Date of this Version
Studies in numerous species provide evidence that diet during development can partially control physiological changes necessary for puberty (Frisch, 1984). Energy balance or plane of nutrition influences reproductive performance in heifers and cows (Butler and Smith, 1989; Randel, 1990; Robinson, 1990; Short and Adams, 1988; Swanson, 1989). Numerous studies have reported inverse correlations between postweaning growth rate and age at puberty (Arije and Wiltbank, 1971; Ferrell, 1982; Short and Bellows, 1971; Wiltbank et al., 1966, 1969, 1985) and pregnancy rates in heifers were shown to be dependent upon the number displaying estrus prior to or early in the breeding season (Byerley et al., 1987; Short and Bellows, 1971). Thus, rate of postweaning growth was determined to be an important factor affecting age of puberty, which influenced pregnancy rates. This and other research conducted during the late 1960s through the early 1980s indicated puberty occurs at a genetically predetermined size, and only when heifers reach their target weight can high pregnancy rates be obtained (reviewed by Patterson et al., 1992). Guidelines were established indicating replacement heifers should achieve 60 to 65% of their expected mature body weight by breeding. Traditional approaches for postweaning development of replacement heifers used during the last several decades have primarily focused on feeding heifers to achieve or exceed an appropriate target weight, and thereby maximize heifer pregnancy rates. Substantial changes in cattle genetics and the economy have occurred over this time, indicating traditional approaches should be re-evaluated. Intensive heifer development systems may maximize pregnancy rates, but not necessarily optimize profit or sustainability. These systems require significant use of fossil fuels and cereal grains, and high capital investment in equipment and facilities. Cereal grains, often used as an energy source in heifer diets, detract from the system’s sustainability due to growing demand for human food and ethanol production. Furthermore, almost all studies on heifer development conducted over the last half century have focused on production to first calving with little information concerning effects of heifer development systems on lifetime productivity.