If you are a food writer with a taste for the depredations of industrialised agriculture, then the modern ground zero for your investigations seems to be California’s Central Valley.

Philip Lymbery begins his book Farmageddon with a visit to the valley, "the fruit bowl of America, and home to perhaps the biggest concentration of mega-dairies in the world".

And just how many cows are we talking about?

There are no fewer than 1.75 million dairy cows in California.

When the great cookery writer and campaigner Mark Bittman, of the New York Times , toured the valley for a magazine article last year, he pulled over at the Harris Ranch in Coalinga and "had the experience of getting out of the car and being hit by what felt like a giant hairdryer blowing 120-degree, manure-scented air".

The Harris Ranch is the feedlot home to no fewer than 100,000 cattle.

In Bittman’s article, a local writer, Mara Arak, summed up the Central Valley like this: “This land and its water have gone mostly to the proposition of making a few men very wealthy and consigning generations of others, especially farm workers, to lives in the dust.”

Philip Lymbery's book is concerned with the effect factory farming has on animal welfare – he is the chief executive of the charity Compassion in World Farming, founded in England by Peter and Anna Roberts in 1967.

Relentless suffering
Yet what comes through on page after page of Farmageddon is not simply the relentless suffering of animals – farmed fish, caged chickens, abused pigs, cows knee-deep in excrement –but the relentless suffering of the exploited people who work in these industrialised systems, and the suffering of people who live near to or beside these mega-complexes.

Lymbery tours the world – Peru, Taiwan, Central Valley, Mauritius, Scotland, Argentina, Mexico and elsewhere – and he finds the same thing everywhere he goes: the depredations of industrialised agriculture.

For factory farming is predicated on one factor and one factor alone: cost. The animals do not count, and nor do the people in or near the factories.

Both are consigned to living lives not so much in the dust, as in the shit – California’s cows produce more excrement than the entire human population of the United Kingdom.

Farmageddon is sobering and sad, not just because industrialised agriculture is cruel, incredibly inefficient and the biggest polluter of the natural world.

It is also sobering because we have known for more than 50 years – ever since the publication in 1962 of Rachel Carson's landmark book Silent Spring – what the consequences of chemicalised pollution really are.

And since 1975, when the philosopher Peter Singer published Animal Liberation , we have known the consequences of ill-treating animals.

The chemical and agricultural industries have found a way to counter the arguments of Carson and Singer: you move the problem out of sight.

Put the pigs where they can’t be seen; put the chickens in cages; put the cattle in feedlots; pack the fish into sea cages. If we can’t see the problem, then where’s the problem?

This may seem to be only a moral issue, but it isn’t – it’s a health issue.

And it’s not simply a health issue for the unfortunates who live near or work in these factories.

'Fat production'
Lymbery quotes Prof Michael Crawford, of London's Institute of Brain Chemistry and Nutrition: "The intensification of animal farming has virtually destroyed the nutritional quality of our food," says Crawford, who describes industrial chicken production as "fat production, not meat production. Such chickens are no longer a protein-rich food, but a fat-rich food. The explanation is simple, namely that they are fed largely on cereals."

Is there any consolation to be found in this picture of global suffering and pollution?

As I read Farmageddon , the image that kept coming to my mind, ironically, was the cow in the Irish field, chewing the cud, and the sheep on the low hill, grazing relentlessly.

This staple of our culture is precious beyond words, and the freedom to roam and graze should be extended to every animal that we choose to eat – every chicken, every pig, every fish, every cow and every sheep.

Farmageddon is published by Bloomsbury

John McKenna’s website is guides.ie