Sources of variability in lactational performance

Abstract

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 Table 1. Sources of variability in lactational performance classifid according to animal, environment and time

Milk production and composition in captive Iberian red deer (Cervus elaphus hispanicus): effect of birth date.

“…The greater production in this study compared with Garcia et al (1999) might be attributed to an increase in production from the second to the third lactation, as it has been observed in cattle up to the fourth lactation (Oldham and Friggens, 1989), to a lower estimate of double weighing compared with milking or to an increase in BW between studies (our unpublished data). However, the latter hypothesis does not seem very likely because milk yield reported here did not depend on hind BW.…”

“…A number of factors should be considered when assessing lactation performance, and changes in maternal BW and milk composition are the most important (Oldham and Friggens, 1989). In deer, one of these factors is birth date: Hinds shift from BW gains to losses during lactation, the later they breed, and their calves gain more weight at the peak of lactation, the earlier they are born (Adam and Moir, 1987).…”

Lactation, maternal behavior and infant growth in common marmoset monkeys ( Callithrix jacchus ): effects of maternal size and litter size

“…So while small mothers did draw more heavily on reserves (as suggested by weight loss), the reserves drawn upon were inadequate to meet the lactation demand, resulting in lower milk energy output. This result is similar to that found in other mammals (Mattingly and McClure 1985;Roberts et al 1985;Oldham and Friggens 1989;Dewey 1997;Rogowitz 1998). It provides further support for the proposal that, across a fairly wide variety of mammals, maternal investment as reflected in milk yields is sensitive to maternal energy balance.…”

Effect of reproductive rest on the subsequent breeding in Iberian red deer hinds (Cervus elaphus hispanicus)

“…Although dominance has been related to reproductive success in red deer (Clutton-Brock et al, 1984;1986b;1988) and a few other species (Silk, 1983), it is well established that one of its most important components, fecundity, in cervids (Sadleir, 1987) and cattle (Oldham and Friggens, 1989) is directly related to female body weight. Thus, hinds that have reproductive rest at the preceding year might gestate earlier and provide the next year's offspring with a better start in life.…”