- A series of factorial experiments (4 x 4 and 4 x 4 x 4) was designed
to investigate the effects of selected combinations of some
major and trace elements upon in vitro rumen cellulose digestion.
Inocula, varying in ratios of clarified rumen fluid (strained through
No. 50 cheesecloth and centrifuged at 365 x g for 2 minutes) to basal
mineral medium to phosphate buffer, were incubated at 39 degrees C
with purified cellulose and the corresponding treatments of test elements.
After a 24-hr incubation period, cellulose digestion was calculated.
In addition, the mineral composition of rumen fluid and
rumen bacteria was studied in relation to the mineral composition of
the diet of a fistulated steer.
Mineral analysis for 6 major and 15 trace elements clearly
indicated that, when compared to the diet, concentrations of these
elements in the clarified rumen fluid were several times lower,
whereas concentrations in rumen bacteria were several times higher.
In general, rumen bacteria appeared to concentrate trace elements
to a greater extent than major elements with approximately the following
overall distribution: elements concentrated in excess of 20
times included Co; 10 to 20 times- -Se; 5 to 10 times- -Na, P, Al, Fe,
Mo; 1 to 5 times- -Ca, K, Mg, S, B, Ba, Cd, Cu, F, Mn, Ni, Sr,
and Zn. Chromium was the only element found to be in lesser concentrations
in rumen bacteria than in the diet.
Significant interactions (P< .05) affecting in vitro rumen cellulose
digestion were found amongst the following combinations of elements:
Mn-Zn, Ca-Zn, Mg-Ca, Mn-Fe, S-Mg-Ni, P-Mg-Ni, Mg-Co,
P-Mg-Co, Mg-Co-Ni, and Cu-Mo-S. Additions of non-toxic levels
of Mn partially but significantly reversed the inhibition in cellulose
digestion caused by additions of excessive Zn. This protective effect
remained significant when ratios of clarified rumen fluid to basal
mineral medium to phosphate buffer were altered. Similarly, additions
of Ca to the incubation medium significantly (P<.05) protected
against the toxicity of excessive Zn. The significant (P <.05) depression
caused by excessive Ca, on the other hand, was partially but
significantly reversed by additions of Mg. Added Fe (only at levels of 10 [mu]g/m1) had some protection against the depression in cellulose
digestion caused by excessive Mn.
Additions of 0.1 percent NaCl were effective in counteracting the
depression in cellulose digestion caused by excesses of KCl, however,
additions of larger quantities of NaCl in the presence of 1.0 percent
KCl resulted in total suppression of cellulose digestion.
Significant interactions (P<.05) were found between S, Mg and
Ni. Additions of non-toxic levels of Mg partially reversed the severe
depression in digestion caused by excessive Ni with additions of S
or P significantly enhancing this protection. Non-toxic levels of Mg
were also successful in alleviating the depressing effects of excessive
Co. Additions of S or P, however, were ineffective in augmenting
the protection offered by Mg. When Mg, Co and Ni were tested
simultaneously, Mg offered the already established protection against
Ni and Co individually as well as against combinations of the two elements.
However, as the concentrations of Ni and Co increased, the
depression in cellulose digestion also increased, but the protection
offered by Mg decreased.
Copper was found to be highly toxic to rumen microorganisms
(additions of 0.5 [mu]g/ml practically stopped all microbial activity).
Neither Mo nor S or both were effective in alleviating the depression
caused by excessive Cu. In lieu, the simultaneous presence of the
three elements, in concentrations in which individually were non-deleterious
to cellulose digestion, proved toxic to cellulose digestion.
Nonetheless, additions of Mo significantly (P<.05) increased digestion
in the absence of added Cu and S.
A S requirement by rumen microorganisms for optimum cellulose
digestion was established in three separate experiments. A
level of 8.4 [mu]g/ml S--present in the inoculum--did not support adequate
cellulose digestion, whereas, additions of l0 [mu]g/ml S (18.4
[mu]g/ml total S) to the inoculum resulted in maximum digestion.