CHICAGO, Jan 19 (Reuters) - Farmers have ramped up construction of swine farm buildings in Iowa, home to about a third of U.S. hogs, as they take advantage of cheap feed and strong demand for pork from countries including China and Mexico.

Some are also looking to add space for the heavier pigs currently favored by customers and to cater for extra piglets as sows become more fertile, experts said.

But with hog prices languishing near six-year lows and profits slashed from the boom year of 2014 when Porcine Epidemic Diarrhea virus (PEDv) ravaged supplies, adding numbers to the U.S. herd could cut incomes further this year from 2015, experts say.

Last year, Iowa farmers built 280 hog barns capable of holding more than 1,250 head, over 60 percent more than 2014 but shy of the 347 constructed in 2012, according to data recently released by the state’s Department of Natural Resources.

“There is a demand from our customer for heavier market weights, 5-10 pounds heavier than two years ago,” said Allen Whiley, who oversees raising hogs at Iowa Select Farms, one of the state’s biggest producers.

“At the same time our sow productivity has increased, meaning there are more pigs weaned and marketed per sow,” he added. U.S. sows on average gave birth to a record 10.53 piglets in a litter in the September-November quarter.

Josh Flint, a spokesman with pig farmer and pork producer The Maschhoffs, said the company had contracted with some Iowa family farmers for new buildings where pigs are fattened up for slaughter, called “finishing barns.”

The company itself, which says it produces enough pork to feed 16 million Americans a year, did not expand in 2015 and has no plans to do so this year.

The U.S. hog inventory hit 68.3 million head as of Dec. 1, the highest since 1988 when the U.S. Department of Agriculture started collecting data.

“Generally, I think the farmer who partners with us to build a finishing barn is looking to diversify their source of farm income,” said Flint.

Farmers view building a hog barn as an effective way to increase income without having to spend a lot of cash to expand their corn and soybean operations.

But they may be disappointed. Pork profits are already weak and could head lower this year, Iowa State University economist Lee Schulz said.

ISU estimates that Iowa producers who take hogs from birth to slaughter made a profit of $7.93 per head in 2015 - little better than a tenth of the $61.85 per head in 2014 when PEDv-constricted supplies sent hog prices climbing.

This year, farmers could struggle to make any profit in the first half, the university predicts, with annual earnings sliding to $5.41 per head.