Planting a shelterbelt is the only option open to some New Zealand livestock farmers for reducing the adverse effects of wind. Wind reduces pasture productivity by mechanically agitating the leaves and results in inhibition of plant cell expansion. It causes greater evaporation and physical damage to the plants and reduces pasture yield. At any given temperature, wind raises the lower critical temperature for an animal, and this is potentially wasteful in terms of energy utilisation, and it increases the risk of hypothermia and mortality in particularly sensitive stock such as newborn lambs and recently shorn ewes. (see Table 1-the impact of wind speed on air temperature)
In NZ, when it rains it is usually windy as well. It has been estimated that wind speeds during periods of rain are, on average, 30% higher than the long term mean. In Southland, southerly and south-westerly are usually the most effective rain-bearing winds and so this gives rise to combinations of cold , wet and windy weather which is particularly conducive to excessive heat loss in livestock.
Windbreaks (shelterbelts) reduce wind velocity. They do this by deflecting the wind upwards, causing a reduction of wind speed in the lee of the shelterbelt. Shelterbelts also filter wind, depending on their porosity, also slowing wind speeds. A well designed permeable shelterbelt can reduce open wind speed by up to 50% for a distance of around 10 x the height of the belt and up to 25% for a distance of around 15-20x the height. Open wind speed develops again at a distance from the belt of about 40x the height. The general rule of thumb is that an effective zone of shelter extends for 10-15x the height of the shelterbelt.
This reduction in wind speed has some important benefits for sustainable agriculture.
– Reduction in lamb mortality and abortions. – Improved growth rates and ovulation rates in cattle and sheep.
Increased pasture and crop production.
Good shelter offers possibilities of more flexible shearing dates and improves efficiency in use of expensive supplementary feeding.
Soil conservation and wind erosion control.
Improved wool growth rates.
Well designed shelter systems also increase biodiversity on the farm. They provide shade for livestock in sunny weather and improve landscape and amenity values.
Disadvantages can include excessive shading, blocking of tile drains and the cost of maintenance.(side trimming). Most of these can be controlled with appropriate management procedures.
The effectiveness of shelter belts is based around four main principles. These are:
Orientation- Windbreaks are most effective when planted at right angles to the prevailing or damaging wind direction.
Height The size of the sheltered area ahead and behind any shelter planting is directly related to its height. The higher the trees the larger the area that will be sheltered.
Length Windbreaks should be as long as possible (at least 25x height) and continuous. Wind velocities increase around the ends of shelterbelts and through gaps.
Permeability-The most effective shelterbelts act as a wind filter and not as a solid barrier. Semi-permeable belts (40-60% porosity) provides the smoothest airflow and maximum zone of effective shelter. Impermeable (dense) shelter belts are ideal for young or vulnerable stock for short periods in cold, wet and windy weather. However, dense belts will not encourage stock to move out into grazing areas. Appropriate permeability can be achieved by selecting different species, correct spacing and side trimming.
To achieve successful and effective shelter systems, having a Wind break Tree planting Plan is strongly recommended. Environment Southland Land Sustainability Officers provide assistance with the preparation of WBTP plans. A comprehensive plan will delineate soil type boundaries, the direction of prevailing winds, watercourses, drainage systems, existing and proposed subdivision, water and power services and areas of natural and existing planted shelter among other things. The plan should have an appropriate scale (1:10000 is often used) which allows for accurate estimation of windbreak lengths. A plan enables new shelter plantings to be prioritized and costed. A programme of planting might cover several years and materials can be budgeted for and seedlings ordered in advance.
Various types of plantings should be contemplated such as stock and crop shelter, riparian plantings, erosion control plantings, shade, amenity and woodlot plantings. A planting plan is flexible and should be reviewed regularly. It often provides the impetus to carry out regular plantings and thus achieve a comprehensive and effective shelter system.
Reproduced from a study for Environment Southland
You might find the following Links useful.
Attracting beneficial insects and birds as well as providing Bee Fodder is an important consideration when planning your Shelter