We thank the ARC Institute for Agricultural Engineering in South Africa who made their manual on sheep production and facilities available to the readers of ProAgri Zambia.
Any building will exercise an influence on the relative moisture content, quality of the air, the temperature, as well as the amount and quality of light in the building compared to conditions outside. The purpose of ventilation is mainly to create a healthy environment by replacing stale air with fresh air and at the same time removing moisture, dust, heat, odours and harmful gases.
Since sheep are adapted to handle reasonable temperature variations, the aim of ventilation is to create a dry, draft-free building rather than influencing the internal temperature.
Temperature in the building must never be manipulated by closing some or all ventilation openings. Such a situation will lead to enormous moisture build-up and eventual condensation against the roof and walls. This condensation not only reduces the isolation capability of the building material, but is a quick recipe for causing pneumonia in sheep.
Ventilation requirements are a function of, among others, the external temperature, construction material, and the group sizes housed in the building. It is therefore expected that some measure of ventilation control should be built in.
Ventilation can be provided in two ways, namely natural ventilation and artificial ventilation.
Natural ventilation is supplied by placing inlet and outlet openings in the wall and roof. These can be permanent or adjustable vents such as hinged or movable panels. A building with natural ventilation will usually be only a few degrees warmer than the outside during winter and a few degrees cooler during summer. Figure 1 shows a typical situation of natural ventilation in summer and winter conditions.
If the building is open on one side, the open side must be placed away from the prevailing winter wind direction as shown in the figure above. Ventilation requirements will vary according to the dimensions of the building, and will also depend on the position of internal obstructions inside the building.
The following directives are applicable to natural ventilation conditions and specific values are based on a building width of 18 m and a minimum height of 3 m.
Vents can be permanent or adjustable and can include windows, wall openings or panels, ridge openings, roof edge openings, et cetera.
Permanent wall openings must not be placed lower than 1,5 m from the floor surface. This will prevent a direct draught on the animals during winter. Inlet openings can be adjustable panels or doors on the one side, with a permanent continuous opening of 100 to 150 mm in the opposite wall.
Alternatively, a minimum of 0,035 m² per ewe space should be provided. Assume that the building is designed with an occupation of 2 m² per ewe, then at least 0,07m² ventilation opening per ewe must be provided.
The ridge opening should be about 450 mm to ensure a good inlet at all the openings. The ridge opening must be in total about 50% of the total inlet openings provided.
In areas where it is necessary to heat a building artificially, artificial ventilation is provided. Thermostatic control systems can be connected to the fans to ensure correct air movement for different external temperatures and occupations or group sizes.
Two types of artificial ventilation systems can be distinguished:
Positive pressure systems:
Air is forced into the building by fans, and leaves the building through outlet openings provided. The disadvantage of pressure systems is that moist air is forced into the shed. This moist air can condense and moisten insulation material with a resulting decrease in the insulation capacity of the material.
Negative pressure systems:
With these systems, air is extracted from the building by extractor fans. Fresh air enters the building through permanent or adjustable inlets.
Directives for artificial ventilation systems for an occupation of 2m² per ewe:
The insulating value (U-value) of building materials should be 1,4 and 0,7 W/m² for the walls and ceiling respectively.
Provide an airflow of at least 2,5 ℓ/s per ewe to prevent moisture build-up. Fans can be chosen to provide a basis of 25 ℓ/s per 1 000 kg live weight of the animals. An additional 80 ℓ/s per 1 000 kg must be introduced in steps as the temperature rises.
Inlet openings of at least 275 mm² per ℓ/s airflow or fan capacity must be provided. Alternatively air vents must have a minimum surface of 0,1 m² per 300 ℓ/s fan capacity.
With positive pressure systems, the air vents can be calculated as 1 360 mm² per 10 ℓ/s.
Although lambs can withstand low temperatures when they are dry, heating can be provided for new-born lambs by means of a 250 W infrared lamp. Good protection against shocks and burning is necessary.
Next month we shall look at one of the most important aspects of sheep handling which is often overlooked: The production and handling of sheep waste and the value it contains.
Published with acknowledgement to the ARC Institute for Agricultural Engineering for the use of their Sheep Facilities Manual. Visit www.arc.agric.za for more information.