Farmers are being advised to avoid feeding pregnant and lactating cows on swedes grown from herbicide tolerant seeds following the deaths of hundreds of cows in Southland last spring.

DairyNZ is recommending that farmers do not feed herbicide tolerant swedes to cows in spring and is also advising caution if farmers are considering other leafy varieties.

Photo: File.

The industry body says that feeding maturing swede crops increases the risk of ill-health in cows. This follows results from analysis of Southland swede plants collected last season.

Autopsies undertaken by Southland veterinarians at the time showed in some cases, severe liver damage and occasionally kidney damage, associated with the deaths.

Significantly elevated liver enzymes and compromised kidney function was also revealed by blood testing of some affected animals, including all ages and classes of stock.

According to PGG Wrightson Seeds website, seed of HT brassica cultivars are bred to be tolerant to the sulfonyl urea herbicide, DuPont's Telar which is 'a broad-spectrum herbicide that provides excellent control of broad-leaf weeds in brassica crops with Cleancrop Brassica System technology”.

Last September DairyNZ arranged for swede sample analysis from 11 Southland farms following issues with cows becoming ill and some dying after feeding on swedes.

Staff collected swede samples, dissected plants and froze the samples in liquid nitrogen to stop any spoiling of the plant material and break down of the glucosinolates ( known as GSLs), the naturally occurring compounds in brassicas that have been linked to cow health problems.

Swedes were dissected into up to six plant parts so that each section could be analysed separately. Up to 150 plant parts were analysed from three swede varieties across the 11 different farms.

DairyNZ Southland-South Otago regional leader, Richard Kyte, says the new plant data supports the current DairyNZ advice that farmers should be very cautious when feeding swedes.

'This analysis confirms that feeding maturing swede crops increases the risk of ill-health. We're continuing to advise farmers to focus on managing a number of factors involved in feeding swedes this season, including the proportion of swede that makes up the diet of their cows.

'And these new plant results back up our earlier farmer survey findings that feeding swedes on the milking platform (farm) in spring when cows approach calving and early lactation increases the risk of ill-health.

'The farmer survey we released earlier indicated that in spring 2014, there was a higher risk of ill-health for Herbicide Tolerant (HT) swedes compared with other varieties of swedes in spring.

'Given those findings and now the plant analysis, we're recommending that farmers do not feed HT swedes to cows in spring when the animals are in late pregnancy or early lactation and when the risk of all the factors that can lead to ill-heath and potential cow deaths can rapidly combine.”

DairyNZ's full farmer advisory on the plant analysis is available at: www.dairynz.co.nz/farm/adverse-events/southland-swedes/