It doesn't take much to get called green in the desert, but the opuntia species stand out as not only green, but the round "pads", also called cladodes, could be described as lush (for arid lands) even in the most dire straights of a drought. Texas and Mexico are the largest users of opuntia commercially, but South Africa, India, and many other countries make great use of this niche plant.
Cattle, sheep, and goats are the main consumers of opuntia, but occasionally pigs are also fed it. Cattle can be fed 15-60kg per day. This can increase to 90-120kg per day under drought conditions. On average, a 400kg cow will need to consume 40kg daily to maintain itself. Sheep and goats need to consume about 3-5kg per day to meet their requirements.
55kg opuntia and 2.5kg cornmeal were required to produce 1kg of meat.
Older cladodes are more turgid and less palatable. A grazing rotation can be used to control the age of various stands. The fourth year growth in a Texas study documented 194,200kg of fresh opuntia growth that consisted of about 170,000kg of water.
Diets consisting of more than 50% opuntia run the risk of inducing diarrhea in the animals. For this reason, under non-emergency conditions, opuntia should be supplemented with straw or hay. The inclusion of straw or hay will increase the relative consumption of opuntia and counteract potential nutrient deficiency.
Younger cladodes have a more pronounced laxative effect, while the older pads are tough. A good compromise is feeding 2-3 year old growth.
Allowing stock to graze opuntia directly can save time, but generally will result in more wastage than a labor intensive cut and carry strategy. While the spineless opuntia may grazed without any other preparations, it will need to be fenced to prevent unwanted grazing. The more common thorny variety will need to have the spines removed before most animals will eat it. This can be accomplished easily through the use of a white gas flamethrower used to singe the spines off. These called "pear burners" in Texas.
Sheep and goats are sometimes willing to eat the cladodes with spines still intact. This may lead to gastrointestinal damage and infection.
Standing opuntia can provide a living safety net of emergency feed that doesn't degrade over time as with traditional "hay in the barn". On the contrary, opuntia stands will increase allowed.
The food, and more importantly, water -- containing up to 97% --, in the dry months can be a great way to tide over hungry stock. Cows and sheep have lived up to 500 days with no additional water in the most extreme circumstances.
The fruits are browsed before the pads.
If you have excess opuntia, there are several methods which to store it and use it as fodder. The first, easiest, and most common tactic is to simply dry and chop up the pads. The process of mechanically chopping the opuntia should also be enough to break the spines to the point where there are no longer a concern. This preparation method is very forgiving and long lasting. The fresh cut cladodes can be safely ignored, if left in the shade, for 6+ months without any loss in nutrient content. These old cuttings are even still perfectly viable to plant!
Stock prefer the dried and chopped form over fresh. A sheep will eat less fresh opuntia than it needs to survive, but when dried sheep increase intake to meet daily requirements.
To create a more nutrient rich feed, silage can be created. If there are fruits on the opuntia, they may provide enough sugar, but molasses is generally added to the mix.
Strict cut and carry without drying or preserving the opuntia is discouraged under anything but the most precarious of situations. Moving opuntia is like hauling water around, not cost efficient.
This is only a small part of even the food use of opuntia. Humans can eat the fruit and cladodes as a vegetable, and not as emergency food. Both fruit and vegetable are regularly sold commercially in may Mexican and southwestern USA grocery stores as well as grown in home gardens and orchards.
Opuntia's usefulness continues to expands beyond its usefulness as a feed, solidifying its position as one of the best tools a permaculture designer can have in arid (and some not-so-arid) environments. Living fences, soil stabilization/anti-desertification, and even biomass production are only some of the additional benefits one can harness by utilizing opuntia. Check out the other opuntia tagged articles or spend some time searching the databases for more specific information.