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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996
of 27.5 to 33 mg/kg dietary dry matter and that predicted DMI not be adjusted when lasalocid or laidlomycin propionate are added to the diet.
The second case relates to adjustments for use or nonuse of growth-promoting implants. As with ionophores, statistical evaluation of the 185 data points indicated no basis for adjustments to predicted DMI if growth-promoting implants were used. On the other hand, considerable research and field evidence suggest that such implants increase feed intake. Hence, the subcommittee suggests that values suggested by Fox et al. (1992) be used as a guideline for adjustments to predicted DMI in cases where implants are not used (e.g., 6 percent decrease in predicted DMI when implants are not used).
The third case deals with effects of forage allowance. Data presented in Predicting Feed Intake of Food-Producing Animals (National Research Council, 1987) relative to forage availability were reevaluated by Rayburn (1992). He constructed a quadratic regression of relative DMI on available forage mass. The resulting regression equation was
where FM is available forage mass =1,150 kg/ha. The FM value of 1,150 kg/ha represents the maxima of the quadratic equation (first derivative), and relative DMI is assumed to be 100 percent for FM greater than this maxima. For application to grazing situations, the subcommittee suggests that this relationship be used in two steps. First, the daily forage allowance (FA) should be determined
where grazing unit is the pasture size in hectares and SBW is in kg. If FM is =1,150 kg/ha, or FA is four times the predicted DMI (expressed as g/kg SBW), no adjustment should be made to the predicted DMI. If neither of these conditions is true, relative DMI should be calculated from the equation shown above, and the predicted DMI should be multiplied by the relative DMI (expressed as a decimal) to adjust predicted DMI for the effects of limited FM. This adjustment procedure should be applied to all types of grazing systems; however, rotational or other intensive grazing systems with heavy stocking rates will result in more rapid changes in FM than continuous systems with lower stocking rates. This necessitates careful attention to FM in intensive grazing systems and frequent reevaluation of relative DMI.
BEEF COWS: DIETARY ENERGY CONCENTRATION
The preceding edition of Nutrient Requirements of Beef Cattle (National Research Council, 1984) includes an equation for feed intake by breeding beef females similar in form to an equation for growing and finishing beef cattle; DMI is described as a function of SBW0.75, and linear and quadratic effects of dietary NEm concentration (DMI, kg/day=SBW0.75 * [0.1462 * NEm-0.0517 * NEm2-0.0074]). As with the growing and finishing equation, the description of how this equation was developed was inadequate in that publication. Predicting Feed Intake of Food-Producing Animals (National Research Council, 1987) provides an alternative equation for beef cows that described DMI as a linear function of dietary NEm concentration:
To further evaluate the relationship between dietary NEm concentration and intake by beef cows, an approach similar to that described previously for growing and finishing cattle was used. Treatment means for DMI were obtained from a variety of sources. Data were obtained from articles in the Journal of Animal Science (1979 through 1993), unpublished theses, and unpublished data that were solicited from individual scientists. The 153 data points used in the analysis represented treatment or breed×year means for DMI by nonpregnant beef cows or by cows during the middle and last one-third of pregnancy. As with growing and finishing beef cattle data, the beef cow data base contained a mix of full and shrunk body weights; the subcommittee assumed SBW in developing these equations. The data base was not sufficiently detailed to allow incorporation of information about body condition scores or frame sizes of the cows; and for some data points, only information on dietary NEm concentration and DMI per unit SBW0.75 was available. Dietary NEm concentration (range=0.76 to 2.08 Mcal/kg) was taken from the data source or calculated based on tabular values (National Research Council, 1984) for feeds. Total NEm intake was calculated as the product of DMI and dietary NEm concentration and expressed per unit SBW0.75 (average SBW0.75 during the intake measurement period). Data were then subjected to stepwise regression analysis (SAS Institute, Inc., 1987), with dummy variables included to account for the specific physiological stage of the cow.
It should be noted that data points were not included in the regression analysis when an obvious nutrient deficiency existed. This exclusion primarily impacted data points from beef cows fed low-quality forages that were deficient in crude protein. In such cases, only data from protein-supplemented cows were included in the data set. Hence, the resulting equation would not be applicable when the user wants to predict intake of a protein-deficient forage. Alternatively, the resulting equation would be applicable when the user wants to estimate total intake (e.g., forage plus supplement).
The relationship between dietary NEm concentration