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Problems encountered in estimating the rate of stocking of "natural pastureland" in a tropical zone

G. BOUDET *

(*) G. Boudet: de l'Institut d'Elevage et de Médecine Vétérinaire des Pays Tropicaux.


Basis for estimating rates of stocking
Necessity of control of the use of pasture by animals
Conclusion


SUMMARY

An estimation of the rate of stocking is the necessary conclusion to a pasture survey.

In intertropical countries, this rate is most often assessed on the sole basis of the forage production of the pasture: the edible forage production or simply the biomass, measured at the end of the active period.

The available estimation methods are described. and the necessity for an experiment using stocking trials with cattle is demonstrated. These exploitation trials of pasture must be performed with techniques suitable for popularisation.

Although an area of pasture may have been identified and its productivity estimated, the most suitable system for its exploitation cannot be proposed until after an evaluation of the rate of stocking has been made.

The rate of stocking of pasture represents the amount of grazing stock that the pasture can support without deterioration; the livestock should remain in good condition, if not gain weight or produce milk, while it is on the pasture.

This rate of stocking will depend partly on the quantity of forage produced and partly on the quality of the forage available to the livestock to enable it to produce economically satisfactory results.

Basis for estimating rates of stocking

The energy value of pasture is frequently the only factor taken into consideration. The potential productivity of pasture is expressed in feed units per hectare, and forage evaluation is also calculated in feed units.

This assessment is acceptable when it is possible to add a nitrogenous supplement for the stock during critical periods.

Edible forage yield is often used in estimating rates of stocking; it is based on the concept of an " average living-space quota " for domestic herbivores.

The daily consumption of a head of cattle is usually estimated at 2.5 kg of dry matter per 100 kg of live weight. This consumption may decrease if the dry matter content is low, or if the nutritional value of the forage is low; and conversely it may increase if the forage is rich. With regard to sheep, consumption follows a similar pattern as with goats kept in sheds, although goats may consume up to 5 kg of dry matter per day on natural pasture.

Provided that forage production is calculated in dry matter and not in " green" forage, the rate of stocking can be estimated in the number of days of feed per hectare for 100 kg of live weight:

This may similarly be expressed as a yearly rate:

kg of livestock/ha/year 365

and as a seasonal rate:

where n is the number of days in the season under consideration.

The quantity of edible forage produced during the year, in particular for perennial graminaceae, can be estimated on a small area or " placeau " 16 or 25 square meters in size, cut to a level of 5 cm from the ground, at intervals of 30 days, at the period of optimum growth:

Kg DM of edible forage/ha/year =(k1 + k2 +... + ki +... kn)

The potential yield of pasture is revealed in the biomass of herbaceous cover produced during the growth period when the pasture is protected from herbivores. It may be calculated by means of protected placeaux, which are pre-cleared after the end of the rains. The analysis of a partial sample will provide in addition an estimate of the amount of nitrogen that might reasonably be derived from this kind of pasture.

Such a potential yield is never entirely edible, and it is essential to link assessment of potential yield with assessment of the yield of edible fodder for each kind of pasture: this must be achieved through parallel observations.

Nevertheless experiments have shown that approximately half of the biomass produced at the end of the growth period vanishes from small protected areas during the dry season, in the case of both the annual herbaceous cover and the perennial graminaceae cover. In addition, traumas that the plant suffers, caused by browsing during the growth period, decrease yield by interrupting photosynthesis, and this must be compensated for by the liberation of part of the reserves contained in the roots. The trampling of livestock causes losses, and it is necessary to ensure the maintenance of at least a minimum cover of plant life to protect the soil against the various forms of erosion. The fact that potential yield is consumed at a rate of only 50 percent should also be taken into consideration.

Sample Calculation

Hyparrhenia diplandra pasture (Wakwa - Cameroon).

Characteristics:

Growth period: April to the end of October (210 days).
Production of edible forage: 3,100 kg/ha in 3040 days of growth.
Potential yield: 4,000 kg/ha of dry matter.

Rate of stocking estimate:

- taking edible forage yield: stocking for the year:

kg of live weight/ha

stocking for the growth period (April - end October):

kg of live weight/ha

- taking potential yield:

usable yield:

kg/ha

stocking for the year:

kg of live weight/ha

stocking for the growth period:

kg of live weight/ha

In this experiment, the stocking calculated on the basis of potential yield is probably underestimated; experiments with livestock will be necessary to produce firm estimates.

Utilization of Stock Units

The expression of stocking in kg of live weight is straightforward, although it may appear curious to users who are unfamiliar with it.

Since the average weight of adult cattle in the tropical zones varies from 200 to 400 kg, a tropical livestock unit may be defined as an animal with an average weight of 250 kg. This is the UBT (Unite Bovin Tropical), equivalent to the LSU (Livestock Standard Unit) of English-speaking countries.

Its daily consumption would normally be 6.25 kg of DM, which would enable a stocking rate to be established, expressed in LSU.

Taking the preceding example, at 4,000 kg of potential yield:

- annual stocking: 219 kg/ha/year of live weight

eitherLSU/ha/year

orha/LSU/year

- seasonal stocking during growth period (rainy season): 380 kg/ha of live weight

eitherLSU/ha/rainy season

orha/LSU/rainy season

This concept of a standard unit, particularly useful in the calculation of rates of stocking based on forage production, may equally be used in assessing the total forage yield by pasture or on a regional scale.

In temperate countries, total forage yield is expressed in UF by converting the entire production of livestock feed (fodder, forage, crop residues, industrial by-products). In this case all consuming animals are evaluated in UGB (Unité-Gros-Bétail), which is equivalent to " a cow of 550 kg of live weight, which has been on the farm for 12 months and produces 3,000 litres of milk per year, against an approximate intake of 3,000 feed units (UF)."

In tropical zones, the UBT/LSU may serve as a point of reference. The needs of cattle of 250 kg average weight are estimated at 2.8 UF and 155 g of digestible nitrogenous substances (dNS) per day, hence 1,022 UF and 56 kg of dNS per year.

Apart from UF it would appear essential to mention nitrogenous substance requirements, as they are very often the limiting factor in animal production in tropical regions.

Taking the example of temperate countries, where the different domestic herbivorous animals may be represented in UGB or LSU, it is possible to adopt the following equivalents:

1 adult head of cattle = 1 horse = 1 camel = 1 UBT/LSU
1 sheep = 1 goat = 0.12 UBT/LSU
1 calf = 0.7 UBT/LSU

Necessity of control of the use of pasture by animals

Our estimates of rates of stocking remain theoretical and need to be checked with actual livestock; measurement of the performances of animals would either confirm or fail to confirm the preceding estimates.

The animals used would preferably be males gathered in homogeneous groups, between the ages of 18 to 30 months, at the height of their development, and would be weighed every month. The pasture would be exploited, with the stocking rate determined by the preliminary calculation, and using a system both adapted to the type of pasture and appropriate to future widespread use.

At the same time the utilisation of pasture would be controlled by the measurement of dry matter yield, with the intervals between cutting corresponding to the pattern of grazing.

On small areas of grazing land, a placeau would be set aside and cattle prevented from grazing on it by means of a barrier. It would then be cut when the cattle had left, together with a similar placeau that had been grazed. In this way it would be possible to estimate the yield as between the two placeaux, as well as the waste, all the results being expressed in dry matter. Two new placeaux would be demarcated each time the cattle moved.

The annual consumption of an UBT/LSU would then be in round figures: 1,000 UF, 56 kg of dNS, 2,300 kg of DM.

At the same time, measurement of the primary yield of the herbaceous cover would be carried out, at similar intervals, on a small protected area, with a separate estimate of live and dead yield as between the two series of measurements, for the interval of grazing and also for the period beginning at the start of the rains.

In these experiments, the small protected area would be " rejuvenated " and " cleared " at the same intervals and with the same treatment as the pasture used for grazing.

At the end of the seasonal or annual experiments, yield provided by the stocking test livestock would be converted into energy (UF) and nitrogenous (dNS) values, according to the estimates of the requirements of the livestock (support and gain in weight). These estimates would then be compared with the estimate of pasture yield evaluated by cutting, weighing and analysis.

Conclusion

The assessment of the yield of pasture and thus its rate of stocking is the necessary culmination of a study of pasture. This yield cannot be determined from plant yield alone, but requires a minimum of control points on actual pasture by the consumers - the animals to which we would like to give a privileged place in the pastoral ecosystem - our herbivorous domestic animals.


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