A lot of pellets are alfalfa based, which is believed to be not ideal for adult pigs due to the nutritional content of legumes, which is what alfalfa is, rather than a grass. I have three 9month old girls who are on Burgess Excell pellets (alfalfa based) but I'm planning on swapping them to a grainless (and alfalfa-less) pellet once they're grown up. I'm sure a very many pigs are fed alfalfa pellets their whole lifes without any problems