Sources of variability in lactational performance.

Variation in lactational performance comes about as a result of variation in the nature of the animal, its environment, or with time. Table 1 lists a variety of sources of variability, classified under broad headings of variation with the animal (species, breed and individual), with time and with various aspects of the environment. The list is probably not comprehensive, but even so, it contains more issues than can be dealt with in depth here. So we have chosen to select for comment a few issues in which we have a particular interest and which are likely to interest members of this Society. An important feature in any consideration of lactational performance must be to note that ‘lactational performance’ has many dimensions. There are elements of milk secretion, change in maternal state, the growth and well-being of offspring and, in species which display lactational oestrus, the initiation of a subsequent cycle of reproduction. Each of these elements of lactational performance varies depending on species, environment and time, but not usually independently of each other. So, for example, animals which are thin at parturition might adjust both the amount of food they choose to eat and their rate of milk secretion in relation to the extent and state of their body tissue reserves. There may also be metabolic adjustments which alter the likelihood of renewed conception. It is the concerted set of events which represents lactational performance. The orchestration of the various elements of overall performance is subject to a variety of controls which deal both with tactical (homeostasis) and strategic (homeorhetic) aspects of metabolism (Bauman & Cume, 1980). A thorough appraisal of variability in lactational performance requires knowledge of each of the different contributing elements. Unfortunately, many reports of lactation concentrate only on the milk secretory process and tend to ignore other, associated, changes. We have tried here to recognize and emphasize the multi-dimensional aspect of lactational performance, and to emphasize the important interactions between the nature of the animal and its environment in determining temporal changes in overall lactational performance. A number of important issues are dealt with superficially or not at all. In addition to papers in this symposium, the following are useful reference sources which deal with variability in lactational performance in relation to evolutionary strategies, the thermal