Diets should be managed to reduce nitrogen (N) losses. Protein is the chief N source in the diet, and N is the nutrient we are most concerned with. If a growing pig requires 22% protein in the diet and is fed 25% protein, then the excess (containing N) is going to be lost. Some N is going to be lost in the feces, and some that is absorbed is going to be lost as urea in the urine. Conversely, if a pig requires 22% protein, and is only fed 18% protein, then that animal’s production will be limited to the 18% level. In this case other nutrients in the diet will be in excess and cannot be utilized efficiently. Nitrogen feeding strategies are different for all livestock species.
Ruminants (cow, sheep, goat, etc.) have a requirement for proteins that are quickly degraded in the rumen, called degradable intake protein (DIP). They also require proteins less quickly degraded or undegradable in the rumen, undegradble intake protein (UIP). If too much UIP protein is fed, then some of that excess will probably be excreted in the feces. On the other hand, if too much degradable protein is fed, there will be too much absorption of nitrogen into the blood supply and it will be lost in the urine as urea. Most research has shown that lactating dairy cows require about 32% to 38% undegradable protein in the diet, with the remainder being made up as degradable protein.
To learn more about protein for cattle see the following Livestock and Poultry Environmental Stewardship (LPES) Curriculum lesson sections:
With non-ruminant animals, like chickens, horses, and pigs, individual amino acids are the basis of diet protein formulation. (Protein is composed of individual nitrogen-containing amino acids). A ruminant has a microbial population that produces essential amino acids in the rumen, so there is less focus on amino acids for them. In the case of pigs, horses, and chickens each individual amino acid is important. Lysine is usually the first limiting amino acid when feeding pigs and horses, and methionine is usually first limiting with chickens. Commonly used feeds are limiting in these amino acids and must be supplemented through balancing with other ingredients or by adding commercially-available crystalline amino acids to the diet.
To learn more about protein for non-ruminants, see the following Livestock and Poultry Environmental Stewardship (LPES) Curriculum lesson sections: