Effect of nitrate upon the digestibility of kikuyu grass (Pennisetum clandestinum)
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Date
1985
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Abstract
The factors affecting the accumulation of nitrate in kikuyu
grass pastures and the effect of elevated nitrate levels upon
digestion in the ruminant were investigated. A high potassium
level in the soil seems to be the major factor stimulating
the accumulation of excessive amounts of nitrate in kikuyu
grass, when the nitrate content of the soil is also high. The
continuous elongation of kikuyu grass tillers allows constant
exposure of high nitrate containing stem tissue to the
grazing ruminant.
Digestibility studies in vitro showed that nitrite, formed
during the assimilatory
reduction of nitrate to ammonia,
reduces cellulose digestion, but the degree of reduction also
depends upon the presence of readily available carbohydrates
and protein in the digest.
Studies in vivo showed that the microbial population can
adapt to metabolise high concentrations of nitrate (500 mg%
N, m/m) in fresh kikuyu grass, without the accumulation of
nitrite in the rumen. However, introduction into the rumen of
nitrite in excess of the capacity of the nitrite reducing
microbes, causes nitrite accumulation. Nitrite has no direct
effect upon rumen cellulase activity. Due to the affinity of
rumen carbohydrases for the substrate, attempts to isolate
these enzymes by means of isoelectric focusing and other separation techniques met with limited success.
Nitrite strongly reduces the xylanolytic, total and
cellulolytic microbial numbers with a concomitant decrease in
xylanase and cellulase activity of the digest. Decreased
microbial numbers could not be .attributed to a less negative
redox potential of the digest in the presence of nitrite, nor
could the effect upon the cellulolytic microbes be attributed
to an effect of nitrite on branched chain fatty acid
synthesis required for cellulolytic microbial growth. A study
of the effect of nitrite upon the specific growth rate of
pure cultures of the major cellulolytic bacteria,
Ruminococcus flavefaciens strain FDI, Butyrivibrio
fibrisolvens strain Ce 51, Bacteroides succinogenes strain S
85 and Ruminococcus albus strain 22.08.6A and the
non-cellulolytic bacterium Selenomonas ruminantium strain
ATCC 19205 revealed the extreme sensitivity to nitrite of
some of these bacteria and the relative insensitivity of
others. Growth inhibition seems to depend primarily upon the
extent to which these microbes derive their energy from
electron transport-mediated processes.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1985.
Keywords
Minerals in animal nutrition., Trace elements in animal nutrition., Kikuyu grass., Nitrates., Theses--Biochemistry.