Question

I've been told that second-cutting clover hay is more likely than first-cutting to cause slobbers in horses. Is there any truth to this statement? If so, what are the reasons?

Answer

Characterized by profuse salivation, the condition known as slobbers results when horses consume Rhizoctonia leguminicola, the fungus that causes blackpatch disease in red clover and occasionally other legumes. The fungus produces the mycotoxin slaframine, which triggers hypersalivation in horses. Resolution of slobbers typically coincides with removal of infected forage.

Symptoms of blackpatch disease on red clover include concentric lesions on leaves that may be gray, tan, bronze, or dark brown. Other signs of disease include similar stem lesions and growth of fungal threads on the plant. In severe cases of fungal disease, plants may die.

The fungus thrives in stressful conditions (high humidity, drought), making slobbers a common occurrence during summer grazing months when fresh forage is consumed. Slaframine can remain viable on hay made from infected plants for a prolonged period of time, well beyond normal storage times. Later cuttings of hay are likely to contain higher levels of slaframine due to growing conditions and the presence of the fungus.

Trivia: In reading about slobbers, two synonyms for hypersalivation popped up: ptyalism (`tī•ə•li•zəm) and sialorrhea (sī•ə•lə•`rē•ə).

Two classic papers about black patch disease include:

Berkenkamp B. 1977. Blackpatch of forage legumes. Canadian Plant Disease Survey 57:65-67.

Gough, F. J., and E. S. Elliott. 1956. Blackpatch of red clover and other legumes caused by Rhizoctonia leguminicola sp. nov. West Virginia University Agricultural Experiment Station Bulletin 387T.

A more contemporary review paper:

Kagan, I.A. 2016. Blackpatch of clover, cause of slobbers syndrome: A review of the disease and the pathogen, Rhizoctonia leguminicola. Frontiers in Veterinary Science 27.