The objective of this chapter is to integrate the knowledge of feed quality assessment with that of grassland science to optimize nutrient fluxes in the plant-animal system. Although animal performance is the most accurate answer to define appropriate feed quality, in vitro laboratory methods are often used to evaluate feed quality by feed composition, digestibility or degradability of defined groups of substances. This is possible because of the standard laboratory conditions, the small amount of sample required, and the ability to analyze multiple samples simultaneously. The estimation of the energy value of forages is becoming increasingly important and the digestibility of organic matter plays an important role in determining the energy value of forages. High milk yields are possible and justifiable if the animals can be supplied with energy and nutrients to meet their performance requirements, a major part of which comes from forage intake.