Agronomy Advice
April 12, 2023

The Rising Value of Good Silage

Never has silage been more important than right now. With costs spiralling, developing a good quantity of high quality silage is what will get you through to next year.

Silage harvest in a field
Silage harvest in a field

If you take a look at AHDB’s grass input calculator, it’s clear that fertiliser still provides a great return on investment, even at the current price point.

With more dairy farms increasing the number of silage cuts they do, it’s important to consider the nutrient balance after each cut. Your 1st cuts will already be heading to the silage clamp so the next thing to consider is how to maximise grass growth before your 2nd cut. Fertiliser application in the days that follow your 1st cut will set you up for success, but what nutrients does your grass truly need at this point in the growing season? And how much nitrogen should you be applying?

The truth is that it’s a fine balance. If you apply too much nitrogen, grass will have less sugar content and higher ammonia and butyric acid levels making it unpalatable. But not enough nitrogen will affect yield and protein levels.

Here’s our advice:

  1. For swards with good yield potential, apply 90-100kg/ha of total N.
  2. If the interval between cuts is less than six weeks, nitrogen requirements will be different. Assuming the rate of N uptake is 2.5kg/ ha/day then multiply this by the number of days between your 1st cut and expected 2nd cut harvest dates.
  3. If you apply slurry, use RB209 to estimate the amount of N you’re likely to be applying. Alternatively, you can get slurry tested by a lab for an exact nitrogen content reading.
  4. Top up the remaining requirement with fertiliser.
  5. As sulphur is not mobile within the plant, a constant supply of it throughout the growing season is necessary, so spreading a sulphur containing fertiliser on 2nd cuts is definitely worthwhile.

Don’t forget Phosphate and Potash requirements

Fields that are regularly cut for silage will have a higher requirement for P and K due to the high removal of these nutrients by the crop. K is particularly important to maximise your grass yields so we would advise you to prioritise slurry applications on the silage area to replenish the K that was removed in previous harvests.

To get the best grass growth and maximise its quality, we need to precisely calculate the crop’s requirements with what we know the soil can provide and the slurry can add (use RB209 or send a slurry test to a lab). Any shortfall then needs to be made up with fertiliser to optimise quality grass growth.

Here’s an example of that calculation where the soil index is 2 for P and K:

  N P K
Crop requirement kg/ha (units/acre) 100 (80) 25 (20) 90 (72)
22 m3/ha of cattle slurry (by splash plate) provides kg/ha 14 26 54
Fertiliser required kg/ha (units/acre) 86 (69) 0 36 (29)

Get the timing right

This balance of nutrients is not just about what you apply to the field and grass but when you apply it. We would advise you to apply slurry as soon after your 1st cut as possible and follow up with your fertiliser application a week later. If you’re not using slurry, spread the fertiliser as soon as possible. Any delay will reduce your 2nd cut yields.