Date of this Version
Published in Beef Research Program Progress Report, No. 4, Part 2 (May 1993)
Animals require nutrients for maintenance and production. A large part of the calculated nutrient requirements is based on body weight, which includes the contents of the gastrointestinal tract (gut). Ruminants have a large gut capacity, and for a 1000 Ib steer, gut contents can account for 50 to 250 Ib of its body weight. These contents are not a part of the animal and should not be considered when calculating maintenance requirements. Therefore to translate nutrient requirements for each unit of empty-body weight (body weight minus the weight of gut contents) gain into requirements per unit gain in body weight, we need an accurate method of estimating the weight of gut contents. Several systems have been proposed to estimate emptybody weight. The National Research Council and the AgriculturalResearch Council used equations to calculate empty-body weight as a constant fraction of shrunk-body weight, or a constant fraction of body weight within three discrete dietary classes, respectively. Results of previous research have demonstrated that in addition to body weight there is a continuous relationship between weight of gut contents and dietary characteristics such as percentage of dietary concentrates and neutral detergent fiber (indigestibleand slowlydigested fractions of the feed). Other work has also shown that weight of gut contents is much higher when animals consume hay vs silage prepared from the same forage source. Our objective was to develop and evaluate a method to estimate weight of gut contents and use this estimate to convert body weight to empty-body weight. To achieve this objective a model was developed to predict weight of gut contents in cattle as a function of forage neutral detergent fiber, physical form of forage dry matter (hay vs silage and pasture), proportion of dietary concentrates and body weight.