Abstract: Ruminants are fed a water-insoluble polymer, which may be swollen (i.e. having absorbed water to form a gel) or unswollen. At least 0.04 kg of (swollen) polymers per day per 100 kg body weight is administered, and the polymer has not necessarily been used as a substrate for the preparation of feedstuff.Such methods are particularly advantageous when the animal is being fed on a diet causing acid conditions in the rumen, such as a diet of silage.A useful polymer is ammonium polyacrylate cross-linked by about 500 ppm methylene bisacrylamide ("MBA").The polymers enhance feed conversion rates and hence lower the effective cost of feeding the animals.