This isn't efficient at all though (and the reason why industrial farmed animals fed on crops aren't efficient either):

https://en.wikipedia.org/wiki/Ecological_efficiency#Ten_perc...

> When organisms are consumed, approximately 10% of the energy in the food is fixed into their flesh and is available for next trophic level (carnivores or omnivores). When a carnivore or an omnivore in turn consumes that animal, only about 10% of energy is fixed in its flesh for the higher level.