What do seedless grapes and much of the rice planted this year have in common? Gibberellic acid. For several decades, gibberellic acid has been well-established as a growth stimulant for plants. For example, it is used to make Thompson seedless grapes larger.
As for rice, Richard Dunand, a scientist at the LSU AgCenter's Rice Research Station at Crowley, started studying use of the plant growth regulator 18 years ago.
Dunand established its effectiveness at improving rice seedling emergence and increasing the percentage of seed germination for fields planted by drill seeding. Now he's continuing his research to determine if new rice varieties respond favorably to treatment with gibberellic acid.
Gibberellic acid was approved for rice seed treatment in 1990, but back then there wasn't much demand for it in southwest Louisiana, since very few farmers were drill-seeding rice, Dunand recalled. Most farmers in the area preferred instead to plant seeds by airplane over flooded fields.
On the other hand, Dunand's research did benefit farmers in other rice-growing areas such as California and Arkansas, where drill seeding was a more common practice, he said.
Since 2001, with the commercial release of the Clearfield rice variety developed at the LSU AgCenter's Rice Research Station, many farmers in Louisiana have changed their planting techniques, leading them to start drilling with gibberellic acid-treated seed.
Farmers nationwide planted 200,000 acres of Clearfield last year, and that figure is expected to increase this year.
“You're going to have a lot of farmers drill seeding this year for their first time,” said Steve Linscombe, the LSU AgCenter's regional director in southwest Louisiana and a noted rice breeder.
Dunand said he started looking at the role of gibberellic acid in 1986 after the release of semi-dwarf rice varieties, 3 feet tall or so, that have limited growth rates throughout the rice season. “It's semi-dwarf all through its life cycle,” Dunand said.
A shorter plant is less likely to lodge, but that advantage of short stature once the plant matures is a disadvantage for the plant in the first stages of development. Getting the sprout to push through the soil surface in the first few days of germination is essential, Dunand said.
He said the amount of natural gibberellic acid found in rice and other plants is limited in semi-dwarf rice plants. The seeds for semi-dwarf plants need a boost to get the sprout growing quickly out of the soil.
“We found it was a very effective seed treatment for improving emergence,” Dunand said, adding that using gibberellic acid in agriculture isn't new.
Gibberellic acid is sprayed on Thompson seedless grapes to stimulate growth, Dunand said. “Without gibberellic acid, the grapes would get no larger than your little fingernail,” he said. It also is applied to flowering plants for larger blossoms, he said.
Gibberellic acid for agricultural use is derived from fungus grown in huge culture vats, Dunand said. Because it's organic, it's not dangerous like powerful acids, such as hydrochloric acid.
Seeds are pre-treated by soaking them in a small concentration of gibberellic acid.
If seeds are pre-sprouted before water seeding, Dunand said, gibberellic acid is removed by soaking, and seed companies are releasing all rice in treated form, making the assumption that all Clearfield will be drilled.
In drill seeding, the treated seed is planted deeply enough, perhaps 1 or 2 inches, within existing soil moisture, preventing a farmer from having to spend money to run a pump to flush a field.
Drill seeding with treated seed allows farmers to reduce the amount of seed required to get a stand of at least 10 plants per square foot, the LSU AgCenter scientist said. That's the minimum density of plants needed to get an acceptable crop, but he said it's good to get a slightly higher density as a cushion just in case something goes wrong.
The recommended seeding rate for drilled Clearfield 161 is 60 pounds to 90 pounds of seed per acre, compared to 90 pounds to 125 pounds per acre for water seeding. At the lower seeding rate, gibberellic acid has even more importance, Dunand said.
In fields where farmers have not been able to afford laser-leveling — those fields that might be plagued with low areas — gibberellic acid-treated seeds in potholes are able to emerge quickly instead of being inundated with flooding.
A study Dunand conducted with Linscombe and Russell Dilly Jr. determined that growers could plant gibberellic acid-treated Clearfield 121 and Clearfield 141 at a seeding rate of 25-30 percent less than the rates for untreated seed. The study also found that Clearfield emerged four to five days earlier with gibberellic acid treatment.
Gibberellic acid also helps fledgling seeds cope with cold weather which often accompanies early planting, Dunand said. By getting a jump start on the growing season, farmers likely will have an earlier harvest and an earlier second crop.
“The sooner you can get the ratoon crop growing, the more favorable harvest you will have,” he said. The goal is to get the ratoon growing before the days get shorter and cooler, he said.
Dunand's work continues, even though gibberellic acid's advantages have been well-established. “We're still working with it and other plant growth regulators,” he said.
Studies are ongoing to determine its effectiveness at improving panicle development and to see whether it can directly increase yields. Dunand also is conducting a seedling vigor study that will look only at experimental breeder varieties' response to gibberellic acid. “There are six rows here, and each row has a different variety,” he said during a visit to a test plot.
In one of the rows with gibberellic acid-treated seed, small green threads of rice are starting to pop out of the ground. In the next row, sprouts have yet to emerge. “See how short they are? That's with no gibberellic acid.”
In general, long-grain rice is more responsive to gibberellic acid than medium grain, Dunand said, and semi-dwarf varieties are more responsive than taller varieties.
The LSU AgCenter researcher also is studying the effects of early planting with gibberellic acid, compared to planting the same varieties in April.
Dunand said many of the things he does in the test plots, such as planting seed 3 inches deep, are not typical. “I would never recommend it to a farmer. That would be heart-attack material. But I do it because I want the most stressful planting situation,” he explains. “I figure that if the gibberellic acid can help the rice emerge from 3 inches, it can probably help under most planting conditions the farmers will experience.”
Dunand said he appreciates the risks farmers take every year, and he hopes his work can help put some of the odds in their favor. No two growing seasons are alike, he said, and unforeseen variables create new challenges.
“I like the unknown. That's what makes me get up every morning to go out to the test plots and work on research.”
Bruce Schultz writes for the LSU AgCenter. (337-788-8821 or firstname.lastname@example.org).