CORVALLIS - Why is it right to kill the mouse and not the cow?

This question is central to a study of bioethics that explores the moral foundation of a strictly vegetarian, so-called vegan diet. The research, by Steven Davis, a professor of animal science at Oregon State University, adds a new perspective to a millennia-old debate: Is it right for people to kill animals in order to feed themselves?

Davis turns that question on its head. How many animals must die, he asks, in order for people to feed themselves?

To address the question, Davis applies a principle used by moral philosophers to measure the least amount of harm an action might cause, called the Least Harm Principle.

Davis's research focuses on the work of Tom Regan, a philosophy professor from North Carolina State University and founder of the contemporary animal rights movement. Regan argues that the least harm would be done to animals if people were to adopt a vegan diet - that is, a diet based only on plants, with no meat, eggs, or milk products.

What goes unaccounted for in Regan's vegan conclusion, according to Davis, is the number of animals who are inadvertently killed during crop production and harvest.

"Vegan diets are not bloodless diets," Davis said. "Millions of animals die every year to provide products used in vegan diets."

Davis presented his research last fall at a meeting of the European Society for Agriculture and Food Ethics, in Florence, Italy. There he questioned the conclusions of animal rights proponents and offered alternatives using the Least Harm Principle. Central to his argument is the unseen mortality that accompanies the production of row crops and grains, staples of a vegan diet, in agricultural systems large enough to sustain the human population.

"Over the years that I have been studying animal rights theories, I have never found anyone who has considered the deaths of - or, the 'harm' to - animals of the field," Davis said. "This, it seems to me, is a serious omission."

Consequently, Davis asks what is the morally relevant difference between the field mouse and the cow that makes it okay to kill one but not the other so that humans may eat.

Few studies document the losses of rabbits, mice, pheasants, snakes and other field animals in planting and harvesting crops. Said one researcher: "Because most of these animals have been seen as expendable, or not seen at all, few scientific studies have been done measuring agriculture's effects on their populations."

Davis has found evidence that suggests that the unseen losses of field animals are very high. One study documented that a single operation, mowing alfalfa, caused a 50 percent reduction in the gray-tailed vole population. Mortality rates increase with every pass of the tractor to plow, plant, and harvest. Additions of herbicides and pesticides cause additional harm to animals of the field.

In contrast, grazing ruminants such as cattle produce food and require fewer entries into the fields with tractors and other equipment. In grazed pastures, according to Davis, less wildlife is lost to the mower blades, and more find stable habitat in untilled fields. And no-till agriculture also helps stabilize soil and reduce run-off into streams.

"Pasture-forage production, with herbivores harvesting the forage, would be the ultimate in 'no-till' agriculture," Davis said.

Davis proposes a ruminant-pasture model of food production, which would replace all poultry, pig and lamb production with beef and dairy products. According to his calculations, such a model would result in the deaths of 300 million fewer animals annually (counting both field animals and cattle) than would a total vegan model. This difference, according to Davis, is mainly the result of fewer field animals killed in pasture and forage production than in the growing and harvest of grain, beans, and corn.

Applying the Least Harm Principle, Davis argues that people may be morally obliged to consume a diet based on plants and grazing ruminants in order to cause the least harm to animals.

Davis's work goes beyond the vegan debate to grapple with issues of animal cloning, genetic engineering, and ethical treatment of production animals. Through the OSU Agriculture Experiment Station and a regional project on animal bioethics, Davis is part of a team of biological and social scientists from throughout the West who are working to integrate ethics and moral reasoning into the work and study of agriculture.