Approximately 100 Hereford cows had been grazing a light soil, native pasture paddock on a property east of Narrabri, in the foothills of the Nandewar Ranges. The mob had just started calving, and was expected to continue calving over the next couple of months.
One cow was found dead on 17 July 2011. It was estimated that she had died about 24 hours earlier. Another (Cow 1) was depressed, lethargic, inappetant and had watery, brown diarrhoea. She was isolated from the mob and had not improved 24 hours later.
It was known that mother-of-millions (Bryophyllum spp, also known as Kalanchoe spp) was present in a 2 ha wooded area of the paddock, so the mob was moved on the morning of 18 July. One cow (Cow 2) was reluctant to move with the mob, and when pressured became excited and collapsed and died. A further three cows were lethargic.
Cow 1 was examined on 18 July in the yard where she was isolated. She appeared dehydrated, depressed and had profuse, melaenic, watery diarrhoea. She was sham drinking, and while she showed some interest in hay, she did not eat.
The paddock from which the cows had been moved was examined. The majority of the paddock had been cleared and consisted of dry, standing pasture composed of mostly native grasses. There was a wooded area extending in a strip approximately 20m wide along the boundary. There were a large number of mother-of-millions (MOM) plants throughout the wooded area, with many plants in early flower (Figure 2). Many of the plants had been chewed off (Figure 3).
Cow 1 died on 19/7/11. There were two further deaths in the mob within 9 days of mortalities commencing. In total five of the hundred cows died.
A diagnosis of cardiac glycoside toxicity due to ingestion of MOM was made after consideration of the history, clinical signs and post-mortem findings. The producer was advised to ensure adequate nutrition for the mob and to minimise stressors over the next three weeks in an effort to reduce deaths. It was anticipated that the stress of calving might cause further deaths. The area infested with MOM was to be treated with a herbicide. It was also to be permanently fenced off from the paddock as in the author's experience the herbicide was unlikely to be completely effective and there was a high likelihood of reinfestation from across the boundary fence. Burning infested areas is also sometimes recommended (Qld DPI fact sheet), however, in this case this was not appropriate due to the proximity to the boundary.
Bryophyllum spp contain cardiac glycosides of the bufadienolide type. These toxins cause heart arrhythmias leading to ventricular fibrillation and arrest (Seawright, 1989). While a number of species are susceptible, toxicity has only been recorded in cattle (Dowling and MacKenzie, 1993). Poisoning tends to occur more commonly between May and October when the plants are in flower (Dowling and MacKenzie, 1993). The flower heads contain five times as much toxin as the stems and leaves (Seawright, 1989). The clinical signs and post-mortem changes seen in this case are consistent with those listed by Seawright (1989).
Seawright (1989) indicates that fatally affected animals die within 5 days. However, deaths have been recorded up to 3 weeks after ingestion (Shaun Slattery, SDV North West LHPA, pers comm.). In this case, deaths occurred up to 9 days after the first case although the later cases were not investigated.
MOM plants are common in many areas of eastern Australia and are frequently seen in the North West LHPA growing in lighter soils along roadsides resulting from dumped garden waste. They are also commonly seen as 'escapees' from abandoned gardens on rural properties. Cases of toxicity occur in most years, mainly over winter and are most often seen in introduced cattle or those unfamiliar with these plants.
In this case, the cows had been grazing the infested paddock for many weeks before toxicity occurred, and had grazed the paddock in past years. It is suspected that MOM may have become more attractive to the cattle after pasture species 'dried off' following a number of consecutive frosts during a period of low rainfall. Also, this ingestion of MOM occurred while the plant was at its most toxic stage of the growing cycle (flowering).