Overall, increasing amino acid supply, whether by changes in protein feeding or by postruminal infusion of protein or amino acids or intravenous infusion of amino acids, has variable effects on the secretion of milk fat. However, it is well known that supplements of methionine often increase the yield and concentration of milk fat and this occurs when the methionine is added to the diet, in rumen-protected form (Overton et al., 1996) or as methionine hydroxy analogue (MHA) (Huber et al., 1984), or infused postrumi-nally (Varvikko et al., 1999) or intravenously (Chamberlain and Thomas, 1982). It has been suggested that the effects of methionine on the secretion of milk fat might be linked to a stimulation of the synthesis of lipoproteins and hence increased transport of triglycerides to the udder (Pullen et al., 1989).
However, the effects of amino acid supply on the secretion of milk fat are not limited to methionine. Diets deficient in histidine led to increased concentrations of milk fat (Chamberlain et al., 1992). Moreover, intravascular infusion of amino acid mixtures deficient in histidine markedly increased the yield and concentration of milk fat (Kim et al., 1999, 2001b; Cant et al., 2001) and reintroduction of histidine returned the fat concentration to the starting level (Kim et al., 1999, 2001b; Fig. 20.8). It is difficult to know whether the effects of amino acid supply are due to specific actions of particular amino acids, such as methionine and histidine, or whether they relate to amino acid balance more generally. It is known that, in the experiments of Kim et al. (1999, 2001b), histidine was the first-limiting amino acid for secretion of milk protein and the infusion of amino acid mixtures lacking histidine would therefore fit the classical definition of an amino acid imbalance (Harper et al., 1970). In many of the experi-
Basal AA -His AA+His
Fig. 20.8. The effect on the secretion of milk fat of the intravenous infusion of a mixture of amino acids with or without histidine. The infusions consisted of (g day 1): methionine, 8; lysine, 28; tryptophan, 2.5 (AA-His); and histidine, 6 (AA+His). (Data of Kim et ai., 2001b.)
merits in which adding methionine to the diet has increased the yield or content of milk fat, it was not the first-limiting amino acid for protein secretion, as judged by the absence of any effect on the yield of milk protein (e.g. Varvikko et al., 1999). In these circumstances, addition of methionine might be expected to induce an amino acid imbalance. A mechanism whereby increases in fat secretion might arise from supplying the mammary gland with an unbalanced mixture has been suggested by Cant et al. (1999). From the observation by Bequette et al. (1998) that a deficiency of histidine led to a marked increase in mammary blood flow, Cant et al. (1999) suggest that the increased blood flow might lead inevitably to an increased uptake of fat precursors by the gland. That an increased fat secretion would follow automatically from an increased mammary blood flow is supported by the finding that the mammary uptake of fat precursors seems to be regulated primarily by their arterial concentrations (Nielsen and Jakobsen, 1994). Whether the increased mammary blood flow is a response specifically to deficiency of histidine is not known but it is pertinent to note that a similar tendency was observed during an experimentally imposed deficiency of leucine (see Bequette et al., 2000), raising the possibility that the effect might be more general.
Other circumstances in which an enhanced supply of amino acids stimulates secretion of milk fat may also be related to effects on amino acid balance. As already mentioned (see above), abomasally infused hydrolysates of casein or corresponding mixtures of free amino acids were less effective than whole casein in stimulating the secretion of milk protein. It was suggested (see above) that the inferior response to the hydrolysates and free amino acids might relate to effects on the balance of amino acids in portal blood. It is noteworthy that the lower response of milk protein was accompanied by very marked increases of milk fat concentration and yield (Choung and Chamberlain, 1993b, 1995a), again suggesting a link between amino acid balance and the secretion of milk fat. More information is needed before reaching firm conclusions but it is tempting to speculate that changes in milk fat might be a useful indicator of the optimum balance of amino acids in infused mixtures. Extending this idea to the evaluation of practical diets might be more difficult because of the intervention of the other dietary factors known to affect milk fat. But, at the very least, we should add amino acid balance to the list of dietary factors that affect the concentration of milk fat and attention should be given to defining how the various factors interact. When added to the results of the infusion studies described here, the finding that intraperitoneal injection of a mixture of branched-chain amino acids (BCAA) and arginine offset milk-fat depression in cows given a low-fibre diet in early lactation (Hopkins et al., 1994) suggests that amino acid supply might have a more pronounced influence on milk fat secretion than hitherto suspected.
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