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Fertilising for a Crop of Quality Silage

07 April 2021
Type Media Article

Austin Callaghan, Drystock Advisor, Teagasc Claremorris

On many farms producing average to poor quality silage is somewhat the norm. Cattle or sheep fed this silage will have reduced growth rates and reproductive performance of breeding females will be below optimal levels. It is surprising then there isn’t a greater effort made to improve the quality of silage made.

Fields for silage should be closed by the 10th of April each year. On wetter farms this may be a week later. Where there is a butt of old grass this should ideally be grazed before closing. This can be difficult on cattle only farms, especially in wet weather conditions, but every opportunity should be taken to graze from mid-February each year, up to the first week of April. Well managed grazed grass has a similar feeding value to barley and the aim in making silage is to maximise the energy value of this grass ensiled in a silage bale or a pit.

To do this the silage crop needs to grow quickly in a 6-7 week period. For this to happen the crop needs to be adequately fertilised with nitrogen, phosphorus, potassium and sulphur. It will take the following per acre unit applications, nitrogen (N) 80-100, phosphorus (P) 16, Potassium (K) 100, Sulphur (S) 16 to grow a crop of quality silage in 7 weeks.

Many crops are not receiving this level. What happens in this scenario after 6-7 weeks when the crop is inspected and the silage yield is low? The silage crop is allowed to grow for another 1 – 2 weeks. In doing this the crop will bulk up but the unit energy value will be diluted. The grass will now have more stem and more seed heads.

Is your silage crop receiving adequate fertiliser applications? Slurry will per 1000 gallons supplies 5-11 units of N, 5 units of P and 30 units of K. A slurry application prior to closing is vital in supplying a substantial portion of the P and K requirements.

Applying 2500 gallons per acre should supply 12 units of P and 75 units of K per acre. Watery diluted slurry will have lower P & K content. Depending on the weather and the application method this 2500 gallons of slurry could supply 10 – 25 units of N per acre.

The chemical fertiliser will then need to supply 70 – 80 units of N, 4 units of P and 25 units of K. A suitable application in this case would be 3 bags of Cut Sward (24. 2 ½. 10) per acre. To supply the required sulphur us a similar compound that also includes sulphur.

The message is don’t skimp on the fertiliser for silage. This is false economy because if you have to let the crop grow well past the 7 weeks then you will produce a silage crop lower in energy. This silage will have a lower feeding and monetary value and will result in reduced animal performance.

Having the silage grown and cut allows for a quicker return of the field for grazing or closing for 2nd cuts.  Quality Silage is money in the bank. It gives you options. Making poor silage is a very expensive feed.

Other Silage making points.

SOIL TEST:  You need to soil test your silage fields every 3 years because many of these fields have below the required pH (lime), P & K levels. Acidic soils will give a poorer response to all slurry and chemical fertilisers. Where soil P & K levels are low, additional applications of P & K to those recommended above will be required to build these levels.

NOTE: There are limits on chemical phosphorus usage and for this reason it is also important to soil test regularly.

NO Slurry spread: Where no slurry applied before closing you may need to apply 1 bag of 0.7.30 per acre to supply adequate P & K.

SLURRY: Take care not to spread slurry at in excess of 3000 gallons per acre.

SILAGE ANALYSIS: Have a silage analysis done on a yearly basis and use this to improve your understanding of the process of making better silage.

RESEEDING: To produce real top quality silage you probably need to be reseed silage fields.

CUTTING DATE: Aim to cut silage from the 20th of May onwards. Be ready and avail of suitable weather conditions when they arrive. This silage will be good feed for all growing cattle, autumn calving suckler cows and cows that calf over the period December to February and are calved for many weeks prior to going to grass.

For advice on Silage Quality contact your local Teagasc adviser.