More on Triguera Negra, or poisonous grass, the animal killer

Normal grass on left
(Photo: Paula Creaghan-Smith)

There seems to be a measure of panic, or disease of the week syndrome, about Triguera Negra, the poisonous grass that has apparently killed several (18?) horses in the area; if your horses have been affected, panic is naturally understandable. As a result we have consulted expert horse people (who wish to remain anonymous) and are told that this is nothing new. They attribute the panic created to the fact that the grass itself, which is indistinguishable from most normal grasses to the naked eye, is rare further downhill than La Hoya; most previous cases, such as those in the early 1990s have been during droughts. We are also told that danger arises when it rains because the horse pulls out the roots, which contain cyanide, an instant killer in sufficient dosage.  But it hasn't rained! we hear you say. True, but>>>
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according to our own research, the plant's concentration of cyanide is increased substantially under drought pressure. The effects of ingesting or breathing in cyanide-containing materials depend on the concentration. Small amounts are simply broken down and passed out through urine. In toxic concentrations, cyanide blocks the cells from receiving oxygen, in effect suffocating the person or animal (Read more: Natural Source of Cyanide in Plants | eHow.com (http://www.ehow.com/about_5201273_natural-source-cyanide-plants.html#ixzz1p6XwlaXX) There is nothing to be done once the animal has eaten the root, death is almost instantaneous and very distressing.

Signs can occur within 15-20 min to a few hours after animals consume toxic forage. Excitement can be displayed initially, accompanied by rapid respiration rate. Dyspnea follows shortly, with tachycardia. Salivation, excess lacrimation, and voiding of urine and feces may occur. Vomiting may occur, especially in pigs. Muscle fasciculation is common and progresses to generalized spasms before death. Animals stagger and struggle before collapse. Mucous membranes are bright red but may become cyanotic terminally. Death occurs during severe asphyxial convulsions. The heart may continue to beat for several minutes after struggling and breathing stops. The whole syndrome usually does not exceed 30-45 min. Most animals that live ≥2 hr after onset of clinical signs recover, unless continuous absorption of cyanide from the GI tract occurs. (Merck Veterinary Manual) 

We were also told what to do about preventing it: If you have a dry paddock, with absolutely no grass on it all, put the horse there. If it must be out in a grassy field, muzzle it and feed it like that. An other alternative is to not let the animal feed outdoors at all. These is particularly important after the first rain for the reasons mentioned above. The Merck Veterinary Manual contains some Treatment, Control and Prevention information about the animal, though not about how to control the grass, alas.

We have been unable to find the Latin name for the Triguera Negra (a local nomenclature) but have good reason to believe that it is probably called Johnson grass in the US (See Rutgers University on the subject, which may not be correct if it's the wrong grass for the Triguera Negra). If you know the Latin name, please let us know, so we can do some more research!