THE CHEMICAL BASIS OF NUTRITION. The need for animal digestion experiments. Historical development of digestibility methods. Digestion experiments are needed with all feeds. NATIONWIDE COOPERATION IN FEED EVALUATION. Apparatus for the animals. Mangers and feeding arrangements. Supplying drinking water. The collection of feces manually. Special buildings for digestion trials. A metabolism stall or crate. FECES BAGS AND HARNESSES. Animal comfort. Separation of feces and urine. Digestibility cages for laboratory animals. WORK TO DO BEFORE AN EXPERIMENT. Advanced preparation of records. Preparation of apparatus. Selection and preparation of feeds. SELECTION AND PREPARATION OF ANIMALS. Daily routine in digestion trials. Records. Regular daily feeding. ROUTINE FECES COLLECTION. Weighing of animals at specific at times. Sampling feeds. Preparing feces samples. PROXIMATE CHEMICAL ANALYSES. General considerations regarding chemical methods. The heat of combustion. FEED CONSUMPTION AND REFUSED FEED. Preliminary periods. Collection Periods. Feces markers. CALCULATIONS OF DIGESTIBILITY. Digestibility by difference. The indicator method. Pasture consumption and digestibility. Identification and description of feeds. TRUE DIGESTIBILITY AND APPARENT DIGESTIBILITY. Selection of animal species for digestion experiments. Can sheep be used to determine digestibility for cattle?. Breed or subspecies differences-should digestion trials be with zebu or Other cattle?. FACTORS AFFECTING DIGESTIBILITY. Stage of maturity affects digestibility. Associative digestibility. Associative digestibility. Effect of feed processing. EFFEC OF FROST ON DIGESTIBILITY. Effect of exercise on digestibility. Effect of the age of the animal on digestibility. EFFECT OF FREQUENCY OF EATING AND AMOUNT OF WATER DRUNK. Effect of ambient temperature on digestibility. Sources of errors in digestion experiments. Estimating digestibility's of feed samples without animals. AN EVALUATION OF TDN AND DIGESTIBILITY DATA. Summary of TDN evaluation. Publishing digestibility data.