Know your limits with pits

Excessively high silage pits are becoming an all too common trend on some farms and pose a significant risk to the contractors and farmers working on them. While cow numbers (particularly dairy) have increased on some farms, increasing pit space has been neglected on the majority.

Safe pit heights and calculating capacity

Pit walls typically run between 2.1m (7ft) and 3m (10ft) in height. Pits of any wall height should not exceed 3.6m (12ft) of settled silage. The settled height will be between 1 and 2m lower than the height the pit was clamped at, with grass of a lower dry matter closer to the 2m mark and high DM grass closer to 1m.

Grass of 25% dry matter has a density of approximately 1 tonne grass: 1.39 cubic metres (at settled capacity). To calculate pit capacity, several calculations and measurements need to be taken. The pit width, length and wall height need to be considered.

Firstly, a calculation has to be completed to deduct capacity lost from the front of the pit, as pits angle up from the ground. If we are working off a pit height of 3.6m, we will see the pit reach full height 3.6m from the front (assuming the pit is sloped from the ground up at a 45° angle). A simple way to calculate this is to work off an average height of 1.8m for this first 3.6m of pit and multiply by your length.

Secondly, the pit will be angled from the wall to the top of the silage mound, again at a 45° angle for safety of operator, which will result in lost capacity.

If we are working off a 3m wall and a 3.6m high pit, the sides of the pit will have to be sloped for the 0.6m difference between wall height and pit height to create our 45° angle. Again, using an average height of 0.3m (half our difference), we can multiply this by our full height pit length to sufficiently deduct this loss of capacity.

Example

We will take a silage pit of 3m wall height (H) which is 15m in width(W) and 30m in length(L) and fill it to a max settled height of 3.6m. To calculate our pit capacity, we first subtract the wedge in the first 3.6m; 3.6m L x average H of 1.8m x W of 15m= 97.2m³

Then, we calculate our loss running along each side of the pit wall as mentioned above; 0.6m W x average H of 0.3m x 26.4m L (subtracting the first 3.6m of our wall length) = 4.75m³ per side with both sides equalling 9.5m³ loss.

Now, by multiplying the pit height, width and length and subtracting our two deductions above we can calculate actual pit capacity.

To calculate this as tonnes of grass, divide the answer by 1.39, as 1 tonne of 25% dry matter grass has a capacity of 1.39m³. The above pit therefore has a capacity of 1,088 tonnes of silage. Higher dry matter grass will increase the pit capacity.

While heavy crops are likely not as much as a problem as other years for first cuts for first cuts, excess grass should ideally be round baled or pitted elsewhere as opposed to overloading silage pits.

As mentioned in the main article, wilting grass to a 30% dry matter will aid in increasing pit capacity.

Dairy cows require 1.6 tonnes of feed per month. Assuming a five-month winter (bales fed at shoulders of the year) each cow requires eight tonnes of feed.

Dividing individual cow requirements (eight tonne) by our pit capacity (1,088 tonnes) in the example given, the pit should be sufficient to store five months of silage for 136 cows. Higher cow numbers than this will have to be serviced by baled silage or a second pit.