The use of oilseeds is an important option for increasing the energy density of ruminant diets, in addition to supplying dietary protein to support high rates of production. Ruminal degradation of dietary feed CP is an important factor influencing ruminal fermentation and AA supply to dairy cattle. Ruminal protein degradation is described most often by first order mass action models. One of the more complex of these models is the Cornell Net Carbohydrate and Protein System (CNCPS). In the model, feed CP is divided into five fractions (A, B1, B2, B3 and C) which sum to unity. The five fractions have different rates of ruminal degradation. Fraction C contains proteins associated with lignin and tannins and heat – damaged proteins such as the Maillard reaction products. The remaining B fractions represent potentially degradable true protein. Glucosinalate levels in the rapeseed meal reduce its feeding value as it makes meal unpalatable and toxic to animals. Several studies suggest that certain oils primarily through changes in peripheral deiodination of the inactive hormone thyroxine (T4) to the active hormone triiodothyronine (T3).