Fertilising for first cut of silage

With the recent spell of dry weather and increased growth rates, conditions have been ideal for grazing off silage ground and getting ready to apply fertiliser.

Two common questions that are often asked in relation to first-cut silage are: should I graze off the silage ground first before I apply fertiliser; and what fertiliser should I apply?

The answers to both questions will be farm-specific and will be dependent on a number of variables such as grazing management in the autumn/spring period; ground conditions; grass cover; and soil fertility. Grass silage has a large nutrient demand, and adequate Nitrogen, Phosphorus and Potassium is essential for maximising grass yield and producing sufficient winter feed.

Maintaining soil pH at 6.3 to 6.5 for optimum grass production is also essential to maximise the availability of soil N, P and K that is applied as either organic manure or artificial fertiliser. Nitrogen (N) is the key driver of grass silage yield. Grass swards with high levels of perennial rye grass will use N more efficiently than older swards.

Recently reseeded swards (zero to three years) will have 25pc higher N demand. A crop of grass silage (5t/ha of dry matter) will require 125kg 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 N utilisation.

Make adjustment for fertiliser N applied for early grazing. For example, assume ~25 per cent of early N applied will be available for the silage crop.

For example, where 40 units/ac of N is applied for grazing, reduce N applied by 10units/ac for grass silage crop. Phosphorus (P) and Potassium (K) are essential to maximise grass silage yields; therefore, adequate supply of these nutrients in the soil is critical.

Organic manures are an effective source of N, P and K and can provide a large proportion of crop P and K requirements at relatively low cost. As a simple rule of thumb, you can assume that for every 1,000 gallons of cattle slurry you spread, the nutrient content of that slurry will be approximately six units of N, five units of P and 30 units of K. Cattle slurry is the most common manure applied to silage fields and can vary in nutrient content depending on its 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. In other words, a larger quantity of diluted slurry will be needed to supply the same levels of P and K as undiluted slurry. It’s important to take account of slurry DM content to reduce the risk of under-fertilising silage crops. The slurry hydrometer is a tool than can be used to assess the DM of slurry, helping you to predict the nutrient content more accurately.

For example, 3,000 gallons/ac of good-quality cattle slurry (seven per cent DM) will supply sufficient P and K levels to grow a crop of grass silage.

The maximum amount if Potassium (K) you should be applying should be 90units/acre.

Luxury amounts of K may be taken up by grass where more than 90kg/ha K are applied. This can reduce fertilizer K efficiency and may upset the Potassium:Magnesium:Sodium balance in the herbage.

Where more than 90 kg/ha is advised, only 90kg should be applied in spring, and the remainder to the aftermath or in late autumn.

As a general guide on Index 3 soils, the nutrient requirements for a good first cut of silage will be approximately 80 to 100 units of N/acre; 16 units P/acre and 90 units of K/acre, which can be supplied by a combination of organic and chemical fertiliser.

It is important to refer back to your most recent soil sample results, and also to your nutrient management plan to ensure maximum return from fertiliser as well as adhering to the legal limits for both N & P fertilisers.



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