SORRISO, Brazil (Reuters) - Farmers in Brazil’s Mato Grosso, the country’s top soy state, are shunning once-heralded, genetically modified soy varieties in favor of conventional seeds after the hi-tech type showed poor yields.

“We’re seeing less and less planting of GMO soy around here. It doesn’t give consistent performance,” said Jeferson Bif, who grows soy and corn on a large 1,800 hectare farm in Ipiranga do Norte, near the key Mato Grosso soy town of Sorriso.

He said he obtained average yields of 58 bags (60 kg) per hectare with conventional soy last season while fields planted with GMO soy in the same year yielded 10 bags less.

Growers began illegally using genetically modified varieties of soy even before Brazil passed a biosafety law around four years ago permitting their use, in the hope of gaining higher yields and reducing production costs.

Around half of Mato Grosso’s soy is estimated to be genetically modified but the tide is turning against it.

Part of farmers’ disappointment over the performance of the modified soy may stem from misunderstanding of the specific conditions in which its altered characteristics can bring rewards.

Uptake of GMO soy was fast in the state Rio Grande do Sul because of its resistance to glyphosate, which is used to kill the weeds that flourish there. But this feature is much less useful in Mato Grosso, where weeds grow much less thickly.

Farmers in Mato Grosso also benefit from better support from cooperatives and government bodies which provide advice and technical assistance and help them maximize yields even with conventional soy.

Another reason for Mato Grosso’s ongoing shift away from GMO-soy is that trading houses and meat processors, conscious that some consumers strive to avoid GMO foods, prefer conventional soy and will pay a premium for it.

Soy is a key component of many cattle feed products.

Alexsander Gheno, agronomist at APAgri consultancy, said GMO soy may have other uses which could indirectly boost yields. By rotating planting of conventional soy with GMO soy, it could help break the cycle of diseases afflicting the crop.

But Gheno said the momentum that GMO crops have gained may see them chase out conventional soy in the long run, even if growers don’t prefer the high-tech varieties.

“Companies have been focusing their research on GMO soy more than on conventional ones. So in 10 years we could have 100 percent of the area planted with GMO soy not because this was farmers’ choice exactly but because development of new conventional varieties is getting scarce,” he said.