The growth of sheep fed forage brassica crops is lower than would be predicted
from the chemical composition of the crops, which are generally highly digestible and
contain moderate concentrations of carbohydrate and protein. The problem has been
attributed to low voluntary food intake (VFI) and among potential reasons for this is
the presence, in the herbage, of secondary plant metabolites. The fate and physiological effects of two groups of compounds, the glucosinolates and S-methyl cysteine
sulphoxide (SMCO) were studied in a series of in vivo and in vitro experiments.
The glucosinolate breakdown products allyl cyanide (ACN) and allyl isothiocyanate
(AITC) were continuously infused for 21 days into the rumen of sheep fed either fresh
forage rape or dried grass pellets. The VFI of forage rape by ACN-infused sheep (2.4
mmol / d) was reduced, although not significantly, while AITC (2.4 mmol /d) caused
no VFI reduction. Neither compound affected VFI when infused (4.8 mmol / d) into
sheep fed the dried grass pellet diet. Thyroid hormone concentrations were unaffected
by treatment on the dried grass diet but plasma T3 concentrations were reduced by
AITC on the forage rape diet.
In a further experiment, 3 levels of ACN (0, 4.8 and 9.6 mmol / d) were infused
intra-ruminally into dried grass-fed sheep for 63 days. Voluntary food intake was
again reduced by treatment and liver damage was indicated by elevated plasma gamma
glutamyl transpeptidase (GGTP) concentrations. Clinical indicators of kidney function
(plasma creatinine, plasma urea) indicated no renal effects. Hepatic cytochrome
oxidase activity was significantly depressed at the highest rate of ACN infusion at the
end of the treatment period indicating chronic cyanide toxicity.
Rumen degradation of glucosinolate breakdown products was examined by
measuring the stability of ACN and AITC in rumen fluid in vitro. ACN was degraded
by rumen fluid from cabbage-adapted sheep but not when the donor had been offered
dried grass pellets. In a further experiment, rumen fluid samples taken at intervals
from sheep consuming cabbage for 30 days had variable ACN-degrading activity with
little evidence for a cumulative increase in activity over time.
The digestive fate of sinigrin was examined by dosing sheep with either sinigrin,
AITC or ACN and analysing for urinary metabolites of AITC in the urine. Preliminary
results indicated the proportion of sinigrin broken down to AITC in vivo to be 0.53.
In a final experiment potential interactions between AITC, ACN (0 or 10 mmol /
d) and the rumen product of SMCO, dimethyl disulphide (DMDS) (0 or 25 mmol /
d) were studied by dosing sheep with combinations of compounds for 35 days.
Combined administration did not increase the effects of individual compounds. The
dual presence of DMDS and ACN reduced overall effects and this was attributed to
changes in metabolic fate of the compounds emphasizing the importance of the
composition of the whole diet in determining the ultimate effects of individual constituents.