- High concentrations of glucosinolates (GSL; a family of compounds found in brassicas) can cause brassica toxicity. Leaves and flowers of swedes have higher GSL concentrations relative to the bulb/crown.
- All cultivars of swedes can cause liver damage in cattle. The severity of toxicity is influenced by multiple factors.
- The higher disease incidence reported in Southland and South Otago in 2014 was largely caused by feeding swede crops that were reproductive (‘bolting’). Herbicide Tolerant (HT®) variety swedes, shown to have higher concentrations of GSL, exacerbated the problem.
- Air temperatures in winter and spring of 2014 were warmer than normal for the area, which probably increased the proportion of leaf in swede crops and caused crops to bolt earlier than normal.
Brassicas known to cause toxicity
Brassica crops are a valuable source of supplementary forage for dairy cows in pasture-grazing systems, particularly as winter feed in cooler regions of New Zealand. However, brassicas contain glucosinolates (GSL), many of which are pre-toxins and break down to actively toxic products when plant tissue is damaged. When ingested by cows, they may cause a variety of brassica-associated diseases.
Reports of sick cattle spark investigation
During the winter of 2014, dairy farmers across Southland and South Otago encountered unusual patterns of illness and deaths of cattle grazing on swede crops. Sporadic reports and cases of dead or sick animals were first received by veterinarians from mid-July. DairyNZ collaborated with other organisations to:
- support affected farmers through the rest of the season
- understand the cause of problems, and
- develop management strategies to mitigate the risk of brassica toxicity.
Researchers initially suspected brassica toxicity was the cause of the problems given that clinical signs closely matched descriptions in the veterinary literature. The basis of the research hypotheses to be tested was:
- the unusual patterns of illness and death in cattle grazing on swede crops was due to liver damage from toxic compounds formed following the consumption and digestion of GSL
- concentrations of GSL were higher in the reproductive parts of the swede
- the Herbicide Tolerant® (HT®) swede variety could be a contributing factor.
The DairyNZ-led research plan fell into three main phases:
- collection and analysis of animal (i.e. blood) and plant samples from the event
- review of scientific literature and an in-depth farmer survey
- timely dissemination of findings and recommendations for farmers and rural professionals.
Results of the investigation
The analysis of blood samples collected from affected and unaffected herds indicated there was subclinical disease with all swede varieties, irrespective of whether or not cows suffered ill health. This result was deduced from the concentration of the enzyme, g-glutamyl transferase (GGT), which is an indicator of liver damage. When damage occurs, GGT is secreted at higher amounts from the bile duct linings. As GGT concentrations in blood were elevated in cows fed swedes, this backed up the theory that the disease outbreak was due to toxins causing liver damage.
An important result from the farmer survey was a strong correlation between disease incidence and consumption of the HT® swede variety when cows were fed swedes on the milking platform during calving or in early lactation. Weeks on crop did not appear to be a major contributing factor to the disease incidence.
Climatic conditions were also thought to have influenced growth and maturity of swedes, with autumn through spring 2014 being much warmer than normal; at times and in places by as much as 1.5°C. Also more rain fell than usual in April, May and July. Farmers reported visual differences in their crops compared with other seasons, with stems elongated and plants in the reproductive (bolted) phase (Figure 1).
Analyses of swede plant samples indicated significant differences in GSL concentrations between different plant parts and between HT® and non-HT® swede varieties (Figure 2). For all plant parts, except bulb/crown, the average GSL concentration was greater in HT® than non-HT® swedes. In non-HT® swedes, the bulb/crown and lower leaf GSL concentrations were similar, but there was a steady increase in GSL concentration moving up the plant from lower stem, upper leaf, upper stem to the flower. In HT® swedes, there was an increase from bulb/crown, but upper leaf, upper stem and flower were similar for GSL concentrations.
The now common practice of feeding swedes during calving and early lactation was, in the researchers’ opinion, a major contributor to this disease outbreak as it increased the risk of feeding crop with higher concentrations of GSL.
Over the last 10-20 years, swede use has changed from being primarily a wintering crop to one now often used to fill the feed deficit on the milking platform in early lactation. As swede crops for both scenarios are sown at the same time, cows will be exposed to a more mature crop during calving and early lactation, and their GSL intake will be higher. In 2014 this effect was probably exacerbated by crops reaching maturity more quickly due to the unusually warm winter with fewer frosts.
DairyNZ recommendations to farmers
- Do not feed HT® swedes on the milking platform in late August/early September (i.e. late pregnancy, early lactation). This period is when many of the factors that lead to ill-heath and potential cow death (warmer temperatures, new leaf growth, bolting) can rapidly combine.
- Do not feed swede crops in their reproductive growth phase, which can be recognised when the stem of the swede elongates, new growth appears and the swede plant develops flowers and a seed head.
- Be cautious when grazing animals on swede crops in autumn, before the first frosts, as they may eat more leaves than bulbs as the bulbs are hard and difficult to eat.
- Be cautious, at any time during the season, when grazing animals on swede crops with a high leaf to bulb ratio as cows may preferentially graze leaf.
- Observe the physical characteristics of the crop being fed, monitor the health of cows and adjust their feed management if ill-health is observed. For more information around feeding management go to
dairynz.co.nz/swedes and refer to Advisory #11.
- Follow PGG Wrightson Seeds’ advice regarding HT® swedes and their use.
These recommendations are based on the following factors:
- Warmer weather will stimulate swedes to enter the reproductive phase.
- HT® swedes have a higher concentration of GSL in the plant parts where re-growth occurs. This may occur with other leafy swede varieties.
- Cows that are under metabolic stress, due to late pregnancy and early lactation, are less able to cope with toxins arising from high concentrations of total GSL in their diet.
Managing risk in the future
Simplify winter feeding systems to minimise the transitioning requirements for animals as they change feeds (i.e. pasture to crop; crop to crop; crop to pasture). For more information go to dairynz.co.nz/swedes and refer to Advisory #12.
Use farm management practices (e.g. mob age structure, feeding frequency, and break dimensions) that reduce the potential for individual cows to graze proportionately more leaf.
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