|[Image: zinc definciency test plot]|
Has the cause of the mystery malady been solved? LSU AgCenter agronomist Dr. Dustin Harrell has a suspect in the form of zinc deficiency. But the jury is still out.
"For the past two years I’ve been able to go out to one location that had a severe case of ‘mystery malady,’" Harrell said.
The field had classic signs of the disorder, including dying plants, loss of tillers and bronzing leaves. He suspected a lack of zinc in the soil, so he conducted tests with rates of 0, 5, 10, 15 and 20 pounds of zinc per acre using granular zinc sulphate fertilizer per acre.
"We did see a response to zinc," Harrell added.
The first year the optimum amount was 5 pounds per acre, and this year plots that received 10 pounds per acre showed the greatest benefits from the supplement.
Harrell said different sources for zinc also will be studied. In addition, it remains unknown whether a field will have to be treated in subsequent years, he said.
"We don’t know how much residual is left from one year to the next," Harrell explained.
He said farmers historically have treated the mystery malady by draining fields and then re-flooding after the soil dried and the plants recovered, but that requires expensive pumping.
"The rice with the problem this year is the worst I’ve ever seen," Harrell said. "In the plots with no zinc, we had almost a 100 percent stand loss."
Harrell said fields with high soil pH, above 7, have more zinc problems. Soil containing less than 1 part per million of zinc should be considered deficient in the nutrient, he said.
In another field trial in Vermilion Parish, however, Harrell said a rice field with a soil pH of 6.8 and 2 ppm of zinc showed little to no response to an application of zinc.
Harrell is expected to make a presentation on his work at the Rice Technical Working Group in Biloxi, Miss., in February.
He also is working with researchers from the University of Arkansas, Texas A&M and Mississippi State University on a nitrogen soil test for rice. "We’ve found there has been a good correlation with the soil test and nitrogen rates on silt loam soils," he said.
Harrell said Arkansas scientists have calibrated the soil test to determine the amount of nitrogen that should be applied.
That calibration will be validated in field trials during the 2010 and 2011 growing seasons in Louisiana, Harrell said. If the results validate that the calibration can be used in Louisiana, verification testing will be done beginning in 2012 on farmers’ fields, he said.
"It appears that we don’t need as much nitrogen as we thought to maximize yields," he said.
The study in Arkansas has shown that the lush, emerald green appearance of a plant may not be necessary, he said. In fact, plants with a light green coloration may even yield the same or better than plants with a robust green color.
"We’ll try to see if that’s the case here in Louisiana," Harrell said.