When a calf drinks milk or a milk replacer based from skim milk, it goes into the abomasum. Within ten minutes, the milk forms a clot in the abomasum from the coagulation of milk protein or casein, the enzymes rennin and pepsin, and the hydrochloric acid in the abomasum. Other milk components, primarily whey proteins, lactose and most minerals separate from the curd and rapidly pass into the small intestine (as much as 200 ml per hour). The lactose is digested quickly and, in contrast to casein and fat, provides immediate energy to the calf. The clot is then slowly absorbed by the blood stream over the next 12–18 hours.
Ammonia is used as a nitrogen source for microbial growth and VFAs absorbed from the rumen are a key energy source for the cow. Increasing the rumen-available energy content of the diet in the form of sugar and starch stimulates papillae growth, improving VFA absorption. While rumen fermentation allows good use to be made of fibrous feeds that could not otherwise be digested, it does mean only around 70-85% of the energy in the feed is available to the animal - 6-15% commonly being lost as gases (mainly methane) and 6-7% as heat.
Betaine has been proposed to increase leanness and enhance feed efficiency in finishing pigs, but research data has shown mixed results to support this claim . Betaine is a choline metabolite that is involved in osmotic regulation, methionine metabolism, and serves as a methyl donor. The impact of betaine in pigs is dependent on the sulfur amino acid and energy concentrations in the diet with the response to betaine being greater in diets low in energy and marginally deficient in sulfur amino acids (methionine, cystine). Betaine’s role as a methyl donor could have a sparing effect of methionine when it fills this role in diets that are marginally deficient in methionine .