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Influence of the intensity of exploitation on the productivity of grasslands

M.I. CISSE * and H. BREMAN **

(*) M.I. Cissé, Centre Pédagogique Supérieur, Bamako, Mali.

(**) H. Breman, Associate Expert, UNESCO.


Introduction
Conclusion
Bibliography


SUMMARY

The primary productivity of a pasture in the transition zone between the savannah and the Sahel is enormously influenced by exploitation during the rainy season. This is shown by simulation of grazing in the form of mowing. Mowings at intervals of 2 and 4 weeks, at a height of 5 cm, decrease the total biomass produced during the season by 85 percent and 70 percent respectively. These treatments kill the perennial grass Andropogon gayanus. Even one simple mowing in the middle of the growing season decreases productivity from 5 to 2 tons/ha of fodder.

Introduction

Situated in the Sudano-Sahelian zone and with a well-established pastoral tradition, Mali is a stock farming country par excellence.

While the Soudanian zone is characterized by sedentary stock farming, the Sahelian zone is known for extensive breeding characterized by the transhumance of large herds.

The recent drought that has seriously affected the Sahel makes us wonder whether the grasslands of the zone are rationally grazed. We shall now set out to determine when, and the rate at which, pasturelands should be exploited to maximum advantage.

1. Study of site

1.1. Location

The grassland studied is at Niono, in a transition zone between the Sahel and the Sudanian savannah. The perennial grass Andropogon gayanus could be found there at the extreme north. It is an important component of the pastures of the lowlands (IEMVT, 1970), but its existence is threatened by over-grazing and prolonged drought.

1.2. Climate

The Niono region has a tropical Sudano-Sahelian climate characterized by high temperatures and only one rainy season, from June to September.

The average annual temperature is 29.8°C. It reaches its maximum in May at 40.1°C and its minimum in January at 12.2°C. The normal rainfall calculated over a period of 36 years is 569 mm, with a standard deviation of 127 mm. With a recording of 382.7 mm in 1974, the rainfall was deemed below normal.

1.3. Vegetation

The pastureland studied is part of a vegetation group that Boudet named,, Pterocarpus lucens and Andropogon gayanus variegated savannah,, (IEMVT, 1970). This group occupies the lowlands and the lowest parts of the lightly undulating sandy mantle.

Though at the study site the woody species, composed mainly of Acacia seyal, were sparse, the relatively dense herbaceous cover had 3 strata:

- A high stratum composed of Andropogon gayanus;

- A medium stratum formed by Diheteropogon hagerupii, Loudetia togoensis and Cassia mimosoides;

- A low stratum with Borreria Polycarpea corymbosa, Elionurus elegans, Schoenefeldia gracilis, Fymbristylis hispidula, and Blepharis linariifolia.

2. Methodology

Before the rainy season, 52 plots of 2 X 2 m were marked and cleared of their entire production of the previous year.

Periodic samples taken made it possible to assess the influence of the intensity of grazing on their productivity. The samples were made in three series referred to as P2, P4 and P8, on each of the 4 plots mowed every 2, 4 and 8 weeks, respectively.

Special samples were taken in 10 series from 4 plots tagged, U2 to U20, which were treated respectively at the 2nd, 4th, 6th, 8th, 10th, 12th 14th, 16th, 18th and 20th weeks. This not only made it possible to follow the growth of the vegetation but also to know the impact of the time of mowing on the total annual productivity, which we assessed through fresh cuttings at the end of the experiment (final observations). The productivity was assessed on the basis of the biomass of the above ground parts.

To do this, the production of each plot, cut with a pair of clippers fitted with a device that allowed for cutting at a height of 5 cm from the ground, was either oven-dried under 70-80 °C heat or dried under the sun.

An average productivity was assessed for each of the series, calculated on the basis of the average biomass established from the biomass of the 4 plots of the series.

3. Results

The experiments took place from June to November 1974. Plots P2 were cut 9 times, plots P4, 6 times and P8, 3 times. Plots U continued to be treated until growth was considered to have halted, in the light of the biomass. The final observations were made 2 weeks after the mowing of plots U20.

Fig. 1. - The productivity is expressed in terms of grams of dry matter per m² and per day during the rainy season.

3.1. Influence of cutting on productivity

Frequency of cutting has a great influence on productivity as shown in Fig. 1.

If maximum productivity was obtained under the 4 types of treatment, it should however be noted that on plot P. this was obtained well before the end of the rainy season, and that optimum productivity on plots U was obtained at the end of the rainy season.

3.2. Influence of cutting on the biomass

The different yields (3.1) occur as a result of the evolution of the biomass of the various plots (Fig. 2). The total biomass produced varies with the intensity of exploitation. Thus the highest total biomass was produced on plots U and the lowest on plots P2. Fig. 3 seems to reflect an exponential evolution of the biomass according to the number of cuttings. It shows clearly that the biomass is reduced with each exploitation during the growth period.

3.3. Influence of cutting on the biomass of annual and perennial species

A reduction of the biomass under intensive exploitation (3.2) does not necessarily result in a deterioration of the value of the rangelands. This is why we assessed separately, the biomass of the annual and perennial species. Its appears from the diagram (Fig. 4), showing the biomass at the end of the rainy season, that cutting diminishes especially the growth of Andropogon gayanus, in such a way that the annuals become relatively more important.

Fig. 2. - The evolution of the biomass in grams of dry matter per m² during the rainy season.

The nutritive value is thus probably increased, considering the presence of Cassia mimosoides, Blepharis linariifolia and Borberia, but this is only an apparent increase, because the total quantity of the annual species decreases.

This change in the proportion of annuals and perennials will certainly remain for a long time because of the extinction of perennials on plots P2 and P4. This decimation has even reached 100 percent on P2 (Fig. 1). The re-growth of perennials was about 25 percent in the case of the land under study. he land under study.

Fig. 3. - The total biomass produced in relation to the number of cuttings during the growth season.

Fig. 4. - The total biomass of the perennial and annual species in relation to the number of cuttings during the growth season.

Table 1 - Influence of mowing on Andropogon gayanus

Plots

Number of live tufts on:

16 m² at the time of maximum development

At the end of the experiment

P2

40

0

P4

49

30

P8

41

41

U

48

48

3.4. Influence of the time of cutting on the total productivity

Fig. 5 shows that it is not only the intensity of exploitation that influences productivity (3.1) but also the time of grazing on the land.

We have estimated the total productivity on the basis of the total biomass produced on plots U, by establishing for each type the total biomass produced after the first cutting and that of the final observation (see 2). Thus, it will be noted that any grazing before the end of the growth period reduces the productivity and consequently the food available. The influence is greatest during the 10th week of the rainy season.

Fig. 5. - Influence of the time of grazing on the total biomass available. One cutting during the growth season followed by a second at the end.

Conclusion

These results show that the productivity of the grasslands is enormously influenced by the method of exploitation. With grass cut to a level of 5 cm from the ground, maximum productivity is obtained if this is done only once every year. Intensive and frequent grazing kills Andropogon gayanus, the most important component of that grassland. As regards the time of exploitation, the end of the rainy season, when the biomass is maximal, seems the best time. Future studies will, however, prove the validity of this conclusion in the case of higher cuttings; in other words, for a less intensive grazing.

Bibliography

1. BELIME, E.L. - Les travaux du Niger. Paris, Martin-Mamy/Crouan et Roques, 1940.

2. BOUDET, G. - Désertification de l'Afrique tropicale sèche, Adamsonia (sér. 2: 505-524) 12: 1972.

3. CLAUDIN, J. - Etude Phytoécologique du Hodna. Algérie. M.A.R.A./F.A.O., 1973.

4. I.E.M.V.T. - Etude Agrostologique pour la création d'une Station d'embouche dans la région de Niono, no. 29, le Mali, 1970.

5. I.E.M.V.T. - Esquisse Pastorale et Esquisse de Transhumance de la région du Gourma, le Mali, 1971.

6. I.E.M.V.T. - Aménagement du berceau de la race N'Dama dans le cercle de Yanfolila, no. 36, le Mali, 1972.

7. I.E.M.V.T. - Projet de Développement de l'Elevage dans la Région de Mopti, no. 37, le Mali, 1972.

8. Mali. - Annuaire Statistique 1970, Service de la Statistique générale de la Comptabilité Nationale et de la Mécanographie, 1970.

9. Mali. - Rapport sur l'Etude Sectorielle concernant l'Approvisionnement en eau du pays. D. Balfour et Sons, 1973.

10. RATTRAY, J.M. - The Grass Cover of Africa, F.A.O., 1960.


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