Appropriate methods for the evaluation of tropical feeds for ruminants

The nutritive value of a feedstuff is dependent on the intake and on the amount of nutrients absorbed from the ingested feedstuff during its passage through the gastro intestinal tract. This paper, however, focuses only on the potentials and limitations of some methods found appropriate to estimate the feed intake, energy value and the protein value of feedstuffs. It was concluded that experiments should be made to establish the potential feed intake of different tropical feeds. A system based on the physical fill of feed in the rumen is appropriate as the dominant limitation for intake in ruminants is physical fill of partly digested plant fibers in the rumen. The degradation and passage rates of NDF in the rumen is a potentially useful method to predict the physical fill of fibrous feeds. The potential digestibility of a wide variety of feeds can be determined with reasonable accuracy by using rumen fluid in vitro. The energetic value of concentrate feeds can moreover be determined by enzymatic methods which do not require rumen fluid. For very poor roughage, in vitro methods are less reproducible and should be supplemented with the nylon bag method. The traditional chemical analyses, e.g. for crude fibre, NDF or ADF do not predict the digestibility well, except for feeds where the relationship between the chemical analysis and the digestibility has been established beforehand. The protein value of a feed depends on rumen degradation as well as the amino acid content and digestibility of the undegraded protein passing to the small intestine. The nylon bag method is the only available tool to generally predict the rumen degradation but the between laboratory variation is too big and figures obtained can only be used within laboratory. To improve reproducibility and reduce costs an alternative method is required, e.g. an enzymatic method. The intestinal digestibility of undegraded protein can only be predicted with the mobile bag technique and also here there is a need for alternative methods. Too little research is concerned with quantitative and qualitative protein need of the rumen microbes.