An objection to not farming non-human animals that is common in Australia is that most Australian farm animals are raised in pasture land or arid land that would otherwise not be suitable for growing crops. Therefore, if everyone in Australia was vegan, we would starve. Or something like that.
When people think of Australian farmed animals, they usually think of cattle, sheep and goats, which are more likely to be pasture raised (but not always). People rarely think about the chickens and pigs, which are much more commonly kept in factory farms and fed a diet of grain and other farmed plant food. Using the average from 1994-2016 from FAO, in Australia there are 27.4 million beef cattle, 95.3 million sheep, and 2.7 million goats. For animals mostly raised in factory farms and fed grain, there were 2.5 million pigs, 1.1 million turkeys, and 87 million chickens (although note 551 million chickens were slaughtered in 2012 – this dichotomy is due to their short lifespans). Once we examine the statistics, Australia doesn’t quite seem like the land of pasture farming anymore.
One might reasonably suppose that it is no mistake that the type of farmed animal Australians are most familiar with are cows and sheep. If Australians knew what happened in chicken/pig farms (even free range, which are usually as bad), and all slaughterhouses, they might not eat animals. The pasture raised animals are the ones we see in advertisements of struggling farmers, not the chickens stepping over the decaying bodies of their fellow species in ‘free range’ farms.
It takes many kg of plants to make 1 kg of animal flesh (often at a 7 to 1 or greater ratio). We should be able to assume that these plants could be consumed by humans as well, but even if that is not the case, if we didn’t grow whatever plant it was, we could grow crops for humans in their place. If we assume that most of the plants fed to non-humans in factory farms in Australia are sourced in Australia (I think this is reasonable), we should still have enough food to feed Australians even if we eliminated all animal farming in Australia. We wouldn’t even need to repurpose arid land to grow crops that are suited to those climates (e.g. almonds and hemp), although we may want to do this anyway.
To put this another way, we would have so much spare land for growing crops for people if we stopped farming chickens, pigs and turkeys that it would almost certainly make up for the lost ‘food’ from farming cows, sheep and goats, and then some.