A grazing beef cow will drink about 7-10 gallons during the day. If it is hot, they will drink 25. Wheat yields around 1LB for 130 -150 gallons over the growing season. An avocado (1lb) requires about 90 Gallons over the growing season.
There are a lot of things to factor into this, crop yields aren't static and there are a lot of other things that can change the yields. Cows have legs, so you don't have to 'irrigate' them, they just have to be pastured within a couple thousand feet of water, and you can set up temporary solar pump + water trough if you have water within a couple miles.
Beef Cattle will give you 5-600 lbs of meat at about 1200 Calories per pound, with an average grazing time of 12 months. works out to 150cal/gallon.
Avocado has about 700+ cal per pound so about 8cal/gallon. (I'm not as sure on this, it could be lower as a 'medium' avocado is about 1/3 of a pound, but I've never grown avocados so I don't know the exact here.)
Calories in a whole grain wheat is about 1500/lb, so 1500/150 is about 10cal/gallon.
So it's kinda relative, but the only way you're going to harvest edible nutrients from scrub land is buy herding or hunting grazing animals that process the stuff with 4 stomachs.
There's a larger question of the value that grazing provides and if we need that as a food source. Before modern farming and shipping, if you lived in a semi-arid scrubland style climate, you were very much dependent on some type of animal to turn that stuff into a digestible food source for you.
