Seed vigor — a seed's ability to make a stand and grow to maturity — is at least as important as germination rate, says Rick Cartwright, Arkansas Extension plant pathologist. Germination rates, which are based on standard tests performed under optimum germination conditions, don't tell a grower anything about the seeds' vigor, he says.
But recent research in northeast Arkansas, led by Plant Pathologist John Rupe, found that a technique called accelerated aging is a reliable guide to seed vigor. The research was sponsored by the Arkansas Soybean Promotion Board.
The goal of accelerated aging is to stress seeds before doing germination tests. Researchers collected about 440 seed samples from Arkansas growers' planting stocks. Seeds were heated in an incubation chamber to 105.8° F for 72 hours. Then the seeds were wrapped in special paper designed to promote germination and maintained at optimum germination conditions for seven days.
Afterward, the samples were evaluated to see how many seeds germinated successfully. Seed samples stressed by accelerated aging were also planted in test plots to see how they performed under field conditions.
CARTWRIGHT SAYS germination rates did not decline much from April through July, even when seeds were stored before planting. But vigor, as measured by the accelerated aging test, did decline. “It became apparent that two seed samples with the same germination rating didn't necessarily start with equal vigor,” he says.
“Germination rates are pretty stable,” says Jeremy Ross, Arkansas Extension soybean agronomist, “but vigor is sort of a moving number. Because the seed is a living organism, it has metabolic processes that continue even during storage. These processes may reduce vigor, especially if storage temperatures and humidity stress the seed.”
Accelerated aging was standardized and approved by the American Association of Official Seed Analysts and the International Seed Testing Association in the 1990s. Unlike germination tests, though, vigor tests are not required by the USDA.
“Seed companies are likely to use seed vigor tests voluntarily,” Cartwright says. “Because their reputation depends on the quality of their seed, they want to know all they can about their product.”
However, farmers don't always have easy access to that information, Cartwright says. “But, if they know what to ask for, some companies are willing to provide it to them.”
If you have seed with reduced vigor, is there anything you can do to improve stands?
Adjusting seeding rates, planting dates or using seed treatments to ward off pathogens may help, Rupe says. Especially for late planting dates in a double-cropping system, Ross says, it's important to get the stand up and the leaf canopy closed to help shade out weeds. If you know seed vigor is poor, he says, “you may want to increase the seeding rate, wait until planting conditions are closer to ideal or select a higher quality seed with good vigor.”