Giant piles of black manure towering over cornfields, while rancid-smelling effluent from thousands of cows spills onto the land -- this is the price of a glass of milk in China today.

Large-scale dairy farms have boomed in the Asian giant, as its near 1.4 billion consumers overcame centuries of cultural reluctance to embrace the white fluid.

An economic boom and government backing transformed dairy into a $40-billion-a-year industry, shifting production away from small-scale producers towards massive megafarms with up to 10,000 cattle -- and a lot more waste.

"The smell of the manure... in the summer it's very intense," said Ren Xiangjun, a farmer in Gannan county.

Pointing at a stream of green water escaping from under a grey brick wall at the giant farm owned by agro-conglomerate Feihe International, he added: "You can see how it flows right out of the farm.

Dodging packets of animal medicine and syringes littered nearby, he explained: "The rubbish left after injections is just thrown here. My land is directly affected."

When the Feihe farm opened in 2012 in the grassy hills of the northern province of Heilongjiang it said it had 10,000 cows.

In Daxing village next door, a woman also surnamed Ren said: "You can see the manure piled up like a mountain. There are no advantages for us. There is just pollution and noise."

The dairy industry in China has posted average yearly growth rates over 12 percent since 2000, due to rising wealth and desire for the health benefits of calcium.

The ruling Communist party fanned the expansion, with former Premier Wen Jiabao in 2006 expressing a "dream" that Chinese children should enjoy a daily 0.5 kilograms of dairy products.

But a 2008 scandal over baby formula tainted with the industrial chemical melamine saw six children killed and more than 300,000 others affected, shaking confidence in the industry.

The crisis was blamed on small-scale farmers using chemicals to inflate the protein content of their milk as they scrambled to meet demand.

The Chinese government responded by demanding the creation of large-scale milk production units.

"They thought if we have scale farms they are easier to regulate and inspect," said David Mahon, founder of a Beijing-based investment firm specialising in dairy.

- Unbearable -

There are large scale farms in other countries, such as New Zealand, but rarely with more than 3,000 cows at a single facility.

By 2014 China boasted 56 farms with 10,000 cows or more, according to state media -- 80 percent of the global total -- creating a string of pollution problems in several provinces.

Estimates say that just 3,500 cows can produce 100,000 tonnes of fluid waste and effluent a year. Chinese farms are required to process it into fertiliser, but regulations are often flouted.

"There are some areas of China that it's better to visit in winter, because of the small hills of effluent. Once it thaws it's unbearable," said Mahon.

"China is learning about dairy farming and the lack of experience has resulted in such things."

In Gannan residents alleged that local officials profited from the farm and took no action against polluters. AFP was not able to verify the claims and local food officials could not be reached for comment.

But attitudes may be starting to shift. The vice-head of China's state-backed Dairy Association, Yang Liguo was cited in 2014 as saying "The bigger the scale, the bigger the environmental, pollution and biosecurity problems".

Mahon said there had been a "genuine rethink" in Beijing and the Chinese government was "looking more and more to 350 head farms".

- Like a mountain -

Packing more animals together increases the risk of illnesses such as brucellosis, which can spread to humans and cause arthritis.

Feihe employee Wang Dali, who once mucked out cowsheds at the farm in Gannan, contracted brucellosis in 2012, leaving him unable to work, and now suffers near-constant pain in his joints.

He blames his infection on poor sanitation.

"The cows were packed very close together," he said, estimating each had about 12 square metres. "There was no way to treat the manure. We dug a big hole close to the facility... now it has piled up like a mountain".

Feihe denied the residents' allegations, with a woman who answered the phone at its Gannan office saying: "These things are impossible."

Dismounting from a tractor a stone's throw from the manure piles in Daxing, one farmer said: "The pollution hasn't been cleaned up well. Of course it has an impact."

Pointing to corn stalks growing beside syringes, he added: "We don't eat these ourselves. We sell them to the market".