New Delhi: Genetically modified (GM) pest-resistant crops may not be the panacea they are made out to be, a new study shows, with specific reference to Bt cotton.

The field trial by scientists in Nagpur shows that the soil the plants are grown in matters almost as much as insect-killing genes and pesticide sprays.

The finding could significantly increase the amount of money farmers spend in buying and spraying pesticides. It could also mean lower yields than those promised by manufacturers of GM seeds.

The study, published in the peer-reviewed journal Current Science, says that the quantity of toxin exuded within a Bt cotton plant sowed in deep soil was nearly three times as much as that of shallow soil and there was marked reduction in the amount of toxin (which combats pests) that was present in the plant during the months of October and November, when most of the cotton is matured and most vulnerable to pest attacks.

On the ground: A Bt cotton field. The study says that the quantity of toxin exuded within a Bt cotton plant sowed in deep soil was nearly three times as much as that of shallow soil.

“...This decline coincided with the peak boll formation stage. At this stage, the toxin concentration was 0.48–2.40 µg/g. The toxin concentrations were, in general, less than the critical concentration of 1.90 µg/g..." wrote the authors D. Blaise and K.R. Kranthi of the Indian Institute of Soil Science, Bhopal, and Central Institute for Cotton Research, Nagpur, respectively. The tests were performed over the monsoon of 2006 and 2007 over 21 test plots in the Nagpur institute’s cotton fields.

At least 60% of India’s agricultural land is rain-fed, the extent of moisture in the plant swings between extremes, and according to the study, exposes cotton to pest attacks at a level much greater than previously imagined.

“There is no doubt that we need Bt cotton. But in regions like Vidarbha which is rain-fed and has a lot of shallow soil, Bt cotton wouldn’t work as well as in other parts of the country. The study just points out that you need different kinds of cotton in different regions. A one-size-fits-all approach can’t work," said Kranthi.

Bt cotton now accounts for over 90% of the country’s cotton acreage and has been credited with tripling the yield since 2006 and making India a net exporter as well as world’s second largest producer of the commodity.

It’s largely due to the success of Bt cotton and its acceptance among farmers that several companies and agricultural research institutes have been trying to integrate the Bt gene into food crops in India.

India’s major cotton producing states are Gujarat, Andhra Pradesh, Maharashtra and Punjab which all have varying soil topographies and varying exposure to monsoon rainfall.

Genetically modified cotton today uses one or more genes from a soil bacterium called Bacillus thuringiensis that trigger an insecticidal protein.

These toxins are usually fatal only to a bug called the American bollworm, considered the chief cotton pest and, as a result, the target of most sprays.

Though Bt cotton seeds are costlier than their non-Bt counterparts, its proponents claim that seeds engineered in this way dramatically reduce the sprays—and, hence, costs—in protecting cotton crop. It is estimated that pests cause losses worth $120 billion ( 5.9 trillion today), of which losses worth 60,000 crore take place in India.

Pesticides worth $8 billion are used every year in India, with cotton accounting for nearly $3.8 billion of this. GM technology is expected to reduce at least 50% of the expenditure on pesticides.

Last year, Kranthi had pointed out several “unforeseen" consequences of the widespread adoption of Bt cotton. In a report, he said 90% of the current GM cotton hybrids appear susceptible to mealy bugs and whiteflies (also considered a minor cotton pest) and that insecticide use in cotton, as measured by value, appears to have increased from 640 crore in 2006 to 800 crore in 2008.