Many growers have been hesitant to plant varieties with resistance to soybean cyst nematode (SCN) because of the potential for so-called yield drag. This has been especially true for fields with no visible signs of SCN damage.
But according to a recent on-farm study at the University of Illinois conducted with funding from the Illinois Soybean Checkoff Board, nearly all growers in the state would probably benefit from planting SCN-resistant varieties.
"There has been a lot of concern by growers because of the history of resistant varieties not producing well on ground that does not have SCN," said Greg Noel, USDA nemotologist in the Department of Crop Sciences at the U of I. "The reality is that there are probably very few fields in Illinois that do not have at least some cyst nematodes in them."
Noel has conducted two years of on-farm trials at seven locations in central Illinois and western Indiana on fields ranging from no infestation to moderate levels of infestation by SCN. Each field was planted with a susceptible variety, a variety with low levels of resistance from the Fayette line, a variety with higher levels from the same line, and a variety with the Peking line of resistance.
"These trials were carried out on farmers' fields, and all operations were done with their own equipment," Noel said. "Each variety was planted the full length of the field, randomized and replicated three times. Yield was determined with a weigh wagon, and moisture was corrected to 13 percent."
Although yield-drag up to 10 percent from resistant varieties was confirmed in fields with no nematodes, the study pointed to a major difficulty in making the determination that nematodes are not present without extensive testing.
The study further confirmed that planting resistant varieties most often paid off in fields with even low levels of SCN infestation.
"At some locations where we found no nematodes prior to planting, we sampled that same site at harvest and found fairly large numbers of nematodes," he said. "That means there is a lot of error in undertaking the common methods used in sampling for nematodes."
He notes that growers can have yield losses as high as 10 to 15 percent from nematodes in fields where there are no outward signs of damage. In most cases, those fields appear to the eye to have completely filled-in rows and to be growing perfectly well.
"Part of the dilemma that growers face is the fact that there is no accurate way of telling what numbers of nematodes they have in a particular field," Noel said. "In our experiments, we took four samples of 20 cores in each round, for a total of 48 samples and 960 soil cores in each field.
He points out that current recommendations for sampling just do not adequately represent what's in the field.
"It's not anyone's fault," Noel said. "In order to rectify the situation, a grower would have to spend a lot of money and take a lot of samples, which is just not worthwhile economically."
According to Noel, growers who have any history of SCN problems should always plant SCN-resistant varieties. Even those with no clear signs of a problem should also strongly consider planting a resistant variety as a form of no-cost insurance.
"Although yield drag does occur in fields with zero or low nematode populations, the problem is trying to determine whether or not you have a low population," he said. "The troubling thing is that you can have a fairly significant yield loss from SCN and not even know you have it. In those cases, a resistant variety would certainly pay off in higher yields."
Despite the problems with sampling, Noel suggests that growers can take some simple steps to improve their chances for selecting the proper variety for their fields.
"One way is to monitor the yields as you move through a field," he said. "By checking the yield monitor, you can look for drop offs and then flag the area. Then you can come back to those areas and take samples for SCN during the fall."
Noel adds that valuable information is also available from the Varietal Information Program for Soybeans (VIPS) maintained at the U of I. This database provides unbiased information on soybean varieties from a wide range of companies, including evaluations of resistance against various nematode populations. The varieties entered in the trials were tested at 13 different sites around the state of Illinois.
There were 134 conventional varieties and 661 Roundup-resistant varieties from 70 companies in the 2003 soybean trials. Besides those entered by participating companies, the total number of soybean varieties included 244 that were nominated by Illinois farmers and entered directly by the Illinois Soybean Checkoff Board.
"Growers can easily use this invaluable resource to help make an informed decision on what specific varieties to plant," Noel said. "I would suggest finding the test location near where a grower lives and identifying a resistant variety that has the highest level of resistance and the highest yield."
The information from the VIPS database is available on the Internet at http://www.vipsoybeans.org/.