Last September, DairyNZ senior scientist Dawn Dalley was one of a team of industry and DairyNZ specialists who began looking into the swede crop issue which caused ill health and deaths in cows.
“We have not seen such a widespread outbreak like this before, with lots of cows or multiple herds. It seems like conditions aligned to cause a particularly bad year,” says Dawn.
A full study, run by DairyNZ, is looking at plant and animal samples, previous research and an in-depth survey of 120 dairy farmers and 40-50 graziers.
Dawn says early work was about preserving information.
“Farmers were taking cows off swede crops and even those with some left were talking about ploughing it all under,” says Dawn. “Some samples were better than none, so we quickly got samples from four different types of farms.”
Those were farms with HT (herbicide tolerant) swedes – both crops associated with sick cows and those not – and farms with non-HT swedes (again, with sick cows and without). Unfortunately, time was against the team to capture samples from the handful of non-HT farms with sick cows.
However 18 farms provided plant samples, with the team collecting the swede's bulb, crown, upper stem, lower stem, old leaf, new leaf and flowers (if flowering).
“Each plant sample was snap frozen in liquid nitrogen and packed in dry ice until deposited in a -80°C freezer. That’s the gold standard for collection of plant material and stops the breakdown of samples,” says Dawn.
Vets collected blood samples at 10 farms, from 10 randomly selected cows (100 samples in total).
Because affected cows presented with symptoms suggesting liver damage, the blood testing looked at liver and kidney function.
The disease condition appears to be consistent with known forms of liver damage associated with cows grazing brassica forage crops, but with clinical signs that are more severe.
Previous experience suggests that the toxin that may cause liver damage is likely to be linked to plant compounds, known as glucosinolates.
“There are over 120 individual glucosinolates. These compounds are modified as they enter the cow’s rumen and all act differently, so finding those that specifically cause liver disease is difficult. The other diet information (obtained through farmer interviews) will also help understand the cow’s rumen condition and how glucosinolates could act once ingested.”
For example, a cow with low rumen pH could mean a more toxic effect when combined with the compounds in highly digestible swedes.
Levels of glucosinolates in plants can be affected by plant growth conditions and are generally highest in mature plants, flower heads and regrowth.
“There could have been a higher level of glucosinolates in swedes this year – with the mild conditions, there’s been more leaf and the cows eating less bulb. Some farms had trouble with cows not eating the bulbs and gave bigger breaks to compensate, which might mean more leaf eaten.”
Additional information needed
Information was also gathered through farmer surveys and previous research.
“DairyNZ ran two farmer surveys – an initial, brief questionnaire prompted 400 responses and provided basic information,” says Dawn.
“A follow-up interview helped understand the situation on each farm and with the cows, with one-on-one interviews of 60 affected farmers and 60 non-affected farmers, chosen at random.”
Where to from here?
The plant and animal samples, along with farmer information, will be analysed to identify potential causes of the animal health issues.
When the outbreak occurred, there was no accredited laboratory in New Zealand to analyse the plant samples. The method is being developed now.
Preliminary results will be released as they become available and after being reviewed by a working group – whose members include DairyNZ, Ministry for Primary Industries, New Zealand Veterinary Association, Federated Farmers, Beef + Lamb NZ, local veterinarians, Rural Support Trust and PGG Wrightson Seeds.
The final report will be released in autumn 2015.
This article was originally published in Inside Dairy February 2015