With a move to more frequent silage cuts on many dairy farms, it won’t be long before first cuts are underway. With this in mind, it is worth reviewing your second cut fertiliser application plan and ensure that you have the right type and quantity of fertiliser ready for application within the first week after harvesting.

1. Nitrogen

Nitrogen (N) drives grass growth and yield, so getting the right N rate is important. We tread a fine line with N applications; too much produces grass with lower sugar and higher ammonia and butyric acid levels, which can make silage less palatable, however, too little N compromises yield, and protein levels can be lower.

For swards with good yield potential, we would advise 90–100 kg/ha of total N for second cuts. In multi-cut systems, this rate may need to be adjusted where the cutting interval is less than 6 weeks.

Calculate the rate by multiplying the rate of N uptake (assume 2.5 kg/ha per day) by the number of days between the first cut harvest date and the expected second cut harvest date.

Where slurry is applied for the second cut, we can estimate the necessary N by using the book value from RB209. Another option is to have a slurry sample tested by a lab.

Using RB209, the available-N per 11 m3/ha of cattle slurry applied during the summer months is:

  • 7 kg/ha if applied by splash plate
  • 9 kg/ha if applied by trailing shoe

Using a fertiliser containing sulphur is worthwhile on second cuts as soil supply is unlikely to meet crop demand. Since sulphur is not mobile within the plant, a constant supply over the growing period is necessary.

2. Phosphate and Potash

Phosphate (P) and potash (K) recommendations should be based on recent soil test results.

Fields that are regularly cut for silage have a greater requirement for P and K due to the high removal of these nutrients by the crop, for example, a 30% dry matter (DM) silage contains 2.3kg of P and 9.9kg of K per tonne of fresh material.

The addition of K is particularly important to maintain grass yields; slurry applications should be prioritised on the silage area in order to replenish the K removed in the previous year’s silage crops.

Table 1 outlines the P and K recommendations for second cut silage in RB209.

Table 1: P and K recommendations for second cut silage

Soil index P or K

0 1 2 3 4

Phosphate kg/ha 25 25 25 0 0

Potash kg/ha 120 100 90 40 0

If we haven’t had a sample tested in a lab, then we can use the RB209 book value for 6% DM cattle slurry. This gives us the following P and K values for each 11 m3/ha applied.

Where the soil index is 2 or greater for P and K:

  • 13 kg/ha of P
  • 27 kg/ha of K

Where the soil index is 0 or 1 for P and K:

  • 7 kg/ha of P
  • 25 kg/ha of K

Example 1: second cut fertiliser recommendation where P and K index is 2.

N P K

Crop requirement kg/ha 100 25 90

22 m3/ha of cattle slurry (by splashplate) provides kg/ha 14 26 54

Fertiliser required kg/ha 86 0 36

Example 2: second cut fertiliser recommendation where P and K index is 1

N P K

Crop requirement kg/ha 100 25 90

22 m3/ha of cattle slurry (by splashplate) provides kg/ha 14 14 50

Fertiliser required kg/ha 86 11 40

So to conclude, slurry should be applied as quickly after the first cut as possible, before applying fertiliser a week later. If slurry hasn’t been used, then spread the fertiliser as soon as possible and delays will risk reducing second cut yields.