Field pea toxicity or "pea mania" is a relatively rare condition in cattle grazing field peas. It has been described by Reardon and McKenzie (2002) but to date the toxic principal is unknown, as are the conditions that lead to the occurrence of toxicity.
In mid-October 2012, the NW LHPA was phoned regarding the acute deaths of 15 mature cows grazing a pure stand of field peas. There were 58 cows with calves at foot and they were part of a group of cull animals that were to be sold when their calves were weaned. They had been grazing the crop of field peas for 7 days and in total 16 cows were found dead. Another two animals were affected by neurological signs including mouth chomping, agitation, aggression, circling, back peddling and general manic behaviour. These animals subsequently recovered over a period of 3 days when moved off the crop. Young weaner animals on another paddock were also seen to be affected by neurological disease but recovered when removed from the crop. Two weaners were found dead in this paddock also.
One animal was autopsied in this case. Histopathology was performed on the brain and other organs. Samples were collected to determine the rumenal pH and calcium, magnesium and ammonia levels on aqueous humour.
Biochemistry results were within normal ranges and histopathology revealed only some non-specific changes.
References to toxicity by ingesting field peas are few although in the past 3 years there have been four reported cases in the north west of NSW, and three in the last 6 months. In 2010, the district veterinarian at Moree witnessed a case and subsequently this year there have been cases at Nyngan, Narrabri and now Inverell. Most reports have occurred in crops that are at the pre-flowering stage but the case at Nyngan was in a mature crop and the Inverell case was in crop that was in the flowering and early pod stage. There was one overseas report of pigs poisoned by pea seeds.
Traditionally field peas have been grown as a human food source mainly exported to countries such as India. However, recently they have been exploited as a good fodder crop for cattle and a good alternate winter legume crop.
Unique to this case is the high level of mortalities experienced. Previous cases have generally involved animals displaying neurological signs that recovered when removed from the paddock. Mortalities have occurred but were generally due to misadventure. The differential diagnoses in this case were:
Bloat, enterotoxaemia, lead poisoning, hypomagnesaemia, urea/ammonia toxicity, nitrate toxicity, polioencephalomalacia and anthrax.
An aqueous dipstick test for nitrates was negative. The aqueous magnesium level was within normal ranges as was the ammonia level when considering time post-mortem. Both lead and PEM have similar histopathological brain changes and none of these were detected. Lead was not specifically ruled out by testing but considering rapid recovery of animals removed from the crop, the fact that animals in different paddocks were similarly affected, and the lack of a lead source, helped to rule it out as a cause. Enterotoxaemia seemed unlikely due to neurological signs and age of animals affected. There were no lesions consistent with anthrax in the carcass and the blood clotted. There was no evidence of froth in the rumen.
Based on previous cases, it has since been hypothesised that stress on the plant is a predisposing factor to toxicity. Also light rainfall events prior to exposure to the pant have been suggested as possibly significant. Fertiliser application, soil type and previous crop history seem to play little if any part. Similarly, the variety of field peas is inconsequential to the development of toxicity. In this case, two varieties were planted with significant differences in size and flowering stages but toxicity occurred on both.
After previous cases of this disease, it has generally been suggested that although toxicity can occur, it is rare and field peas would still be considered a good choice of fodder crop for cattle. Following this recent case near Inverell there is a need for further work to establish the predisposing factors if not the toxic agent in order to continue to have confidence in field peas as a good source of fodder for cattle. This case also highlights that grazing monocultures of any plant species can be dangerous. In the recent case at Narrabri the field peas were co-sown with oats but the oats were preferentially grazed until a monoculture of field peas remained. This case had few clinically affected animals and the fact that other plant species were available may have reduced the amount of clinical disease.
Previous cases had shown some cerebral oedema but none was detected in this case. A diagnosis of field pea toxicity was made based on history, clinical signs and the absence of evidence to suggest another cause.