Fertilising 1st Cut Grass Silage April, 2024
Silage reserves are running low due to poor weather, extended housing / feeding period and there is a real need to replenish quality silage stocks for winter 2024/25. Therefore, the requirement of having a good yield of a high quality 1st cut crop for cutting in mid-May is essential. This cutting date will ensure subsequent cuts aren’t delayed thus maximising annual yield rather than looking for a “bulky” 1st cut which will ultimately not only reduce the quality of the feed available but also the subsequent yield from a 2nd cut crop. While also doing away with the possibility of a 3rd cut where forage deficits are identified. Cutting date should not be delayed in light of poor conditions in February or March, this will only add to any forage shortages next winter.
Grass silage has a large nutrient demand and adequate N, P & K is essential for maximising grass yield and producing sufficient winter feed as we head into the most efficient time for nutrient use (April & May).
Nitrogen (N) is the key driver of grass yield. Grass swards with high levels of perennial rye grass will use N more efficiently than older swards. Recently reseeded swards (0-3 years) will have 25% higher N demand, especially when reseeded after a tillage rotation. A crop of grass silage (5t/ha of DM) will require 125 kg N/ha (100 units/acre). Grass silage will take up on average 2.5kg/ha/day of N (2units/day), therefore apply N at least 50 days before cutting to ensure full crop N utilisation. Adjust N rates depending on target cutting date for example 40 days growth requires 80 units N/ac (40 x 2 units/day).
Phosphorus (P) and Potassium (K) are essential to maximise grass yields therefore adequate supply of these nutrients in the soil is critical. Consult the most recent (3 to 5 years) soil test reports to determine the P and K requirements (in organic manure and fertiliser) for silage fields. A crop of grass silage will remove approximately 4kg P and 25kg K /tonne of grass DM. Fertiliser silage crops based on targeted grass yield.
Organic Manures
Cattle slurry is the most common manure applied to silage fields and can vary in nutrient content depending on its dry matter (DM) content. Diluting cattle slurry with water is beneficial for ease of agitation and can help to improve the N availability in the slurry, however it will also dilute the P and K content of the slurry (i.e. a larger quantity of diluted slurry will be needed to supply the same levels of P and K as undiluted slurry). The slurry hydrometer is a useful tool than can be used to assess the DM% of slurry helping you to predict the nutrient content more accurately and adjusting application rates.
| Table 1: Available N, P & K values for cattle & pig slurry applied by LESS | |||
| Available N, P & K (units/1,000 gals) | |||
| Manure type | N | P | K |
| Cattle slurry (6% DM) | 9 | 5 | 32 |
| Dilute cattle slurry (4% DM) | 6 | 3 | 21 |
| Pig slurry (4% DM) | 19 | 7 | 20 |
In 2024 due to un grazed silage fields aim to supply crop N, P, K & S requirements by selecting a suitable fertiliser and apply cattle slurry after 1st cut. For example, 3,000 gallons/ac of good quality cattle slurry (6% DM) will replenish soil P and K reserves. Table 2 shows the recommended rates of N, P & K at different soil P & K indexes (1 to 4) required for 5t/ha grass dry matter (10 tonnes fresh grass / acre).
Maximum K Application 90kgK/ha (In spring time)
Luxury amounts of K may be taken up by grass where more than 90 kg/ha K are applied. This can reduce fertilizer K efficiency and may upset the K:Mg:Na balance in herbage. Where more than 90 kg/ha K is advised; only 90 kg/ha should be applied in spring, and the remainder to the aftermath or in late autumn.
| Table 2:- 1st Cut Grass Silage N, P & K Requirements (5t/ha DM) &
Suggested Fertiliser Programmes |
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| Soil Index | N kg/ha (units/ac) | P kg/ha (units/ac) | K kg/ha (units/ac) | Suggested Fertiliser Options3,4 | |
| No Slurry1,4
(bags/ac) |
+ Cattle Slurry 4,5 3,000gal/ac | ||||
| 11 | 125 (100) | 40 (32) | 175 (140) | 3.5 bags/ac 13-6-20
1.4bags/ac ProUrea |
1.9 bags/ac
ProUrea + S |
| 21 | 125 (100) | 30 (24) | 155 (120) | 3.5 bags/ac 13-6-20
1.4bags/ac ProUrea |
1.9 bags/ac
ProUrea + S |
| 3 | 125 (100) | 20 (16) | 125 (100) | 3.0 bags/ac 13-6-20
1.6bags/ac ProUrea |
1.9 bags/ac
ProUrea + S |
| 42 | 125 (100) | 0 | 0 | 2.5bags/ac ProUrea | 2.5 bags/ac ProUrea |
| 1Index 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…
2Index 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. Avoid applying cattle on Index 4 soils. 3For new / older swards with higher / lower yield potential reduce N, P, K by 25 kg N, 4kgP & 25kg K per tonne of grass dry matter (DM). 4 Protected Urea + S = Protected Urea (Urea 38% N + 7.5% S) 5Cattle slurry (6.3% DM) assumed to be applied by low emission techniques (LESS) – Slurry N-P-K = 9-5-32 |
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Timing of N, P & K application
Apply crop N, P & K requirements when closing silage fields. In wetter soil conditions fertiliser N can be split 50:50 for example 50% in early April and the remainder 7 to 10 days later to reduce the risk of N losses.
Don’t Forget Sulphur (S) Sulphur deficiency is most lightly on light sandy / free draining soils with low soil organic matter. Current research shows that S deficiency is not just confined to light textured soils and S deficiencies are occurring on heavy textured soils in early spring. Grass silage crops have a requirement of 20kg S/ha per cut. The application of 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 to be used more efficiency. Apply S with main N split as N +S (e.g. CAN +S / Protected Urea +S).