The problem doesn't seem to be energy efficiency though, the problem is finding a grey wizard able and willing to chain cast Mockery of Death over and over on cows or horses or what have you, that's just not viable. It's suicidal for the vast majority of wizards, and those for who it isn't have far better things to do with their time (outside of extreme situations like Mathilde is in now).
On the other hand, give an enchanted item to someone with a bit of training and they can spend their day packing animals. It may be far slower, in the worst case I could see a single cast every dusk and dawn so two animals a day, and it may be expensive, but it's turning a previously impossibility into a reliable possibility. Whether it's worth it to spend all that time slowly loading up the animals, or to spend that much money as it may take to buy the artefact (or several) to solve the problem is another matter, but if it does work then that's just a matter of testing whether it can be done economically. And we've still provided an option that may come in useful at some point in the future if there's ever an extreme need.
Assuming an reusable enchanted item of MoD for baselines is possible, of course.