Apparent Total Tract Digestibility

Apparent digestibility includes endogenous fecal losses (intestinal epithelial cells, bile, enzymes, microbial residues) that originated from the animal, not the diet. True digestibility corrects for these endogenous losses, resulting in higher values. Endogenous losses are relatively constant and proportional to nutrient intake, making them more apparent at lower dietary levels.

Yes, individual animals show variation in ATTD due to differences in gut microbiota, intestinal surface area, nutrient absorption capacity, and metabolic efficiency. This is why digestibility studies typically use multiple animals (minimum 4-6) per treatment to estimate population averages. Genetic selection for improved digestibility efficiency is possible but slow.

Partially. Regression equations developed from published digestibility data can predict ATTD from feed chemical composition (e.g., crude fiber, neutral detergent fiber content). However, these predictions are feed-type-specific and less accurate than measured values. Novel feeds or those with unusual composition should be measured directly.

Feed processing (grinding, pelleting, extrusion, heat treatment) often increases ATTD by disrupting physical barriers and improving access to nutrients. However, excessive processing or heat can damage nutrients (proteins, vitamins) and paradoxically reduce ATTD. The optimal processing depends on feed composition and animal species.