Fertilising 1st cut Grass Silage

By Sean Mannion, B&T Drystock Adviser, Teagasc Galway/Clare

In the coming weeks the decisions made on farm will determine the cost of production for up to 6 months next winter. Conserving high quality silage will ensure good daily weight gain on weanlings and also reduce meal inputs on finishing cattle next winter. On sheep farms where ewes are winter housed silage feed value has been ranked as the third most important factor affecting profitability in mid-season sheep production systems (Keady and Hanrahan, 2006).

Recent weather has enabled favourable conditions on many farms to graze fields that are planned for silage this year. It’s important to remove the dead butt of grass by grazing out properly to 4cm prior to closing for silage. This will have a detrimental effect on silage quality. In particular, fields that were not grazed in late autumn/early winter. This is easier to achieve on farms with a sheep.

The first week of April is usually when many farmers start closing fields for 1^st cut grass silage. What fertiliser should I use on my silage ground? A frequently asked question by many farmers recently. Ideally farmers would have a fertiliser plan prepared for their farm including a liming plan, where to apply your slurry/FYM and what fields to target with specific fertiliser based on soil test results.

Nitrogen (N) is the key driver of grass yield. Grass swards with high levels of perennial ryegrass will use N more efficiently than older swards. Recently reseeded swards (0-3 years) will have 25% higher N demand.

A crop of grass silage (5 t DM/ha) needs 100 kg N/ha (80 units/ac). Grass silage takes up, on average, 2.5 kg/ha/day of N (two units/day), therefore apply N at least 6 weeks before cutting to ensure full crop N utilisation. Don’t overdo it with chemical N as this can cause lodging of the crop pre-harvest which will negatively impact digestibility. If your P & K soil index is at index 4 then 2 bags of protected urea 40% N + 6% S per acre is sufficient (without slurry).

Phosphorus (P) and Potassium (K), you can’t manage what you don’t measure. Accurate soil analysis is the basis of determining what rate if any of these nutrients are required to grow your crop of grass and to get the optimum from fertiliser and or organic manure you apply. See table 1 for P & K advice, depending on your soil P & K index. Since January 2023, all farmers with a grassland stocking rate above 130kg N ha before N exports, and all arable land sown from this date, must have valid soil samples. Where soil samples are not taken for the above holding type, then Index 4 for Phosphorus will be assumed until soil tests are taken.

This area is assumed as index 4 and has no tolerance for Phosphorus therefore 18-6-12 or alternative P products should not be used or you will breach regulations. Its best practice for all farms who purchase fertiliser to have their land soil sampled every 4 years at minimum.

Each 1 tonne of grass silage will remove 4kg P and 25kg K per tonne of grass dry matter, respectively. Cattle slurry has the correct P : K ratio for fertilising the grass silage crop, therefore silage fields should be prioritised for slurry spreading as crop removal takes its toll on P & K and they should be replenished. Slurry & farmyard manure offers a huge role in this replacement followed by chemical fertiliser. Remember Low-Emission Slurry Spreading (LESS) is mandatory on all farms with a grassland stocking rate above 100kg/N/Ha.

Silage crops remove more P and K from fields than grazing.

Sulphur is often forgotten about when it comes to fertiliser planning. There is no soil test to determine the level of sulphur in soil, you can however test the herbage.  Sulphur deficiency is most likely on light, sandy or free draining soils with low soil organic matter. 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 young leaves that are most affected due to the poor mobility of sulphur in the plant. In contrast, N deficiency symptoms are most strongly expressed in the oldest leaves as nitrogen is very mobile. Applying S to soils where it is required will improve grass DM yields and quality as it helps to maintain an optimum N:S ratio and N will be used more efficiently.

Grass silage crops require 20 kg S/ha (16 units /acre) per cut. Many of the protected urea products contain S therefore, many fertiliser compounds are available with S included. For example, Urea 40% N +6%S (protected), 30-0-15+S (protected), 16-6-12+S.  

Table 1. N, P & K requirements for 1^st cut grass silage and (5t/Ha DM) and suggested fertiliser programmes

¹ Index 1, 2 & 3 soils apply P & K balance to build/maintain soil P & K levels to after grass for example apply as 18-6-12 / 0-7-30 / Cattle slurry / etc.. For swards reseeded in the last 3 years apply an extra 25kg N/ha

² Index 4 soils omit P for 2/3 years & retest, Index 4 K omit for 1 year and revert to index 3 advice thereafter until next soil test. Don’t apply cattle on Index 4 soils.

³ For re-seeded/older swards with higher/lower yield potential increase/reduce N, P, K by 25kg N, 4kg P & 25kg K per tonne of grass dry matter (DM) increase or decrease in target yield.

⁴ ProUrea + S = Protected Urea (Urea 38% N + 7.5% S + NBPT or 2-NPT or NPPT)

⁵ Cattle slurry (6.3% DM) assumed to be applied by low emission techniques (LESS) – Slurry N-P-K equivalent to a 50kg bag of 9-5-32 fertilizer