Energy and Nutrient Balance
Apparent total tract digestibility and nutrient retention
For nutrients, such as minerals, that are not metabolized by microbes in the hind gut, apparent total tract digestibility (ATTD) values are usually measured. Likewise, for energy and nutrients where absorption may occur in the hind gut, the ATTD are also measured.
In the Stein Monogastric Nutrition Laboratory at University of Illinois, values for ATTD are measured by placing pigs in metabolism cages that allow for the total, but separate, collection of urine and fecal materials. The cages have an expanded metal floor that allows urine and fecal materials to fall through the floors as they are voided. A screen based floor under the metal floor will collect the fecal material, but the urine will flow into pans that are installed under the screen-floor.
There is a feeder installed in each cage and pigs are usually meal-fed. A daily amount of feed equivalent to 2.5 or 3 times the maintenance requirement for energy (i.e., 106 kcal ME per kg BW0.75) is offered in two equal meals. A screen is placed under the feeders to collect orts that are subsequently weighed. The quantity of orts is then subtracted from the total amount of feed offered to the animal for accurate estimation of the daily feed intake. Water is available at all times from nipple drinkers that are installed in the metabolism cages.
Pigs are usually fed the experimental diets in periods of 14 days. The initial 7 days of each experimental period is considered an adaptation period to the diet. In the morning meals on days 8 and 13, 0.5% ferric oxide, or another marker, is mixed into the diet. Fecal collections are initiated as the marker appears in the feces for the first time after day 8 and will cease when the marker appears in the feces for the first time after day 13.
