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 (Short and Adams, 1988; Butler and Smith, 1989; Swanson, 1989; Randel, 1990; Robinson, 1990). Numerous studies have reported inverse correlations between post-weaning growth rate and age at puberty (Wiltbank et al., 1966, 1969, 1985; Short and Bellows, 1971; Arije and Wiltbank, 1971; Ferrell, 1982), and pregnancy rates in heifers were shown to be dependent upon the number displaying estrus prior to or early in the breeding season (Short and Bellows, 1971; Byerley et al., 1987). Thus, rate of postweaning growth was determined to be an important factor affecting age of puberty, which in turn 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 stating replacement heifers should be fed to achieve 60 to 65% of their expected mature body weight by the time breeding starts in order to reach puberty. Therefore, 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. However, 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. Developing heifers in this manner requires significant use of fossil fuels and cereal grains, and high capital investment in equipment and facilities. The fuel requirement to harvest feed and deliver it to cattle creates high energy demands in this development system. Cereal grains, often used as a major energy source in heifer diets, detract from the system’s sustainability due to growing demand for human food and ethanol production.
Since the inception of the target weight guidelines, subsequent research demonstrated the pattern of growth heifers experience prior to achieving a critical target weight could be varied. This provides an opportunity to decrease feed costs by altering rate and timing of gain, creating periods of compensatory growth and/or allowing producers to limit supplementation to critical periods of heifer development (Clanton et al., 1983; Lynch et al., 1997; Freetly et al., 2001). For example, delaying heifer gain until 47 or 56 d prior to the breeding season did not negatively influence reproductive performance, but reduced the amount of feed needed (Lynch et al., 1997). In one year of this study, puberty was delayed in heifers fed to achieve lower early gains, but first-service conception rate tended to be improved in these same heifers. Similarly, Freetly et al. (2001) found delaying gain until the later part of the post-weaning period reduced total energy intake, but calving rate, age at calving, postpartum interval, and second year pregnancy rate were not impacted. This suggests total energy intake, and possibly heifer development costs, may be reduced by limiting heifer gain early in the post-weaning period followed by accelerated gains before the breeding season.