Despite higher fertiliser prices, grazed grass will still be the cheapest feed that can be offered to cattle.

Every farm has different circumstances in terms of location, rainfall, stocking rate, soil and sward type, so response to organic and chemical nitrogen will differ.

When it comes to making best use of spring fertiliser, outlined are common questions the livestock team have fielded in the last few weeks with some advice on for the year ahead.

1 Which fields should I target with slurry this spring?

Slurry can be a great source of P and K. Both nutrients are important for maximising silage yields and replacing nutrient offtake in such crops.

Therefore, direct as much slurry as possible to silage ground in early spring. Silage crops can then be topped up with bagged nitrogen when closing up for first cut.

The thing to remember with slurry is that the availability of nitrogen is much lower when it is applied during the summer and autumn.

So you’ll get the best bang for your buck by using slurry in the spring. Just make sure heavy grass covers have been eaten off first before spreading.

Also, using LESS equipment to spread slurry can increase nitrogen availability by 25% to 30%.

At current fertiliser prices, this makes using such equipment more cost effective.


If grazing starts in late March or in April, there is merit in spreading 1,500 to 2,000 gallons/ac on grazing paddocks at P and K index 1 to 2.

Get slurry applied around the start of March and hold off on bagged fertiliser for a couple of weeks.

At index 3 or above for P and K, do not apply slurry to grazing swards. The nutrients will be wasted or leached from soils.

Leave at least three weeks after spreading before grazing, otherwise utilisation will be grass poor.

Remember, slurry is high in K, so avoid grazing freshly-calved cows on paddocks that got slurry, as there is a risk of animals taking grass tetany.

2 When should I go out with fertiliser this spring?

If turnout is normally in mid to late March, then you need to get grass growing. Fertiliser should be applied by the outset of March to help covers build.

With the first dressing, target fertiliser to paddocks with low and medium covers. Graze off heavy covers before applying fertiliser.

However, if grazing does not start until April, then hold off from spreading bagged fertiliser until late March.

This keeps grass growth in line with livestock demand, helps manage sward quality and improve grass utilisation.

3 Which paddocks should I target with fertiliser first?

There are plenty of farmers planning to use less fertiliser this year. But in paddocks that have been reseeded in recent years, spreading less nitrogen is a false economy.

Young, productive swards will give the best economic response to every kilogram of nitrogen applied.

Limiting fertiliser can result in a reseed crashing or weed grasses getting established.

For a March fertiliser application, target all reseeds and productive swards.

For paddocks with older, natural grasses, delay spreading fertiliser until April, when growing conditions are normally more favourable.

4 What fertiliser application rates should I go with for grazing and silage?

On grazing swards, apply 25 to 30 units/ac of nitrogen with each dressing in spring.

With first-cut silage, aim to apply 100 units of nitrogen/acre.

Applying 3,000 gallons/ac of slurry should provide enough P and K for first-cut silage, as well as 25 to 30 units of nitrogen.

Top up with three bags/ac of CAN and consider a product with sulphur.

5 Should I go with a single or split fertiliser application for silage?

If the aim is to cut silage in May or early June, fertiliser is normally applied by early April.

Weather conditions can still be marginal at this time, with frosts common.

With one big fertiliser application, there is a greater risk of losing nitrogen through surface run-off following heavy rain, atmospheric losses or leaching when soil moisture levels increase.

Therefore, it may be more beneficial to split the silage fertiliser over two applications. Aim for two bags/ac with the second dressing.

As a guide, in good growing conditions, grass will use two units of nitrogen per day.

Spreading two bags of CAN is 54 units, so the second dressing should be applied one month before target cutting date.

6 Can I rely on clover only to drive grass growth?

Under the right conditions, clover can deliver up to 150kg/ha (120 units/ac) of nitrogen.

It is also more tolerant to drought conditions, high in trace minerals, high crude protein levels and it can also increase dry matter intakes.

However, clover struggles to grow in spring. So you will need to make use of organic or chemical fertilisers through March and April to boost grass growth.

Chemical nitrogen levels can be reduced thereafter. Maintaining soil pH around 6.5 is crucial for a productive clover sward.

It is also important to keep grazing tight to ground, letting the plant access sunlight.

Carrying heavy grass covers of ryegrass will shade out clover, making it a less reliable source of nitrogen.

7 Will spreading lime fix soil fertility and reduce nitrogen requirements?

Spreading lime is always recommended and at pH 6 to 6.5, there is more N, P and K available in the soil.

But as we move towards March, do not spread lime on silage swards, as there is a risk of bringing it back into the silo and inhibiting fermentation.

Lime applied to grazing is OK. Limit to 2t/ac and allow a gap of three weeks before grazing.

However, spreading ground limestone in spring will not correct pH for at least four or five months. Therefore, you will not see the benefits until late summer.

In case you missed it

For more advice, watch our fertiliser and slurry webinar here.

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