Hmm, they used llamas as pack animals, if memory serves. The presence of equids in the Americas prior to Columbus is (ahem) controversial. There used to be someone around here who argued that I must think the tribes were dumb to not realize they could ride horses which escaped from the Spanish — but that their tribal ancestors hadn’t, eating them all instead.
I’ve seen Llamas described as the goats of South America. Never thought of them as serious beasts of burden. When one considers the vast extent of agriculture evident around Cusco for example, what with the little a Llama can carry, the numbers required would probably have resulted in very little produce reaching the market...not to mention the effort required in creating the layered growth medium (drainage, compost, fertiliser?)
I’m thinking slaves maybe.
“... the Incas were magnificent engineers and agriculturists.
In places terraces were built up dangerously precipitous hillsides with massive inclined walls supporting a metre deep fill of stones for drainage and a metre of excellent soil, often brought in from elsewhere.
Clay was used for foundations to retain water and to encourage roots to decay so that the biological activity would keep the soil a few degrees warmer than the chilly mountain air all year round.
Incan terraces still occupy about a million hectares of land but three quarters of them are abandoned...
http://www.ipsnews.net/1997/05/peru-ancient-inca-irrigation-works-restored-to-raise-record-crops/
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The amount of physical labour required is simply astonishing