Ryegrass endophyte-related heat stress in cattle

Heat stress in Northland cattle has been shown to be similar to tall fescue toxicosis as described in south-east USA, but incidence has not been correlated with the presence of tall fescue on farms. Tall fescue toxicosis results from grazing endophyte-infected tall fescue, and is caused by the alkaloid ergovaline. Cases are described of cattle suffering typical symptoms of ergovaline poisoning, though they had negligible access to tall fescue. Pasture surveys have shown ergovaline levels in ryegrass pastures to often be sufficient to cause toxicosis. Ambient temperatures interact with ingestion of toxin to cause heat stress. It is suggested that usual weather conditions in New Zealand temper the negative effects of the toxin. Higher temperatures increase the levels of ergovaline in pasture, and increase the sensitivity of livestock to it. Heat stress in North Island cattle probably usually results from the interaction of particular environmental conditions with the grazing of perennial ryegrass. Keywords: endophyte, ergovaline, fescue toxicosis, heat stress, Lolium perenne