For a number of applications there is a need to use a common unit to describe livestock numbers of various species as a single figure that expresses the total amount of livestock present – irrespective of the specific composition. In order to do this, the concept of an "Exchange Ratio" has been developed, whereby different species of different average size can be compared and described in relation to a common unit. This unit is 1 Tropical Livestock Unit (TLU).

Various methods of obtaining exchange ratios among species have been used, but none has been completely satisfactory. Different formulae for estimating TLUs may be utilised in different parts of the world, depending on common livestock varieties. (e.g. 1 TLU = Camels 1.0; Cattle 0.7; Sheep/Goats: 0.1).  However a single formula for estimating TLUs in this way is unable to account for different livestock varieties - which may differ significantly in size - and a different approach is required.

If the feed eaten is reasonably the same for both species being compared, the ratio of metabolic weights provides the best means of comparison. This relationship expresses that the fact that smaller animals produce more heat and consume more food per unit of body size than do larger animals (Heady, 1975).
 

Basal metabolic rate (energy expenditure per unit body weight per unit time; i.e., kcal heat/weight/day) varies as a function of a fractional power of body weight, usually determined to be body weight raised to the 0.75 power. Loss of protein from the body also varies by a similar fractional power of body weight and is presumed to be related by the same exponential power of body weight.

Under resource driven grazing conditions the average voluntary feed intake amongst species is remarkable similar, about 1.25 times maintenance requirements (1 for maintenance, 0.25 for production = growth, reproduction, milk etc.). Metabolic weight is therefore considered as the best unit for aggregation of animals from different species, whether this is for the total amount of feed consumed, manure produced, or product produced.

TLUs and Exchange Ratios

The standard used for one Tropical Livestock Unit is one cattle with a body weight of 250 kg.

Boxes 1 to 3 present the exchange ratios for animals with different body weights in Tropical Livestock Units based on metabolic weight. It shows that 5 sheep or goats of 30 kg will consume as much as 1 cow of 250 kg. Similarly, two buffalo of about 425 kg will consume as much as 3 cattle of 250 kg. However, strictly speaking, they can only be compared in this way when the different species consume the same feed, something that is often not the case.

Exchange Ratios for livestock in Tropical Livestock Units
based on Metabolic Body Weight.
 

Body Weight (kg)	Metabolic Body Weight (kg 0.75)	T L U
5	3	0.05
10	6	0.09
15	8	0.12
20	9	0.15
25	11	0.18
30	13	0.20
35	14	0.23
40	16	0.25
45	17	0.28
50	19	0.30
60	22	0.34
75	25	0.41
100	32	0.50
125	37	0.59
150	43	0.68
200	53	0.85
250	63	1.00
300	72	1.15
350	81	1.29
400	89	1.42
450	98	1.55
500	106	1.68
600	121	1.93
700	136	2.16

Using Tropical Livestock Units

A number of points need to be noted when using TLUs:

• In respect of grazing pressure there are differences amongst species in grazing / browsing behaviour and in feeding abilities that will alter the exchange ratios. The optimum number of each species in a pasture depends upon the amount of grass and browse available, not upon the total number of TLUs and total biomass.

• Species differ in grazing behaviour and abilities - resulting in different opportunities to utilise vegetation (see also species). For example, when feed density is low (onset of rainy season, poor fallow vegetation, early re-growth of perennial grasses etc.) sheep can find enough feed to grow. For cattle the equivalent density will still be too low even for maintenance. Camels can reach browse at higher levels than goats, while goats can reach browse between rocks and on hill slopes, feed that camels are unable to reach.

• The impact of grazing and browsing on the composition of the vegetation is different. Repeated grazing will result in more browse and repeated browsing in more pasture (Staples et.al., 1942). Early grazing by sheep can reduce grass production - notably when the growing season is short. This can reduce feed availability for cattle more than the equivalent of the feed consumed by sheep.

In the case of communal pastures under mixed grazing management systems the relationships of TLUs per hectare with the condition of the pasture and its carrying for livestock are in very weak.

Tropical Livestock Units do, however, provide a reliable estimate of the amount of feed consumed and of manure produced. For these purposes the assessment of total number of TLUs can be useful.

Further Reading

Grazing Systems

Heady, H.F. 1975. Rangeland Management. McGraw-Hill Book Company, USA.

Staples, R.R. Hornby, H.E. and Hornby, R.M. 1942. E. Afric. Agric. J. 8, 62.
 
 

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