Ruminant production plays an important role in the predominantly agricultural economy of Vietnam, especially in mixed animal-crop production systems (Trach 1998). Like in other developing countries in the region, large ruminants in Vietnam mainly rely on rice straw with addition of other crop residues and grass from uncultivated land. These feeds are imbalanced in essential nutrients and low in digestibility, thus production levels are low. One way of improving the quality of the diets is to supplement them with other feed resources which are richer in energy and protein and/or superior in digestibility or intake (Bhat and Bansil 1999).
Several approaches have been made to improve the utilization of rice straw such as treatment with ammonia and supplementation with urea-molasses blocks, green forage and sources of bypass protein (Chenost and Sansoucy 1989). The value of bypass protein lies in its effects on increasing efficiency of use of absorbed nutrients and on increasing voluntary intake (Preston and Leng 1987). Stated in another way, supplementing a diet of crop residues with a bypass protein is considered to be a way to improve the protein:energy (P:E) ratio in the nutrients absorbed (Leng 1991) This has a large influence not only on the level of production but also on the efficiency of feed utilization (i.e. the amount of feed required per unit of milk production or growth, is lowered).
A good candidate for supplementation of fibrous crop residues is cassava foliage (Reed et al 1982), as it is considered that part of the protein in the leaves is in a form which by-passes the rumen (Wanapat et al 1997). A low level of tannins in foliage, as is the case for cassava, can increase the utilization of crude protein by protecting it from digestion in the rumen thereby increasing the flow of essential amino acids to the small intestine for absorption (Barry et al 2001).
High-producing or rapidly growing animals require more high quality protein than that provided by rumen microorganisms (Leng 1991). This protein can be supplied by increasing the amount of dietary protein escaping degradation in the rumen. Various chemical and physical methods of treating proteins have been used to reduce their degradation in the rumen. Of these methods, the most successful physical treatment has been heating (Waltz and Stern 1989).
Klopfenstein et al (2001) indicated that accurate measures of forage protein degradability are necessary to predict animal performance or to appropriately supplement ruminants to meet metabolizable protein requirements when high amounts of forages are included in the diets. The methodology for determining forage protein degradation is evolving, and includes in vivo, in situ and in vitro methods (Elwakeel et al 2006). One system that has received much attention is the "Cornell Net Carbohydrate and Protein System (CNCPS)" model.
The objectives of the present studies were to: