Look at the Poo!

A ruminant has multiple specialized "stomachs" which allow the animal to digest fibrous materials. The one of special interest is the rumen which is quite large and is where bacterial digestion occurs to break down material that would otherwise be indigestible. So you need to think of ruminant digestion as a bacterial digestion factory instead of like us humans (digest chemically, with only a little help from beneficial bacteria). What this means is, it is possible to mess up the bacteria in the alpaca's gut and rumen and they will starve to death with a full stomach. How would this occur? You could overadminister antibiotics. Your vet will tell you how to administer antibiotics. If you think a little more might be better, you might wipe out the gut bacteria along with the bad ones that you wanted to kill. There are other ways, but normally, you will never encounter this problem. The rumen is designed to house and take care of its bacteria population.

Changing pastures

With many ruminants, moving onto fresh pasture is the beginning of runny stools. The bacteria get a sudden slug of different feed with a different moisture content, and they can't digest it very well. Typically, after a few days, a slightly different population of bacteria that is better suited for the food coming in will develop and the diarrhoea will go away. Alpacas are not nearly as sensitive to this as are sheep and cattle. This is a good thing.

...it is a good practice not to shock their system with changes in feed.

Avoid sudden changes

Though alpacas are not as sensitive to feed changes as other ruminants, it is a good practice not to shock their system with changes in feed. When moving from dry lot to fresh pasture, make sure they have a ready source of the hay they are used to. They will naturally eat some hay as well as the fresh pasture and give their rumen and bacteria time to adjust.

Analyzing digestion problems

The most important steps you can take to make sure everything is going well are the ones made in your pastures. Check the poop piles. Yes, your neighbours and friends will think you're crazy. Do it anyway.