Richard T. Dunand, Steven D. Linscombe and R. Russell Dilly Jr.

Treating rice seed with gibberellic acid has improved rice production in the northeastern rice-growing area. On semi-dwarf varieties gibberellic acid improves seedling vigor by hastening emergence and increasing seedling population. With the advent of herbicide-tolerant rice, this treatment offers an opportunity to improve rice production in the major rice-producing region of southwestern Louisiana.

Gibberellic acid is currently not used much in southwestern Louisiana because rice treated in this way must be sown in a dry field. This requires a planting method called drill seeding, which involves using a seed-grain planter that places the seed just below the soil surface.

Most rice in southwestern Louisiana is sown in water or on wet soil, with seed being placed on the soil surface. Unlike drill seeding, water seeding allows early flooding and uses water management to suppress red rice, a noxious weed for which there had been no postemergence herbicide control before herbicide-tolerant technology.

Herbicide-tolerant rice can withstand certain herbicides that kill rice varieties and red rice. This new technology will permit more drill seeding in southwest Louisiana. Depending on seed supply and governmental approvals, herbicide-tolerant rice will be a useful technology for controlling red rice. It is possible that several thousand acres of herbicide-tolerant rice will be drill seeded in Louisiana and other southern rice-growing states in 2002. Since herbicide-tolerant rice is derived from semidwarf parents, it is semidwarf in nature, prompting evaluations of the response of herbicide-tolerant rice to seed treatment with gibberellic acid.

Small Plot Tests
Two imidazolinone-tolerant (Clearfield) entries (designated CL-121 and CL-141) and two glufosinate-tolerant (Liberty Link) entries (designated LL0001 and LL0401) were evaluated for their response to seed treatment with gibberellic acid in 2000. Two studies were planted about one month apart—one on March 17 and the other on April 18. Seed of CL-141 was limited and included only in the first study. Each study was an identical comparison of seed treated with a standard fungicide and insecticide and seed treated additionally with gibberellic acid. Seed were drill planted at a rate of 100 pounds an acre and to a depth of 2 inches below the soil surface into a prepared seedbed. At this depth, seed were placed into moist soil containing sufficient moisture for germination. Seedlings grew and emerged, relying solely on existing soil moisture present at planting and rainfall following planting. There was no surface irrigation (flushing), which is a normal practice in rice production. Five rainfalls were spread evenly over the four-week evaluation period after the March planting and totaled 2.5 inches. Rainfall after the April planting consisted of 3.5 inches that fell during a four-day period slightly more than two weeks after planting. It was associated with a late spring cold front.

Two parameters, emergence and final stand, were measured to evaluate seedling vigor. Emergence was the number of days from planting until a seedling population of 10 plants per square foot was reached. This population density is considered the minimum necessary for optimum production. Final stand was the number of plants per square foot at 25 days after planting.

Seedling Vigor Improvement
Seedling emergence of the four herbicide-tolerant entries was quicker when seed treatment with gibberellic acid was used, and the Clearfield entries were more responsive to gibberellic acid than the Liberty Link entries (Table 1). In general, the Clearfield entries emerged four to five days earlier with the gibberellic acid seed treatment compared with two to three days earlier for the Liberty Link entries. There were two exceptions. Emergence of both Liberty Link entries planted in March was not significantly affected by gibberellic acid.

Final stand was improved by the seed treatment with gibberellic acid in much the same way as seedling emergence was (Table 2). The Clearfield entries were more responsive than the Liberty Link entries, and LL0401 was the least responsive. CL121 and CL141 planted in March had 25 percent to 30 percent higher final stands from the seed treatment with gibberellic acid. Also, CL121 planted in April had a twofold increase in final stand with gibberellic acid; without gibberellic acid, final stand was less than optimum (fewer than 10 plants per square foot). In the Liberty Link entries, stand was increased by 60 percent in LL0001 planted in April. Final stand was not significantly changed by seed treatment with gibberellic acid for LL001 planted in March and LL0401 planted in March and April.

The Clearfield entries were long-grain types, and the Liberty Link entries were medium-grain. In previous research with standard commercial rice varieties, semidwarf long-grain varieties have typically shown a higher response to seed treatment with gibberellic acid compared with semidwarf medium-grain varieties. The herbicide-tolerant entries appear to follow this same trend.

Benefits to Rice Industry
These results show that gibberellic acid will allow rice growers to plant Clearfield rice at seeding rates 25 percent to 30 percent lower than currently recommended for drill-seeded rice not treated with gibberellic acid. A reduction in seeding rate can provide two benefits: lower costs and more acreage with less seed. A lowered seeding rate can reduce the cost of the herbicide-tolerant technology, because its cost will be based primarily on price of seed, which will be higher than standard varieties. Using gibberellic acid to reduce seeding rate will allow more acreage of herbicide-tolerant rice to be planted. As with the release of any new rice variety, initially a limited amount of seed of the herbicide-tolerant rice will be available. Drill seeding at a reduced seeding rate, facilitated by seed treatment with gibberellic acid, will allow more acres of herbicide-tolerant rice to be planted.

(This article was published in the winter 2002 issue of Louisiana Agriculture.)