NEW ZEALAND – Laboratory sampling has revealed a wide range of factors that increase the risk of feeding swedes to cattle, with the major finding centring on not feeding mature swedes.
Swede toxicity depends on herbicide tolerance, the part of the plant eaten and plant age as these affect glucosinolate levels, the naturally occurring substance in brassicas linked with cow health issues.
This is according to levy board DairyNZ, which has said latest analysis results have supported previous advice given to producers earlier in the year that mature swede plants should be treated with caution.read more
A study of three swede varieties from eleven different farms found that glucosinolate levels were higher in herbicide tolerant plants, particularly in the upper leaf and stem.
While variation in GLS type and concentrations varied between species and within different parts of individual swedes, all herbicide tolerant swedes pose a particular threat when approaching flowering and bolting stages.
DairyNZ states: “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.”
A year ago, a scientific inquiry was launched after South Island New Zealand farmers reported ill-health and deaths in cow grazing swedes towards the end of the grazing season.
Warm weather and a lack of frosts across Southland in 2014 had caused advanced stages of swedes to available cow late-gestation and early lactation cows. A farmer survey was distributed by DairyNZ to assess the similarities between cases.
Important Points for Farmers
- 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 (farm) 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.
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