Even the ancient Romans knew there was something in some soils that made legume crops grow better, according to Evers.
"They didn't know what it was, but when they planted alfalfa on land that they had conquered, they knew to bring soil from where the crop grew well," he said.
Today, instead of importing soil, rhizobia are applied to the seed right before planting. A recently available alternative is to buy "coated" seed. The coated seed is pre-inoculated, then coated with a water-soluble clay, lime or talcum to protect the rhizobia from heat and sunlight, Evers said.
Agronomists have prescribed for decades that farmers inoculate their clover seed and other legumes at planting in the fall to ensure the best bacterial strains are present, Evers said. Though the bacteria can occur naturally, many soils are deficient or don't contain the most efficient strains, and there is not a simple test to determine how much or what type is present.
Today, many farmers still don't understand the science any more than the ancient Romans did and many either don't do a good job of inoculating seed or don't do it all, he said.
"Hence, the rationale for coated seed," Evers said. "There's also the labor-saving aspect of having the seed pre-inoculated."
Seed companies posit that seed germination and seedling vigor are greater for coated seed and therefore yields would be similar, according to Evers.
"To my knowledge, no one else in the U.S. has tested this theory," he said.
However, preliminary results of Ever's study indicated that when planted at the same seeding rate, coated clover seed produced lower yields than seed that was inoculated at the time of planting.
At issue is the number of seeds per pound of product, he said.
"The kicker is that the coating comprises 25 to 45 percent of the weight of a bag of seed," he said.
Therefore, because farmers pay about the same per pound for uncoated as coated seed, they may be getting only from three-quarters to half the number of seed per bag of coated seed compared to a bag of uncoated seed. But seed companies recommend the same planting rates in terms of pounds per acre of coated seed as with uncoated seed, he said.
In the first year of the study (2008-2009), using a standardized research design, Evers did not find any yield differences. But in the second year, per-acre yields for uncoated seed were significantly higher than coated in the first harvest for hay. There was no significant difference in yields for the second cutting.
Evers noted that the first year of the study, when yields didn't differ between un-coated and coated seed, was abnormally dry. The second year, when there was a significant advantage to using un-coated seed, there was adequate moisture.
Evers originally only planned to do a two-year study. But as the first year wasn't representative of average conditions, he plans to continue the study for a third year and plant this fall.
"That is, if Mother Nature gives us enough moisture," he said.
To ensure similar yields when using coated seed, farmers should calculate how many pounds of seed are actually present per 50-pound bag. The coating is listed on the seed bag label as the percentage "inert material." For an example, if inert material is listed as 25 percent, then a 50-pound bag will contain only about 75 percent seed or 37.5 pounds of seed.
"And they should adjust their planting rate accordingly," he said.
Alternately, farmers can save money by inoculating their own seed -- if they do it correctly, Evers said.
Too often, farmers fill up the seed boxes of their planters, sprinkle on some inoculant, then stir it around, he said.
The best method is to use a portable cement mixer, Evers said.
"Put a 50-pound bag of seed in the mixer, then turn the mixer on," he said. "With the mixer turning, pour in one to two pints of water. Let the mixer run until all the seed are moist, then add the bag of inoculant."
The mixer should run for 30 seconds to a minute in order to thoroughly coat the seed with the inoculant, Evers noted.
"Then put the inoculated seed back in the bag and let it dry before you put it in the planter," he said. "If you don't, it will not flow smoothly through."
--From Texas AgriLife news release
Related resources:
View AgriLife's YouTube video on how to inoculate clover seed.
