Feed evaluation with emphasis on fibrous roughages and fluctuating supply of nutrients: A Review

In this paper, it is argued that in the long run, both for economic and environmental reasons, ruminants should be, as far as possible, be fed on fibrous feeds that are not in competition with human food. New feed evaluation systems are needed that are especially suitable for roughages, including feed intake as an integral part of the system. A system has been suggested and some evidence provided for the development of an expression of feed potential that gives an estimate of the intake of digestible energy, and the performance that can be achieved, if the feeds are offered ad libitum. The feed potential of roughages depends on the size of the soluble fraction and of the insoluble but fermentable fraction, on the rate at which the latter is fermented, and on the rate at which the unfermented residues are eliminated from the rumen. It is suggested that it may be possible to select ruminants with greater gut volume and greater capacity to digest and utilise roughages. It also seems possible to select cereals for the feed potential of their straw without sacrificing yield and quality of their grain. Ruminants have adapted to seasonally fluctuating pasture forage quality and (or) availability by conserving energy from the good season in the form of fat stores. Fat stores can be manipulated to fuel growth, lactation and maintenance. It is argued that storage of energy must be contrasted to other methods of feed preservation. Fat storage generated by high intakes of high quality grass may sometimes be energetically more efficient than conservation of the forage. Fat can be reduced to match consumer demand without loss, or even with gain of carcass weight by feeding ruminally undergraded proteins sources. Finally, evidence is provided to suggest that during periods of low level nutrition, one should try to meet about one-third of the energy required for maintenance in order to avoid the excessive lean loss that otherwise occurs, to alleviate glucose deficiency. (C) 1998 Published by Elsevier Science B.V.