Maize for Silage
Technical
Insight 320

# DESIGNING A MAIZE SILAGE BUNKER OR STACK

## INTRODUCTION

There are 10 rules for good silage stack or bunker management. They are:

• Determine the proper bunker or stack dimensions.
• Drain rainwater run-off away from the stack site.
• Harvest the crop at the correct drymatter content.
• Use a proven maize silage specific inoculant.
• Cut the crop to the correct chop-length.
• Ensile the crop rapidly.
• Compact. Compact. Compact.
• Seal cover edges and joins securely.
• Fence the area to keep livestock away from the stack.
• Ensure the correct method and rate of feed-out.

Determining the correct storage structure is important since silage must be fed-out fast enough to stay ahead of aerobic (oxygen-present) deterioration. The face area of the silage stack is calculated as the width multiplied by the height. An ideal silage stack has a small face area which allows rapid feed-out. Ideally, at least 15-20cm of the stack face is removed every second day.

The top of the face must be easily reached from ground level using the feed-out machinery that is available. To ensure a good compaction, a bunker must always be at least twice the width of the vehicle that will be used to compact it. Stacks higher than 4m may pose a significant Health and Safety risk if an overhand develops during feed out. Overhangs develop when the feed-out machinery cant reach to the top of the stack.

Below are eight simple steps to help you to work out how high, wide and long your silage bunker or stack needs to be. There is a worksheet at the end of the bulletin that will help you determine your own stack or bunker dimensions.

## DETERMINING BUNKER OR STACK DIMENSIONS

### 1. Determine the total amount of drymatter that will be fed daily (on average)

Example:
200 cows x 5 kgDM per cow per day = 1,000 kgDM per day

### 2. Determine the cubic area that will be removed from the face each day

Pioneer measured the density of 25 maize silage bunkers and stacks. The average densities of maize silage in a bunker (with walls) or a stack (no walls) were as shown in Table 1 below:

Table 1: Maize Silage Stack & Bunker Average Densities

Storage System

Number of Sites

Average Density(kgDM/m3)

Density Range (kgDM/m3)

Stack

13

200

150– 74

Bunker

12

225

167–288

Example:

Average density maize silage in a bunker = 225 kgDM/m3

1,000 kgDM per day / 225 kgDM/m3 = 4.44 m3 removed per day

### 3. Determine the desired feed-out rate

Table 2: Target Maize Silage Face Removal Rates Example:
Target feed-out rate is 0.20 m (or 20 cm) per day

### 4. Divide the cubic area removed (Step 2) by the desired feed-out rate (Step 3) to calculate the square area of the face removed each day

Example:

4.44 m3 removed per day / 0.20 m per day = 22.2 m2 removed per day.

### 5. Determine maximum width and height dimensions

Determine the desired bunker height or width and divide by the face square area (Step 4) to calculate the desired width or height. Note the actual height and width can be lower than the numbers calculated but ideally should not be higher.

The only exception is if the mixer or feed-out wagon is to be loaded at the stack face without the tractor having to exit the bunker for each bucket load a wider face may be necessary. Work out the minimum width taking into account feed-out machinery dimensions and consider using a face shaver or block cutter to keep the face tight

Example:
Assume tractor bucket can reach 2.5 metres so desired height = 2.5 metres.
22.2 m2 (from Step 4) / 2.5 metres = 8.88 metres wide maximum
OR
Assume maximum width of bunker is 8 metres (must be 2 x tractor wheel width).
22.2 m2 (from Step 4) / 8.0 metres = 2.775 metres high maximum

Plastic roll width should be taken into consideration when planning dimensions. If you are building a stack and you have no height or width restrictions, aim for a maximum width of:

Number of Cows

Recommended Stack Width (metres)

200

6

300

7

400+

8

### 6. Calculate the total silage tonnage that will be stored

Either calculate the total amount of feed required (by multiplying the number of cows by the feeding rate by the number of days fed) or calculate the total tonnes harvested (crop yield per hectare x hectares harvested).

Example:

200 cows x 5 kgDM/cow/day x 100 days = 100,000 kgDM (100 tonnes DM)

OR

4 hectares x 25,000 kgDM/ha (25 tDM/ha) = 100,000 kgDM (100 tonnes DM)

### 7. Calculate the total cubic area that will be required

Divide the tonnes of silage to be stored (from Step 6) by the average density (Table 1).

Example:

100,000 kgDM stored / 225 kg/m3 = 444 m3 storage space required

### 8. Calculate stack length

Divide the total cubic area by the actual face area (actual height x actual width)

Example:

Divide the total cubic area by the actual face area (actual height x actual width).

Example:

From Step 5. Maximum Height = 2.5 m       Desired Height = 2.0 m

Maximum Width = 8.8 m        Desired Width = 7.0 m

444 m3 storage space / (7.0 m wide x 2.0 high) = 444/14= 31.7 m length

## FARMER BUNKER OR STACK DIMENSION WORKSHEET

### 1. Feed required per day

Number of cows x feed-out rate (kgDM/cow/day) = (kgDM/day)  (A)

### 2. Cubic area removed per day

Feed required/day (A) / stack density (Table 1) = m3/day  (B)

### 3. Desired feed-out rate

Desired feed-out (from Table 2) (C)

### 4. Face area removed per day

Cubic area removed/day (B) / desired feed-out rate (C) = m2 (D)

### 5. Stack width and height dimensions

Choose a desired height or width:

Face area removed/day (D) / desired width (m) = height (m) maximum

OR

Face area removed/day (D) / desired height (m) = width (m) maximum

Note: The figures that you have chosen are maximum dimensions. You may choose to reduce either the height or width (or both) giving a smaller face area. However the face area (height x width) must be no greater than that calculated (D)

Actual height (m) (E)

Actual width (m) (F)

### 6. Tonnage of silage to be stored

No of cows x days fed silage x feeding rate (kgDM/day)= kg  (G)

OR

Crop area (ha) x crop yield (kgDM/ha) =  kg (G)

### 7. Cubic area required

Tonnes of silage to be stored (G) / stack density (Table 1) =  m(H)

### 8. Calculate stack length

Actual height (E) x actual width (F)= face area (m2)              (I)

Cubic area required (H)/ face area (I) = stack length (m)            (J)

### Final stack or bunker dimensions

Height (E)
Width (F)
Length (J)

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© 2021, Genetic Technologies Limited. No part of this publication can be reproduced without prior written consent from Genetic Technologies Limited.

Revised: Jan 2020
Expires: Jan 2022