Plant factors limiting roughage intake in ruminants
The Rowett Research Institute, International Feed Resources Centre, Bucksburn, Aberdeen Introduction A debate on plant and other factors limiting roughage intake in ruminants is both relevant and timely. It is relevant because in the last decade we have learned a great deal more about it.It is timely because in the last decade or two it has been recognized, at least in countries in which environmental constraints (e.g. dry seasons and winters) cannot be buffered by high level of concentrate feeding, that static feed evaluation systems whether based on starch equivalents, total digestible nutrients, metabolizable energy or grain units have a limited relevance since none of themcan predict voluntary feed intake. As a result these systems are of limited use for farmers who want to predict the production capacity of pastures or roughages and to assess their exchange rates to other forages. They are of limited value also for the planners of livestock production who need to know the potential feed intake of farm animals in order to predict if only approximately, the potentiallivestock production in a region from the available feed resources. The problem of intake is of course not a new realization, many researchers have paid attention to this. Crampton (1957) attempted to predict intake from digestibility and chemical composition and found no good relationships and he suspected that degradation rate was an important factor though had no means of measuring it. VanSoest (1982) made a great contribution by attempting to divide the plants chemically in order to determine intake. While this was perfect within plants i.e. predicting intake at different stages of maturity, it was not accurate when divergent plants like legumes and grasses were involved. Balch (1969) attempted to predict intake by the
Plant factors limiting roughage intake in ruminantsnumber of chews required per unit of feed. Teller et al. (1993) advanced the hypothesis that the animals had a finite capacity to chew whether it be eating or rumination and that the total would not exceed about 16 h/day. Minson (1990) lists a whole range of factors plant and animal including grinding resistance to predict forage intake. Lechner Doll et al. (1991) described the importance ofparticle density in the rumen and its effect on rumen retention time which in turn could affect intake. There are no doubt differences in the capacity of different ruminants to digest roughages. Ruminants that are most selective usually have the smallest rumen volume Hoffman (1989) Mould et al. (1982) demonstrated large differences between breeds of cattle in Bangladesh and Britain. Even within the sameanimals the gut volume is affected by pregnancy and lactation as discussed by Kay (1990). Hoffman (1989) discussed the seasonal variation in gut volume as response to quality of diets. Animal factors relating to intake will be discussed in more detail in another paper but here we must conclude that it is unlikely that description of plant factors can predict intake under all circumstances. Onecould hope that it would accurately predict ranking as there will be additional effects of season, breed, physiological state etc. In this article I will review briefly the plant factors and plant dependent animal factors, which determine the intake and digestibility of roughages by ruminants and so the value of roughages in terms of animal production. This has been our main objective at theInternational Feed Resources Unit at the Rowett Research Institute. I will trace the stages by which, making full use of roughage degradability studies in vitro and in vivo, we have been able to define a feed potential index which provides a simple integrated measure of the value of roughage for animal production. Rumen Environment Definition of conditions In order to pursue these lines of thoughts we...