Organic farming may yield up to a third less of some crop types, according to a study proposing a hybrid with conventional agriculture as the best way to feed the world without destroying it.

Organic farming seeks to limit the use of chemical pesticides and fertilisers, but critics suggest lower crop yields require bigger swaths of land for the same output as conventional farms.

This would mean parts of forests and other natural areas being turned into farmland, undoing some of the environmental gains of organic tilling methods, they say.

The new study by Canadian and American researchers, published in Nature Wednesday, found that organic yields are indeed as much as 34 percent lower for some crops -- 25 percent less overall.

Fruit and oilseed were the best performers -- yielding just three percent less, in ideal farming conditions, than conventionally grown crops that benefit from chemical pest killers and nutrients, the researchers found.

Organic farming of cereals and vegetables, however, yielded up to a third less produce.

"Today's organic systems may nearly rival conventional yields in some cases ... but often they do not," said the report.

The findings contradict those of earlier studies that organic farming matched, or even exceeded, conventional yields.

High agricultural productivity is becoming ever more important as the world's population grows, and food demand with it.

An international expert panel said last November that global food production must rise by up to 80 percent by 2050.

Study co-author Verena Seufert of McGill University in Montreal told AFP the findings pointed to a mix of organic and conventional farming for the future.

"We identify, for example, legume crops and perennial crops as performing better in organic systems than annual and non-legume crops. We also see that organic systems do much better if the farmers apply good management practices," such as crop rotation and effective pest and nutrient control.

"We identify the situations where organic does well and we also identify the situations where it does not do so well, for example under irrigated conditions where the conventional yields can be just so high that organic agriculture can't match these yields."

A single system of "either organic or conventional is much too simplistic," said Seufert.

"We should try to learn from those systems that perform well in terms of yield but also environmental performance and just adopt the systems in those places where they do well.

"At the same time we need to address the problems that organic systems have shown."

The study also found that organic yields rose over time as soil fertility and management skills improved.