FIGURE 4–2 Relationship of fetal weight to day of gestation in cattle.

The Role of the Placenta

Functions of the placenta include exchange of metabolites, water, heat, and respiratory gasses. The placenta also serves as a site of synthesis and secretion of numerous hormones and extensive interconversion of nutrients and other metabolites (Munro et al., 1983; Battaglia, 1992). Placental transport of oxygen, glucose, amino acids, and urea and placental clearance of highly diffusible solutes increase during gestation as indicated by net fetal uptake or loss in both sheep and cattle (Bell et al., 1986; Reynolds et al., 1986). Because of the numerous metabolic functions of the uterus and placenta (uteroplacenta), oxidative metabolism is extensive throughout gestation. Even in late gestation when the fetus is several times larger than the placenta, energy consumption of the uteroplacenta is about equal to that of the fetus (Reynolds et al., 1986). Similarly, uteroplacental net use of glucose is at least 70 percent of gravid uterine glucose uptake, even in late gestation. Likewise, a major proportion of the net use of amino acids taken up from the uterine circulation is metabolized by the uteroplacenta (Reynolds et al., 1986; Ferrell, 1991b). An increase in maternal metabolism is also required to support the requirements of pregnancy. Thus, of the total increase in energy expenditure associated with pregnancy, about one-half may be attributed to metabolism of tissues of the gravid uterus and about one-fourth maybe attributed to the fetus per se (Kleiber, 1961; Ferrell and Reynolds, 1987).

Energy Requirements

Energy accretion in the gravid uterus of Hereford heifers bred to Hereford bulls has been reported by Ferrell et al. (1976a). The equation used to describe the relationship of energy content of the gravid uterus (Ye) vs day of gestation (t) in kcal, was

Eq. 4–1

Similar values can be calculated from the data of Prior and Laster (1979) who used crossbred heifers bred to Brown Swiss bulls, and from the data of Jakobsen (1956) and Jakobsen et al. (1957) who used Red Danish cattle. Other information related to bovine fetal growth and weight change of the pregnant cow is available (Winters et al., 1942; Ellenberger et al., 1950; Eley et al., 1978; Silvey and Haycock, 1978).

Eq. 4–1 was associated with a predicted calf birth weight of 38.5 kg. Scaling Eq. 4–1 by birth weight yields the following equation (kcal):

Eq. 4–2

This equation may be differentiated with respect to t to estimate daily energy accretion in the tissues of the gravid uterus, yielding (kcal/day):

Eq. 4–3

The gross efficiency of metabolizable energy (ME) use for accretion in the gravid uterus of cattle averaged 14 percent (Ferrell et al., 1976b). Other estimates with cattle and sheep average about 13 percent (Graham, 1964; Langlands and Southerland, 1968; Lodge and Heaney, 1970; Moe et al., 1970; Moe and Tyrrell, 1971; Sykes and Field, 1972; Rattray et al., 1974; Robinson et al., 1980). Some of the potential reasons for the low estimates of apparent gross efficiency have been discussed previously. Use of

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