Two in situ studies were conducted to examine the
use of odd-chain fatty acid profiles to study microbial
colonization of freshly ingested herbage in the rumen
as well as fatty acid biohydrogenation. In the first
study, fresh perennial ryegrass was subjected to a
range of sample preparation methods before incubation
in the rumen for 2 or 7 h. In the second study, fresh
perennial ryegrass was chopped into 1-cm lengths and
incubated in polyester bags in the rumen for 2, 8, and
24 h. After removal of bags from the rumen, 4 different
washing methods, ranging from manual squeezing to
machine washing, were applied. Fatty acids were extracted
from washed residues and determined, as
methyl esters, by gas chromatography. The main oddchain
fatty acids (with the exception of anteiso C15:0)
were not found in fresh grass and were useful markers
of the effects of incubation time, sample preparation
method, and washing method on microbial colonization/
contamination. The concentration of these and
other odd-chain fatty acids increased with incubation
time in both studies. The results indicate rapid and
continued microbial colonization of freshly ingested
forages, although patterns of odd-chain fatty acids did
not reveal any further information about the types
of bacteria-colonizing herbage. Principal component,
biplot analysis provided a useful overall description of
the processes of microbial colonization and degradation
of plant fatty acids on fresh herbage incubated in
the rumen. Bolus formation during mastication and
ingestion results in extensive damage to herbage; none
of the techniques (cutting, crushing, and drying/grinding)
investigated in this work was able to replicate the
effects of bolus formation in the animal. The study
provided further evidence of loss of unfermented feed
particles through polyester bag pores, especially when
feeds are dried and ground. Biohydrogenation of the
polyunsaturated fatty acids of fresh herbage was used principally by solid-associated bacteria to enable them
to take up high levels of trans-11 C18:1 and C18:0 fatty
acids. Although trans-11 C18:1 was strongly associated
with bacterial markers (odd- and branched-chain fatty
acids), its precursor (cis-9, trans-11 C18:2) was not associated
with bacterial variation, suggesting that its production
in the rumen under these conditions was
mainly extracellular.
