The physiological bases of nutrient responses during lactation.

The lactating dairy cow responds to dietary changes according to the nutrients provided and her propensity to produce milk. The allocation of nutrient use to milk production or body gain is generally termed ‘partition’ and has been shown to be a function of both the animal and its diet. As feed intake is increased, the lactating cow responds with progressively smaller increments of increased milk yield and progressively larger increments of body tissue. This ‘diminishing return’ is not associated with a change in efficiency of any physiological process. It is the result of changes in the rates of synthesis of milk components and of body tissue. The individual rates of milk synthesis and body tissue synthesis may be influenced independently. It is therefore somewhat awkward to speak of control of partition as though it were a single physiological process. We are concerned with the synthesis of specific milk constituents and the role of specific metabolites in influencing the rates of synthesis and, thereby, milk yield and composition. It is unfortunately very difficult to tell if milk production is limited by nutritional state, i.e. nutrient supply, or by the hormone balance or other physiological factors involved in milk synthesis. There is very clearly a close interaction between nutrient supply and physiological control of milk synthesis. Bauman & Currie (1980) used the term ‘homeorhesis’ which they credit to Kennedy (1967) to refer to the coordinated changes in metabolism of body tissues necessary to support a physiological state such as lactation. It is clear that nutrient response is altered by physiological state and that for a given physiological state, milk production will be influenced by nutrient supply. Recent reviews have extensively discussed the endocrine control of partition and milk biosynthesis (Hart, 1983; Akers, 1985)’ mammary-gland differentiation (Houdebine, 1985), uptake and utilization of amino acids by the mammary gland (Clark, 1975; Mepham, 1982), and glucose homeostasis (McDowell, 1983). It is apparent that the control of milk synthesis or partition involves both substrate availability and endocrine control. The effects of substrate availability may be presumed from experiments in which dietary manipulation has caused a change in rate of milk synthesis and a corresponding change in body-tissue energy balance.