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Rating resistance

Buying soybean seed based upon soybean cyst nematode (SCN) resistance has been as foggy as a London morning.

You know the process. You wade through the resistant, moderately resistant, moderately susceptible, and susceptible SCN classifications and still wonder about a variety's SCN status. Those broad categories can make it difficult to decipher SCN resistance levels in each variety. At times, the current system can leave you crossing your fingers, praying that SCN will spare your beans.

Many times, it doesn't. SCN is the top-damaging soybean pest in North America; it causes lost yields of $1 billion/yr. The fact that SCN field populations consist of many races compounds the problem. Major races are Races 1, 2, 3, 5 and 14. Race 3 is considered the most predominant in the Midwest, followed by Race 1.

There's a better way to select for SCN resistance, according to Pioneer Hi-Bred International. Rather than using the resistant/susceptible rating, this year the company has introduced a numerical rating of 1 to 9 (1 is most susceptible, 9 is most resistant). This scale is the same one used to measure other maladies, such as iron chlorosis.

“It more accurately pinpoints the relative level of SCN resistance,” says Jeff Thompson, a soybean research scientist for Pioneer.

Here's the plan. The move is part of a three-year plan to assign numerical ratings for each Pioneer soybean variety. In 2001, only the varieties defined as “resistant” under the old system will contain the numerical rating for several races. In 2002, these same varieties will contain ratings for Races 1, 2, 3, 5 and 14. In 2003, all Pioneer varieties will have SCN ratings for the five major races.

Like the old system, the new system uses the Index of Parasitism (IP) to pinpoint the degree of SCN infestation. A variety's IP is determined by the amount of SCN reproduction compared with a susceptible benchmark variety, such as Lee or Lee 74.

The old IP scale consists of four levels: <10% IP is resistant (R); 10 to 30% IP is moderately resistant (MR); 31 to 60% IP is moderately susceptible (MS); and >60% IP is susceptible (S).

A wide range between categories is the old scale's flaw, Thompson says. Under the old system, Variety A with 11% IP and Variety B with 29% IP were both considered MR. However, Variety B had nearly three times the SCN reproduction level compared with Variety A.

The new numerical system still uses the IP. However, the gaps between classes are narrowed to 7 to 9% IP scores.

The rating switch will not affect price, says Dennis Judd, senior marketing manager for Pioneer. “The only way the price could be impacted is that if this adds any value,” he says. “Historically, there hasn't been any increase in price for SCN resistance.”

Other takers? Thompson has pitched the Pioneer rating system to several Midwestern nematologists. Thus far, though, other soybean seed companies plan to maintain the old system.

Stine Seed Company officials have discussed converting to a numerical system but plan to retain the present system for now. “We have some concerns about the consistency of the results and how we would apply numbers to those results,” says Chuck Hansen, production manager for the Adel, IA, company.

Monsanto also plans to retain its current system. It uses only the R and MR rankings. Any variety falling outside that range is unlabeled for SCN resistance.

“If a variety falls into the MS range, there's not much benefit to ranking it for resistance,” says Tim Schlueter, soybean product manager for Monsanto-branded seed.

However, Pioneer is committed to its new rating system. Thompson states, “It's consistent with our approach to provide growers with the right tools to make the best varietal choices for their farms.”

Pioneer soybean cyst nematode resistance scale

Index of Parasitism (IP) score

New 1 to 9 resistance scale

Traditional description

0 to 7%

9 — Outstanding resistance


8 to 15%

8 — Outstanding resistance

Resistant to moderately resistant

16 to 23%

7 — Very good resistance

Moderately resistant

24 to 32%

6 — Good resistance

Moderately resistant to moderately susceptible

33 to 40%

5 — Average resistance

Moderately susceptible

41 to 50%

4 — Below-average resistance

Moderately susceptible

51 to 60%

3 — Susceptible

Moderately susceptible

61 to 70%

2 — Susceptible



1 — Highly susceptible


Courtesy of Pioneer Hi-Bred International

Sidebar: Mark that gene

If soybean cyst nematode (SCN) is slicing your soybean yields, take heart. A tool called a “molecular marker” will help Pioneer Hi-Bred International researchers bring SCN-resistant varieties to market as much as two years faster than in the past.

“Think of a molecular marker as a road sign,” says Jeff Thompson, a Pioneer soybean research scientist. “It flags a segment of DNA for the genes for a specific trait.”

Once researchers identify molecular markers, they can use DNA analysis early in product development to screen for the presence of these traits. This reduces extensive trait-specific field testing, which can be very time-intensive and is less precise.

“Field characterization and identification of a complex trait can be tricky,” says John Soper, director of soybean research for Pioneer. “The ideal conditions — the combination of pathogen, environment and plant genes — to determine whether or not the trait is present do not always exist. This can extend the testing process and delay commercialization of products with valuable characteristics.

“If we know the gene with resistance is in the variety before we yield test, we have a greater probability of identifying higher-performing, resistant varieties in our field plots,” Soper continues. “We're now able to move our experimental lines into the field with complete confidence that they contain the traits we want. This allows us to focus our field-testing efforts on what we really need to do — test for yield and performance across a broad range of growing environments and conditions.”

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