Metabolic Limits to Milk Production, Especially Roles of Growth Hormone and Insulin

Abstract

Potential of the bovine mammary gland to synthesize milk probably is determined fully at parturition. Realization of that potential depends on the gland receiving adequate amounts of nutrients in the correct proportions. Provided the cow is fed adequately in relation to her potential yield, supply to the body of those nutrients that are absorbed directly from the gut is not likely to limit milk synthesis. Glucose and long-chain fatty acids, however, are not absorbed from the gut in adequate amounts, and metabolic factors (primarily hormonal regulation) determining the supply of these may be important limits to milk synthesis. Even when amounts and proportions of metabolites entering circulation are adequate for maintenance and maximum milk synthesis, the cow's potential cannot be realized fully unless nutrients are channeled selectively towards the mammary gland. This occurs naturally in the high-yielding cow in early lactation and probably can be maintained artificially into midlactation by use of growth hormone. Consequences of this on milk output, health, and reproductive performance are considered.

Effects of Exogenous Growth Hormone on Milk Production and Nutrient Uptake by Muscle and Mammary Tissues of Dairy Cows in Mid-lactation

“…Overall, the data obtained in the present study provide the first direct evidence that exogenous GH alters the partition of nutrients in the lactating cow as has been proposed previously (see Bauman and Currie 1980;Bines and Hart 1982). Further, evidence for altered tissue utilization of nutrients was obtained.…”

“…Further, in studies of this type, exogenous GH has not altered digestibilities of dry matter, energy or nitrogen (Peel et al 1981;Tyrrell et al 1982). Thus it appears that GH alters the partition of nutrients such that nutrient supply to the mammary gland increases, perhaps at the expense of maintenance and/or growth of other tissues (see reviews by Bauman and Currie 1980;Bines and Hart 1982; see also McDowell et al 1987).…”

Formaldehyde-treated proteins for dairy cows – effects on blood hormone concentrations

“…By calculation, based on work by the Agricultural Research Council (1980), neither rumen degradable (RDP) nor undegraded dietary (UDP) protein supply was limiting for milk production, even for the untreated casein diet. The increases in GH, resulting from an increased supply of amino acids to the intestine, were small in comparison with increases in GH known to elicit improvements in milk yield (Bines & Hart, 1982). On these grounds the lack of a response in milk production to feeding formaldehyde-treated proteins was to be expected.…”

“…(1981) were likely to have had a high starch content -but so too did the diets used in our work. Alternatively, the discrepancy may have resulted from differences between experiments in the stage of lactation in which the experiments were conducted, or in basal levels of GH in the experimental animals (Bines & Hart, 1982) or possibly in assay specificity (Cowie et af. 1980).…”

“…When this has been done by abomasal infusion of casein the magnitude of response has been about 8% in milk yield and about 14% in milk-protein yield (Oldham, 1981) We have previously reported that increasing amino acid supply to the intestine, by abomasal infusion of casein, results in increased concentrations of growth hormone (GH) in blood of lactating goats (Oldham et al 1978). In view of the known effects of changing GH status on milk yield (Bines & Hart, 1982) this might suggest that part of the response in milk output to increased amino acid supply to the intestine is mediated via changes in GH status.…”