Other commercial enzymes described as “cellulase,” consisting of preparations from P. funiculosum, A. niger, T. reesei, and T. viride, increased the rate of evolution of gas from both grass and corn silages in vitro (Table 4).
As with enzymes A and B, the response increased as the concentration of enzyme increased (P <0.001 in most cases). The response also tended to be greater with grass silage than with corn silage (Table 4; P = 0.071). For example, the responses to added cellulases at 11.1 mL/L of the various enzymes were 22.7 and 39.8% for corn silage and grass silage, respectively.
As with enzymes A and B, significant responses were only seen at the highest concentrations of enzymes. The finding that cellulases purchased commercially,
which were not intended for use as feed additives, can function in the same way as feed additives A and B suggests that there is nothing particularly unusual about A and B, and that many other sources of fibrolytic enzymes might be expected to have a similar effect.The other conclusion that can be drawn for the data presented thus far is that ruminal fermentation of corn and grass silages is indeed enzyme-limited. Bhat et al.
(1988) concluded that the rate of adhesion of cellulolytic microorganisms did not limit the rate of degradation of barley straw in the rumen. Dehority and Tirabasso
(1998) manipulated the population size of cellulolytic
bacteria in the rumen of sheep by changing the diet, but
the rate of digestion of alfalfa cellulose was unaffected,
indicating that “the concentration of cellulolytic bacteria
is not the limiting factor in the digestion of cellulose
in the rumen.” Other factors, such as the specificity of
enzyme activity that might benefit ruminal fermentation
most, have not been investigated. However, it is
clear from the present experiments that increasing enzyme
activity in ruminal fluid does increase the rate of
fermentation, albeit at concentrations that cannot be
provided by present feed supplements.
The next aim was to attempt to identify which, if
any, of the enzyme activities that were present in the different preparations was rate-limiting in ruminal fermentation of the silages.
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