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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996
after weaning. Based on MARC data, breeding bulls are assumed to be 67 percent heavier at maturity than cows, giving an SRW of 800 kg, which is the mature weight of a bull with the same genotype as the 1984 NRC medium-frame steer.
The EQSBW computed from the SRW/FSBW multiplier is then used in Eq. 3–7 to compute the NEg requirement. If Eq. 3–1 or 3–6 is used, SBW is adjusted to EBW with Eq. 3–4. Alternatively, the equation of Williams et al. (1992; EBW=full BW * [1-gut fill], where gut fill is 0.0534+0.329 * fractional forage NDF) can be used to predict EBW from unshrunk liveweight. Predicted gut fill is then corrected with multipliers for full BW, physical form of forage, and fraction of concentrates.
Because a table of requirements can be generated for any body size using the computer disk provided, only one example is shown (533 kg FSBW to represent the average steer in the United States). A similar table can be computed and printed for any body size with the computer disk containing the model. In this representative example, an FSBW change of 35 kg alters the NEg requirement by approximately 5 percent. Heifers and bulls with similar parents as the steers represented in this table have 18 percent greater and lesser, respectively, NEg requirements at the same weight as these steers. This system requires accurate estimation of FSBW. Most cattle feeders are experienced with results expected with feedlot finishing on a high-energy diet of backgrounded calves or yearlings that
Representative Example of Requirements
This example, a 320-kg steer with an FSBW of 600 kg (or herd replacement heifer with an MW of 600 kg) has an EQSBW of (478/600) * 320=255 kg. A 320-kg heifer with an FSBW of 480 has an EQSBW of (478/480) * 320=319 kg. The predicted SWG for the 320-kg steer consuming 5 Mcal NEg is (Eq. 3–7); 13.91 * 50.9116 * 255-0.6837=13.91 * 4.337 * 0.02263=1.365 kg/day. The SWG of the heifer consuming the same amount of energy will be 13.91 * 50.9116 * 319-0.6837=1.17 kg/day. To compute NEg requirement in this example 320-kg steer using Eq. 3–1 (0.891 * SBW to compute EBW and 0.956 * SWG to compute EBG): 255 * 0.891=227 kg EBW; 1.365 * 0.956=1.305 EBG; RE=0.0635 * 2270.75 * 1.3051.097=0.0635 * 58.5 * 1.339=4.97 Mcal. Assuming NEm requirement is 0.077 SBW0.75, the NEm requirement is (0.077 * 3200.75)=5.83 Mcal/day. Net protein requirement for gain is then (Eq. 3–8); 268-(29.4 * (5/1.365)) * 1.365=147 g/day. This value is then divided by the efficiency of use of absorbed protein to obtain the metabolizable protein required for gain (0.83-(0.00114 * EQSBW)), which is added to the metabolizable protein required for maintenance (3.8 * SBW0.75) to obtain the total metabolizable protein required. For the 320-kg steer, MP=147/(0.83-0.00114 ((478/600) * 320))+(3.8 * 3200.75)=560 g.
have received an estrogenic implant. Guidelines for other conditions are
reduce FSBW 25 to 45 kg for nonuse of an estrogenic implant,
increase FSBW 25 to 45 kg for use of an implant containing trenbolone acetate (TBA) plus estrogen,
increase FSBW 25 to 45 kg for extended periods at slow rates of gain, and
decrease FSBW 25 to 45 kg for continuous use of a high-energy diet from weaning.
A variety of anabolic agents are available for use in steers and heifers destined for slaughter to enhance growth rate, feed efficiency, and lean tissue accretion. Trade names, active ingredients, and restrictions on animal use for products currently available in North America are given in Table 3–3. With the exception of melengestrerol acetate (MGA), which is added to the feed, these products are implanted into the ear. They have been approved for use by the Food and Drug Administration in the United States and the Bureau of Veterinary Drugs in Canada, although not all of the products listed in Table 3–3 are approved in both countries. The mode of action of anabolic agents is not completely understood but, in the final analysis, they enhance the rate of protein accretion in the body (National Research Council, 1994). Effects of these agents on growth, body, and carcass composition have also been reviewed (Galbraith and Topps, 1981; Unruh, 1986).
These products enhance rate of gain and feed intake. Rate of gain is usually enhanced more than intake, and feed efficiency is also improved. Their effect on nutrient utilization is minimal, so their impact on requirements can
TABLE 3–3 Anabolic Agents Used for Growing and Finishing Cattle in North America