by Peter Treacy, Grassland Agro Specialist Advisor
It is difficult to talk about silage for 2018 when some stock are still housed in the early days of May. However planning must commence to refill all the empty pits around the country. The problem is heightened by the fact that every shear grab of silage in the country has been cleared out. Even dubious quality silage has been fed during a very tough spring. The window of opportunity to build contingency silage stocks has also been impeded by the difficult spring with slow growth and limited windows to spread silage fertiliser.
With regards to nitrogen, farmers should be aiming between 50-60 units if targeting a harvest date by the end of May. The rule of thumb is that a crop of silage will use N at an average rate of 2 units per day. One method is to work back from a proposed harvest date and work out the amount of N to be applied based on the number of days between application and the proposed harvest date. The P and K requirements of a silage crop is where things can get complicated.
Watch your K
With the large amounts of slurry after going out on some fields due to the delayed turnout of animals, it is important not to overdo the amount of chemical potassium (K) in fertiliser. The nutrient content of thick slurry is 5-5-30 units per 1,000 gallons, so an application of 2,500 gallons would supply 75 units of K. Excess K application going onto ground in a short period can be problematic due to luxury uptake which can increase the risk of grass tetany due to reduced magnesium uptake in grass. High K levels in silage can also lead to difficulties over winter with milk fever.
Soil Index Matters
A good crop of first cut silage will take about 100 units/acre of K out of the field. However, applying all this in the spring can be excessive, especially with later applications. Target 70 units/acre of K if going with slurry and fertiliser close together. After 1st cut silage look to get additional slurry and/or fertiliser out and also aim to correct any soil P and K deficiencies based on soil samples. For example, if 2-cut silage ground is index 3 for P aim for 30-35 units of P over the year. If soil P is Index 2 aim to add on an extra 8-10 units/acre for build-up. For K, at Index 3, aim for 160-180 units/acre over 2 cuts, and add an extra 25 units/acre if in soil Index 2.
Splitting fertiliser applications is a good way to maximise nitrogen efficiency. When spreading nitrate-based products (e.g. CAN), once the granule is broken down, it will start feeding N to grass immediately. However, nitrate N can also be prone to losses out of the soil. Even in a late spring like this year, splitting the N application helps to reduce losses of N, especially from soils that are wet. The idea of splitting the N is to apply a primary application and come back in again with a ‘top-up’.
One option would be to spread 2 bags of 24-2.5-10, or similar product, and follow on ten days later with extra N, e.g. 1 bag of Sulfammo 30 (30% N + 8% S). Sulfammo 30 is an enhanced technology nitrogen called N-Process. It is phased release, and therefore safer in the soil, and also contains plant stimulants that make the plant more efficient in the conversion of N to protein, resulting in a higher protein content and quality in the silage. It also reduces the potential for free nitrate issues in grass when ensiling, and gives more flexibility with harvesting date when high nitrate grass is an issue.
Don’t forget your Sulphur
Sulphur (S) is something which many farmers overlook but is very important to have in order to grow quality silage. The balance of N and S has a big impact on the efficiency and nitrogen conversion to protein in the plant. Many farmers who don’t use sulphur will fail to reach the silage quantity and quality that they are after. Advice is to apply 10-15 units of sulphur for first cut silage on responsive soils.