America's powerful agribusiness lobby has hit back at Michelle Obama's decision to make her new White House kitchen garden entirely organic, urging her to consider the use of appropriate "crop protection products".

Wearing her finest Jimmy Choo boots, Mrs Obama broke the ground on her vegetable plot on the White House lawn last month, enlisting the help of local schoolchildren to help make a point about the need to tackle childhood obesity.

The kitchen garden is White House's first since Eleanor Roosevelt "dug for victory" in the Second World War, and pictures of the photogenic First Lady getting to work gained massive worldwide coverage.

To the anger of Big Ag, however, Mrs Obama has aligned herself with the growing movement of "locavores", people who grow their own fruit and vegetables at home or try to buy only locally-grown food. The principles of organic gardening, which focuses on building healthy soil, mean that she will not be able to use chemical products to tackle pests or give her plants a boost.

Shortly after she got to work on the plot, Mrs Obama received a letter from the Mid-America CropLife Association (MACA), which represents the companies producing the pesticides and fertilisers underpinning "conventional" American agriculture.

Addressed to "Mrs Barack Obama", the letter congratulated the First Lady on "recognising the importance of agriculture in America". Farming is America's largest industry, generating 20 per cent of GDP and directly or indirectly employing 22 million people.

The letter does not mention the word "organic", nor even "pesticide" or "fertiliser" but highlights the role played by farmers in preventing soil erosion and the massive yields achievable thanks to technological advances - technologies that can see a single acre produce almost 20 tonnes of strawberries of 110,000 heads of lettuce in a season.

"Today, an average farmer produces enough food to feed 144 Americans who are living longer lives than many of their ancestors. Technology in agriculture has allowed for the development of much of what we know and use in our lives today," MACA wrote.

"If Americans were still required to farm to support their family's basic food and fibre needs, would the US have been leaders in the advancement of science, communication, education, medicine, transportation and the arts?

"We live in a very different world than that of our grandparents. Americans are juggling jobs with the needs of children and ageing parents. The time needed to tend a garden is not there for the majority of our citizens, certainly not a garden of sufficient productivity to supply much of a family's year-round food needs."

The letter "respectfully" encourages Mrs Obama to recognise the role played by conventional agriculture in feeding America's growing population and is carefully worded not to be provocative.

But Bonnie McCarvel, the MACA executive director, was not so diplomatic in an e-mail forwarding the letter to MACA supporters and members, in which she said: "While a garden is a great idea, the thought of it being organic made (us) shudder."

More than 100,000 people have signed an online petition supporting Mrs Obama and asking MACA to stop its "propaganda about pesticides".

"Stop asserting that the First Lady is somehow disserving our nation's citizens by encouraging them to grow their own food locally, sustainably and without your industry's chemicals," the petition says. "We know better and you should, too."

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