While opuntia has a plethora of positives associated with it, there is no free lunch. A number of negative features of opuntia must be listed to give the full story. Like the positives it is up to the designer to accentuate the desired uses while diminishing the negatives. In permaculture it is often said that "the problem is the solution." Often what looks like a problem from one perspective can be quite the opposite from another.
The limited use, on a broad or commercial scale, of opuntia results, of course, in limited statistics. Often the negatives get much more attention because of their perceived urgency. Within the Americas there are less concerns because of the presence of natural predators. More caution should be taken in exotic (for opuntia) and habitable climates.
An obvious economic drawback to transporting opuntia is its water content. It opuntia can and should be used on-site, the cost of hauling mostly water is uneconomic. Even still, some farmers in Mexico have transported opuntia, sometimes great distances, to be used as feed for dairy cattle. This seems to be a consequence of the removal of wild opuntia for sale to foreign markets - an illegal practice that continues today - and from increased use as fodder.
One of the earliest economic uses of opuntia was to use it as a host for cochineal insects, which are used to create a number of red dyes. There was a global monopoly in the late 1700's by Spain and Portugal (via the Americas). The English wanted their cut and deployed colonies of cactus and insect, taken from Brazil, to establish opuntia/cochineal insect plantations in Botany Bay, now Sydney, and South Africa.
During 1961 Cuba planted 13km (8mi) of opuntia along the Guantanamo Bay Naval Base's border to prevent Cubans from fleeing to the United States of America. This living wall was dubbed 'The Cactus Curtain' after its Iron and Bamboo counterparts half a world away.
The Great Escape
After Britain introduced opuntia to Australia and South Africa's moister environments, the genie was out of the bottle. Usually kept in check by harsher desert conditions or natural predators, opuntia exploded across the landscape of the comparably lush and predator-free Sydney and South Africa. From its initial initial establishment in Australia, opuntia expanded to cover 259,000km^2 (100,000mi^2). It wasn't until the 1920's that the invasion was brought under control, after infesting 23.5 million hectares, by the larvae of a South American Cactus Moth - cactoblastis cactorum. Today almost 1 million hectares remain infested. At its peak in the early 1920s, opuntia would spread over an additional 400,000ha per year.
In South Africa opuntia has been a blessing and a curse. Over 2 million hectares were infested by the British introduction. The same South American moth was used to control the cactus, but farmers, particularly those in the cooler regions, continue to farm the spineless varieties for fodder and fruit production. Despite the presence of the cactus moth, today 1500 hectares (XXX acres) are being cultivated for fruit and another 3000 hectares (XXX acres) are used for fodder production.
A Tarnished Image
While opuntia is highly regarded as a forage and/or fruit crop in places like Brazil, Tunisia, and Italy, in Australia it has not garnered such a positive response. Beyond the already invasive nature of it in places like Sydney, opuntia also damages the wool of sheep, a large industry down under.
Opuntia's once prized and highly utilized forage via pear burners has eroded in the minds of Texans, who now it is seen as competing with grasses. This concern is questionable though. Turning productive land into an opuntia plantation would not be a great idea, but in Texas, specifically western, the environment does not include very productive grasslands. This marginal land should be used for vegetation more suitable to it - opuntia.
When sheep and goats are not provided enough food, they will begin to eat Opuntia that still has spines. The spines can cause gastrointestinal issues and/or lesion. Some forms have common names referencing their danger to stock, such as O. mycrodasys or blinding prickly pear. Again, this behavior occurs primarily when animals do not have enough safe food to eat (Merrill et al.,1980; Migaki et al., 1969) (De la Cruz, 1994).
As Stan Lee reminded us, "With great power comes great responsibility." Opuntia is a perfect example of an extremely powerful tool in the permaculturists' bag of tricks. Learning from the successes and failures of past generations reveals that opuntia provides an extremely hardy and exceedingly useful plant in marginal arid lands, where the climate can keep it from growing too quickly. On the other hand, if you are willing to take responsibility for playing with fire, opuntia can produce a ever increasing yields as one moves into more habitable regions, primarily measured with increased rainfall.