The role of shelterbelts in protecting livestock: a review

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 this results in inhibition of plant cell expansion. It causes greater evaporation and physical damage to the plants, which also reduce 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. Ewes that are shorn pre-lambing seek protection from the wind and there is a greater likelihood of them lambing within shelter if it is available. The tendency to lamb in isolation away from shelter depends on breed and weather conditions. For most purposes, the best shelterbelt is one which has between 40 and 60% porosity that is distributed evenly throughout its length and height and protects the maximum area of pasture from the wind. Under cold conditions shelter has been shown to improve pasture yield, and growth rate, ovulation rate, and wool growth rate in cattle and sheep, and to reduce lamb mortality and abortions that are induced by hypothermia. Providing shelter from the sun during hot conditions has been shown to improve milk yield, milkfat yield, freedom from mastitis, and conception rates in dairy cattle, and growth rate in fattening cattle. The welfare benefits from shelterbelts and shadebelts are implicit in their greater use during cold-windy and hot-sunny conditions and they relate to the thermal comfort of animals. The purpose of snow shelterbelts is to filter the wind and trap snow without forming snowdrifts in which stock are Rely to be trapped. Their effectiveness depends largely on the amount of snow falling relative to the size and porosity of the shelterbelt. Trees-on-pasture systems provide protection from wind and raise the minimum grass temperature, but because of the competition introduced with the trees they are usually less productive in terms of livestock yield than pasture alone. There are a number of disadvantages in growing shelterbelts but many of them can be controlled with appropriate management procedures.