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Is rice seeding rate really that critical?

PORTAGEVILLE, Mo. — Unlike many other crops grown in the Mid-South, rice varieties have been developed mostly by university breeding programs rather than private companies.

The introduction of herbicide-tolerant (Clearfield) and hybrid (RiceTec) rice varieties has brought about the privatization of variety development and has led to increases in seed costs of some varieties. Registered conventional rice seed (Wells, Cocodrie, Cheniere) in 2004 at recommended seeding rates cost growers anywhere from $20 to $30 per acre; whereas CL161 (Clearfield) cost growers around $41 per acre. Hybrid rice, on the other hand, cost growers nearly $69 per acre at a seeding rate of 30 pounds per acre.

Increases in seed costs have led some growers to re-evaluate seeding rates.

Until recently, seeding rate hasn’t been given much thought because it wasn’t one of the more expensive inputs in rice production. I asked a grower in Arkansas County, Ark., about his seeding rate. He told me, “I plant 2.2 bushels per acre.” Why was for that particular seeding rate? “That’s what Daddy planted.”

He then went on to mention that he was planning to reduce that some this year. The upside to a planting rate like this is that he never had to replant. He did mention that he sprays fungicide for sheath blight nearly every year.

Arkansas, Louisiana, and Texas recommend a seeding rate of 40 seed per square foot for conventional varieties to achieve an optimum final plant population between 15 and 20 plants per square foot.

Research indicates that optimum rice yield with some varieties at low seeding rates can be achieved by increasing nitrogen fertilizer rate.

Other research has shown that in a dense rice stand as a result of excessive seeding rates, the incidence of sheath blight may increase.

Rice has the innate ability to compensate for voids in the canopy by producing more reproductive tillers. Since the introduction of hybrid rice, producers who have grown it have become accustomed to sparse stands after emergence, and then watched as the canopy was filled and sometimes unheard-of yields were obtained.

In a recent three-year study in Arkansas, we found that seeding rates ranging from 5 to 40 seeds per square foot did not affect yield of Wells, CL161, or XL8, indicating seeding rates for these varieties can be reduced while not sacrificing yield.

We are not suggesting that growers set their drills to plant 5 seeds per square foot. We are suggesting, however, that growers evaluate their current seeding rates and try a lower seeding rate on a small acreage. The benefits include a reduction in seed costs and savings in fungicide applications due to potentially less sheath blight.

With researchers from Arkansas, Louisiana, and Mississippi, we are evaluating the relationship between seeding rate and nitrogen rate for several new rice varieties on various rice soils. We hope to gain an understanding of the relationships among these factors so that more-precise, variety-specific seeding rate recommendations can be made in the future.

Brian Ottis is a rice agronomist with the University of Missouri Delta Center at Portageville, Mo.

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