Be vigilant when feeding swedes to cows

This page outlines some important things to consider when feeding swedes to cows.

The advice was pulled together after the winter of 2014, when Southland and South Otago dairy farmers encountered unusual patterns of illness and deaths of cattle grazing on swede crops across the region.

Warning signs to look for

Observe the physical characteristics of the crop being fed, monitor the health of cows and adjust their feed management if ill-health is observed.

• Cows

  • Weight-loss and ill-thrift
  • Poor crop utilisation – leaving crop behind especially the bulb
  • Cows not putting on condition despite adequate crop allocation
  • Signs of photosensitivity
  • One or two cow deaths
  • 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.

• Crops

  • New leaf growth
  • Swede neck elongating
  • Bolting swedes or flowering heads
  • 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.

Recommendations

• 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 – see Advice for feeding swedes.
• 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) -  see 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.
• 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.
• Follow PGG Wrightson Seeds’ advice regarding HT swedes and their use.

Investigation into swede toxicity

An investigation into why cows became ill or died after eating swedes in the winter of 2014 found it was the result of liver damage, possibly caused by higher levels of certain nitriles. These compounds can be formed as breakdown products following the consumption and digestion of glucosinolates (GSL) found in all brassica species.

• Key findings for the investigation

  • High concentrations of GSLs 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.

Advisories

• Advisory #12 (24 September 2015)