Correct fertiliser application rates and cutting dates for first-cut silage

What happens on farm over the next few weeks will have a significant impact to silage stocks on dairy farms for next winter. James Dunne, Teagasc Dairy Specialist, offers key advice on fertiliser application rates in this article.

'Silage in the yard is money in the bank' - this is an age old idea, but it remains a core principle of managing risk on dairy farms. A rolling reserve of one month of feed (equivalent to about two bales) above normal winter feed requirements should be in place to cope with adverse weather. From speaking to advisors and farmers around the country, any silage reserves that were traditionally carried from year to year have run low, whether that being the effect of the drought last July/August or the wet March that farms have just experienced. Therefore, the requirement of having a good yield of a high quality, first-cut crop for cutting in mid-May is essential. This will ensure subsequent cuts aren’t delayed, thus maximising annual yield.

What does a good quality first-cut crop require?

Grass silage has a large nutrient demand and adequate nitrogen (N), phosphorous (P) and potassium (K) is essential for maximising grass yield and producing sufficient winter feed as we head into the most efficient time for nutrient use (April and May). With a relatively dry week forecast, farmers should target to get silage fertiliser applications spread this week. We take a look at the advice this spring from Mark Plunkett, Soil and Plant Nutrition Specialist, Teagasc Johnstown Castle.

Nitrogen 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. A crop of grass silage (5t/ha of DM) will require 125kg N/ha (100 units/ac). This traditionally will be a combination of slurry and chemical nitrogen. 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. Make adjustments for fertiliser N applied for early grazing (assume ~25% of N applied will be available for the silage crop). For example, where 40 units/ac of N were applied for grazing, reduce N applied by 10units/ac for the grass silage crop.

Phosphorus and Potassium 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/t of grass dry matter (DM). 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. For example, 3,000 gallons/ac of good quality cattle slurry (6% DM) will supply sufficient P and K levels to grow a crop of grass silage where soil fertility levels are optimum.

Maximum K application

Luxury amounts of K may be taken up by grass where more than 90kg/ha of K are applied. This can reduce fertiliser K efficiency and may upset the K:Mg:Na balance in herbage. Where more than 90kg/ha is advised, only 90kg should be applied in spring and the remainder to the aftermath or in late autumn.

Table 1: 1st Cut Grass Silage N, P & K Requirements (5t/ha DM) & Suggested Fertiliser Programmes

Timing of N, P and K application

Apply crop N, P and K requirements when closing silage fields. 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 (e.g. 50% in early April and the remainder 7 to 10 days later to reduce the risk of N losses).

Building soil P and K in silage fields

Apply additional P and K (soil build-up rates) to index 1 and 2 soils after first-cut silage or in late summer. For example, 16% P, 50% K or 0-7-30 are very suitable fertilisers for building soil P and K levels to the target index 3 over a number of years. Slurry / farmyard manure can replace fertiliser where available.

Don’t forget Sulphur

Sulphur (S) deficiency is most likely 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).