Effects of Supplementary Amino Acids on Milk Production in Dairy Cows Consuming Diets Low in Histidine

1. Experiments in this thesis were conducted to investigate effects of supplementary amino acids on milk production in dairy cows consuming grass silage and a supplement containing feather meal which is low in histidine. The primary object was to identify a basal diet showing a clear deficiency of the first-limiting amino acid for milk production, such that supplementing the supply of this AA, either direct into the abomasum or intravenously, would evoke sizable increases in the yield of milk protein so allowing the efficiency of incorporation into milk protein to be calculated. 2. A series of four experiments was conducted to identify the first-limiting AA for milk production, and to determine the magnitude of the response of milk production to supplementation with the limiting AA. Experiment 1 examined responses of milk production to continuous intravenous infusion of 9.7 g/d histidine, 9.1 g/d methionine, 30.0 g/d lysine and 2.6 g/d tryptophan (4AA); infusion of the AA mixture without histidine (- His); and infusion of the AA mixture without lysine (-Lys). Since methionine has been shown to be not limiting in a previous experiment with this basal diet, infusion of the AA mixture without methionine was not included in the experimental treatments. The yields of milk protein were 740, 874, 715 and 851 g/d (SED 32.8) for the basal, 4AA, -His and -Lys treatments respectively. The response was not diminished by omission of lysine but was cancelled by omission of histidine, so indicating that histidine was the first-limiting amino. In Experiment 2 a single dose (9.7 g/d) of histidine was given intravenously but there was no significant effect of histidine supplementation on milk production. Experiment 3 was conducted to examine responses of milk production to increasing intravenous doses (3, 6 and 9 g/d) of histidine alone. All levels of histidine increased the yield of milk protein over the basal treatment, the respective values being 670, 700, 781 and 699 g/d (SED 22.2) for Basal, 3, 6 and 9 g/d of histidine supplementation. In Experiment 4, increasing intravenous doses of 0, 3, 6 and 9 g/d of histidine were supplemented with the combination of 8 g methionine, 28 g lysine and 2.5 g tryptophan. The addition of the other acids was aimed at ensuring that histidine remained first-limiting as its level of infusion was increased. Increasing doses of histidine produced linear increases in the yield of milk protein [727, 785, 826 and 879 g/d (SED 18.6) for the 0, 3, 6 and 9 g/d levels, respectively]. 3. Two experiments were conducted to test the hypothesis that an increase of milk production would follow from delivery of extra histidine into the abomasum as a result of dietary addition of an effective rumen-protected form of the AA or from dietary addition of proteins rich in histidine and of low rumen degradability. In Experiment 5 effects of intravenous and intra-abomasal administration of two levels (3 and 6 g/d) of histidine were compared. None of treatments affected the yield of milk or milk constituents. Experiment 6 examined responses of milk production to progressively substituting avian blood meal (rich in histidine and poor in methionine) for part of the feather meal in its supplement. Blood meal cubes were substituted for 0, 0.10, 0.20 and 0.40 of the feather meal cubes in the supplement. Substitutions of blood meal cubes increased the yield of milk protein [593, 679, 681 and 701 g/d (SED 25.2) for 0, 0.10, 0.20 and 0.40 of blood meal substitution for feather meal, respectively] but there was no further increase beyond the first level. 4. Two experiments were conducted to characterize more fully the dietary 'model'. Experiment 7 was designed to determine the sequence of limitation of the AAs for milk production when each of three AAs was omitted in turn from a continuous intravenous infusion supplying a mixture of the group of probable next-limiting AAs after histidine (methionine, lysine and tryptophan), histidine being retained in all infusates. The yields of milk protein were 771, 872, 784, 812 and 897 g/d (SED 27.5) for the basal, 4AA, methionine-free, lysine-free and tryptophan-free treatments, respectively. The results indicated that methionine was likely to be the co-limiting AA or the very close second- limiting AA. Experiment 8 was carried out to determine how much of the milk production response to addition of all the essential AAs could be obtained by histidine alone or the group of the next 3 most-limiting AAs. The four treatments were the basal treatment (Basal), continuous infusion supplying a mixture of essential AAs (EAA), infusion of a AA mixture of histidine, methionine and lysine (3AA), and infusion of histidine alone (His). The yields of milk protein were 518, 625, 626 and 578 g/d (SED 24.4) for the basal, EAA, 3AA and His treatment, respectively. No extra response was seen to EAA over that seen with 3 AA and histidine alone accounted for about 56% of the maximum response.