Alex Goudy, Agriculture Victoria, Warrnambool.
As we move into the harvest season, using fertilisers to boost yield should be front of mind. Potassium is an important nutrient in spring that affects both plant and animal health.
Too little in your soil means reduced pasture growth, however too much can lead to metabolic problems in cattle.
Therefore, it’s important to think about your soil fertility levels and potassium fertiliser use.
Being over generous with potassium fertiliser in spring has the potential to cause milk fever and grass tetany next calving season.The worst-case scenario from this is the loss of cows, or in less severe cases an impact on production and profitability.
When soil potassium levels are high, pasture plants can accumulate excess potassium which can cause a mineral imbalance in cows.
A high potassium diet reduces the uptake of magnesium and calcium in the gut which leads to milk fever and grass tetany. Cows are most susceptible to mineral imbalances prior to and up to three months after calving.
A lot of potassium is removed in a hay or silage harvest, usually between 60 to 100 kg per hectare.
Therefore, many fertiliser recommendations in spring aim to give a boost to pasture growth using nitrogen and replace potassium, phosphorus, sulphur and nitrogen removed in harvested pasture.
However, if soil potassium levels are already adequate, the pasture can accumulate additional potassium beyond what is needed for extra growth.
This is called ‘luxury uptake’. It can result in hay that if fed to susceptible animals, will predispose them to metabolic problems. Some caution around adding extra potassium fertiliser to paddocks with a high soil level is therefore necessary.
The following recommendations can help with potassium fertiliser decisions:
If potassium levels are high (Colwell K levels greater than 250mg/kg) additional fertiliser won’t increase pasture production. Cutting back on potassium fertilisers (and monitoring with soil tests) will save you money.
When using nitrogen to boost spring harvest yields, consider using straight nitrogen fertilisers if soil fertility is adequate and nutrients aren’t limiting.
Cows susceptible to milk fever and grass tetany should be on a low potassium diet, fed rough hay or straw prior to calving, and kept off feed and paddocks with high soil potassium levels, prior to and soon after calving.
Magnesium supplementation pre and post calving will also reduce the chance of grass tetany if you are grazing susceptible pastures.
Be conscious of where the hay being fed out was made. If it was on paddocks with high potassium levels avoid feeding this hay to springers or dry cows.
Remember silage and hay removes a large amount of potassium. Be sure to replace this on these paddocks.
Regularly soil test to monitor the potassium (and other nutrient) levels of your paddocks. Potassium levels can change quickly due to leaching and fodder conservation, so make sure tests are done regularly to plan fertiliser use.
Testing farm management zones assists to monitor the changes in fertility of certain areas of the farm that are managed differently.
Current guidelines for fertility target levels can be found at Dairy Australia’s Fert$mart website: http://fertsmart.dairyingfortomorrow.com.au/dairy-soils-and-fertiliser-manual
By considering current fertility levels and fertiliser practices potassium fertiliser can be used with confidence on your farm.