FERTILIZER ASSOCIATION URGES AGAINST SEEKING ‘BULKY’ FIRST CUT OF SILAGE

The Fertilizer Association of Ireland has issued its guidance on the best strategy to employ ahead of the first cut of silage.

According to the association, silage reserves are running low due to poor weather, extending the housing/feeding period and therefore there is a real need to replenish quality silage stocks for winter 2024/2025.

“The requirement of having a good yield of a high quality first-cut crop for cutting in mid-May is essential,” the Fertilizer Association has stated.

This cutting date will ensure subsequent cuts are not delayed therefore maximising annual yield.

The advice is not to aim for a ‘bulky’ first-cut which will ultimately not only reduce the quality of the feed available, but also the subsequent yield from a second-cut crop and also eliminate the possibility of a third 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,” the association said.

Grass silage has a large nutrient demand and adequate nitrogen (N), phosphorus (P) and potassium (K) is essential for maximising grass yield and producing sufficient winter feed.

NUTRIENTS FOR FIRST-CUT SILAGE

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.

The Fertilizer Association of Ireland has said that 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 125kg N/ha (100units/ac).

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, e.g., 40 days growth requires 80units N/ac (40 x 2units/day).

P and K are essential to maximise grass yields therefore adequate supply of these nutrients in the soil is critical.

The advice is to consult the most recent (3-5 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/t 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 Fertilizer Association has said that the slurry hydrometer is a useful tool than can be used to assess the DM percentage of slurry helping farmers to predict the nutrient content more accurately and adjusting application rates.

In 2024 due to ungrazed silage fields, the association is advising farmers to aim to supply crop N, P, K and sulphur (S) requirements by selecting a suitable fertiliser and apply cattle slurry after the first cut.

E.g., 3,000gal/ac of good quality cattle slurry (6% DM) will replenish soil P and K reserves.

Table 2 (below) shows the recommended rates of N, P and K at different soil P and K indexes (1-4) required for 5t/ha grass DM (10t fresh grass / ac).

MAXIMUM K APPLICATION

Luxury amounts of K may be taken up by grass where more than 90kg/ha K are applied.

This can reduce fertiliser K efficiency and may upset the K:Mg:Na balance in herbage, the association has said.

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

TIMING OF SILAGE STRATEGY

The Fertilizer Association has advised to apply crop N, P and K requirements when closing silage fields.

In wetter soil conditions, fertiliser N can be split 50:50, e.g., 50% in early April and the remainder 7 to 10 days later to reduce the risk of N losses.

Meanwhile, sulphur (S) 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, the association stated.

The advice is to apply S with main N split as N+S (e.g., CAN+S / protected urea+S).