Swede cow death findings
Do not feed herbicide tolerant (HT) swedes to cows when the animals are in late pregnancy or early lactation.
Last week DairyNZ circulated a third advisory note from the Southland Swede Working Group* warning it would not have “clear conclusions in time for your 2015 crop planting decisions”.
The theory is elevated glucosinolates in the crops, notably those of the HT variety, caused the widespread problems ranging from ill-thrift and agitation to photosensitivity and death.
Initial estimates of 200 fatalities due to the condition are now suspected to be conservative and the long-term impact of liver damage in clinical and sub-clinical survivors remains to be seen.
DairyNZ general manager extension, Craig McBeth, says a preliminary survey found problems “in pretty much all the dairy areas of Southland with no obvious geographical drivers for the clinical photosensitivity or other symptoms seen”.
Samples of swedes from paddocks where problems were reported, and where they weren’t, are awaiting analysis, and while some blood samples taken from affected and non-affected cows on swede crops have been analysed, he wouldn’t reveal results last week. “We’re not prepared to release details on those as they are only initial results,” he told Rural News.
However, he did say they would be available “by the end of the month when we can put confident interpretation and messaging to them”.
Specialist vet Gwynn Verkerk has been contracted by DairyNZ to work on the case and the levy body is coordinating information and support for farmers affected, and collating results from veterinary practices that independently investigated the problem.
A second survey of farmers with affected cows is being prepared with input from vets and seed companies. McBeth says it will drill down into the detail of the paddock and crop history, and management of the stock, looking for common factors as to the cause.
The initial survey didn’t ask whether crops were the HT-type or not but the second one will.
McBeth says he thinks the widely reported estimate of 200 deaths due to the problem is “probably conservative” but was unable to give more detail on the numbers of fatalities or clinical cases. “The iceberg beneath all this is what the sub-clinical impact will be in the cows that appear to be okay but sub-clinically may have suffered some impact.”
Blood tests are revealing liver damage and whether that will impact on production and reproduction is the unknown.
Mark Bryan of Vet South, who has been leading much of the local response to the problem, says the “working hypothesis” that unusually high intakes of glucosinolates in leafier-than-normal crops caused the problem is still the best theory. The fact that far from all cows on the crops were affected is explained by the likelihood some cows prefer roots to leaves, and vice versa, he suggests.
However, blood tests on cows showing no symptoms have found elevated levels of the liver enzyme GGT indicating some damage. What that will mean for production or reproduction isn’t known but in the North Island where facial eczema causes similar increases in GGT there are often long-term effects, he notes.
While none of the clinical cases attributed to the swede problem would have been classed as acidosis, Bryan acknowledges it could also be a factor as breakdown of the glucosinolates into the liver damaging toxins is favoured by a lower pH rumen.
A lesson emerging from the problem is that stock transfers between all crops, even of the same species, should be considered a transition and treated with the normal caution. “Because of the differences in geography, topography and climate, you can’t assume swedes are going to be the same in one area to another, or even paddock to paddock.”
• More on page 60
