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Why are sugar and nitrate levels in your silage important?

It’s that time of year again when the shopping bags of frozen grass samples start to arrive at the doors of Teagasc offices all over the country.

As silage can make up to 6 months of an animal’s annual feed and thus weight gain, it critically important we get the preservation right. Two important factors in getting a good preservation in silage are the sugar and nitrate levels in the grass at ensiling.

When ensiling silage the aim is to get the bacteria in the silage to convert the sugars available as quickly as possible to lactic acid to drop the pH of the silage to around 4, this is where the silage is stable. The higher the sugar content the more food for the bacteria the quicker the pH drops. The target sugar content to ensure good fermentation is 3% or higher and this is measured in Teagasc offices using a refractometer.

On the flip side high Nitrate levels in grass are bad as they increase the buffering capacity making it more difficult to get the pH level down. As a rule of thumb, as grass grows it uses 2 units of Nitrogen per day, so if you applied 100 units per acre, it will take on average 50 days for this fertiliser to be used up. However, this is dependent on how fast grass is growing which in turn is dependent on many factors such as, is it old or new pasture, what is the soil fertility, is the weather cold, or too dry etc. To know for sure, get the Nitrate level tested using test strips and this will give you a very good indication of where the Nitrate level is.

If your nitrate test reading is high, the most likely cause is that not enough time has elapsed from the spreading date.

To add one other complication to the mix on average, grass digestibility decreases by 2 to 3 % units per week from the second-half of May. This decline reflects the increasing proportion of stem in the grass plant as the crop matures, typically:

  • A leafy sward with little or no stem should typically give a 75-80 DMD silage
  • On the point of the seed head emerging i.e. some stem typically you should get 70-72 DMD
  • If the seed head is emerging/emerged silage quality will typically be less than 68 DMD

So delaying the harvest can have consequences.

Teagasc work has shown if you have sugar levels above 3% and some nitrate your preservation will be okay. To increase the sugar levels you can, cut in the afternoon/evening when the sugar levels have increased in the grass and wilt the silage to >28% DM to increase the sugar concentration, this will negate the effect of a high Nitrate reading. However if the sugar level is below 3% and the nitrate level is high you would be better advised to wait until the sugar increases and the nitrate decreases.

Anaerobic conditions are essential for initial fermentation and to prevent subsequent spoilage. Fill the silo quickly and roll well to exclude air. Chop length of 1.0 to 1.2cm promotes good compaction. Cover with 2x 0.125 mm polythene sheets. Use vertical sheets along silo walls. Check for 3 - 4 days to ensure seal remains intact as the pit settles. Prevent bird damage.

Further Information

Quality Grass Silage for Dairy and Beef Production Systems - A Best Practice Guide

Making Quality Silage Technical Note PDF