I have seen sporadic mortalities in sheep grazing a range of Brassica/Cruciferous species since 1994 in the Young District. Losses have been seen on grazed-off Canola crops during drought conditions in 1994 and 2002; on spray-grazed Wild Radish (Raphanus raphanistrum); and on forage Brassica during early summer of 2004.
Canola is of course a winter oilseed crop typically sown in May. In drought years a decision to cut or graze it off for fodder is made at the end of September.
Forage Brassica is sown in September/October and after an 8-12 week period is used for high intensity summer grazing. It was first grown in the District in the late 1980’s and is resilient and productive green forage for sheep.
When you consider the range of toxic compounds identified in these plants it is not surprising that problems occur. However, in my experience the specific cause of these mortalities is poorly defined. In addition, photosensitisation on forage Brassica is a common problem. The nett result is that at times some producers are reluctant to graze these crops (similar to the Lucerne /Redgut situation).
According to my reading the range of potentially toxic compounds in Brassica species includes
The common syndromes I see when these plants are grazed by sheep are
1 Sporadic Death Syndrome — seen frequently in mobs of sheep grazing failed Canola crops , particularly as they reach full flowering stage. Occasional losses are seen on self-sown plants in Canola stubbles. In late 2004 similar outbreaks were seen on forage Brassica crops (contrary to my previous general experience). I have also seen one typical outbreak on a heavy infestation of Wild Radish that was spray-grazed by a mob of Merino wethers.
The common scenario is onset of sporadic sudden deaths commencing about 5-7 days or more after grazing commences. It is worth noting that Brassica spp. are quite unpalatable and will generally be one of the last species consumed. Owners do not observe any symptoms prior to death and at autopsy I can’t detect clinical cases in the mob. The mortality rate is low (under 1%) but cumulative losses of 3%-4% over a couple of weeks force owners to remove the affected mob and losses cease.
2 Photosensitisation — seen frequently in sheep grazing forage Brassica crops but not on Canola. This is usually a mild syndrome (’Rape Scald’) reported several weeks after introduction. It generally presents as scaly ears with dry necrosis and occasional ear loss. Obvious symptoms may occur in 10%-30%. Occasionally more severe outbreaks of what appears to be a hepatogenous photosensitisation occur with typical ’bighead’ symptoms requiring immediate intervention.
Despite the fact that sheep are predominately used to graze off Canola crops and Forage Brassica is used to grow out lambs, the absence of reports involving cattle appears to be significant.
Autopsy findings in the sporadic death cases are not remarkable and quite consistently reveal moderate to severe pulmonary congestion; sub-epicardial petechial haemorrhages; and some visceral congestion. There is no evidence of haemolysis or anaemia; aqueous humour is nitrate negative; and the thyroid gland is not enlarged. I presume the cause of death is acute cardio-respiratory failure.
Ad hoc histopathology from a few cases has been unrewarding but a more structured investigation might shed some light. Mostly the post-mortem is to exclude other causes so a diagnosis of ’Brassica Poisoning’ gets me by.
While sporadic mortalities on Brassica species can be relatively easily designated as ’Brassica Poisoning’ it would be of some interest to know more about the pathogenesis.
Some of the technical information is confusing, eg, glucosinlate levels are increased in young leaves; seeds; under moisture stress and with higher sulphate levels but SMCO levels are increased in mature leaf material and effects aggravated by low phosphate levels/intake. Obviously failed Canola crops are moisture stressed but I couldn’t say that for Forage Brassica crops receiving 60 mm/month in November/December 2004.
Some interesting aspects are
All this aside, recommendations can be made to reduce the risk of intoxication. Measures I recommend are