Since it’s still early in the growing season, Mid-South rice farmers may want to check micronutrient levels — particularly zinc — available in their fields. Research out of Louisiana suggests a common problem may be linked to the fertilizer.
When he came to work for the LSU AgCenter nearly three years ago, Dustin Harrell heard plenty about “localized decline” and “mystery malady” from Louisiana rice farmers. Stories abounded about an unknown menace taking out plants and tillers early in the season.
“The basic complaint: early-season stand declines that, in many cases, had no apparent cause,” says the assistant professor and rice agronomist at the Rice Research Station in Crowley, La. “Once a permanent flood was put on — delayed flood rice, about four weeks into the season — complications in the field would increase. That led to many plants dying, thin stands and, ultimately, low yields.”
One of the more common symptoms was brown lesions on the plant.
Other researchers had already tried to find the cause of the problem. “Work had looked at silica and other nutrients. A high uptake of iron in the plant tissue was observed in a lot of cases.”
However, after that research, nothing was definitely fingered as the culprit.
After studying the situation, Harrell says it appears localized decline is more than one problem. “One operation may have one problem issue while another has a different nutrient problem. All of it was being thrown together under ‘localized decline.’”
Harrell’s belief was bolstered after recent plot work. In one south Louisiana location, “I reviewed soil test information, checking the pH. The pH was high at 7.8. I theorized that this wasn’t a mystery malady but a zinc deficiency. Zinc had been overlooked a little even though it’s an important fertilizing nutrient for rice.
“So, I put together a zinc study at the field. Zinc ratios of 0, 5, 10, 15 and 20 pounds per acre were applied through zinc sulfate. As the season progressed, sure enough, the check plots showed bronzing symptoms and the death of tillers. Those spots had very thin stands.”
However, where zinc was applied the stands were healthy. At the end of the season, there was also a positive yield response. Tissue samples of the poorly-performing plots showed the plants were, in fact, zinc-deficient based on tissue concentrations.
“So, my thoughts are that a number of things are causing stand decline and being blamed on ‘mystery malady.’ However, it seems a large portion may be due to forgetting the importance of micronutrients like zinc. There have been such high prices, fertilizer applications have been cut.”
Last year’s prices certainly didn’t help slow the trend. Harrell knows some farmers who are “cutting back on P and K. Understandably, if they’re cutting those, the micronutrients are paid even less mind. Those will be the first knocked off the list.”
Harrell is careful to say that micronutrients — zinc in particular — aren’t the problem in all the localized decline fields. In a good portion of them, though, “I think that is the problem. Low zinc and high pH soils aren’t a good mix with rice.
“If a farmer has a high pH soil — say, above 7 — even though a soil test says the nutrient is available, the high pH will cause it to be insoluble and unavailable to the plant.”
Was there any difference between the 5-, 10-, 15- and 20-pound zinc rates?
“No. The 5 pound-per-acre rate of actual zinc (about 14 pounds of zinc sulfate), took care of the problem. Higher rates of zinc increased uptake but didn’t improve yields.”
Before this research what were farmers trying to correct the situation?
“All kinds of things!” says Harrell. “There were many chemical companies pushing products. Different tests were done.
“I had a pretty good idea there was a zinc connection. The symptoms were typical for that and soil pH was high. What I did differently is apply the nutrient before the problem arose. We shut it down. Everyone else was trying different things after the problem was evident.”
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