With many farmers around the country focusing on getting their silage in with the good weather over the last week, fertilising with potassium (K) should be the next thing on their minds.
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 and if farmers are targeting a third cut of silage, K in the soil will certainly boost growth.
This year we could see K being a key contributor to fodder supplies, as many farmers fear of running short this winter and a third cut may be crucial.
Nutrient offtakes
If K is to be a key element in bulking up of third cut silage supplies and turning your fodder deficit around, we need to know the nutrient offtakes from cutting silage.
The nitrogen (N), phosphorus (P) and K offtakes from certain covers are as follows:
Pre-cutting yield (kg DM/ha) Bales/ac N units/ac P units/ac K units/ac 1,500 3 30 4.8 30 2,000 4 40 6.4 40 2,500 5 50 8 50
There is an opportunity to apply K with fertiliser N for next seven weeks and if you don’t have a P allowance there are zero P products with N, K and sulphur (S) in them.
K should not just be used to boost your third cut silage but 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 in grazed swards
Spreading K in August or September 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.
There is simplified K requirements of grazed swards on dairy farms 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:
Soil K index <130 kg/ha Org N 131-170 kg/ha Org N 171-210 kg/ha Org N >210 kg/ha Org N 1 85 (68) 90 (72) 95 (76) 100 (80) 2 55 (44) 60 (48) 65 (52) 70 (56) 3 25 (20) 30 (24) 35 (28) 40 (32) 4 0 0 0 0
Cattle slurry is an excellent source of K, as there are approximately 30 units of K in 1000gal of cattle slurry and can be used to meet some of these requirements.
Potassium for silage
However, the best way to re-cycle slurry, is to spread it back out onto silage fields, but, spreading high levels of K fertiliser in spring can lead to suppression of magnesium uptake leading to grass tetany.
If there is slurry in the tanks at home, getting out after second cut will see a good response and will go towards securing a good third cut.
P and K rates of fertiliser is shown as kg/ha (units/acre in brackets) and their requirements of silage are as follows:
Cut once Cut once 2nd and 3rd cuts 2nd and 3rd cuts Soil index P (kg/ha) K (kg/ha) P (kg/ha) K (kg/ha) 1 20 (16) 120 (96) 10 (8) 70 (56) 2 20 (16) 120 (96) 10 (8) 55 (44) 3 20 (16) 120 (96) 10 (8) 35 (28) 4 0 0 0 0
The above rates shown are total requirements, before deductions for organic fertilisers and the rates of P shown in the table above are guideline P rates advised for silage swards by Teagasc.
Allowable application rates will vary depending on the farm specific circumstances and the P allowances under the Nitrates Regulations, as some farmers may need to adjust these rates, so they don’t exceed their limit.
There is no doubt that K application can be the hidden gem this autumn in securing a decent third cut silage and to see good farm covers on the grazing platform until the housing period.