Livestock grazing comparison is a method of comparing the numbers and density of livestock grazing in agriculture. Various units of measurement are used, usually based on the grazing equivalent of one adult cow, or in some areas on that of one sheep. Many different schemes exist, giving various values to the grazing effect of different types of animal.
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Use
Livestock grazing comparison units are used for assessing the overall effect on grazing land of different types of animals (or of mixtures of animals), expressed either as a total for a whole field or farm, or as units per hectare (ha) or acre. For example, using UK government Livestock Units (LUs) from the 2003 scheme a particular 10 ha (25-acre) pasture field might be able to support 15 adult cattle or 25 horses or 100 sheep: in that scheme each of these would be regarded as being 15 LUs, or 1.5 LUs per hectare (about 0.6 LUs per acre).
Different species (and breeds) of livestock do not all graze in the same way, and this is also taken into account when deciding the appropriate number of units for grazing land. For example, horses naturally graze unevenly, eating short grass areas first and only grazing longer turf if there is insufficient short grass; cattle graze longer grass preferentially, tending to produce a uniform sward; goats tend to browse shrubs if these are available. As these feeding styles are complementary, a pasture may therefore support slightly more units of mixed species than of each species separately. Another consequence of different grazing styles is variation between species in the number of units that can lead to overgrazing – for example, horses may overgraze the short parts of a pasture even when taller grass is still available.
Livestock grazing comparison units are used by many governments to measure and control the intensity of farming. For example, until 2004 the UK Government had an extensification scheme which paid additional subsidy to farmers who kept their livestock at less than an average of 1.4 LUs per hectare.
Schemes
Although different schemes have similar aims, they vary in complexity and detail. For example, some schemes give no value to a young calf, but an additional value to a cow together with her calf at foot. Some give values to different-sized animals of the same species, or different values to the same species in different regions. Most schemes use a calculation based on the weight of the animal. Some use figures for animals of different sizes which are directly proportional to their weight – for example the 2006 UK Government scheme uses a figure for ruminants of the animal's weight (in kilogrammes) divided by 650. Others include an adjustment for the proportionally higher metabolic rate of smaller animals, according to Kleiber's law, which states that the metabolic rate of most animals varies according to their weight raised to the power of approximately 0.75. For example, the Food and Agriculture Organization's Tropical Livestock Unit is based on the weight of the animal raised to the power of 0.75, compared with the equivalent figure for a "tropical cow" of 250 kg (550 lb).
The following is a summary of some schemes in common use, using the most closely comparable categories: