Signal grass,Brachiaria brizantha orBrachiaria decumbens, is native to tropical Africa but is now widely distributed in tropical Americas. It gives high productivity under intensive use with low ground fertility and is resistant to pests and disease. In Jamaica it may be found in the hilly, cooler parts of the island and is sometimes the primary source of forage for ruminants.
Depending on soil characteristics, signal grass has a moderately high nutritive value and intermediate to high digestibility. It is, however, associated with liver damage and photosensitization in cattle, sheep and goats.
The hepatotoxicity appears to be multifactorial in origin. The grass produces steroidal saponins which undergo metabolism in the rumen to hepatotoxic substances which result in the formation of lithogenic biliary crystals causing a severe cholangiohepatitis, biliary obstruction and periportal hepatocyte necrosis. A saprophytic fungus associated with the grass which produces a toxin called sporidesmin may also be a factor.
Hot, wet conditions, allowing increased grass growth, and possibly fungal growth may increase the risk of toxicity*.
Clinically this will result in malaise, weight loss and severe icterus. Liver disease impedes the metabolism of phylloerythrin which is a product of the breakdown of chlorophyll by anaerobic rumen bacteria. Increased levels of phylloerythrin in the blood may result in photosensitization, with hyperkeratotic, ulcerative dermatitis occurring on exposed areas such as the face, ears and dorsum. Facial oedema may also occur. Renal impairment and neurological signs such as incoordination, head-pressing, “star-gazing” or circling may occur in later stages. Death is the ultimate result.
Post mortem findings include generalized icterus, an enlarged, firm, bronze-coloured liver, with swollen mottled kidneys.
Treatment is generally unrewarding unless diagnosed early. It may include supportive medications, reduced sun exposure, the use of skin protectants, removal of the offending grass, and hay feeding to reduce chlorophyll intake. Grazing a mixed pasture which includes at least 30% of another grass may reduce or eliminate the problem*.