This should form your basis for deciding the correct levels of slurry and fertiliser application rates in order to deliver the correct balance of nitrogen (N), P and K for good yields.
On average silage fields tend to have lower levels of P and K for a number of reasons. Firstly, these fields are continuously cut, thus large quantities of P and K are removed each year.
Secondly, these fields tend to be furthest from the farm yard and may not necessarily receive an annual application of organic manures due to the longer travel distances.
The productivity of grass swards very much depends on maintaining soil pH within the optimum range of pH 6.3 to 6.5.
This pH range is required for the survival of perennial rye grass in the sward and is essential for the release of soil nutrients (especially N, P and K).
Where fields have lower than optimum soil pH a strategy for liming must be put in place. As the date for closing silage fields is fast approaching, it is now best to delay lime applications until after first cut silage. Ideally leave at least a minimum of three months between lime application and closing for silage, as if lime gets into the pit it interferes with the ensiling of grass.
Nitrogen is the key driver of yield but too much of it in the grass at harvest will make it difficult to ferment as it reduces the grass sugar levels and dry matter content.
In contrast, too little nitrogen will reduce grass growth and overall yield and delay harvest date. Grass swards with high levels of perennial rye grass will use nitrogen more efficiently than older swards.
Swards recently reseeded in the past three years will have a 25pc higher nitrogen demand, especially when reseeded after a tillage rotation.
High yielding first cut grass silage delivering 5 to 6t/ha of DM will require 125 to 150 kg N/ha or 100-120 units/ac. A silage crop will uptake, on average, 2.5kg/ha/day of N or two units/day. Therefore it is necessary to apply the nitrogen fertiliser at least 50 days before harvesting to ensure full N utilisation.
Where fields received nitrogen applications within the last month for grazing assume up to 30pc of this nitrogen will be available and deduct from the above nitrogen total for the silage crop.
Reseeded swards will respond better to nitrogen than old permanent swards.
Phosphorus (P) and Potassium (K)
Phosphorus and potassium are essential to maximise grass yields so adequate supply of these nutrients in the soil is critical. Assess the most recent soil test reports to determine the P and K requirements for silage fields.
A crop of grass silage will remove approximately 4kg P and 25kg K/t of grass dry matter (DM). Therefore a 5t/ha DM crop, with a fresh grass silage yield of around 10 t/ac at 20pc DM, will remove 20kg P/ha and 125kg K/ha by harvest time.
Where insufficient P and K are applied for silage swards, soil P and especially K levels will decline rapidly due to the high off-takes of these nutrients by the silage crop.
Mark Plunkett and David Wall are soil and plant nutrition experts with Teagasc in Johnstown Castle, Co Wexford
* Timing is key to deliver growth and the best time to apply crop N, P and K requirements is when closing off silage fields in late March or early April with a mid-May target harvest date.
Where cattle slurry is applied, delay the top-up fertiliser applications for one week. In wetter soil conditions fertiliser N can be split 50:50 for example 50pc in late March or early April and the remainder within 10 days to reduce the risk of N losses.
Where the soil P and K status of silage fields has been run down over a number of years, put a plan in place to build these up and restore their high grass yield potential.
Apply additional P and K to build-up towards index 1 and 2 soils after first cut silage, or later in summer.
Additional slurry could be targeted on these fields as a cost effective P and K source.
Alternatively fertiliser products such as straight 16pc P or 50pc K, or compounds such as 0-7-30, 10-10-20 and so on are very suitable for building soil P and K levels to the target index 3.
This strategy will generally take a number of years, however, this will be rewarded with higher grass yields of more consistent silage quality.
Another factor that many can forget is the impact of sulphur deficiency. It is most likely to occur on light sandy and free draining soils with lower soil organic matter levels.
Grass silage crops have a requirement of around 20kg S/ha per cut. On soils where it is lacking the application of fertiliser S will increase grass DM yields and quality as it helps to maintain an optimum N:S ratio. This will also improve the efficiency of N use by the grass.
The best method is to apply S with the main N split as N +S, for example CAN +S or Urea +S.