Variability in the ranking of the three most-limiting amino acids for milkprotein production in dairy cows consuming grass silage and a cereal-basedsupplement containing feather meal

Previous experiments showed that histidine was clearly first-limiting for m ilk protein production in cows consuming grass silage and a cereal-based su pplement containing feather meal. Two experiments were carried out to inves tigate further the responses of milk production to the intravenous infusion of amino acids in dairy cows consuming this basal diet. In Experiment I, f our cows received the following treatments: (1) the basal diet (Basal); (2) Basal plus jugular infusion of 114 g d(-1) of essential amino acids contai ning 6 g d(-1) of histidine (EAA); (3) Basal plus histidine, methionine and lysine in the amounts supplied by EAA (3AA); and (4) Basal plus 6 g d(-1) of histidine (His). The EAA and 3AA treatments increased (P < 0.05) the yie ld of milk protein by 106 g d(-1) the corresponding increase (P < 0.05) for the His treatment was 60 g d(-1). This result confirmed those of earlier e xperiments in showing histidine to be clearly first-limiting. In Experiment 2, five cows received the following treatments: (1) basal diet (Basal); (2 ) Basal plus jugular infusion of (g d(-1)) histidine, 9.0, methionine, 10.0 , lysine, 25.5, and tryptophan, 4.8 (4AA); (3) Basal plus 4AA minus methion ine (-Met); (4) Basal plus 4AA minus lysine (-Lys); and (5) Basal plus 4AA minus tryptophan (-Trp). Milk protein yield was increased (P < 0.05) only b y the 4AA and -Trp treatments; the increases were 101 and 126 g d(-1) respe ctively. These results indicate that methionine and lysine were also Limiti ng. Taking the results of these experiments, together with those published previously, suggests that, despite all reasonable attempts to maintain a co nstant diet composition, the ranking of the three most-limiting amino acids varied between experiments. (C) 2000 Society of Chemical Industry.