Previous studies have indicated that supplements to high quality grass silage based on molasses and sodium bicarbonate can enhance microbial efficiency and milk output. However these experiments showed that there was still a need to include preformed protein in the supplement to maximise the response. Also, work at this Institute (M. Gill, unpublished data) had indicated marked responses to branched chain volatile fatty acids (vfa) in high forage diets, but only in the presence of protein supplements. The aims of the experiment were to firstly examine whether preformed proteins in high forage diets could be replaced by either protected or unprotected amino acids (methionine and lysine) in dairy cow diets and secondly, to investigate the interaction between branched chain acids and the supply of protein.

Eighty British Friesian cows in their second and subsequent lactation were offered grass silage ad libitum together with 4 kg of molasses and sodium bicarbonate (Alkacarb, ICI plc) at 30 g/kg silage dry matter (DM). They were given a protein supplement consisting of soyabean meal and fishmeal in a 3:1 ratio at either 2 (L) or 3.2 kg/day (H).