I think all of these numbers are off. We have a cheap food policy that encourages over-production of grains in an average year, so that in a bad year, there is still a surplus. The only way to do that is to provide a market to the farmer for the surplus. Surplus grains become food aid, ethanol, and animal feed. The grain is going to be grown as a result of the cheap food policy - the only question is where the surplus will go.

Dog food is made from grain and meat products - typically the stuff that can't otherwise be sold at a higher price for human consumption. What would we be doing with surplus meat and grain otherwise? If you eliminated all dog food, would you eliminate any grain production? Likely not. Livestock production? Maybe a little. So should we be thinking of 'average footprint' of grains and meat, or 'incremental footprint' of the specific product?

Eliminating conventional meat production will reduce the CO2 & methane emissions from cattle, sheep, and hogs. But the grains would otherwise be left to rot somewhere because there is no market for them, emitting the same CO2 and greater methane. The land being farmed for livestock feed would still be farmed - only the pasture land that is being carved out of forest would go back to it's native vegetation.

Depends on the number of mice it catches.

Dear Alan F Rawle thank you for this rather unusual technical question. As you know we are inorganic chemists, so I'm far from being a proven expert in this field. However, with respect to your questions "Can anyone provide primary data? Is this comparison (cat/BMW) reasonable?" I would suggest the following research article entitled

The Ecological Paw Print of Companion Dogs and Cats

https://academic.oup.com/bioscience/article/69/6/467/5486563

The paper is Open Access and can thus be freely downloaded from the general internet.

According to the following link "meat-eating by dogs and cats creates the equivalent of about 64 million tons of carbon dioxide a year, which has about the same climate impact as a year’s worth of driving from 13.6 million cars."

The truth about cats’ and dogs’ environmental impact

https://newsroom.ucla.edu/releases/the-truth-about-cats-and-dogs-environmental-impact

Frank T. Edelmann

Reason is that we have 5 cats and no BMW’s...

I see, already have the cat so now hoping for the BMW...

Dogs, Cats And Climate Change: What's Your Pet's Carbon Pawprint?

https://www.forbes.com/sites/jeffmcmahon/2017/08/02/whats-your-dogs-carbon-pawprint/?sh=2c5262b213a6