greater with high- than with low-quality forages (Galyean and Goetsch, 1993).
Grinding feeds can affect intake, but effects depend on the type of feed. With forages, fine grinding can increase intake, presumably through effects on digesta passage (Galyean and Goetsch, 1993). With concentrates, fine grinding often decreases feed intake. Adjustments to intake predictions for finely processed diets as a function of dietary NEm concentration have been suggested (National Research Council, 1987). Fermentation of feeds by ensiling generally has little effect on DMI unless the silage is unusually wet or dry and undesirable fermentation has occurred (National Research Council, 1987). Intake of wilted grass silages is usually greater than that of direct-cut silage, but reasons for the decrease with direct-cut silages are not fully understood (Minson, 1990).
The approach used to develop prediction equations for feed intake involved reevaluating relationships suggested in the previous edition of Nutrient Requirements of Beef Cattle (National Research Council, 1984). Equations presented in the previous edition have been used extensively in practice; however, description of the data base used and statistical validation of the equations were inadequate. Hence, efforts will be made to fully describe the approach used to develop prediction equations for growing and finishing cattle and beef cows. No attempt was made to develop prediction equations for intake by nursing calves; readers are referred to Predicting Feed Intake for Food-Producing Animals (National Research Council, 1987) for a proposed equation. It also should be noted that the focus of prediction in each case was average DMI over an extended feeding period. Although prediction of feed intake for shorter periods is highly desirable, no data base exists from which to develop such prediction equations for the wide variety of production situations and feeds available to beef cattle producers.
As noted previously, the Nutrient Requirements of Beef Cattle (National Research Council, 1984) provided an equation to predict DMI by growing and finishing beef cattle. This equation describes DMI as a function of dietary NEm concentration, with adjustments for frame size or sex. The base NRC 1984 equation is
where DMI is expressed in kg/day, SBW is expressed in kg, and NEm concentration is expressed as Mcal/kg dietary dry matter. Data from the published literature were used to reevaluate the relationship between dietary NEm concentration and DMI by growing and finishing beef cattle (Figure 7–1).
Data were obtained from experiments conducted with growing and finishing beef cattle and published in the Journal of Animal Science from 1980 to 1992. Each of 185 data points extracted from the literature represented a treatment mean for average DMI throughout a feeding period. Feeding periods varied from 56 to 212 days. Approximately 48 percent of the cattle were implanted with a growth-promoting implant, and approximately 50 percent were fed an ionophore. Information on frame size (small, medium, or large), sex (steer, heifer, or bull), age (calf or yearling), and initial and final SBW was recorded. Because this data contained a mix of full and shrunk body weights, the subcommittee assumed SBW in developing these equations. Dietary NEm concentration was calculated from tabular values (National Research Council, 1984); however, actually determined NEm values were used, when available. Because of the limited number of observations, bulls were classed as large-frame steers and large-frame heifers were classed as medium-framed yearling heifers. Total NEm intake was calculated as the product of DMI and dietary NEm concentration. Total NEm intake was then divided by average metabolic body weight (average SBW0.75 in kg). The intake of NEm per unit SBW0.75 was analyzed by stepwise regression procedures (SAS Institute, Inc., 1987) with dietary NEm concentration, NEm2, length of the feeding period, and dummy variables used to account for effects of sex and frame classes as possible independent selections.
The relationship between NEm intake per unit SBW0.75