Optimal Fertilizer Applications for High Quality Silage Production

Author: 
Dr D. Davies, IBERS

Preparation of silage fields should already have been considered. One key factor in the preparation of silage fields is that of fertiliser application which can have major implications on the quantity and quality of silage produced.  

Nitrogen supply is particularly critical, too much and the silage produced is likely to have a high ammonia and butyric acid concentrations; too little and yields and crude protein content could be poor.  Nitrate-N in the grass at harvesting should be below 0.02% of total N.

The key factor that should be assessed prior to making any nitrogen applications is how much N is available from the soil. Ideally all soils on the farm should be tested on a 3 – 5 year cycle. However, if they are not then ‘a rough guideline’ on soil nitrogen supply can be made from the previous land management. 

With soil N supply being high from grassland with over 250 kgN/ha last year, moderate from grassland receiving between 100 – 250 kgN/ha in the previous year, or with substantial clover content, and low in grassland receiving less than 100 kgN /ha in the previous year.

Taking the above factors into account and concentrating just on 1st cut silage (which due to better growth conditions has a higher N requirement than second cut silage) then the requirements will be for 150, 120 and 120 kg N/ha for low, moderate and high soil N supply for high quality 70D silage.  It is worthy of note that if early spring grazing has been done then these amounts can be reduced by 25 kgN/ha.

Finally, if this is all too much then a very generalrule of thumb is: Grassland will utilize 2.5 kgN/ha/day (equivalent to 2 units of N/acre/day) under ideal weather conditions, being sufficient water, sunlight and warmth for optimal growth. 

As we all know ideal weather conditions do not occur everyday between fertilizer application and harvest and so this is the absolute maximum application rate and in my view everyone should apply less based on their own local knowledge of their farms’.  It is preferable to be able to harvest the grass a few days early, when the weather is good, knowing all the nitrogen has been utilised.  Rather than waiting for it to be used and finding the next 2 weeks are wet!

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