The environmental movement has advised us to reduce our eating of animal products (especially red meat and dairy) or, better still, become vegetarians.
For instance, the Australian Conservation Foundation, on its GreenHome website, advises this. So does Beyond Zero Emissions, although it thinks a shift to eating feral animals would be a good thing.
Such groups say that producing animal foods takes up a lot of land, uses a lot of water, is inefficient (especially if grain is fed to animals), results in methane production (especially from ruminants such as cattle), harms our soils, and is energy-intensive (transport, refrigeration etc).
Other groups, such as Greenpeace Australia, are more concerned about threats from genetically engineered foods, synthetic chemicals, and big corporations. It advocates "ecological farming" that is organic-based, small-scale and tries to imitate natural processes.
Such farming would have to include grazing and other animal-based farming enterprises if it was to mimic natural processes, where plants and animals exist in synergistic relationships.
For instance, on an organic vegetable farm, where would fertilisers come from if animal manures were not available? Could cover crops, green manures and composted plants alone do the job?
In short, the more I learn about agriculture, the less impressed I am by environmental arguments for a vegetarian diet.
Adults need to eat about 2500 calories a day (depending on age, sex and activity).
If we don't get our calories from an omnivorous diet, we have to get them - as well as all the proteins, vitamins, minerals and whatever else keeps us healthy - from plants and fungi.
Vegetarians are usually keen to point out that all the nutrition we need is available from plants alone, although it is accepted that a person has to be more careful in their menu planning to ensure they get their full nutrition.
However, as ABC TV's Catalyst program highlighted on October 24 and 31, saturated fats are probably not the nasties we have been led to believe, and the nutrition "industry" (and its pharmacological cousin) needs a thorough colonic irrigation.
A plant-based diet might not be as healthy as many people think, especially when it comes to carbohydrates, sugars, fats and oils.
After watching the two-part TV program, people might need to think again about their ideal diet.
In my reading, advocates against meat-eating tend to underrate the environmental harm of plant-based agriculture and overrate the harm of grazing - or at least fail to acknowledge the possibility of sustainable grazing.
Sustainable grazing tries to mimic wild-herd behaviour, with benefits to soils, pastures, water resources, and biodiversity.
By sequestering carbon in soils, and building topsoil, such grazing may even have a net greenhouse benefit to the atmosphere.
In fairness, the Australian Conservation Foundation does acknowledge that meat-eaters can buy local, grass-fed animals rather than those that are "grain-fed", (although you are left wondering about its knowledge of the feedlot system in Australia and to what extent grain-fed animals are also grass-fed).
The net environmental effects of a vegetarian diet versus an omnivorous diet are not as easily calculable as some believe.