* Brazil's share of soy exports to China rose to 53 pct in 2017
* Analysts, industry see that growing again this year
* Brazil soy often cheaper, contains more protein
* U.S. exports hit with tougher quality rules from this year
* China needs soy to help feed its massive livestock herds
BEIJING/SINGAPORE, Jan 25 (Reuters) - Brazil's share of soybean exports to China, the world's top buyer of the commodity, grew to the largest on record in 2017 and looks set to expand again this year, helped by competitive prices and the high protein content of its crops.
That's another potential blow to rival exporter the United States as it grapples with tougher quality rules on its shipments to China from 2018, as well as with global markets flooded over the last few years by bumper supply of the oilseed.
China, which imports 60 percent of the soybeans traded worldwide, bought 50.93 million tonnes from Brazil in 2017, accounting for 53.3 percent of total purchases, according to customs data released on Thursday. Chinese buyers mainly use soy to churn out cooking oil and ingredients for animal feed.
U.S. sales came in at 32.9 million tonnes, or 34.4 percent of China's imports, the exporter's lowest share since at least 2006.
"Soybean imports from Brazil to China are expected to keep growing in the new year ... Brazilian beans will have an advantage in prices and protein (content)," said Tian Hao, senior analyst with First Futures in the Chinese city of Tianjin.
Brazil took over from the United States in 2012 as the top supplier to China, with its exports often attractively priced as it relies on overseas markets for sales due to limited domestic demand and storage. Brazilian beans also boast higher protein levels than U.S. supplies, making them more attractive for animal feed producers.
A weaker real also helped lure buyers in 2017.
Brazilian soybeans for March, including freight to China, are currently around $428 a tonne, compared to $433 a tonne for U.S. supply, according to trader data.
"From early May, we were mostly crushing Brazilian soybeans," said a manager at a crushing plant in southern China, declining to be identified as he was not authorised to speak with media.
"Last year, about 65-70 percent of the beans we crushed were from Brazil."
The United States is the world's biggest soybean exporter after Brazil, with the two countries accounting for roughly 80 percent of global shipments, which are valued at around $50 billion annually.
Brazil's share of the export market is on track to keep growing in 2018 as it gears up for a crop of around 114 million tonnes, matching last year's all-time high production.
Larger harvests mean Brazil's marketing season, which begins in May, has started to extend into October-November rather than ending around September as is traditional. That has eaten into the period when U.S. supply has typically dominated markets.
"Soybean inventories everywhere are so large that there is nothing like U.S. marketing season or South American marketing season (any more)," said a veteran soybean trader at one of China's state-run trading companies.
"Brazil has started harvesting new-crop beans while it is still not done selling last year's crop."
And Brazil's slice of the Chinese market will likely get another boost in 2018 as restrictions imposed by Beijing on U.S. shipments from Jan. 1 bite.
China introduced the stricter import standards, which have reduced the amount of foreign matter allowed in the most widely-traded soybean variety to 1 percent from 2 percent previously, after raising concerns about weed seeds.
"They are penalising U.S. beans," said Roy Huckabay, an analyst with Linn & Associates, a futures brokerage in Chicago.
Meanwhile, China's overall soy imports are seen climbing in 2018, boosted by growing demand from the nation's massive livestock herds.
With supplies bulging in key exporting countries, benchmark soy prices traded in Chicago averaged $9.75 a bushel in 2017 ($358 a tonne), compared to the $12 a bushel ($441 a tonne) average from 2010 to 2016.
"Chinese importers have been very savvy, the way they have been buying, getting the best quality at the right price," said Phin Ziebell, agribusiness economist at National Australia Bank.
(Reporting by Hallie Gu, Naveen Tukral and Gavin Maguire; Additional reporting by Karl Plume in Chicago; Editing by Joseph Radford)