* El Nino damages maize, rice, wheat yields - study
* Boosts soybean yields, Japanese-led study shows
* Big regional varieties within crop types
By Alister Doyle
OSLO, May 15 (Reuters) - The El Nino weather phenomenon that is likely to strike this year damages world maize, rice and wheat yields but boosts soybeans, according to a study on Thursday that could help farmers plan what to grow.
The Japanese-led report gave what it called a first global set of maps linking yields of major crops to El Nino, a warming of the surface of the tropical Pacific Ocean that can trigger downpours or droughts around the globe.
The maps are meant to help farmers decide which crops or varieties to plant and may give governments a "famine early warning system", the study in the journal Nature Communications said.
Most forecasts show an El Nino emerging in mid-2014, the U.N.'s World Meteorological Organization (WMO) said last month.
El Nino - Spanish for 'the boy' - forms every two to seven years and warning signs emerge months in advance.
Thursday's study said mean maize yields fell 2.3 percent in El Nino years compared to normal in 1984-2004, rice was down 0.4 percent and wheat 1.4 percent.
Soybean yields rose 3.5 percent, with rainfall patterns favouring big U.S. and Brazilian harvests.
In years with a La Nina event, the opposite of El Nino and cooling the Pacific surface, yields for all four dipped, according to the study by scientists in Japan, the United States, Britain, Australia and Denmark.
Lead author Toshichika Iizumi, of Japan's National Institute for Agro-Environmental Sciences, said farmers in Australia were among those who sometimes switched crop plans based on El Nino forecasts. And Indonesia, for instance, advised rice farmers to change planting dates, based on El Nino phases.
"I hope the finding of this study extends such efforts to national governments for controlling food storage, building food trade strategy, and earlier application of food aid in food insecure regions," he told Reuters.
The report found big variations for each crop. Soybean yields gained overall, for instance, but fell in India and parts of China in an El Nino year.
And maize yields, for example, suffered in the southeastern United States, China, East and West Africa, Mexico and Indonesia during an El Nino year, but rose in Brazil and Argentina.
Robert Stefanski, chief of the WMO Agricultural Meteorology Programme, said the regional impacts were most relevant since he said there was "high uncertainty" about global numbers.
"It is difficult to develop and use any reliable global impact on global crop production due to El Nino/La Nina," he told Reuters.
Even in vulnerable regions "more rain for Indian wheat can be beneficial if it falls during the middle of its crop cycle but if it falls during harvesting, it can be detrimental," he said.
Michaela Kuhl, a commodity analyst at Commerzbank, said more information about El Nino's links to crops was welcome. She noted the International Cocoa Organization estimated in 2010 that cocoa yields fall 2.4 percent in El Nino years.
But she cautioned "it's very difficult to show an influence of El Nino because it doesn't work with the same manner and same strength" from one event to another.
The U.N. panel of climate scientists said in a report last year that downpours linked to El Nino may intensify this century. It said there are big uncertainties about whether global warming will affect the frequency of El Nino and La Nina. (Additional reporting by Nigel Hunt in London; Editing by Mark Potter)
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