humane slaughter is not an option

"To me, the most serious objection to veganism - or vegetarianism, for that matter - is that if animals have a good life and are humanely killed, that is better than no life at all. And if they graze pasture that could not be used for agriculture, I can't see any environmental downside either. So it's not easy, on my ethical views, to see why eating such meat is wrong. Of course, there isn't much meat produced today that is like that, but there is some, so it would be possible to eat such meat occasionally." - Peter Singer, 2006 March 11

Arguments

1. This is a hypothetical situation and in the real world it is very difficult to breed animals in numbers which will be painlessly killed.
2. Often the establishment of such a practice eventually leads back to and/or "excuses" the original, more "efficient" but inhumane farming industries.
3. While it is true that in such ideal circumstances the animal may not suffer for the most part, most animals, even those that are "organically raised" or "free range" still have to endure the suffering and the terrors of being slaughtered, of having their eggs or milk stolen, and of seeing their companions taken away to be slaughtered, which makes their enjoyment of their life on a farm questionable.
4. Even with free-range egg-laying hens, the "useless" byproduct, the male chicks, are still killed at birth. This is 50% of the chicken population! Even the females are generally slaughtered at a mere fraction of their life expectancy.
5. Labels such as "free range" are controversial and prone to exploitation: merely leaving the hens open to the outside meets the label's criteria; there is no rule about the size of the yard or the space given to each hen. Many so-called "free-range" chicken or hens (or other animals) are still quite atrociously treated.

References for the Objections

This article is based on a GNU FDL Vegan Wikia article: Animals_can_be_raised_in_conditions_which_allow_them_to_express_natural_behaviours_and_humanely_killed

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