More than 400 cows died and others were left with liver damage in late winter in Southland and Otago while eating swedes.
“This is not something new, we have seen it happen before but not in such large numbers,” Collett said.
“When an animal eats only one single food there can be negative consequences but why liver damage is happening in these cases, we are only speculating.”
Glucosinolates, a natural occurring compound found in all brassica leaves, has been implicated but Collett said nitriles, which can be formed when some glucosinolates are broken down in the gut, have been proven to damage livers in rats, but not yet in cows.
“We’ve got no proof that glucosinolates damage cows’ livers. It is only a hypothesis.
“There is some thinking that as cows are fed more and more highly digestible carbohydrates which decreases the pH of the rumen making it more acidic, more nitriles are being formed when brassicas are eaten.”
He said Massey University was putting feelers out for research funding to investigate further.
“What has happened in Southland this year is a very sudden occurrence and there is nothing in the pipeline yet. Hopefully someone will help.
“But we can’t blame glucosinolates yet. They are not toxic. We all eat cabbages, cauliflower and broccoli and they have certain health benefits. It is the derivatives of the glucosinolates, what they are broken down into, which may be toxic under certain circumstances.”
Collett co-authored a paper published in the American Journal of Agricultural and Food Chemistry earlier this year on whether nitrile derivatives of turnip glucosinolates could damage livers in cattle and concluded further research was required.
Either dosing with the pure compounds or creating rumen conditions so that high concentrations of nitriles were produced might show what the toxic effects are in cows, the paper said.
“Unfortunately the nature of research is not like hitting the one nail on the head and producing a result. Often we have to hit a lot of nails to make a breakthrough,” Collett said.
[BOLD]Warning on feeding summer turnips
The Southern Swede Working Group is cautioning southern farmers to be wary of feeding summer turnips to cows recovering from liver damage from eating brassicas in the winter.
Cows left with subclinical liver damage could be further affected by eating any brassica, including pasture weeds such as wild turnip.
The working group, made up of DairyNZ, Beef + Lamb NZ, Federated Farmers, PGG Wrightson Seeds, Ministry for Primary Industries, Rural Support Trust, NZ Veterinary Association and local vets emailed farmers recommending cows that had lost weight rapidly after calving or had photosensitivity, severe metabolic disease before or after calving or a sudden unexplained drop in milk production should not be grazed on summer turnips.
Cows which had mild signs of the disease could be offered brassicas but farmers should watch their animals for clinical signs of photosensitivity and other symptoms and be prepared to change the diet if needed.
The working group also recommended best practice when establishing and feeding summer turnips including control of wild turnip, using long narrow breaks to encourage intake of leaf and bulb at the same time, transitioning cows slowly onto the crop and not putting hungry cows on it at any time.
Cows should be offered a maximum of 2kg drymatter/cow/day for the first five days increasing to 5kg DM/cow/day over the following five days. Summer turnips should never be more than a third of the daily ration.
For more information go to dairynz.co.nz/swedes
