Time for potassium application on grazing & silage grounds

The good weather last week saw many farmers secure another cut of silage, which means that potassium (K) application for that silage ground and the grazing ground now needs to be factored into fertiliser plans.

A high percentage of K is recycled by grazing livestock. About 90%+ is excreted back in urine onto grazing land, with the most significant offtakes associated with silage crops, which are typically three to four times the K requirements versus grazing.

K increases stem strength, improves drought resistance, cold tolerance, and, most importantly, increases yield of the grass crop.

K is particularly important in silage fields. If farmers are targeting a third cut of silage, K in the soil will certainly boost growth.

Paddocks that have had surplus grass removed as baled silage will also need to return K, as each bale of grass silage removes approximately 10 units of K/bale.

Offtakes

If K is to be a key element in bulking another cut of silage and keeping lush and quality covers ahead of cows, we need to know the nutrient offtakes from cutting silage and surplus bales.

The nitrogen (N), phosphorus (P) and K offtakes from certain covers are as follows:

There is an opportunity to apply K with fertiliser N for the next few months and if you don’t have a P allowance there are zero P products with N, K and sulphur (S) in them, such as Muriate of Potash (MOP) which is 0:0:50.

K should be used for the grazing block as well and products like 29-0-14 can help to replace K on paddocks that have been cut for silage or surplus bales.

Potassium

Spreading K in the second half of the year using muriate of potash (MOP) is an excellent way to build up the level of K within the soil.

The chemical name for MOP is potassium chloride, a compound which contains 50% K and is the cheapest and most commonly used form of straight K.

Farmers should speak with their adviser to identify the fields low in K and will appear as index one (<50mg/l) or index two (<100mg/l).

Index three soils can just be targeted to replace offtakes to maintain their optimum levels.

If they are at index four, there is no K requirement for the year after the soil sample was taken and then just revert to a maintenance application rate until the next sample is taken.

Simplified K requirements for grazed swards on dairy farms is shown in the table below as total K requirements, before deductions for organic fertilisers.

Rates of fertiliser K for different stocking rates are shown as kg/ha (units/acre in brackets) and are as follows:

Cattle slurry is an excellent source of K, as there are approximately 30 units of K in 1000gal of cattle slurry.

The best way to re-cycle slurry is to spread it back out onto silage fields, but spreading high levels of K fertiliser can lead to suppression of magnesium uptake.

This can result in grass tetany as, during high growth periods, the grass can take up more than what it needs.

As well as that, high K levels when closing for silage can lead to excessive K in silage, which is associated with milk fever susceptibility.

If there is slurry in the tanks at home, getting it out after second cut will see a good response and go towards securing a good third cut.

So, a single application is recommended. to not exceed 90kg K/ha, and it should be applied a little and often on grazing land.

Slurry should be used to apply maintenance on silage areas and to apply build up at the end of the growing season.

P and K rates of fertiliser is shown as kg/ha (units/acre in brackets) and their requirements of silage are as follows:

The above rates shown are total requirements, before deductions for organic fertilisers, and are guideline P rates advised for silage swards by Teagasc.

Getting your soil fertility corrected through optimum application of K for the rest of the spreading period will allow for a reduced reliance on chemical nitrogen (N), which will also aid in water quality and emission targets.