The amino acid composition (AA) of the absorbed protein should be as similar as possible to the AA composition of milk protein. This will ensure that as little nitrogen as possible is removed into the environment. Microbial protein (MP) has a more favorable AA composition than most feeds, so we need to maximise MP synthesis. The most limiting AA in MP relative to milk protein is histidine. The AA composition of the feed prior degradation may be very different from the AA composition of the ruminally undegradable protein (RUP). Grasses and lucerne have a very similar AA composition of RUP, which is relatively favorable, but these proteins are poorly digested. In diets with lots of maize silage, lysine and methionine are the first limiting AA. Maize grain is a poor protein source with an unfavorable AA composition of RUP. Maize gluten is a good protein source, but of poor quality because of its low lysine content. Soybean meal is a good source of lysine and histidine, but a poor source of methionine. Rapeseed meal is a poorer source of essential AA (EAA) than soybean meal. Sunflower meal contains more methionine in RUP than any other feed described. In addition, it is a good source of histidine, but somewhat poorer source of lysine. Brewer's grains may have a very favorable AA composition of RUP, but a more detailed analysis is needed. The most limiting AA in a typical diet for dairy cows in Slovenia seems to be histidine.
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