When discussing or planning to plant rice, uniform seeding rates are often cited.
“Maybe you think, ‘I need to plant 70 pounds.’ Or, ‘I need to plant 90 pounds. I need to put out 150 units of nitrogen pre-flood.’ Everyone has these set numbers in their heads,” said Brian Ottis, Missouri Extension rice specialist at the recent Missouri Rice Farm field day outside Glennonville, Mo.
But is the seeding rate really set in stone? Ottis says it isn't.
In conjunction with research conducted with Ottis' counterparts in Mississippi and Louisiana, “what we're finding is the optimum seeding rate varies based on several factors.”
Dates and rates
One factor is the planting date, said Ottis while pointing at plot work behind him.
“This year, we've seen that play a major role. This rice was planted a bit later than most in the state. It went in May 7 to May 10. The majority of Bootheel rice was planted the third week to fourth week of April.”
At the Delta Center (in Portageville, Mo.), “we planted this same test April 17 into a stale seedbed on very heavy Sharkey clay. We didn't get nearly the emergence we got here.”
In Glennonville, the plots closest to the tour trailer were planted in Cheniere and CL131 at 15 seeds per square foot.
“Compare that to (the plots of) 60 seeds per square foot further back. I challenge you to go out there and find a difference between those seeding rates.”
Too often producers don't allow rice to grow to its full potential. “We're worried about having enough seed out. The mindset is often ‘more is better.’ But historically, university recommendations were to plant 40 seeds per square foot. That's aimed at a plant population of 15 to 20 plants per square foot. So they're saying, ‘half the seed you plant won't emerge.’”
Most seed germination is better than that. Under most circumstances, “especially when you consider fungicide treatments and other options, you can expect better than 50 percent germination. That number jumps if the soil temperature is allowed to rise.
“Now, the LSU and University of Arkansas recommendations have backed off to 30 seed per square foot for a final plant population of 10 to 15 plants per square foot.
“I agree with the 10 to 15 plants. That's a good number to shoot for. But do you still need to plant twice as much seed — or three times if you're trying to get 10 plants per square foot — as the plant population?”
With Wells or Cheniere or some of the conventional cultivars, “$7.50 to $8 per bushel of seed may not be such a big deal. But if you're planting CL131 at 60-plus pounds per acre, wouldn't you want to know if you can safely back off to 40 pounds? That would be a considerable savings.”
The disease question
Taking it a step further, Ottis and colleagues wanted to see what impact the seeding rate might have on disease severity and sheath blight incidence. The assumption was that with many plants, disease pressure is worse.
“When you walk into a field of a sheath blight-susceptible rice variety — a variety more susceptible than Wells — the first place you're likely to find disease is on double-planted ends and where fertilizer overlaps. In those areas, there's more vigorous, lush growth. Air circulation is impeded in the canopy and provides sheath blight with nice conditions for development.
“A lot of times when we see that in a field, we grab some Quadris or Stratego or Tilt and hit it.”
A test has been set up to check whether that's necessary. Maybe, suggested Ottis, there's a better way.
Earlier this summer, near midseason, there was a cool and rainy week. A week later, it was very hot and the disease slowed down. Were the sheath blight applications put out during the cool weather really needed?
“The heat might have slowed the disease, allowed the panicle to move up and come out of boot and we might have been able to outrun the sheath blight. Because 70 to 80 percent of the energy that goes into producing the panicle comes from the flag leaf. The flag leaf and the sheath below is what we need to protect for optimum yield. Now, if the bottom of the sheath is rotted out and lodging is looming, you need to treat.”
So applying fungicides in such cases may be unnecessary, especially on varieties like Wells.
“If you look at Wells, it has a very upright leaf angle. That allows air movement in the canopy.”
Also, when the panicle comes out of the boot, the plants seem to quickly increase in height. That can serve as an “escape mechanism. As it grows taller, it's outracing disease. Conversely, short-stature varieties don't have such escape abilities. Their leaves often lay flat out.”
So the researcher wondered if reducing the seeding rate would also lessen sheath blight incidence.
“Unfortunately, that's not the case. We found that because these varieties have the propensity to tiller so well, there's hardly any difference between high seeding rates and low seeding rates.
“With reduced seeding rates, keep in mind planting dates and soil type. Increase them a bit if you're on clay soils. Consider the economics. If it's going to be cheaper to plant 30 seed per square feet versus 15 with a seed treatment, it might be better to try that. A lot depends on the variety.”