There is some controversy among alpaca producers about the feeding of grain. Apparently there is some question regarding whether or not it can be digested by alpacas. Or maybe some see it as harmful. Either way, I will try to explain the place of grain in the alpaca diet.
First, it is important to clarify how grain is digested. Ninety-five percent of all soluble carbohydrates, such as grain, are fermented by microorganisms in the rumen. Only a very small percentage makes it to the small intestine to be digested by the alpaca's own enzymes. Rumen microorganisms ferment grain preferentially to roughage (hay) because it takes less work to break down.
Both the fermentation of grain and roughage yields volatile fatty acids (VFA), which are the predominate sources of energy used by alpacas. Grain yields more energy and faster than roughage. It is almost pure energy. The predominate microorganisms in the rumen will change depending on whether roughage or grain predominates in the diet.
Do/did wild alpacas eat grain? Probably not, or not very often. Would they have eaten it if they had come across it? Definitely. Do/did wild alpacas lose weight during lactation? Yes.
Wild animals normally go through cycles of feast and famine. The mammalian body actually becomes more efficient after a fast. This fact is exploited by cattleman. It also makes it more difficult for humans to lose weight by repeated fasting, and makes weight gain inevitable.
The primary feedstuff in the alpaca diet should be roughage, mainly hays like Orchard grass and Coastal Bermuda grass. Alfalfa should be reserved for lactating females and those which need to gain weight after lactation.
Bagged supplements often contain grain along with vitamins and minerals. To maintain alpacas at a healthy weight and not over-mineralize, it is better to feed grain, as needed, and provide a separate mineral free-choice. The habitual feeding of supplement that contains grain and minerals has led to the current alpaca obesity problem.
A good grain/protein combination such as corn/soybean meal/flax seed, or corn/soybean meal/alfalfa meal, can be fed to gestating females who need to gain weight as well as those which are lactating. Grain/protein mixtures can be custom-made at feed mills.
Alpacas that are overfed have poorer quality fiber, in addition to a greater potential for reproductive and lactation problems.
Feed a grain/protein mixture that does not also contain vitamins and mineral
Separate animals by condition, and feed a grain/protein mixture to those under a body score of 5 and to lactating mother
Regularly evaluate each group and move animals in or out as necessary
Make sure a mineral mix is always available to all groups