Opinion: In no case will a vegan diet be better for the planet than a moderate omnivorous diet, writes Dr Jacqueline Rowarth. She spoke with Jamie Mackay from The Country above about her thoughts.

Veganism will not save the planet from climate change under current population growth scenarios.

The scientific facts are clear. A diet including a moderate amount of meat and dairy products, sourced from efficient (most product for fewest greenhouse gases (GHG)) farmers, delivers the required nutrients per person for least environmental impact.

This includes water use and nitrogen loss as well as the GHG. It also includes the impact of agricultural land use expansion and consequent impacts on biodiversity.

Promoters of Veganuary (avoiding animal products for the month of January) would have you think otherwise.

Promoters also suggest that adopters of the vegan lifestyle will feel so much better that they will never revert.

For some people this might be true. For others it won't. But in no case will a vegan diet be better for the planet than a moderate omnivorous diet.

People claiming that it will, usually base their calculations on feedlot animal production.

The argument is that the land used to grow crops for animals could be used to grow crops for direct human consumption.

This might be possible, but the yields achieved for humans will be lower than those for animals.

Dr Jacqueline Rowarth. Photo / Supplied

New Zealander Eric Watsonholds the world record for producing feed wheatat 17.398t/ha. The average is more like 12 t/ha.

Yields for milling wheat (for human consumption post processing and cooking more GHG) are potentially 12-13 t/ha, but the average is nearer 8 t/ha.

A further factor is that the amount of grain (potentially human food) actually consumed in feedlots is only a small component of lifetime feed.

Researchers from Oklahoma State University have gathered the data, done the calculations, and state that "Regardless of the type of beef production system, the majority of beef cattle's nutrient requirements over a lifetime are met with human inedible feeds.Only 7 per cent of beef cattle's lifetime feed intake is corn grain."

The other 93 per cent of the animal's lifetime diet is generally inedible to humans, and not in direct competition with the human food supply.

In fact, by digesting fibre and converting previously human-inedible feeds into nutritious, human-edible foods, ruminants increase the land available for human food production.

In the US, a considerable proportion of cattle diet is distiller's grains, which is a by-product of alcohol production from corn (either for fuel or human consumption).

The amount of distiller's grains fed to beef cattle has increased rapidly this century as the production of fuel from corn has increased.

The Oklahoma State University researchers suggested that improvements in corn productivity (yield per unit of input, including land) would do more to help the sustainability of land use than tinkering with cattle diets.

Improvements suggested include no-till or conservation tillage practices to reduce soil erosion and increase soil organic carbon, the use of winter cover crops to reduce nutrient run-off and using precision agriculture techniques to apply fertiliser at variable rates across field to minimize nutrient emissions to the environment while improving corn yields. These are practices already in common use in New Zealand.

The argument then becomes human edible crops instead of grass, but that change requires more fertiliser, including nitrogen, per hectare, plus fossil fuels for machines.

On land usage, Veganuary doesn't stack up.

Veganuary proponents also overlook the environmental impact of the manufacture of the dietary supplements that are necessary to maintain health.

Nor do they consider the impact of replacements bamboo shoes instead of leather shoes, for instance.

Bamboo is a crop in this context, and it is possible that the land could be used for growing food instead of material for shoes.

Leather is a component of an animal so when the animal is killed for food, the hide (a co-product) can be used for other purposes e.g., shoes.

Over half a million people have already signed up to participate in Veganuary this year and have a month of "doing their bit for the environment".

Sadly the effect won't be what is intended not calorie for calorie (or kilojoule) nor per unit of protein.

Veganuary might help their own weight, however, by assisting a focus on food consumption and quantity.

There are certainly some people who could eat less meat and dairy products and be healthier. Globally, however, there are at least an equal number that would be healthier if they ate more.

In New Zealand the biggest personal environmental impact we have is in fossil fuel use.This is the component of our national emissions that has almost doubled since 1990.

Covid19 has reduced our transtasman air travel, but not the car trips to the beach and bach.

For those wanting to make a difference, a month of biking will do more to assist the environment than tinkering with diet.

It will probably help health and fitness as well, but of course, impact depends upon starting point a point frequently overlooked by enthusiasts, including those advocating Veganuary.

- Dr Jacqueline Rowarth, Adjunct Professor Lincoln University, has been vegetarian (not vegan) for over 40 years. She is a farmer-elected director of DairyNZ and Ravensdown. The analysis and conclusions above are her own. [emailprotected]

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Dr Jacqueline Rowarth: Why veganism won't save the world - Newstalk ZB