Second Cut Silage - What fertiliser do I need to apply?

By Eddie Webb, B&T Soils & Environment Adviser, Teagasc Galway/Clare

As 1^st cut silage is currently underway, now is a good time to assess the nutrient requirements to support 2^nd cut silage later in the season. Lower yields in first cut have been seen reported which is mainly attributed to the power grass growing conditions in spring. 2^nd cut silage crops will tend to be lighter than first cut ranging from 2.5-3T DM/ha versus approximately 5t DM/ha for 1^st cut.

To maximise a 2^nd cut silage a balanced nutrient supply is essential, with adequate Phosphorous (P) and Potash (K) applications. Nitrogen will drive grass growth and yield and it is important to get N application right to support crop growth whilst also ensuring you are only applying what is required by the crop. Newly reseeded swards with a high Ryegrass content have the potential to utilise the applied N more efficiently.

Much of the Phosphorous and Potash requirement for 2^nd cut silage can be got from cattle slurry. Low emission slurry spreading (LESS) increases the recovery of N by 3 units/1000 gallons and reduces N losses as ammonia. LESS delivers slurry nutrients more precisely across the spread width giving a more targeted nutrient placement.

Table 1: Available N,P,K values for Cattle & Pig Slurry (units/1,000 gallons)

Second cut silage should be fertilised according to crop yield potential. The table below shows the fertiliser requirements based on a grass dry matter yield of 2 to 4t DM/ha (3 bales/acre to 6.5 bales/acre). Applying a 18-6-12 compound in the absence of slurry is more likely to result in a potash deficit which plays an important role in bulking up the crop. 

Table 2: 2^nd cut silage N, P & K Req. (off takes)^3,4 Based on Grass Yield & Fertiliser Programmes

¹Protected urea can replace CAN as N source. ²Apply 4kg P & 25kg K per tonne of grass dry matter (DM). ³N, P & K advice for crop off takes based on grass DM yield at harvest time. ⁴Apply additional P & K for soil fertility build after grass harvest refer to Teagasc Green Book for specific rates. ⁵Fresh grass @ 20% DM.

Sulphur(S) is an important nutrient for grassland production, and is closely associated with nitrogen uptake and efficiency. The response to sulphur fertiliser increases with the rate of nitrogen fertiliser applied .S deposition from the atmosphere has gradually decreased due to improvements in air quality in recent decades.

Amino acids are the building blocks of protein and 20 amino acids contribute to the make-up of plant protein, all containing Nitrogen (N). Two of these essential amino acids (Methionine; Cysteine) contain S and if deficient in soil, it will be the limiting factor in production of plant protein. Protein is associated with chlorophyll (green pigment giving the colour) which is involved with the plant photosynthesis. Sulphur deficiency has a similar appearance to nitrogen deficiency. In both cases, plants will have a pale green or yellow colour. However, with sulphur deficiency, it is the new leaves that are mostly affected due to the poor mobility of sulphur in the plant. In contrast, n deficiency symptoms are mostly expressed in the oldest leaves as nitrogen is very mobile.

There is no soil test to determine the level of sulphur in the soil. Sulphur, like nitrogen is lost through leaching. Sandier well-drained soils with lower soil organic matter can often be low in sulphur whilst poor drained, heavier soils with high OM have less leaching and have greater potential to release sulphur from OM reserves, making it available to meet the demands of crop uptake.

Legumes such as clover, have higher S demands than grass and in this case, S is important in nodule formation and N fixation in clover. This needs to be kept in mind considering the increased inclusion of clover in grass swards. It is not recommended to oversupply S as excess can depress the uptake of Selenium (Se) and reduce the absorption of Copper (Cu) by animals, causing Cu deficiencies. It is best practice to apply N and S at a ratio of 12:1 over the grazing season from March onwards. For 2^nd cut silage grass crops apply 8 to 15kg S/ha (6 to 12 units/acre) per cut.

Road safety should be considered as there is increasing numbers of people of all ages out walking, running or cycling. Silage vehicles and machinery are large and wide and with this in mind extra care and vigilance needs to be taken on narrow country roads as recreational users could be around the next bend. All pedestrians and cyclists should wear high visibility vests, use the correct side of the road, keep in a single file and listen out for road vehicles especially when using narrow country roads. Drivers of the vehicles should always be on alert for dangers and monitor their speed accordingly.