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Sulfentrazone-containing Herbicides may cause Soybean Crop Injury. Integrated Pest & Crop Management Newsletter, University of Missouri-Columbia, April 17, 1998.


http://ipm.missouri.edu/ipcm/archives/v8n2/ipmltr7.htm

Integrated Pest & Crop Management Newsletter
University of Missouri-Columbia
Volume 8, Number 2
Article 7 of 8
April 17, 1998

Sulfentrazone-containing Herbicides may cause Soybean Crop Injury

In the past, the excellent control provided by sulfentrazone-containing products like Canopy XL and Authority Broadleaf has been reported. Donald Null, University of Missouri agronomy specialist, has worked with these products for three years, and has not observed adverse crop responses. However, there have been reports of soybean crop injury. Bob Hartzler, Iowa State weed scientist, and Bill Johnson, University of Missouri weed scientist, recently wrote about it.

Hartzler indicates that there have been two recent developments concerning recommendations for products containing sulfentrazone (Authority). The first involves Authority First/Classic, which is marketed by DuPont. DuPont had planned on limiting sales of this product for use on their Optimum Quality Grain soybean varieties for the 1998 growing season. However, they recently pulled the recommendation for use of Authority First/Classic on these varieties due to field and greenhouse tests that indicated that some (not all) Optimum grain varieties were sensitive to sulfentrazone. Due to the limited acres planted to Optimum grain varieties, this decision will have little impact on the market.

In a separate event, Pioneer Seed Inc. released a list of their soybean varieties that carry a Crop Response Warning. This is the first year that Pioneer has ranked hybrids/varieties according to their tolerance to major herbicide classes. Corn hybrids are evaluated for tolerance to three classes of herbicides. They are:

  1. chloracetamide herbicides (Surpass, Harness, Dual, Frontier, etc.)
  2. growth regulators (2,4-D, Banvel)
  3. sulfonylureas (Accent, Beacon, Basis, etc.)

Soybean varieties are evaluated for their tolerance to metribuzin (Lexone, Sencor) and sulfentrazone. Approximately 20% of Pioneer's soybean varieties carry the Crop Response Warning for sulfentrazone. Their literature states that the greater potential for injury with these varieties may justify considering a different herbicide program or choosing a variety with greater tolerance. DuPont's decision to pull their recommendation was based partially on these ratings since several of Pioneer's Optimum grain varieties carry the Crop Response Warning. Differential responses to sulfentrazone in soybean varieties has been reported in the southern U.S. A half pound an acre of sulfentrazone caused height reductions between 3 and 58% three weeks after planting in a greenhouse study evaluating eight soybean varieties for sulfentrazone tolerance. How do universities rate sulfentrazone for crop injury potential? Hartzler was able to find four states that provided crop injury ratings for Authority Broadleaf/Canopy XL. Two universities (Penn State and Kentucky) rated injury potential with these sulfentrazone products the same as for Lexone/Sencor, whereas two (Nebraska and Illinois) rated Authority Broad- leaf/Canopy XL slightly safer than Lexone/Sencor. Iowa State University has evaluated sulfentrazone in approximately 20 studies during the last two years. While there have been crop responses typical of injury observed with many herbicides, there have not been any cases involving serious injury. Iowa State specialists acknowledge that Authority has the potential to injure soybeans; however, they have not ranked it as a product carrying significantly greater risk than many other products on the market.

Situations with significant herbicide injury usually involve other stress factors that reduce the crop's herbicide tolerance. While it is known that varieties and hybrids differ in their levels of tolerance to many herbicides, in most instances, environmental conditions play a much bigger role in crop responses than differential varietal tolerance. With sulfentrazone, injury has been greatest when rainfall occurs when the soybeans are at the cracking stage, therefore increasing exposure of the growing point to the herbicide. Symptoms include stunting, malformed and crinkled leaves and necrosis. It is difficult to accurately assess the injury potential for herbicides in small plots, even with multiple locations, since the environment plays such a large role in crop tolerance. Because of this, it is necessary to pool as much information as possible regarding crop tolerance when considering using a new herbicide. The information put together by Pioneer and other seed companies concerning variety/hybrid tolerance provides another source of useful information for assessing risks. In deciding whether to try a new product on their farm, growers should consider the benefits a new product has in terms of weed control compared to other strategies, as well as its potential for crop injury. If a new product provides little advantage relative to existing products as far as weed control, Hartzler feels it would be wise to let others experiment with the product and see how it performs on large acreages under a variety of conditions. Finally, it is always wise to evaluate any new product on limited acreage the first year or two until experience is gained with the product under conditions unique to each operation.

Johnson acknowledges the potential problems with sulfentrazone, but thinks that the need for a sulfentrazone product probably outweighs the crop response concern. He thinks that cool, wet, compacted soils dramatically slow soybean growth and contribute to herbicide injury for almost all soil-applied herbicides. Johnson indicates that this was evident in many fields in 1997. He says he did hear of some isolated instances of sulfentrazone-containing herbicides causing injury to soybeans last year. In many instances, the sulfentrazone-containing herbicide had been applied within 7 days of planting followed by cool, wet soil conditions and perhaps a major rainfall event as the soybean was emerging from the ground.

Sulfentrazone was developed by FMC and the first products were cleared for use in 1997 under the Authority trade name. FMC has chosen to develop several package mixes containing sulfentrazone, rather than marketing it as a single active ingredient product. In addition, FMC entered a marketing agreement with DuPont to develop products jointly. These decisions have led to some confusion as to exactly what products are available now or will be in the future. Products that include sulfentrazone are Authority Broadleaf, Canopy XL, Authority One-Pass, Authority First/Classic and Authority First/Synchrony. (Donald Null, 660-564-3363)