In nutrition, what does NPN stand for?

The correct answer is Nonprotein Nitrogen. In the context of dairy cattle nutrition and feed formulation, Nonprotein Nitrogen refers to nitrogen sources that are not derived from proteins but can still be utilized by ruminants for protein synthesis. This type of nitrogen is important because it can be converted by the microbes in the rumen into protein that the animal can then use.

Common sources of Nonprotein Nitrogen include urea and ammonia, which can provide an efficient way to meet the nitrogen requirements of dairy cattle without relying solely on proteins from feed. This can help to optimize feed costs and enhance the overall protein efficiency in the animal's diet.

The other options do not accurately reflect the meaning of NPN in the context of nutrition for ruminants and dairy cattle specifically. Hence, understanding Nonprotein Nitrogen and its role in cattle nutrition is essential for managing their dietary needs effectively.