5- to 10-year-old cows (Beef Improvement Federation, 1990; Gregory et al., 1990). Birth weight of calves born as twins is 25 percent less, but the total weight of twins average 150 percent of the birth weight of calves born as singles (Gregory et al., 1990).

Severe energy or protein underfeeding has resulted in marked reductions of calf birth weight (Hight, 1966, 1968a,b; Tudor, 1972). Inadequate food intake during late pregnancy is also associated with weak labor, increased dystocia, reduced milk production and growth of progeny, and lowered rebreeding performance of the dam (Bellows and Short, 1978; Kroker and Cummins, 1979). Conversely, gross overfeeding during pregnancy can also result in reduced birth weight and subsequent decreased milk production, increased dystocia and neonatal death loss, and poor rebreeding performance (Arnett et al., 1971; Robinson, 1977). The relationship of calf birth weight to cow condition score is typified by data shown in Figure 4–1. Birth weight decreased as cow condition score decreased below 3.5 or increased above 7, but did not change within the range of cow condition scores of about 3.5 to 7. It is suggested that calf birth weight is not substantially influenced by cow nutritional status within a broad range, but may be reduced by extreme over- or underfeeding. In those situations, negative influences on rebreeding performance, dystocia, etc., are of greater concern than calf birth weight.

Effects of Temperature

Although this section is primarily concerned with factors affecting calf birth weight, it is important to note that high environmental temperature during or shortly after conception can significantly increase embryonic mortality in cattle as well as several other species (Bell, 1987). In addition, high environmental temperatures, particularly during early pregnancy, may result in a wide range of

FIGURE 4–1 Relationship of calf birth weight to cow condition score in mature cows of nine breeds.

congenital defects. Limited data are available from well-controlled studies of cattle to characterize the influence of elevated temperatures on calf birth weight (Collier et al., 1982) and, to this subcommittee’s knowledge, no data are available from controlled experiments to characterize influences of chronic cold exposure, although these effects have been well documented in sheep (Alexander and Williams, 1971; Rutter et al., 1971, 1972; Cartwright and Thwaites, 1976; Thompson et al., 1982; Bell, 1987). Numerous data are available, however, to indicate that calves born in the spring are heavier than those born in the fall (McCarter et al., 1991a), calves born in the northern areas of the United States are heavier than those born in southern areas, and that genotype/environment interactions may have important influences on calf birth weight (Burns et al., 1979; Olson et al., 1991). The magnitude of response of calf birth weight to environmental temperature is influenced by severity, duration, and timing of exposure as well as genotype of the dam.

Factors Affecting Fetal Growth

Considerable progress has been made toward understanding how various factors affect fetal growth and the ensuing birth weight. Normal fetal growth follows an exponential pattern (Figure 4–2). In cattle, weight of uterine and placental tissues also increase exponentially (Ferrell et al., 1976a; Prior and Laster, 1979). Growth and development of the uterus and placental tissues precedes fetal growth. Development of those tissues is required to support subsequent fetal growth (Ferrell, 1991b,c). Growth of the fetus is a result of its genetic potential for growth, which is reflected in its demand for nutrients and constraints imposed by the maternal and placental systems in meeting that demand (Gluckman and Liggins, 1984; Ferrell, 1989). The potential of the maternal and placental systems to meet those demands are reflected in uterine blood flow or placental size and functional capacity. The influence of maternal nutrition on fetal development is complicated by the fact that the fetus can be undernourished in well-fed mothers when placental size or function is inadequate to meet fetal demands. Conversely, even though the mother is undernourished, the maternal and placental systems may compensate such that fetal malnutrition is minimal (Bassett, 1986, 1991). Weight and perfusion of uterine and placental tissues are reduced with heat (Alexander and Williams, 1971; Cartwright and Thwaits, 1976; Reynolds et al., 1985; Bell et al., 1987) and with twins as compared with single fetuses (Bellows et al., 1990; Ferrell and Reynolds, 1992). These variables are also influenced by genotype of sire, dam, or fetus (Ferrell, 1991c). Numerous other data are available to indicate that perfusion of uterine and placental tissues and functional capacity of the placenta have central roles in fetal growth (Alexander, 1964a,b; Owens et al., 1986).

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