Protein Requirements of Zebu Beef Cattle

Protein is one of the most important essential nutrients; it is involved in many vital functions in the body, such as growth and tissue repair, enzyme catalysis, transport and storage, coordinated movement, mechanical support, immunological protection, generation and transmission of nerve impulses, and control of metabolism, growth and cellular differentiation. Therefore, ensure adequate supply of protein to animals involve provide an essential nutrient to maintain the homeostasis, allowing the efficient production of beef. Ruminants have peculiarities in terms of protein nutrition; their protein demands are met through amino acids absorbed by the small intestine as in any other animal, however a large portion of digestible protein (50 to 80%) comes from microbial protein synthesized in the rumen (Bach et al., 2005). Despite its importance, true protein is the most expensive nutrient alone in beef cattle diets, and its inclusion in an unbalanced manner results in increased production costs (Cavalcante et al., 2005). Countless studies have been conducted to evaluate the replacement of true protein by alternative and lower cost sources, like urea, in attempt to meet the animal’s demands as economically as possible (Magalhães et al., 2005). In addition to its high cost, an excess of dietary protein results in a reduction in fertility (Elrod and Butler, 1993) and increased urea excretion in urine, which, when converted to ammonia, turns into a compound that can become a serious pollutant, especially in fragile ecosystems (Klemesrud et al., 2000). Consequently, there is a greatly societal pressure on animal production systems, especially intensive systems, to adopt procedures to reduce the environmental impact. In addition, the appropriate formulation of diets, meeting exactly the protein needs of animals, is one of the ways to ensure that excess urea is not excreted into the environment (Todd et al., 2006). Evidently, such procedures can only be taken when the exact protein requirements of animals are known, categorized into maintenance, growth, gestation and lactation beef cattle protein requirements (NRC, 2000).