* Soymeal prices climb almost 40 pct in less than a month
* Buyers, traders on wrong side of surging prices
* Several China buyers yet to fix prices for booked cargoes
* Vietnam buys hand-to-mouth, seen most vulnerable to rally
By Naveen Thukral
SINGAPORE, May 13 (Reuters) - Several Asian traders and end-users of soymeal will be forced to pay sharply more for the key animal feed ingredient after being wrong-footed by an almost 40-percent surge in prices in less than a month.
Crop-damaging rains in major soy grower Argentina and a drop in potential U.S. plantings began pushing prices higher from early April, with the rally fanned as hedge funds flipped a record net short position in March to a 54,000-contract long position.
But, until a U.S. Department of Agriculture report on Tuesday that slashed the outlook for major exporters' year-end soybean inventories, traders and feed mills had largely stuck to the view that world supplies would remain ample.
"A lot of trading houses were calling this rally wrong before the USDA report. Now they have had their faces ripped off," said an Australia-based agricultural commodities analyst, declining to be identified as he was not authorised to speak with media.
Prices of soymeal, used to fatten pigs, cattle and poultry, have jumped around 37 percent to $367.60 a short ton this week from $268.80 at the beginning of April. Soybeans soared to a 21-month high after the USDA report.
Importers in Asia, home to the world's fastest growing population of soymeal consumers, are dependent on supplies from Argentina, Brazil and the United States.
The region consumes half the world's soymeal production and contains seven of the top 10 global buyers.
Millers in China, which saw soybean imports climb 33 percent in April from a year ago, have covered supplies until June, but many buyers have yet to fix prices of the cargoes they have booked, traders said.
Typically, buyers fix the 'basis' of the cargo, or the premium to be paid over a benchmark such as Chicago soymeal futures, at the time of signing the deal but leave the final price to be set later - in some cases just weeks before the shipment arrives.
Vietnam, Asia's biggest soymeal importer, could be most vulnerable to a sharp rise in meal costs as it buys cargoes just a couple of months in advance.
Thanks to rapidly increasing consumption of animal protein, the country has seen imports more than double in the past four years to 4.6 million tonnes in 2015/16.
"I think more than half of the buyers in Southeast Asia have yet to fix prices," said one Singapore-based trader.
He estimated some buyers in Indonesia and Malaysia had booked supplies right up to August, but most shipments had not been priced.
In previous times of tight supplies from South America and the United States, Asian buyers have turned to India, but the South Asian nation has been absent from the market over the last few years as it accommodates its own rising domestic consumption.
"There is reduction in supplies and we have the U.S. growing season to go through," the Australian analyst said. "We don't see prices declining much until September."
This year is likely to mark the first reduction in world soybean output following three years of record production that saw supplies rise by about a third to 319.7 million tonnes in 2014/15.
Reporting by Naveen Thukral; Editing by Gavin Maguire and Joseph Radford