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The search for new chickpea herbicides

One of the barriers to chickpea production is adequate broadleaf weed control. Currently, a
post-emergence application of Sencor is the only broadleaf weed herbicide registered. However, weed scientist Eric Johnson, at Agriculture and Agri-Food Canada at Scott, Saskatchewan, says one new herbicide is on the horizon for chickpea growers.

One of the herbicides Johnson has been working on goes by the chemical name sulfentrazone. It is registered in the US on soybean, tobacco and sunflower, with a Section
18 registration in North Dakota for wild buckwheat control in chickpea and field pea, and kochia control in flax. The US registration on chickpea makes registration in Canada easier, since some safety data and Maximum Residue Limits have been established for sulfentrazone already. “The package has been submitted for registration and we are fairly confident that it will be registered for 2008,” says Johnson, who has been researching the chemistry since 2002.

The registration application was done under the User Requested Minor Use Program (URMUR) and is different than other Minor Use Registrations since sulfentrazone was not registered in Canada on any crops. Most Minor Use Registrations are label expansions of existing products.
A new broadleaf chickpea herbicide will be welcome news for growers. Provincial weed guides rate Sencor control of chickweed, hempnettle, lamb’s quarters, wild mustard, annual smartweed and stinkweed as suppression. Crop tolerance to Sencor is dicey as well.

Sulfentrazone has provided good control of wild buckwheat, red root pigweed and excellent control of kochia and lamb’s quarters. Johnson says the major weakness is with the cruciferous weeds such as wild mustard.

The research on sulfentrazone in chickpea was a Pesticide Minor Use Program priority and tolerance, efficacy, and residue studies have now been conducted. The Pesticide Risk Reduction Program and the Provincial Minor Use Program funded Johnson’s research.
Sulfentrazone is a Group 14 herbicide that inhibits the proto-porphyrinogen oxidase (PPO) enzyme. Group 14 herbicides are not commonly used in western Canada and its use will
also provide chickpea growers with another tool in managing herbicide resistance. The chemistry is a soil applied herbicide that requires moisture for activation and root uptake. As a result, crop tolerance and weed activity are dependent on soil moisture and soil type.
Johnson’s research indicates that chickpea and fababean tolerance to sulfentrazone is excellent. He also screened other crops and found that flax, sunflower, lupin and field pea have good tolerance, while pinto bean tolerance is poor and lentil is very poor. However, as expected, the tolerance varies according to soil type and environment. For example, flax
had good tolerance to sulfentrazone at Scott in 2004, but high rates resulted in crop injury when above normal precipitation was received after application in the spring of 2005.

Wet conditions also occurred at Indian Head in 2005, but injury was acceptable, highlighting the importance of soil type.

Research funded by the Saskatchewan Flax Growers the past two years has helped refine application rates over a number of soil types. “The rates are going to be so soil specific. The
higher the soil organic matter and clay content, the higher the rate will be required for weed control,” explains Johnson.

As a soil active herbicide, residues can carryover to the next year. Re-cropping studies have indicated that with the correct application rates, re-cropping issues should be minimal. Johnson says that his research has found that re-cropping the following year to cereals and flax is acceptable and that canola re-cropping looks good as well. Canaryseed will need at least a two year re-cropping interval and lentils two to three years.

Another chemistry further down the road
Johnson has been working with the isoxaflutole chemistry as well. Isoxaflutole is a Group 27 herbicide that is registered in field corn in eastern Canada. Trials conducted in western Canada indicated that chickpea is tolerant to isoxaflutole; however, broadleaf weed control was inconsistent. Tame buckwheat has also exhibited tolerance to isoxaflutole.

Like sulfentrazone, isoxaflutole is a pre-emergence, soil applied herbicide and requires spring soil moisture or rainfall to activate. Under dry soil moisture, weed control can be dramatically reduced. Isoxaflutole trials found good control of wild mustard, common lamb’s quarters, stinkweed, kochia, red root pigweed, shepherd’s purse, wild tomato and green foxtail. However it does not control wild buckwheat or cow cockle.

Because of the complementary nature of the weed spectrum of the two chemistries, Johnson looked at low rates of sulfentrazone and isoxaflutole in a tank-mix combination. At lower rates, it was hoped that re-cropping restrictions could be minimized, while achieving a broader spectrum weed activity with better control.

Chickpea tolerance to the low rate tank-mix was excellent. It provided excellent control of wild buckwheat, wild mustard and kochia, something that neither product could do on its own. Johnson says that re-cropping studies are underway to see if these low rates also reduce carryover injury to rotational crops. As an example of how important this registration
would be to chickpea growers, yield analysis indicates a 35 to 70 percent yield increase from the application of the combination compared to the untreated check.

Unfortunately, isoxaflutole is not registered on chickpea anywhere in the world, so the researchers and manufacturer are starting from square one in the registration process. Maximum Residue Limits would need to be established and countries around the world would have to agree to the use of isoxaflutole on chickpea, or its use could become a trade barrier. As a result, the timeline for registration on chickpea is hard to predict for isoxaflutole.

Should sulfentrazone be registered for the 2008 growing season, Johnson says that while it is weak in some areas of weed control, it should offer growers the opportunity to improve weed control in chickpea. The different mode of action will also provide an alternative herbicide group to combat resistant weeds such as Group 2 resistant kochia.