Ryegrass staggers is common in horses grazing summer, spring and autumn pastures.   To protect pastures from decimation by Argentine Stem Weevil, plant scientists have intentionally infected some species of ryegrass with a fungus or endophyte.  The downside is the fungus produces a toxin that can effect a horses nervous system.  Affected horses become inco-ordinated. 

The endophyte is present in the stem, leaf and seed of the ryegrass; the better the quality of the pasture, the more potentially toxic it is.

Treatment involves removing horses from infected pasture and feeding a toxin absorbent such as Digest-rite. If the neurological signs are severe, horses should be confined to a 'safe paddock' to eliminate the risk of self-injury.  Full recovery usually takes 2 to 3 weeks.