Don’t feed bolted swedes to livestock

Following the analysis, the industry body is recommending that farmers do not feed herbicide tolerant (HT) swedes to cows in spring when the animals are in late pregnancy or early lactation. 

DairyNZ is also advising caution if farmers are considering other leafy varieties.

DairyNZ arranged for swede sample analysis from 11 Southland farms in September last year after cows became ill and some died. 

Staff collected swede samples, promptly dissected plants and froze the samples in liquid nitrogen to prevent spoiling of the plant material and breakdown of the glucosinolates (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 each section could be analysed separately. Up to 150 plant parts were analysed from three swede varieties from the 11 different farms.

Key findings from the analysis are:

Total GSL concentrations are higher in the HT swede variety than in the non-HT varieties. While there is not much difference in GSL concentrations in the bulb and crown between HT and non-HT swedes, GSL concentrations in the other plant parts are generally higher for HT swedes, with a pronounced difference in the upper leaf and upper stem.

The risk of ill-health and death in cows increases when total GSL concentrations increase as swedes enter the reproductive stage (elongated stem, new leaf, flowers and seed heads).

Different parts of the plants included different individual GSLs.

The concentrations of individual GSLs varied between plant parts.

HT swedes have higher concentrations of GSLs in ‘reproductive’ plant parts, increasing the risk of ill-health for cows grazing swedes with elongated stems and the appearance of flowers.

No single GSL stands out as significantly different between plant variety and plant part.

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

“This analysis confirms that feeding maturing swede crops increases the risk of ill-health,” he says. “Farmers should manage a number of factors 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 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 from herbicide tolerant (HT) swedes than from other varieties of swedes in spring. 

“Given those findings and now the plant analysis, we’re recommending 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 cow deaths can rapidly combine. 

“In spring, air temperatures are expected to increase rapidly, leading to ‘bolted’ swedes; bolted HT swedes have much higher levels of total GSLs. At the time of planting their swedes, farmers have no idea what the following winter/spring is going to be like. Spring is a key risk time as swedes will be closer to going into the reproductive stage,” he says.

However Kyte says the climatic and growing conditions for swedes this season are very different from last year.

“We identified through our farmer survey a departure from the ten year climate average for the region in 2014. The warmer air temperatures and fewer frost days may have enhanced both leaf growth and maturity last year. 

“We’re seeing quite a different growth pattern this season. Plants don’t appear to be bolting. Last season was a bit abnormal,” he says.

“The general advice we’re giving to farmers is that special care is needed with HT swedes and other leafy varieties like Aparima Gold and Triumph, when warm air temperatures from northerly weather conditions cause swedes to regrow and change quickly. By early August last year farmers were seeing the plant in an advanced reproductive state. But that’s not the case this year,” says Kyte.

“This plant analysis has shown that HT swedes, collected in September 2014, had higher concentrations of GSLs in ‘re-growth’ components of the crop, increasing the risk of ill-health for cows grazing swedes that have bolted and have elongated stems. In a nutshell, we’re saying don’t feed bolted swedes to livestock.”

Kyte says it has been a slow process to get the robust plant analysis completed.

“Unfortunately there is no quick test for plants and we’ve spent a lot of time developing and establishing a method to determine GSLs with a commercial laboratory – Hill Laboratories.”

He says DairyNZ is still monitoring the health of a small number of cows this season. “This is the first step in assessing how we can develop baseline monitoring for detecting early signs of ill-health in cows.”

DairyNZ is also running two confidential online surveys to understand more about how farmers use crops during winter and spring in Southland. “This will help us target advice and identify future research needs for managing crops. I’d urge Southland farmers to complete the survey if they get randomly selected.”

DairyNZ will now complete its consolidated report pulling together the results of all the different parts of its study into last year’s issues with swedes. “We expect to release this final report to farmers in October,” says Kyte.

DairyNZ’s full farmer advisory on the plant analysis is available at www.dairynz.co.nz/swedes

Key findings

• Farmers need to focus on managing a number of factors when feeding swedes, including the proportion of swede that makes up the diet of their cows.
• All swede varieties can present a risk to animals depending on how and when they are fed and used as part of the farming systems. However feeding maturing swede crops increases the risk of ill-health in cows.
• The chemistry of glucosinolates (GSLs) for swedes and other brassicas is complex. It changes with swede variety, growing conditions and crop maturity. Further complexity is added when the GSLs are changed during eating and digestion.
• Cow ill-health increased last season as the proportion of swedes fed as part of the diet increased. Feeding swedes on the milking platform in spring when cows are approaching calving or early lactation also increased the risk of ill-health.
• DairyNZ recommends that farmers do not feed HT swedes on the milking platform in spring (late pregnancy, early lactation) when all the factors (warmer temperatures, new leaf growth, bolting) that lead to ill-heath and potential cow deaths can rapidly combine.
• Plant analysis has shown that HT swedes, collected in September 2014, had higher concentrations of GSLs in ‘re-growth” components of the crop, increasing the risk of ill-health for cows grazing swedes that have bolted and have elongated stems. Farmers should not feed bolted swedes to livestock.
• There was a departure from the ten year climate average in 2014 in Southland. The warmer temperatures and fewer frost days may have enhanced both leaf growth and maturity.