KIMBERLY, Idaho—Too much dairy manure can be bad for potatoes and sugarbeets, but those crops can likely handle broader use of the rich nutrient source than growers may realize, new research suggests.
University of Idaho Extension soils specialist Amber Moore has completed the second season of an eight-year rotational study examining how raw manure affects rotations with wheat, potatoes, barley and sugarbeets. Year two was funded with $100,000 from USDA and Idaho commodity groups.
Idaho growers typically avoid manure in rotations immediately before potatoes and sugarbeets, concerned overusing manure could hurt yields and quality in spuds and lower sugar levels while elevating toxins, such as nitrate, in beets.
“I think knowledge is definitely power in this case,” Moore said. “My gut feeling was (manure) did have its place, but we had to figure out where that sweet spot was.”
Moore supplemented each treatment with enough conventional fertilizer to meet nutrient needs.
The largest tuber size and best spud yields came from the highest manure rate, 60 tons per acre, prior to wheat with only conventional fertilizer applied during the fall before potatoes. Yields were 527 hundredweight per acre, 13 hundredweight per acre above fertilizer only for both seasons. The lowest manure rate, 20 tons per acre, applied before both the 2013 wheat crop and this season’s spud crop, was also effective, yielding 521 hundredweight of spuds per acre.
When she applied 60 tons per acre for consecutive seasons, however, spud yields declined, to 452 hundredweight per acre, and more tubers were small and misshapen. She suspects salt accumulation from consecutive years with high manure rates hurt potatoes.
Testing by UI Extension storage specialist Nora Olsen found the study’s spuds had less starch and dry matter after consecutive years with high manure rates, affecting texture and fluffiness. Annual manure treatments also seemed to darken fry color near potato ends, demonstrating stress to the tuber, Olsen said. She cautioned a single year of data can raise questions but is insufficient for drawing firm conclusions.
In beets, a salt-tolerant crop, yields were up by at least 4 tons per acre with every treatment involving manure. At the highest rate—60 tons per acre both following 2013 barley and prior to this season’s beets—sugar concentration dipped to 14.8 percent, likely because late-releasing nitrogen from manure stimulated vegetative growth at the expense of sugar accumulation, Moore said.
Her best overall results came from applying 40 tons per acre of manure before grain and taking a year off before beets. Yields reached 34.9 tons per acre, compared with 30.1 with fertilizer only, and sugars reached 16.5 percent. She also had good results—yields of 34.9 tons per acre and sugars of 16.1 percent—when she applied a low rate of 20 tons of manure per acre in consecutive years.
American Falls grower Jim Tiede prefers dairy compost, reducing the potential for introducing weed seeds, nematodes and disease. He applies 7 tons per acre of compost prior to spuds and lets beets use the residual nutrients the following season.
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