Both age and weight at puberty differ substantially among breeds of cattle (Laster et al., 1972, 1976, 1979; Stewart et al., 1980; Sacco et al., 1987). Within beef breeds, those having larger mature size tend to reach puberty at a later age and heavier weight. Bos indicus heifers tend to reach puberty at an older age than Bos taurus heifers, and heifers from higher milk-producing breeds are generally younger at puberty than those from breeds having lower milk production. Some of those differences are likely the result of direct maternal effects expressed through higher rates of preweaning gain by calves from higher milk-producing breeds.
Numerous data are available that indicate that neither age nor weight is a reliable indicator of reproductive development but that threshold values for both age and weight must be reached before puberty can occur. This conclusion is similar to the “physiological maturity” concept proposed by Joubert (1963) and to the “target weight” concept proposed by Lamond (1970). These concepts have been used by Spitzer et al. (1975), Dziuk and Bellows (1983), and Wiltbank et al. (1985) to suggest that replacement heifers should be fed to reach a preselected or “target” weight at a given age. Heifers of most Bos taurus breeds of cattle are expected to reach puberty by 14 months old or younger, if fed adequately. However, threshold ages of some heifers of Bos indicus breeds may be older than 14 months. Generally, heifers of typical Bos taurus beef breeds (e.g., Angus, Charolais, Hereford, Limousin) are expected to reach puberty at about 60 percent of mature weight. Heifers of dual purpose or dairy breeds (e.g., Braunvieh, Brown Swiss, Friesian, Gelbvieh, Red Poll) tend to reach puberty at a younger age and lower weight, relative to mature weight (about 55 percent of mature weight) than those of beef breeds. Conversely, heifers of Bos indicus breeds (e.g., Brahman, Nellore, Sahiwal) generally reach puberty at older ages and heavier weights (about 65 percent of mature weight) than those of Bos taurus beef breeds (Laster et al., 1972, 1976, 1979; Stewart et al., 1980; Ferrell, 1982; Sacco et al., 1987; Martin et al., 1992; Gregory et al., 1992b; Vera et al., 1993).
Mature weight refers to weight reached at maturity by cows of the same genotype in a nonrestrictive environment (for example, mature weight as determined by genetic potential). In a restrictive environment (high environmental temperature, limited nutrition, parasite loads, etc.), mature weight of cows is often less than that of cows of similar genotype maintained in a less restrictive environment (Butts et al., 1971; Pahnish et al., 1983). Heifer weight at puberty is also reduced, but to a lesser extent than is mature weight. Thus, under those types of conditions, weight at puberty is generally a greater percentage of observed mature weight than described above (Vera et al., 1993).
If the target weight and age to reach puberty are established, and present age and weight are known, rates of gain needed to achieve the target weight and age can easily be calculated. Energy and protein needs to meet those rates of gain can be estimated by use of the previously described net energy and net protein equations for growing heifers. Excessive feeding should be avoided. In addition to increasing feed costs, overfeeding that results in excess fat accretion may have detrimental effects on expression of behavioral estrous, conception rate, embryonic and neonatal survival, calving ease, milk production, and productive life.
Composition of weight change in growing and mature cattle has been discussed in other sections and will not be discussed in detail here. In the mature cow, weight change, with the exception of weight change associated with pregnancy or parturition, primarily reflects change in body condition. In the developing heifer, percentage of body fat and body condition may decrease, even though weight may continue to increase because of skeletal and muscle growth at the expense of body fat. In both the heifer and cow, weight gain associated with pregnancy and weight loss at parturition should not be construed as change in maternal weight or condition. Weight gain during pregnancy and loss at parturition is about 1.7 times calf birth weight and represents weight gain or loss of the fetus, fetal fluids, placenta, and uterus. For many practical purposes, subjective evaluation of body fatness by use of a visual condition scoring system (1=thinnest, 9=fattest) is frequently of benefit. More accurate methods are available for measuring body composition, but their use is generally limited to experimentation because of high costs or amount of labor required.
Death of calves perinatally represents a major production loss for beef cattle. Neonatal mortality is related to birth weight with the greatest losses occurring at low and high birth weights and lower mortality associated with moderate birth weights. Because dystocia, which is positively associated with birth weight, is a major cause of neonatal calf death (Laster and Gregory, 1973; Bellows et al., 1987), some cattle producers have attempted to reduce calf birth weight, particularly in first calf heifers, by underfeeding during the last trimester of pregnancy. As noted previously, malnutrition must be relatively severe to result in substantial reductions in calf birth weight. In nine studies reviewed by Dunn (1980), birth weight was reduced in all but one by severe underfeeding, but dystocia was reduced in only one (Dunn and Moss, 1992); but by underfeeding sufficiently to reduce birth weight, calf survival was reduced. In addition, numerous data (Short et al., 1990; Ferrell, 1991; Dunn and Moss, 1992) indicate the interval from