The latest skirmish in the United States food wars erupted last month, when Washington State University announced it had dumped a common reading programme in which first-year students would read The Omnivore's Dilemma, Michael Pollan's scathing critique of agribusiness.

Officials, who had bought 4000 copies of Pollan's book, a bible for the organic and locally grown food movements, cited budget cuts. The blogosphere erupted, with critics talking darkly of political censorship by agribusiness. When Bill Marler, a local lawyer who has litigated against agribusiness, offered to pay Pollan's fee to speak at the university, it backed down.

But if the grassroots are on fire - Pollan wants President Barack Obama to reform the "entire food system" - then Big Food shows no signs of surrendering. Last year biotech giant Monsanto, which markets genetically modified (GM) seeds and the herbicide Roundup, began an advertising campaign that stressed its sustainable credentials. "How can we squeeze more food from a raindrop?" one ad asked, suggesting the solution to hunger and water scarcity was genetically modified food.

For a war is raging over what we eat, where it comes from, who benefits, and the cascading environmental impacts caused by global food chains. Its footsoldiers are grassroots activists, writers and agribusiness marketers.

It is a war for the hearts and minds of consumers and policymakers, with food a key factor at the connection between climate change, health care, energy use, water scarcity, collapsing ecosystems and hunger.

It is also a struggle over the identity of food. Is it what you buy at the supermarket - Pollan's advice is to never buy food that you've seen advertised - or what you can buy at a farmers' market or grow in your backyard?

In this analysis it's a battle between the grassroots and the corporate food business, with the latter the biggest trencherman at the table. And critics claim that agribusiness is indulging in "greenwash", by dressing up its products with green language, without regard for any negative impacts on the environment

A key word in this debate is "sustainability". It sounds laudable enough. But one man's sustainability, when it comes to this contentious food fight, is another's poison.

At heart it is a clash of two irreconcilable farming systems: one seeks sustainability via harmony with nature; the other seeks to impose a technological fix.

Agribusiness has used sustainability to push those technical fixes that it says are critical to enhancing food production in the 21st century.

Critics, who have growing consumer support, believe sustainable farming necessitates root and branch reform.

In this context words like "sustainable", "natural", and "local" - emblazoned on supermarket products - are indicators of the power struggle over what we eat.

The fight is exacerbated by fears for food security - how can harvests be guaranteed in the face of climate change?; for safety - the H1N1 swine flu pandemic is the latest scare; and by public health concerns, especially links between Western diets and a growing epidemic of diabetes and obesity.

Rebel hopes intensified with Obama's election and expectations that he will tackle health care, energy use and climate change - all issues that spill into the food arena.

Even innocuous events trigger seismic activity between opposing views on agriculture. When Michelle Obama decided to plant an organic vegetable garden at the White House, to emphasise her belief that Americans should eat fresh, unprocessed and locally grown food, rebels were thrilled but agribusiness sensed danger. In a cackhanded PR move, the Mid American Croplife Association, which represents pesticide companies, wrote a letter reminding the First Lady of the role "conventional agriculture plays in the US". Use chemicals, Michelle.

The White House garden quickly assumed iconic status for opponents of agribusiness - the National Gardening Association says seven million more Americans will grow veges and fruit at home this year, up 19 per cent from last year - and was embraced as a harbinger of change by reformers.

And as turbulence rocks the car, energy and financial industries, agribusiness has noted the success of organic foods, which, while they account for only 3.5 per cent of total US sales, rose 16 per cent between 2007 and last year to reach US$22.9 billion ($36.5 billion). The underlying trend is clear: pegging your product as "sustainable" or "local" can attract the green dollar.

Thus, eco-conscious shoppers trolling US supermarket aisles may have been surprised to learn that Frito-Lay potato chips were made locally, a clear bid to attract souls who shop for US grown food. But Frito-Lay's claim was disingenuous; all food is "local" in some sense. Or, as Pollan quipped, "it doesn't come from Mars".

Then there's stealth marketing, when companies that sell organics start non-organic lines with similar packaging. "Our suspicion is that they are trying to ride on the assumption, that consumers have, that the products are organic," says Alexis Baden-Mayer, political director of the Organic Consumers Association.

Or products can mimic a proven product. Witness the brouhaha that ensued when an English producer charged £55 ($142) for small pots of Cornish "manuka" honey - which New Zealand producers dismissed as unlikely to bear much resemblance to genuine manuka honey.

One thing that helps the food marketers - who Pollan says burn through a US$32 billion budget - is that "sustainable" has no official weight with the US Department of Agriculture, unlike the word "organic".

"No one knows what 'sustainable' means. It's not defined," says Marion Nestle, the author of Food Politics and a nutrition professor at New York University. "It's the only part of the food industry that's growing [although the recession has slowed growth]. The food industry is in terrible trouble."

Nestle says there is "enormous distrust" of the US food industry, especially as regards food safety. "In this country there's nothing to reassure people that somebody's minding the store in a serious way."

The US Centres for Disease Control estimates that the bulk of the US$1.5 trillion the US spends on preventable diseases - out of a US$2 trillion annual healthcare budget - involves food-related issues.

"It's too soon to tell what will happen," says Nestle. "But the food industry has caught on to the fact that the only way they can sell food is by having it appear nutritious, good for the environment, or good for farm animals."

But like the battered US car industry, agribusiness may have hit the wall with an unsustainable business model.

"We have twice as much food available in the US as is required by the population," says Nestle.

"On top of that they have to grow every 90 days - difficult when there's a surplus - for Wall Street expects publicly traded companies to post a quarterly profit."

This is a big problem if your product is junk food. "You can't convert it to a health food. You can only make it look like a health food."

All of the companies in the anti-sustainability business want to make it seem as if they are the best and brightest," says Doreen Stabinsky, Greenpeace USA's agriculture campaigner.

"It's an old strategy by industry, to take over the words of alternatives or water them down."

Thus, while food activists blast Big Food for waste and environmental degradation, Monsanto, Syngenta, Nestle and other corporate giants all tout the "S" word on websites.

"I recommend to you the 'Six Sins of Greenwashing'," says Scott Exo, executive director of the Food Alliance, referring to a website hosted by Terra Choice. "It enumerates the common missteps made by marketers. It's a good guide for evaluating claims made by food products."

And while some food companies are "sincere" in their efforts to be more sustainable - Exo cites Starbucks, which releases a carbon footprint report and uses recycled materials in cardboard cups - others, through greed or naivety or both, "put forth inflated [green] claims".

"I haven't seen the word 'sustainable' on food labels here," says Annabel McAleer, editor of Auckland-based Good magazine, billed as "New Zealand's Guide to Sustainable Living".

"It's one of those words that can be co-opted quite easily, because it doesn't have a globally recognised meaning [in the context of food]. 'Natural' is another one of those words. You do see 'natural' a lot here. I think food manufacturers have been using these words for years to claim health benefits. 'Lite' is a perfect example."

Like the US and Europe, New Zealand is seeing signs of a shift in consumer consciousness, as seen in the popularity of farmers' markets, official definitions of terms like "organic", "fair trade" and "carbon neutral", backyard food gardens, and vege box deliveries.

"It's part of a bigger trend where people want to get in touch with where their food comes from," says McAleer.

Any attempt to officially define "sustainable", she says, would have to take into account such elements as packaging, transport and ingredients, then weigh up a product's carbon footprint.

Since the food miles issue erupted, debate has become more nuanced, focusing on the global food system. "A lot of UK supermarkets have sustainability plans around their seafood sourcing," says McAleer. "They're real leaders. They don't buy orange roughy any more. They're trying to include carbon footprint data for products." But while New Zealand consumers are becoming aware, local supermarkets are resistant to change.

So, given the confusion, what is sustainable? Dan McGovern, publisher of US-based Sustainable Food News, says you need "a third-party certification to back up your claim". He suggests the certification handed out to sustainable fisheries by the London-based Marine Stewardship Council as one example of a working scheme.

Given the bitter argument between small producers and food giants to agree on a US organic certification, no one is holding their breath for agreement on a standard for sustainability.

The only working model, by no means universal, is offered by the Food Alliance. It offers a holistic vision that includes labour conditions, impacts on wildlife, soil and water conservation, animal welfare and pest control. It is hard to imagine US agribusiness warming to that menu.

A big challenge is defining exactly what parameters to use. Should the model be narrowly focused or adopt a wider vision, where farming is indivisible from nature?

For the past three decades Big Food has emphasised nutrients, rather than fresh food, as part of the American diet. Processed foods flooded supermarket shelves. The grassroots revolt has placed the emphasis back on food. As Pollan says in his latest book, In Defence of Food: "Eat Food. Not too much. Mostly Plants." Taken literally, this could be a death knell for the industrial food system.

"Obviously, our agriculture system needs to change," says Stabinsky. "It's not sustainable in any way, shape or form." She favours "ecological farming", an approach that bans pesticides and stresses biodiversity and a healthy soil. "That for me is sustainability. How are we going to feed people in a hundred years? If we treat our soil like dirt we wouldn't be able to." In a climate change world Stabinsky says ecological farming will better withstand floods and droughts. "Healthy soil will sequester carbon. Without healthy soil there is no sustainable agriculture."

Monsanto stresses the biotech fix. The company plans to spend US$1.3 billion to bring what it says will be drought-resistant, genetically modified maize seed to five African nations by 2017 in a venture with the Gates Foundation and the Howard Buffett Foundation.

"We've got to drive yield up on a per capita basis," says Kevin Eblen, Monsanto's vice-president for Sustainable Yield Commitment. "That's the most efficient use of our resources, without bringing more land into production."

He says GM seeds will save water, increase production and help over five million small farmers by 2020. Detractors point to biotech's dismal record in India, where thousands of peasant farmers who used GM seeds - which cost more than conventional seeds - have killed themselves when crops failed, leaving them deep in debt and land impoverished.

A New Zealander, Peter Proctor, was among those who dealt with that crisis, in his case by teaching many Indians how to rebuild exhausted soils using Rudolf Steiner's biodynamic composting methods.

Eblen points to the Keystone Group, a think tank of 30 food companies, universities and green groups like the World Wildlife Fund, and "its commitment to sustainable yield".

But "it's Monsanto's front group," says Michele Simon, author of Appetite for Profit: How the Food Industry Undermines Our Health and How to Fight Back. Big Food "just flips" negative attacks through ad campaigns based on greenwash - claiming non-existent environmental virtues - and lobbies to write or weaken regulations, she says..

And while Monsanto and other GM proponents argue that biotech will boost food production, the Union of Concerned Scientists says GM crops have largely failed to deliver.

Monsanto's Africa push mirrors efforts by China, India and other nations to lease or buy African land to grow food or raise biofuel crops. This may not be a sustainable business model if their hosts snatch land back in a food crisis.

Those hoping for change in US food policies took heart at some of Obama's appointments, including Kathleen Merrigan, the Deputy Agriculture Secretary, who helped write the US organic certification standard, and her chief of staff Douglas O'Brien. Both were on a short list circulated by Free Democracy Now, a blog maintained by Iowa farm activist David Murphy, who says 95,000 people signed up to campaign for sustainability.

Rebels may have a foot in the door at the USDA, but Big Food retains enormous clout. In 2008 agribusiness got US$7.5 billion in subsidies. Organic farming got US$15 million.

"It's wishful thinking that agribusiness will change on their own," says Simon. Big Food's business model precludes that, beyond niche products to lure, say, green customers, such as Wal-Mart's organics. "As long as the other guy isn't changing, why should they? It's all a matter of competition and making sure costs are low to maximise profits. Sustainable practices do not fit that model. It's not enough to have Michael Pollan write a few nice books."

When Team Obama, seeking money for child health, tried to cut agribusiness subsidies earlier this year, it was rebuffed by Big Food supporters. "This proposal is ill-timed, ill-conceived, and completely out of touch with the realities of agriculture production," snapped the ranking minority member on the House Agriculture Committee.

Meanwhile, the Global Food Security Act, waved through the Senate Foreign Relations Committee in March, seeks to promote GM crops as a condition of US aid overseas.

This has a troubling parallel with the Bush-era Aids funding that bundled money with messages such as abstinence. "It would force GM on to African countries that are struggling to keep traditional farming alive," says Baden-Meyer. "This is just so cynical. It's horrible."

Critics like Baden-Mayer depict Monsanto, which has bought many independent seed companies, as bent on removing genetic information from public hands so it can sell its patented seeds. "That's why we're consuming most of our calories in high-fructose corn syrup. It's an invidious business model. They're trying to destroy the genetic material we need to survive on this planet."

Foreign Policy in Focus, a US think tank, describes the Global Food Security Act as "a co-ordinated roll-out of the 'new Green Revolution', a project that includes the Gates Foundation's multi-billion-dollar Alliance for a Green Revolution in Africa." Monsanto may preach sustainability, say critics, but how sustainable is reliance on GM seeds?

Yet in the tussle between Big Food and its grassroots opponents, the future of agriculture - and the pressure to feed an estimated 9.3 billion mouths by 2050 - may be decided by increasing concerns about global warming.

"Climate change is the lurking monster," says Murphy. "We don't know how bad the impacts might be. It will have a bottom line impact for farmers all around the world. It's already had a negative impact on crop yields in the US."

And while Monsanto may engineer plants that need less water, the organic camp says this will not resolve soil deterioration from industrial farming. "The problem when you dump a lot of chemicals on soils, it that it eventually decreases the soil's ability to hold water," says Murphy. "It's much drier than no-till or organic soil."

As the US prepares for December's Copenhagen conference on climate change, agribusiness will probably be identified - by activists at any rate - as a major source of greenhouse gas emissions. A 2006 UN report said cattle farming worldwide was a major cause of land and soil degradation and emitted more greenhouse gases than transportation. One way of reducing gases is to shift from grain feedlots, common in the US, to mixed grass pasture.

In a climate change world, food, and the sustainability of production systems, are likely to come into sharp relief. Nations are unlikely to leave this challenge to the market.

"We are going to undergo a transformation in agriculture whether we like it or not," warns Exo. "One fundamental question is whether farmers will have change forced upon them, or whether they will make changes themselves."