Fine tune your second cut fertiliser applications

With a move to more frequent silage cuts on many dairy farms, it won’t be long before 1st cuts are underway. With this in mind, it is worth reviewing your 2nd cut fertiliser application plan and to ensure that you have the right type and quantity of fertiliser in the yard ready for application within the first week post harvesting. 

Nitrogen 

Nitrogen (N) drives grass growth and yield, and for this reason getting the N rate right is important. We thread 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. Too little N compromises yield and protein levels can be lower. 

For swards with good yield potential then we would advise 90 – 100 kg/ha of total N for 2nd 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 or 2 units/day) by the number of days between 1st cut harvest date and the expected 2nd cut harvest date.  

Where slurry is applied for 2nd cut, we need to estimate the N in this, 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 (1,000 gallons/acre) of cattle slurry applied during the summer months is: 

• 7 kg/ha (6 units/acre) if applied by splash plate  
• 9 kg/ha (7 units/acre) if applied by trailing shoe 

Sulphur

Using a sulphur containing fertiliser on 2nd cuts is worthwhile, 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. 

Phosphate and Potash 

Phosphate (P) and potash (K) recommendations should be based on recent soil test results. Fields regularly cut for silage have a higher 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 2.3 kg of P and 9.9 kg of K per tonne of fresh material. The addition of K is particularly important to maintain grass yields, and slurry applications should be prioritised on the silage area to replenish the K removed in the previous year’s silage crops. Table 1 outlines the P and K recommendations for 2nd cut silage in RB209. 

Table 1: P and K recommendations for 2nd cut silage  

How much P and K will slurry provide? If we haven’t had a sample tested in a lab, then we have to use the RB209 book value for 6 % DM cattle slurry. This gives us the following P and K values for each 11 m3/ha (1,000 gallons/acre) applied. 

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

• 13 kg/ha (11 units/acre) of P  
• 27 kg/ha (22 units/acre) of K 

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

• 7 kg/ha (5 units/acre) of P  
• 25 kg/ha (20 units/acre) of K 

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

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

Finally, slurry should be applied as soon after the 1st cut as possible, and then apply the fertiliser a week later. If slurry isn’t been used, then spread the fertiliser as soon as possible. Delays reduce 2nd cut yields.