of resistance
In the 1980s, the late Dr. Roy Smith of the University of Arkansas published groundbreaking research that demonstrated the impact of weed control on rice yield. His 10-year study produced evidence that when weeds are controlled at an early stage of development, there is a payoff in higher yield. This is significant information for today’s grower because there has been a tendency in recent years to apply herbicides later in weed development, rather than when the weeds are small.

“Today’s growers can wait longer to begin their control program because there are new herbicides that can control larger, more mature weeds,” explains Dr. John Leeper, technology leader for RiceCo, a developer and marketer of crop protection products for rice. “However, as Dr. Smith’s research indicated, weeds that are not controlled early in their life cycle can reduce the yield at harvest. It’s better to control weeds early in order to maximize yield.”

Another problem associated with late application of herbicides is that the weeds are harder to kill. Every plant’s metabolism changes as it matures. A grass at the tillering stage is far different, metabolically, than it was at the one- to four-leaf stage. Bottom line: long-term resistance management is easier when growers attack weeds when they are small and easy to control.

Even small weeds may develop resistance over time. Controlling weeds early in their development while managing resistance requires specific tactics that have been developed over years of research and observation by crop scientists and farmers.

Research indicates that the best practice involves using a combination of active ingredients that attack multiple modes of action within the weed. This procedure will help reduce the likelihood of weeds developing resistance to a specific mode of action.

“It is much like a boxer with an effective one-two punch,” says Leeper. “The weed may block the first punch, or it may deflect the second blow, but defending against both is very difficult.”

Controlling weeds increases profitability
From an economic standpoint, weed control and resistance management are absolutely essential. “The most expensive herbicide program is the one that doesn’t work,” says Dr. Ronnie Helms, a rice grower, consultant and researcher in Stuttgart, Ark. “Financially, it is vital to keep weeds under control if you want to be profitable in the rice business.”

Helms says his weed control program typically costs between $50 and $100 per acre.

According to Leeper, American rice growers have a good arsenal of products at their disposal, and most of them have been proven effective over a long period of time. The challenge is finding the right combination for the grower’s particular situation. The answer may vary greatly depending on the growing region, rice variety, whether it is Clearfield or conventional rice, water usage and many other factors.

Making good ‘modes of action’ choices
There are at least a dozen active chemical ingredients registered for use on rice in the United States. These ingredients provide eight distinct “modes of action” against weeds. A mode of action is the way in which a chemical attacks a plant.

ALS inhibitors comprise the largest group of herbicides used on rice in the United States. ALS is an enzyme that is essential to plant growth. Inhibiting the production of this enzyme will kill or stunt the plant’s growth. Other modes of action target other processes in plant growth and development.

The application of a single herbicide will likely do the job and kill a specific group of weeds. However, over time and with repeated use, mutation will occur that can allow these weeds to develop a resistance that renders the herbicide largely ineffective. Leeper cites specific examples of chemicals that were once used extensively to control specific weeds but had become ineffective due to resistance development. These chemicals are still used today, but they are only effective when combined with other products as part of a larger weed-control program.

Developing herbicide combinations has become an important area of research for plant scientists and crop consultants. Companies such as RiceCo specialize in developing combinations of active ingredients with different modes of action utilizing the most modern formulation technology to create synergized products to control weeds. RiceCo also works to educate producers on how best to control weeds in rice fields.

“We will typically make three weed control applications for each rice crop,” explains Helms, who grows about 400 acres of Clearfield rice near Stuttgart. “We’ll use seven different herbicides. We have to use that many active ingredients to handle the wide spectrum of weeds that we have. Fortunately, we get the added benefit of resistance management. The combination of herbicides makes it very difficult for the weeds to develop a tolerance.”

“There are many effective herbicides on the market for rice farmers, and used in combination correctly, they can eliminate most, if not all, weed problems and provide excellent resistance management,” says Leeper.

“To my knowledge, there are few new products in the development pipeline for weed control in rice. We need to use the ones we have wisely in combinations to preserve their efficacy.”

A complete program for resistance management
Herbicides are only one tool for weed control and resistance management. Crop rotation, land preparation and water management are also effective, and necessary, for combating unwanted weeds and managing the development of herbicide resistance.

Rotating between rice and other crops (soybeans, corn, etc.) allows growers to use completely different weed-control programs. This obviously impedes resistance development but also has the added benefits of reducing red rice infestation, fixing nitrogen in the soil and promoting higher yields.

Land preparation and water management also play a vital role in weed control and resistance management.
An effective weed-control program really begins with level ground. For growers who practice no-till farming, this means some minimal tilling is recommended. Rice ground should be relatively smooth and free of ruts and large dirt clods. This helps maximize the effectiveness of pre-emergent herbicides while promoting more uniform watering at the same time.

Water management also helps with weed control, as standing water creates an anaerobic (no oxygen) situation that inhibits weed seed germination. In this way, it acts like a pre-emergent herbicide.

The goal of every smart rice farmer is weed-free rice. With effective agronomic practices and an aggressive resistance management program, producers can take dead aim at weeds and hit their target every time.

RiceCo LLC contributed information for this article.

Free resistance control symposium

Farmers throughout the Southern rice-growing states will have an opportunity to get the latest information about weed control and resistance management at a free series of lectures in March.

RiceCo LLC, Valent USA and Horizon Ag are sponsoring an educational series, which will include meetings in Rayville, La., (March 2), Cleveland, Miss., (March 3), Brinkley, Ark., (March 4) and Corning, Ark., (March 5). The symposiums will feature guest speakers from state universities and technical representatives from Valent and Horizon Ag.

Among the topics of discussion will be cultural practices, crop rotation, herbicide and Clearfield stewardship practices. Attendees will receive CEU credits that will apply to their crop advisor certification status. The event is open to retailers, growers, consultants and others involved in the rice industry.

Each session will start at 9:00 a.m. and conclude at 11:30 a.m., followed by a catered lunch. Those planning to stay for lunch are asked to call 888-835-1313 or email RSVP@ricecollc.com.