achieve the targets. For pregnant animals, gain due to gravid uterus growth should be added to predicted daily gain (SWG), as follows:
where CBW is calf birth weight, kg. For pregnant heifers, weight of fetal and associated uterine tissue should be deducted from EQEBW to compute growth requirements. The conceptus weight (CW) can be calculated as follows:
where, CW is conceptus weight, kg; and t is days pregnant.
Net energy requirement for optimal growth of breeding heifer replacements can be determined for these rates of growth with the primary net energy requirement equations, using expected mature weight as FSBW.
In utilizing available forage, beef cows usually do not consume the amount of energy that matches their requirements for maintenance, gestation, or milk production. Reserves are depleted when forage quality and (or) quantity declines because of weather, overstocking, or inadequate forage management, but are replenished when these conditions improve. In addition, most beef cows are not housed and must continually adjust energy balance for changes in environmental conditions.
Optimum management of energy reserves is critical to economic success with cows. Whether too fat or thin, cows at either extreme are at risk from metabolic problems and diseases, decreased milk yield, low conception rates, and difficult calving (Ferguson and Otto, 1989). Overconditoning is expensive and can lead to calving problems and lower dry matter intake during early lactation. Conversely, thin cows may not have sufficient reserves for maximum milk production and will not likely rebreed on schedule. To maintain a 12-month calving interval, cows must be bred by 83 days after calving (365 minus an average gestation length of 282 days). Dairy cows usually ovulate the first dominant follicle, but beef cows average three dominant follicles being produced before ovulation, depending on the suppressive effects of suckling, body condition, or energy intake (Roche et al., 1992). Both postcalving cow condition score and energy balance control ovulation (Wright et al., 1992). Conception rates reach near maximum at body condition score 5 (Wright et al., 1992). Ovulation occurs in dairy cattle 7 to 14 days after the energy balance nadir is reached during early lactation (Butler and Canfield, 1989). Beef cows in adequate body condition with adequate energy intake may have a similar response because the negative effects of suckling may be offset by the lower energy demands of beef cows (W.R.Butler, Cornell University, personal communication, 1992). Allowing for three ovulations (assuming the first ovulation goes undetected), and allowing for two observed ovulations and inseminations for conception, the first ovulation must occur 41 days after calving. To allow this, the feeding program must be managed so that maximum negative energy balance during early lactation is reached by about 31 days after calving (41 days to first ovulation minus 10 days for ovulation after maximum negative energy balance). If the cow is too fat, intake will be lower and reserves will be used longer during early lactation, resulting in an extended time to maximum negative energy balance. Even if thin cows consume enough to meet requirements by 31 days, a feedback mechanism mediated through hormonal changes seems to inhibit ovulation if body condition is inadequate (Roche et al., 1992). Additional signals relative to the need for a given body condition before ovulation appear to occur in cows nursing calves.
In previous NRC publications, changes in energy reserves were accounted for by allowing for weight gain or loss. However, in practice, few producers weigh beef cows to determine if their feeding program is allowing for the appropriate energy balance. Energy reserves are more often managed by observing body condition changes, and all systems developed since the last NRC publication use condition scores (CS) to describe energy reserves. Body condition score is closely related to body fat and energy content (Wagner, 1984; Houghton et al., 1990; Fox et al., 1992; Buskirk et al., 1992). The CSIRO nutrient requirement recommendations (Commonwealth Scientific Industrial Research Organization, 1990) adapted the 0 to 5 body condition scoring system of Wright and Russel (1984a,b). In their system, a CS change of 1 contains 83 kg body weight change, which contains 6.4 Mcal/kg for British breeds and 5.5 Mcal/kg for large European breeds; this is equivalent to 55 kg and 330 Mcal/CS on a 1 to 9 scale. The INRA (1989) nutrient requirement recommendations use a 0 to 5 system also and assume 6 Mcal lost/kg weight loss, which is equivalent to 332 Mcal/CS on a 9-point scale.
The Oklahoma (Cantrell et al., 1982; Wagner, 1984; Selk et al., 1988) and Colorado groups (Whitman, 1975) developed a 9-point system for condition scoring. The Purdue group (Houghton et al., 1990) used a 5-point scale with minus, average, and plus within each point, which in effect approximates the dairy 1 to 5 system; both are similar to a continuous 9-point scale. Empty body lipid was 3.1, 8.7, 14.9, 21.5 and 27.2, respectively, for CS 1 to 5, which they proposed correspond to CS 2, 5, and 8 on the 1 to 9 scale. Empty body weights averaged 75 kg per increase in condition score, which is equivalent to 50 kg/CS on a 9-point system. The Texas group (Herd and Sprott, 1986) used a 9-point scale and reported 0, 4, 8, 12, 16, 24, 28,