Electric fences for vermin and wildlife: fencing design
FIRST PUBLISHED 1983
by Geoff Burston, farmer, Benambra
Bob Piesse, fencing consultant, Kew East
and Stuart Baud, beef industry officer, Bairnsdale.
The type of electric fence required for controlling vermin and wildlife depends on the type of animals to be kept out and the design and condition of the existing fence. Wombats, foxes, pigs, wild dogs and hares are relatively easily blocked. Kangaroos, wallabies and emus require more complex fences.
Both free-standing fences and additions to existing fences have been used successfully to control vermin and wildlife. An important principle that applies to all these fences is that, because most animals will try to go under or through a fence, the lower wires must be arranged so the animal gets a shock that it will remember.
The operational efficiency of a fence is as important as its design. Animals will soon overcome their fear of one that is not operating efficiently and it may take several weeks of training to 're-block' them. Requirements for high operational efficiency are discussed in the agnote 'Electric fencing to control vermin and wildlife: principles'.
Some examples of successful fence designs are described below.
'BACK-UPS' TO EXISTING FENCES
Electric fences placed on the outside of old netting fences develop fewer faults than those placed on the inside. However, if the fence is placed on the inside, holes in the netting should be left open so the animal can contact the electric fence rather than learning to jump the existing fence and missing the electrified wires. Ragged edges of netting at holes should be rolled up and secured.
The locating of extra posts, stakes or droppers in or close to animal pads (tracks), adds to the firmness and thus to the effectiveness of the wire arrangement. Posts or stakes at these points also help to keep the netting from snagging on the electric fence.
This design (figure 1) proved very successful in stopping all traffic under existing fences, with the exception of rabbits and hares. It should be placed within 300 mm of the existing fence.
Pointed posts 1.0 m by 50 mm by 25 mm are spaced at 10 m maximum. The use of 2.5 mm wire is recommended. No droppers are required unless to firm up wires at a pad.
Six-wire leaning fence
A six-wire leaning fence (figure 2) is one of the few fence designs that has proved 100% effective against emus in low concentrations. It will give almost total relief from kangaroos. However, trip wires on top of the existing fence may be needed as well. They should be supported at 10 m spaces and be tightly strained.
The leaning fence should be placed on the outside of the existing fence.
Pointed posts 1.8 m by 50 mm by 38 mm are spaced to maintain bottom-wire ground clearance. About 40 uprights and 60 droppers (25 mm by 18 mm by 1.1 m) are required per km.
All uprights should be at least 0.6 m in the ground. Droppers may be used between posts to reduce upright spacing to a maximum of 10 m.
Seven-wire free-standing fence
This fence (figure 3) has been very effective against wild dogs and will give some initial success against kangaroos.
Figure 3. Seven-wire fence
The top wire should be live or neutral to save shorts caused by kangaroos twisting wires. (Neutral wires are ones that are connected neither to the energiser nor to the earth). If a short occurs between live and neutral wires, the neutral wires become live. Alternatively, if a short occurs between earth and neutral wires the neutral wire becomes earthed.
For greater effectiveness against kangaroos two neutral trip wires should be added to the top of the fence, increasing total height to 1.6 metres. These trip wires should be fixed on long posts about 10 m apart.
Wombats find gaps under the bottom wire quickly and soon learn to dig under the fence. They can easily be blocked with a piece of netting attached to the bottom earth wire (as shown in figure 4) to ensure that the animal comes up onto an electrified wire. One shock seems enough to keep them back. The netting must be firmly anchored into the ground to avoid short-circuiting with the adjacent live wire.
Figure 4. Using netting to block holes
Eleven-wire sloping fence
The fence design that gives the best exclusion rate for kangaroos and wallabies is a 1.9 m high, eleven-wire fence that leans into the paddock at 45° (figure 5). Most animals push under or through a fence. However, with this fence they are forced into contact with the electrified wires as they top or stand on the lower wires.
Figure 5. Eleven-wire sloping fence
The fence has 1.8 m by 50 mm by 38 mm uprights driven at least 600 mm into the ground. A 2.4 m by 50 mm by 38 mm high-density hardwood post is rested on the ground at a 45° angle and is bolted to the 1.8 m upright.
All live and earth wires are strained to 1.8 kN. The top four neutral wires are strained to 0.9 kN. Post spacings should be a maximum of 30 m apart, with droppers binding the bottom seven wires to form 10 m panels.
Because the 45° angle to the horizontal is maintained in all terrain the length of the upright may vary.
On level ground or over humps the upright is attached to the sloping post between the sixth and seventh wires. Through hollows it is attached between the fourth and fifth wires.
Figure 6. Correct earthing
One of the main reasons for fence failure under dry ground conditions is an incorrect or poor earth system. The energiser should be earthed at the unit with at least two earth stakes driven at least 1.8 m into the ground and spaced at 1 1/2 times their depth of embedment, as shown in figure 6.
A wire should connect these stakes to the fence and be connected to all earth wires (including the old fence if present). At gateways, fence junctions and intermediate strainer posts, the earth wires should be inter-connected with hot-galvanised clamping screw fasteners.
Additional earth stakes should be provided about every 1 km from the energiser for fences that are on wooden uprights and independent of any netting.
If voltage can be felt in the earth wire, further earth stakes are required in suitable locations along the fence for example, in wet gullies.
© 1983 Vic. Dept. Ag