Help Desk

GETTING THE BASICS RIGHT FOR YOUR INTERBLOC SILAGE BUNKER

Good design starts with understanding exactly what you want to achieve. There are also a number of regulations and best storage practices that should influence your end design. Spending the time to get this right the first time will reduce your long-term costs. 

CALCULATING THE BUNKER DIMENSIONS

Below are instructions on working out the ideal dimensions for your bunker. This exercise will provide you with a good understanding of the size of bunker you require. Other factors will need to be taken into account when finalising the design of a bunker, such as location and budget.

HOW MUCH WILL YOU FEED OUT PER DAY?

[Number of cows] x [Amount fed per cow] = [Total amount of kgDM require per day]

i.e. 250 Cows x 5kgDM per day = 1250kgDM day

HOW MANY CUBIC METRES OF MAIZE WILL YOU NEED TO REMOVE FROM THE BUNKER FACE EACH DAY?

[Total amount of kgDM required per day] / [average density of maize stack] = [m3 removed per day]
i.e. 1250 kgDM / 225kgm3 = 5.5m3

WHAT IS YOUR EXPECTED FEED OUT RATE?

Warm Weather - 0.20m

Cool Weather - 0.15m

CALCULATE THE REQUIREMENTS M2 OF FACE REMOVED EACH DAY BY DIVIDING THE CUBIC AREA REMOVED BY THE EXPECTED FEED OUT RATE

[Answer to Number 2] / [Answer to number 3] = [Area of face removed each day]

i.e. 5.5m3 / 0.20m = 27.50m2

WORK OUT THE MAXIMUM WIDTH AND HEIGHT DIMENSIONS
At this stage you should take into account key factors which may limited the dimensions of the bin, for example:

The reach of your front end loader – the top of the bunker should be within easy reach of your bucket.

The width of the tractor – the bunker should be at least double the width of the tractor rolling the maize.

Regulatory requirements – the building act requires any structure over 2.5m to be consented.

Two common ways of calculating the front dimensions of a bin.

USING THE MAXIMUM REACH OF THE BUCKET AS A GUIDE: 

[Area of face] / [Height of bucket] = [Width of bunker]

i.e. 27.50m2 / 2.5* = 11m width of bunker (bunker face = 2.5m High x 11m Wide) *Assume the bucket height is 2.5m

OR, USE THE MINIMUM WIDTH OF BIN AS A GUIDE:

This calculation doubles the width of the tractor rolling the maize as a guide.

Area of face] / [Width of bin] = [Height of Bunker]

i.e. 27.50m2 / 8m = 3.4m high (bunker face = 3.4m high x 8m wide) 

Note: in this example the height of the bunker is over 2.5m and therefore may require a building consent. As such it may be a better option to use option (a).

WORK OUT THE TOTAL STORAGE CAPACITY REQUIRED

There are two options when working this out. Pick the option which best applies to your unique circumstances: Calculate based on the total feed you require:

[Number of Cows] x [Amount fed per cow] x [Total days fed out] = [Total kgDM]

i.e. 200 cows x 5kgDM x 100 days = 110,000 kgDM (110 tonne)

OR

Calculate based on the total tonnage harvested:

[Expected crop yield/ha] x [Hectares of crop] = [Total kgDM]

i.e. 4.4 hectares x 25,000 kgDM/ha = 110,000kgDM (110 tonne) 

Next divide the total amount to be stored by the amount by the average density per cubic metre:

i.e 110 tonne (110,000) / .225 (225 kgDM) = 488 cubic metre of space required.

Finally, WORK OUT THE TOTAL LENGTH OF THE BUNKER:

[m3 of storage required] / [height x width] = [Length]

i.e. 488 / 27.5 (2.5x11 = 27.5) = 17.7m length

OTHER CONSIDERATIONS TO INCLUDE

The Site

Spending some time selecting a site will make a big difference in the future. Here are some guidelines to help you pick the best spot.

• Site the bunker away from hedges, trees, and major drains to ensure unimpeded access.
• The bunker should be accessible all year round, so think about drainage and soil types.
• Pests chew through covers which leads to spoilage – the bunker should be sited away from areas where pests may congregate.
• Siting the bunker close to where you will be feeding out will reduce the cost and time 
• Where the bunker is carved out of a hill side, water should be able to easily drain away from the bunker.

• Most milk companies require feed storage to be sited a minimum distance from the milking shed.
• Councils require bunkers to be sited away from surface water, and water tables.
• Increasingly there is a trend among councils to require silage effluent to be drained to your effluent pond. Considering this when choosing a site now could save you significant money in the future.

The Concrete Foundations 

Ground Preparation

A concrete slab is only as good as the ground it sits on. Good ground preparation is essential to the structural integrity of the pad.

• All top soil, rubbish, and organic material should be removed. Soft spots should also be dug out and filled with compacted hard fill.

• A layer of compact-able hard fill should be spread over the site. Dependant on the ground conditions, CCANZ suggest a layer 75mm thick.

• The sub-base should be levelled to provide the required fall for water runoff. A common fall ratio is 1:50 (20mm every 1m).

• The design should allow for drainage at the lower side of the fall.
• The sub-base must be well compacted.
• The sub-base should be as level as possible. An un-level sub-base will increase the likelihood of cracking.

Concrete Slab

The finished quality of the concrete slab is largely influenced by the ground preparation.

• Reinforcing steel doesn’t increase the load capacity of the pad. It is required for shrinkage control.
• CCANZ suggests slabs that are subject to acid attack, and traffic over 3 tonnes, should be between 125 and 150mm thick.

• Concrete shrinks when drying. This shrinkage causes cracking. Stress cuts are placed in the finished pad to control the cracking in a tidy way.
• The openness area of the farm environment makes slabs especially vulnerable to cracking in the days immediately following a pour. The reasons include the temperature fluctuation between night and day, and the increased likelihood of uninterrupted wind flow, which dehydrates the slab.

REGULATORY REQUIREMENTS

As with most farming activities these days, there are a range of different rules which apply to the design, site location, and management of a silage bunker. This section will introduce you to the main rules. For more information, or specific rules in your area, we recommend you contact your local district or regional council.

LOCAL GOVERNMENT 

Most councils regard silage harvesting and storage as a permitted activity. This means, by and large, you do not have to get a resource consent to grow or store silage. The exact rules differ from council to council. Because maize silage run off is high in nutrients, most rules focus on the proximity of the bunker to water.

Below are a list of general rules which apply across all councils. For more specific information, refer to your local council.

• Silage stacks/bunkers must be located a minimum of 20 – 50m from any surface water, coastal or fresh, and from any bore used for drinking water suppler (both animal and human).
• The base of the stack/bunker must be no less than 1 meter above the highest level of water table.
• There must be no discharge of run off into surface water, or the water table, except very minor seepage.
• There are generally maximum allowed capacities for silage bunkers, if your silage bunker exceeds the maximum allowed capacity a resource consent is required.

MILK PROCESSORS 

Most milk companies have requirements around minimum distances of feed storage from milking sheds:

• Minimum 20m distance for any feed storage from the milking shed if stored on concrete.
• Minimum 45m distance for any feed storage from the milking shed if not stored on concrete.

There are also requirements on the type of materials which can be used to construct feed storage, for example some milk companies restrict the use of treated timber. 

CENTRAL GOVERNMENT 

Central government regulates the design and construction of silage bunker through three main tools:

• Feed storage bunkers can be exempt from the building act as long as they are under 2.5m high.
• The Resource Management Act 1991 forbids the leachate of any contaminant, directly or indirectly, into water ways.
• The Ministry for Primary Industries has issued guidelines around feed storage to prevent cross contamination.