Milk protein is the most digestible protein source for young animals and is always used as the reference protein source. Based on ingredients available and desire to reduce feed costs, feed manufacturers are always looking for alternative protein sources. The true digestibility of soy protein is high, but can be influenced by levels of antinutritional factors, accessibility to digestive enzymes and endogenous protein formulation influenced by protein/carbohydrate structure. The ileal digestibility of different protein sources are shown in the following table:

There have been many studies on the use of soy protein in calf milk replacers. The authors state that practically speaking, the application of a good quality soy protein concentrate does not pose any problems at a 5-6 percent level in milk replacers for fattening or a 15-20 percent level in milk replacers for rearing calves.

The researchers also provided an interesting table that showed the relationship of ages of the pig to their ability to use various protein sources. The data indicated that skim milk is a good protein source for young pigs, however, good quality soy protein is an alternative that can be used.

These results also indicate that the digestibility of soybean meal improves with age of the pig, whereas, a higher protein soy concentrate has a protein digestibility equal to skim milk protein for the very young pig. The bottom line is that there are soy protein products commercially available that are good alternatives to skim milk powders for use in milk replacers for young calves and pigs.

Visser, A. and R. Bremmers. 1999. Soy based milk replacers: Applications for young animals. Feed Management 50(2):19-22.