The research I have done recently (searching through Wikipedia) has yielded that generally cold-blooded animals (Ectotherms) are able to survive on less food that their warm-blooded counterparts, and their low resting metabolic rate allowed them to grow faster.
Thus, would an ectotherm animal be better suited for livestock than an endotherm? Or are there big disadvantages that make Ectotherms less suitable to be used in that role?
The biggest shortcoming would be the growth rate: as in any business, your goal of growing livestock is maximizing profit. If your livestock takes 6 years to reach commercial size, and your competitors grow a different species that takes only 2 years, you will have 1/3 of their revenue, all the rest being the same.
This is why chicken, cows and company get pumped with hormones, plants with fertilizers and so on: to shorten the time it takes to reach commercial size.
Cold blooded animals have generally a slower growth rate, thus it could be more challenging to grow them commercially, especially if there are warm blooded competing species.
The first thing you've got to ask is... are your cold-blooded animals any good at eating plants?
Because efficiently turning plant material into animal material is how a farmer gets the most bang for their buck. A one step food chain, with only one 'conversion ratio' to worry about is best, because energy and resources are lost at each step. So the one step might be:
If your cold-blooded creature is a carnivore, then the food chain becomes:
That's a lot less efficient - you'd be better off eating the cow yourself.
Warm-blooded animals can efficiently digest lots of plant material by keeping their guts warm. Chemical reactions roughly double in speed for every 10 Celsius increase in temperature. So on a 17C morning, a 37C cow (and its gut microbiome) will be digesting its breakfast much faster than a cold-blooded iguana.
Secondly... because their body temperature varies as the seasons pass, you'll need to feed them more in the summer heat and less in the winter cold. That can have advantages and disadvantages. The farmer won't have to put aside quite as much winter fodder for his iguanas as for his goats. But they may be eating him out of house an home in the warmer seasons.
Thirdly... what climates do you want to keep your cold-blooded livestock in? Big cold-blooded herbivores really aren't going to work very well in temperate climates where winters are cold. Little reptiles and amphibians can hibernate. For big ones that's likely to be problematic: a hibernating frog is the equivalent of a shot glass of water which has to be heated up by basking in the sun or by ambient heat. A lizard the size of a cow is in volume like an entire bathtub of water - that'll take much longer. So a frog may only need 10 minutes of basking to become active and go off in search of food, but the lizard-cow may need more hours than there are in the day!
If the lizard-cow burns fat to bring their body temperature up, then they are effectively using a warm-blood trick! Like bears, they'll need a LOT of body fat to successfully do this. Which isn't good for the farmer, because his cold-blooded livestock are using what they eat to survive hibernation, rather than to produce lots of juicy steaks.
