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Potential Risk from High Potassium Straw

17 October 2018

Two samples of pink tinged straw from this years harvest (on right hand side) compared to two samples from harvest 2017. Photo courtesy of Agrii.

The drought conditions of summer 2018 has led to some unusually pink tinged straw at harvest, as a result of high potassium contained within the plant.  This has been particularly evident in the south east of England but has also been reported in the Scottish Borders.  The concern is that the high potassium could be a risk factor for milk fever and other related conditions in dry dairy cows and spring calving suckler cows, where significant levels of straw are included in the diet.

Potassium is a key nutrient for plant growth, with many different functions including protein and starch synthesis and water regulation.  During senescence of the plant prior to harvest, potassium is normally translocated back down into the soil but if moisture is lacking, this process does not occur as efficiently, leading to higher than normal offtakes.

Samples of pink straw have been analysed with potassium levels about three times higher than normal.  Last years levels were about 0.8% in the dry matter but this year, up to 2.4% has been reported by SoilQuest.

For dry dairy cows which may be commonly fed in the region of 3-5kg of straw pre-calving, the higher potassium level and hence higher DCAB (Dietary Cation Anion Balanced) could significantly increase the risk of transition diseases.  Where 4kg of straw is fed, an increase in potassium from 0.8% to 2.4%, increases intake by 55g/cow, increases the level in the overall diet between 0.4-0.5% and raises the DCAB by about 120meq/kg which could easily influence the risk of milk fever and related conditions.

Potassium can lock up magnesium, reducing its absorption in the gut and magnesium is critically important in the hormonal process of releasing calcium from bones pre-calving in order to meet the demand at the onset of lactation.  Retained foetal membranes can also occur due to a lack of calcium.

Feeding of “pink” straw is also a potential risk for spring calving sucker cows fed a silage and straw based maintenance diet over the winter.  The greater risk is where silage is in short supply and more straw based rations with extra concentrates and/or liquid feeds are being used.

There may also be implications for arable farmers on soils with marginal potassium levels. Potassium in the soil may have been depleted more than normal in areas suffering drought conditions.  In addition, livestock farmers that have purchased straw and reapplied the muck to the land may increase the risk of magnesium tetany problems next spring.

Analysis of a small number of wheat and barley straw samples in Aberdeenshire has not returned any extremely high values for potassium (range 0.64-1.29% DM).  However, parts of the country which had a drier summer, or farmers that have purchased straw from drought-hit parts of England, are potentially at higher risk of “pink” straw and it is worth bearing this in mind if dry cow diets are not performing as well as expected.

Lorna MacPherson

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