David Tribe is a Melbourne University scientist and an energetic advocate for GM technology. He also runs a blog called GMO Pundit that has devoted considerable energy to confronting GM opponents. In conjunction with American academic Bruce Chassy, Tribe has launched a detailed attack on one of America's most high-profile GM critics, Jeffrey Smith, whose book Seeds of Deception has been a best seller.

The book, which takes particular aim at Monsanto, claims the company has historically combined some of the most toxic products ever sold with misleading reports, pressure tactics on opponents and collusion with friendly government scientists.

It asserts the company and its competitors now race to genetically engineer and patent the world's food supply, which Smith says profoundly threatens our health, environment, and economy.

Tribe believes that such claims, while popular, are easily debunked and have never been subjected to scientific peer review.

He believes that in the long term, genetically modified organisms will better suit the needs of agriculture, and are the only way to feed an increasing global population amid the uncertainties thrown up by climate change.

''Many of the opponents of GM take that position because they simply dislike large companies, but they are not facing the problem that our agriculture system will need to feed billions more people.

''If global warming is the greatest moral challenge of our time, then we have to confront it. Many of the opponents [of GM] have a fundamentalist ideological vision of the world, and over history fundamentalist ideologies have killed millions of people.''

Sheehan could be seen to be a similar enthusiast. He planted the first crop as soon as the moratorium was lifted and this is the third year in which he has used the seed known as Monsanto's Roundup Ready. Its characteristic is a modification to allow it to survive being sprayed by the herbicide glyphosate, which Monsanto markets under the Roundup brand.

Such spraying is desirable for farmers because it allows them to control other weeds which threaten crop growth, without killing the crop.

The farm business Sheehan manages is called Yaloak Estate, which controls 10 separate properties from its headquarters at Ballan. As well as Ballan and Derrinallum, it also has land near Hamilton.

The enterprise is owned by Melbourne businessman Peter Yunghanns.

Recent drying of the climate has meant the Derrinallum land has become more suitable for crops such as canola and wheat, but it also has a wide variety of weeds in different parts of the property. Consequently, Sheehan uses Roundup Ready canola on about a third of the property, where he needs to control the weed types that Roundup is effective at eliminating.

On the rest of the property, he uses two types of non-GM canola that have tolerance to herbicides useful for different weeds: a relatively new variety called Clearfield and an older variety known as Triazene Tolerant, or TT.

Because it is his third year using Monsanto's GM canola, Sheehan has experience that some of the more recent converts to GM might lack, and he says he is now weighing up whether he will use it in future.

Despite the claims of increased yields from GM proponents, Sheehan has recorded no such increase. What he has noticed is a much higher cost of using GM canola than using TT varieties. Roundup Ready canola costs $25 a kilogram, whereas TT costs about $4 - although this is modified by the amount needed to be planted.

''With TT you have to plant about five kilos to the hectare whereas with Roundup Ready and Clearfield, you only have to plant three.

''But the real problem with Roundup Ready is the very limited window allowed for weed spraying. You can only spray the plants when they have between two and six leaves … There are times when you might need to spray if weeds develop when the plants have 12 leaves, but if you do that you really knock them around. This [limitation] has really taken the gloss off GM.''

Sheehan is not alone among farmers who have tried GM canola and are questioning the alleged benefits. Ricky Miles used it on his property in the far western Wimmera for the first two years it was allowed. But he has chosen not to use it this year because of the costs involved.

''Firstly, there are the licence fees you have to pay to grow it, there is the cost of the seeds, there is the risk of Roundup resistance developing and I found I had to spray two applications of Roundup to knock off the weeds.''

Miles also found there was no increase in crop yields and that there were only a few places where he could sell his GM grain - and at $12 a tonne less than non-GM grain.

''I am not opposed to GM so I will probably give it another go when they have sorted out some of the problems,'' he says.

If it were only simple as one side of the argument being good and the other bad. There are serious health concerns about some non-GM alternatives, too. Triazene is an organic compound that has been widely used as a herbicide since the 1950s. Many farmers who vocally oppose GM use triazene, but it is a controversial chemical and is banned in the European Union.

It is said to have serious effects on non-target flora and fauna, including on amphibians, and because - unlike glyphosate, which readily breaks down - triazene lingers in the environment and can have a serious impact on the productivity of crops for a year or more.

Triazine is used in more than 80 countries and is said to be the world's most used herbicide, but studies now suggest it is an ''endocrine disruptor'' - a substance that mimics hormones in the body and has possible carcinogenic effects. Its long life is believed to cause widespread contamination of waterways and drinking water supplies. Epidemiological evidence connects triazine to low sperm levels in men and has led researchers to call for its banning.

Despite this, the US Environmental Protection Agency decided it was safe enough and in 2006 decided not to ban it.

On the other side, there is concern about a lack of epidemiological studies on the long-term effects of genetically modified food products in our diets. Adelaide-based epidemiologist Judy Carman was commissioned by the previous Labor government in Western Australia to undertake research into the safety of GM canola, but when the government changed, the former moratorium was lifted before her study was completed.

Carman hopes the results of her findings will be released by the Barnett government this year and, although she is unable to reveal what her findings are, she does say she encountered stiff resistance from the Australian biotechnology industry, which was unwilling to provide her with GM canola seeds to study.

''In the end we had to source our material in the United States and do most of our research there. As a result, the study took years to do.''

Andrew Weidemann, a leading member of the Victorian Farmers Federation, is a grains farmer from Rupanyup near Horsham. He has grown canola every year since 1984 and Roundup Ready GM canola since the moratorium was lifted. Indeed, in 2008, Monsanto chose his farm to conduct a field day for farmers to familiarise themselves with the new technology.

Weidemann says that last season he achieved his highest productivity from canola because they were able to sow on time, and the hot dry finish to the season meant that the crop matured earlier. ''In a way we were lucky, but without this [GM] technology we wouldn't have met the growing timelines and would have suffered a yield penalty if we were still maturing at the hot dry end.''

But Gai Marshall, a canola farmer from Berrigan in southern NSW, attended the field trials on Weidemann's property and believes the outcome which showed a better yield from GM canola was achieved by ''smoke and mirrors''.

''For starters, the non-GM canola was planted much more tightly together than the GM which would have given the GM plants a better chance. Also, there was a 20-day difference between when the GM crop was sprayed for weeds and when they sprayed the non-GM. This would have given the GM crop a 20-day start.''