It is estimated that cattle and ewe deaths associated with swedes, particularly a novel strain known as HT (herbicide tolerant) swedes, is well into the hundreds.
VetSouth director Dr Mark Bryan presented findings of the first comprehensive epidemiological analysis of the region’s deaths undertaken by VetSouth, including case studies, blood analysis and mortality data, from farms in Southland and Otago, the areas most affected by the novel disease.
He says determining the cause of cattle illness and disease proved very complex, especially in the initial stages.
“It was essentially an emerging disease, with limited evidence to work from, unusual patterns of illness and death, and a lot of anecdotal comments from farmers. The pattern of disease varied greatly across farms and stock types. To get robust analysis, it was important to drill down to gather really strong data and avoid conjecture.”
Data on the relative proportion of the novel HT swede compared with other swede varieties had not been made available so it was not possible to simply assess mortality and disease rates across the region according to variety planted, says Bryan. However, the fact that 39 of the 41 farms initially investigated used HT swedes and only two used a different variety strongly suggested a link. By the end of spring, these numbers had doubled.
VetSouth analysis of his data shows that there was a significantly higher risk of mortality amongst cattle in Southland grazing on HT swedes during the winter of 2014 than on any other feed type.
He says the case histories and the biochemistry help build a picture of the various health issues that emerged ranging from death, acute liver disease and photosensitivity, through to weight loss and abortions.
It is thought that glucosinolate (GSL) levels in swedes were elevated, most likely due to an unusually mild climate in most of the South. This is thought to have caused severe damage to some cows grazing crop.
Bryan says the analysis of GSLs is the missing link with challenges around finding laboratories able to test for them.
The Swede Working Group’s advice to farmers this season has been to take a vigilant, cautious approach to signs of illness from crops and for improved transition feeding management, which includes a gradual introduction of crops.
Bryan says veterinarians are monitoring the situation for any further issues related to brassica crops over winter. However, it is feared that the disease is not just a dairy cow issue. The NZVA is intending to capture animal health issues and mortality data across all species this winter, to develop a greater understanding of the problem facing farmers.
Swedes are a brassica crop commonly fed to many stock on the lower South Island over winter and have been used as a feeding crop for over 50 years by dairy farmers in the region.
