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Herbicide Residue Carryover

Thursday, February 1, 1996
filed under: Weeds

Residue carryover from certain herbi-cides can be bad news in sunflower country — bad not only for sunflower plant health, but for producers’ pocket-books as well.

So reminds Duane Berglund, extension agronomist for North Dakota State University. Berglund notes that residue carryover from a number of herbicides used on crops in his state can — if fields are not rotated properly — severely injure sunflower seedlings, reduce stands and drastically restrict sunflower yield potential. Among that group of herbicides would be metsulfuron (Ally), triasulfuron (Amber), atrazine (various trade names), picloram (Tordon), clopyralid (Stinger), clopyralid + 2,4-D (Curtail), imazethapyr (Pursuit) and flumetsulam (Broadstrike).

Ally and Amber have been used by North Dakota farmers to control broadleaf weed problems in hard red spring wheat, durum wheat, barley and in chem fallow. But even when used at labeled rates, residue from these herbicides can remain in the soil from 22 to 48 months after application, Berglund notes, and thereby cause problems in sunflower. The rate of Ally or Amber breakdown in the soil decreases as soil pH increases, he adds.

The minimum rotation interval for planting sunflower on Ally-treated land will vary, depending upon geographic area and the amount of cumulative precipitation. Berglund says that in areas west of North Dakota’s State Highway 1 (i.e., the western four-fifths of the state), a waiting period of 22 months and cumulative precipitation of 22 inches is required. East of Highway 1, the requirement is a minimum rotation interval of 34 months plus 34 inches of cumulative precipitation.

Atrazine applied for weed control in corn results in residue the following year or for several years when applied at 1.5 to three pounds (active rate) per acre. Even a one-pound rate often causes crop injury the following year in lighter-textured, high-pH soil if soil moisture has been limiting, Berglund observes. While crops like corn, sorghum and millet are tolerant to atrazine, several other crops are susceptible at various levels. Sunflower ranks among the least-tolerant to atrazine of crops grown in North Dakota.

Symptoms of atrazine residue injury may include stunting of seedlings and yellowing of leaves and leaf margins, followed by browning or leaf tip burning. In severe cases, susceptible plants will be killed.

Accent, a herbicide used on corn, may leave residue problems for sunflower if soil pH is high. Sunflower can be planted 11 months after Accent application if soil pH is below 7.5, Berglund says. However, if soil pH is greater than 7.5, the Accent label requires an 18-month waiting period.

A new corn post herbicide, Permit (halosulfuron), is used to control broadleaf weeds. “Its soil activity and residue can be very damaging to sunflower,” Berglund cautions. Permit’s label recommends waiting at least 26 months after its application before planting sunflower.

Picloram (Tordon) has been used in North Dakota for years at low rates to control persistent broadleaf weeds in hard red spring wheat, barley and oats. Even at the low rate of one fluid ounce per acre (one-fourth ounce active), picloram residue can severely injure sunflower, according to Berglund. He recommends that only grass, small grains or flax be planted in 1996 on fields treated with picloram in the spring or summer of 1995. Sunflower, soybeans, dry edible beans and potatoes are among the crops especially susceptible to picloram.

Picloram also is somewhat mobile, so its residue can be at one- to two-foot depths if leaching has occurred in areas of higher rainfall. This creates a problem, Berglund notes, since sunflower seedlings will not display the usual injury symptoms (stunting, enlarged nodes). But once the plant roots reach the picloram residue zone, the herbicide will be transported to the terminal or top developing bud of the sunflower and may cause abnormal flower initiation and poor development.

Stinger and Curtail (both containing clopyralid) applied in 1995 may leave herbicidal residue in the soil that could cause sunflower seedling injury in 1996. Several crops (e.g., wheat, barley, corn, sugarbeets) have good tolerance and can be planted on land previously treated with clopyralid-containing herbicides. Some other crops (such as sunflower, canola, alfalfa, dry beans, soybeans and safflower) may be planted 12 months after clopyralid treatment — except in soils with less than two percent organic matter and receiving less than 15 inches of precipitation in the 12 months following application. Unless the risk of injury is acceptable, these crops should not be planted within 18 months following treatment of the clopyralid-containing herbicide.

Pursuit (imazethapyr) used on soybeans may carry over in soils and affect sensitive broadleaf crops for more than three years, according to Berglund. He says the following crop rotation restrictions must be observed to avoid risk of crop injury to subsequent crops:

• Field corn, dry edible beans, alfalfa and wheat may be planted 9.5 months after Pursuit application.

• Sunflower, barley, oats and rye, 18 months after application.

• Canola, crambe, flax and potatoes, 26 months after application.

• Sugarbeets, the most sensitive crop for injury, requires a 40-month interval from time of Pursuit application to crop planting.

Broadstrike (flumetsulam) used on corn and soybeans may leave herbicide residue that can injure sunflower for up to 18 months after application. Sugarbeets and canola need to wait 26 months; potatoes, safflower and flax, for 26 months — plus a field bioassay indicating crop safety. Crops like wheat, barley, corn, soybeans and alfalfa can be seeded safely the year following Broadstrike application.

“With most of these herbicide residue problems, various tillage practices would not be a very effective means of reducing potential injury,” Berglund observes. “Planting of tolerant crop species on suspect land is the best management practice to reduce any risk of herbicide residue damage.

“Always check the herbicide labels for rotational restrictions on the use of a product — and adhere to those restrictions,” the North Dakota agronomist concludes.
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