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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649787 |
| Date | January 1991 |
| Creators | Duncan, Alan J. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/27953 |
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