not fat (SNF) was 8.31±1.38 percent (10 studies), and for lactose was 4.75±0.91 percent (5 studies). Energy content (E, Mcal/kg) of milk may be calculated as follows (Tyrrell and Reid, 1965):
Committees of the National Research Council (1984, 1989) concluded that ME is utilized for lactation and maintenance with similar efficiencies; thus the energy content of the milk produced is equivalent to the NEm required for milk production (National Research Council, 1984). Data reported by Moe et al. (1970, 1972), Patle and Mudgal (1976), van der Honing (1980), Agricultural Research Council (1980), Garrett (1980b), Moe (1981), Munger (1991), Windisch et al. (1991), Gadeken et al. (1991), and Unsworth (1991), among others, support this conclusion. Although limited data are available, differences among breeds in efficiency of ME use for milk production appear to be minimal.
Beef cattle are managed under a wide variety of conditions. To a large extent their usefulness lies in their ability to harvest and utilize feed resources available under existing environmental conditions. The large variation in animal genotypes, environmental conditions, and available feed resources presents a challenge in determining and applying nutrient requirement guidelines. Providing nutrients to meet animal requirements is necessary for attainment of maximum production levels. However, it is frequently not economically advantageous to feed beef cattle in the breeding herd to meet their nutrient requirements throughout the year. Production levels to maximize net economic return vary based on interrelationships among numerous factors including, but not limited to, feed resources available, animal genotype, physiological state, costs of supplements, and environmental conditions. It should be recognized, however, that if the animals’ nutrient requirements are not met during part of the year, deficits must be made up during other parts of the year if production is to be maintained.
In grazing, as in nongrazing situations, maximum efficiency of diet utilization is attained by providing nutritionally balanced diets. When energy is first limiting, for example, protein, minerals and vitamins are not efficiently utilized. Supplemental protein, in this case, will be used to meet energy needs until energy and protein are equally limiting. Conversely, if protein is first limiting, provision of additional energy will not improve performance and may in fact depress performance. These concepts are applicable to other nutrients as well, i.e., performance is limited to that which is supported by the first-limiting nutrient. In the grazing animal, the quantity and quality of forages are of primary concern because they provide the nutrient base. The most limiting nutrients are especially difficult to establish for grazing cattle because the quantity and quality of the diets selected by the animal are difficult to assess. This is of less concern when minimal variation in forage quality results in limited opportunity for selectivity, such as occurs most commonly during spring and winter grazing.
The ultimate result of malnutrition of the beef herd is a reduction in the number of viable offspring produced. Influences of malnutrition are seen through effects on attainment of puberty, duration of the postpartum estrus, gametogenesis, conception rate, embryonic mortality, prenatal development, and sexual behavior. Some of these effects will be discussed briefly in subsequent sections. Readers are referred to recent reviews by Hurley and Doane (1989), Robinson (1990), Short et al. (1990), Ferrell (1991a), Dunn and Moss (1992), Schillo et al. (1992), and Patterson et al. (1992) for greater detail.
Age at puberty is an important production trait in cattle because many of the currently used management systems require that heifers be bred, during a restricted breeding system, at 14- to 16-months-old to calve at 2 years old. Heifers that reach puberty early and have a number of estrous cycles prior to the breeding season have a higher conception rate and conceive earlier in the breeding season than ones that reach puberty later. In addition, heifers that conceive early in their first breeding season have a greater probability of weaning more and heavier calves during their productive lifetime.
Underfeeding, resulting in low growth rate of heifers, delays puberty in cattle; and the effects are more pronounced when applied in the early prenatal phase than when applied immediately prepubertal. In an extreme example, Rege et al. (1993) reported age at first calving in White Fulani cattle in Nigeria to be as late as 2,527 days (6.9 years). As an example of more typical conditions in temperate regions, Angus-Hereford crossbred heifers fed to gain 0.27, 0.45, or 0.68 kg/day reached puberty at an average age of 433, 411, and 388 days old, respectively (Short and Bellows, 1971). Although these differences are relatively small, pregnancy rates after a 60-day breeding season were 50, 86, and 87 percent, respectively.