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The productivity, forage value, and dynamics of different cutting intervals of three natural rangeland formations of the Adamaoua plateau in Cameroon

J. PIOT (*) and G. RIPPSTEIN (**)

(*) J. Piot: agrostologue, CRZ de Wakwa, Station Fourragère, B.P. 65, Ngaoundéré, Cameroun.

(**) G. Rippstein, agrostologue, CRZ de Wakwa, Station Fourragère, B.P. 65, Ngaoundéré, Cameroun.


General remarks
Results
Conclusions


SUMMARY

Three formations of natural pasture in the Cameroonian Adamaoua were studied for 5 years in order to describe their behaviour when subject to different cutting rhythms, at 20-, 30-, 40, 60-, and 80-day intervals, and with one unexploited control plot.

The formations studied were:

- pasture on developed red soil on a foundation of old basalt;
- pasture on granitic soil;
- pasture at the base of slopes of old basalt with a predominance of Hyparrhenia diplandra.

Data was collected on the following:

- the pattern of productivity from new growth on the basis of the weight of grass collected at each cutting on each plot;

- the forage value of new growth at different cutting intervals;

- the botanical evolution, noted on the two diagonals of each plot after the last cutting.

At the end of the paper, the conditions of the experiment and dissemination of the results are reviewed.

General remarks

The constituent elements of a vegetation formation react differently according to the intensity of its exploitation; some are favoured above others which might technically be thought preferable.

It was thus important to attempt to approach this problem of plant dynamics, even if it was not possible successfully to carry out experiments on the basis of pastoral conditions, which form a sounder basis than the use of actual pasture.

Natural conditions

The Adamaoua is a vast plateau, with an average altitude of 1,000 to 1,200 metres, situated at a latitude of between 6 and 8 degrees North.

The climate is of the Sudano-Guinean mountain type, with, at Wakwa, more than 1,700 mm of rain in 8 months, and 4 to 5 months that are ecologically dry.

The average annual temperature is 23° C (the absolute maximum and minimum being 35° C in March and 6°C in January). Monthly averages are 32° C in March for the maxima and 13° C in December and January for the minima.

Average relative humidity is 75 % in the rainy season and 40 % in the dry season, and for the absolute minima, it is as low as 10 %, in February. This has serious ecological consequences (slowing or halt of growth of vegetation, very rapid drying up of graminaceae species).

From the geological point of view, the Adamaoua is made up of a granite and substratum with a superficial deposit of sandstone and, in particular, volcanic rocks (most frequently of basalt).

The soil formation of the latter types of parent rock above is generally distinctly richer than the others, especially with regard to the last layers of basalt, which produce a dark, little advanced soil. There are a great many valleys in the Adamaoua, with a good distribution of water courses or marsh; consequently it possesses a fairly extensive area of hydromorphic soil, which is very valuable for grazing animals in the dry season.

The vegetation that covers the greater part of the plateau consists of mixed woodland-graminaceae types and a savannah of shrub and trees, which includes Daniellia and Lophira.

As far as herbaceous vegetation is concerned, it consists basically of Hyparrhenia spp. of different kinds, according to the nature of the soil (H. diplandra and Paspalum orbiculare or H. filipendula and Loudetia kagerensis, etc.), or to the type of exploitation (Panicum phragmitoides, Sporobolus pyramidalis, Andropogon gayanus, Brachiaria brizantha, with H. rufa, H. welwitchii, H. filipendula, H. chrisargyrea, for example).

Methodology

Preparation of experiments

Five plots cut with shears at 4 to 5 cm from the ground every 20, 30, 40, 60 or 80 days are compared with each other. A control plot, burnt at the end of the season for the first three years and afterwards cut, makes up a sixth plot.

From a total area set apart, 19 m x 13 m, the six 5 m x 5 m plots were made, separated by strips 1 m wide in order to eliminate border effects. On each occasion the plot plus a 50 cm strip all round it was cut.

Data gathered

- The pattern of productivity

The pattern of productivity was studied on the basis of the weight of grass obtained at each harvest on each plot. In addition, complete chemical analyses were carried out, which made it possible to express productivity in units of energy, protein, and dry matter. The values calculated, in FU and MPD, are obtained with the use of "Dutch tables ".

The natural formation on red basalt was analysed for one year more than the other types (for six instead of five years).

With regard to Hyparrhenia diplandra, analyses were carried out only for the first year.

At the time of the first harvest, the problem arose as to the definition of the moment when new growth could be said to begin.

This moment was decided upon, taking into consideration the pattern of rainfall. The first harvest was carried out on the plots, at 20 and 30 days, between one and one and a half months after the return of the rainy season, which corresponds to the time when the pastureland was taken over by the Rangeland Station.

The final harvest and the harvest of the control plot were carried out at the end of the growth period of the herbaceous vegetation, that is, between 15 December and 15 January, though later or earlier in exceptional cases.

Botanical evolution was followed by means of botanical analyses carried out on the two diagonals of each plot after the last harvest. This was a strip analysis of the Boudet type (2 cm each side of the analytical axis), which was carried out by gathering species according to the placement of stakes at the angles of the plots.

As the result of analysis we have:

- A summary in the form of a data sheet giving, for each species, frequency and basic areas of cover.

- A table showing the establishment of different species sampled, for each strip and each year. Immediate comparison of the samples makes it possible to follow the evolution of certain species in terms of height, but particularly to correct errors or confusion in analysis.

The formations studied

- Pasture on advanced red soil on a foundation of old basalt (R 17);

- Pasture on granitic soil (G 16);

- Pasture at the base of slopes of old basalt with a predominance of Hyparrhenia diplandra.

Results

Pasture on advanced red soil on a foundation of old basalt (R 17)

Experimental conditions

These are concerned with natural pastureland on old basalt with advanced red soil. This soil is not very rich; laterite just beneath the surface reveals that evolution is very advanced. This is the type of pastureland most frequently encountered on the basalt plateau of the Adamaoua. Much less rich than the pasture of recent basalt, this type is nevertheless among the most intensively grazed areas of pastureland.

Table 1 - Average daily productivity of new growth, per hectare

Cutting

20 d

30 d

40 d

60 d

80 d

Intervals

kg DM/d/ha

kg DM/d/ha

kg DM/d/ha

kg DM/d/ha

kg DM/d/ha

1

17.0

14.6

14.6

18.5

18.6

2

17.0

16.2

10.5

6.6

14.5

3

13.6

15.0

9.9

11.0

7.0

4

12.7

15.0

12.6

9.7


5

7.4

12.4

10.0

3.4


6

7.9

11.6

2.7



7

10.4

12.1




8

12.1

3.1




9

7.4





10

5.6





11

3.7





12

3.2





Averages

9.83

12.50

10.06

9.84

13.37

Table 2 - Average annual productivity per hectare

Kg DM

2,770

3,100

3,140

3,120

3,510

FU

1,660

1,850

1,850

1,590

2,000

Kg DNM

152

133

116

94

95

Table 3 - Average forage value of new growth

Intervals

20 d

30 d

40 d

60 d

80 d

FU as % 100 kg DM

60

59.3

59

51

57.3

DNM as % of DM

5.5

4.3

3.7

3

2.7

DNM/FU

92

72

63

60

47

DM as % of VM

23.7

25.2

26.6

25.3

28.7

Productivity and average forage value of new growth (of 3 years, 1965, 1968 and 1970).

Average daily yield during the active period of the vegetation is at its highest level at a cutting interval of 80 days and 30 days: respectively, 13.37 and 12.50 kg/day/ha. It may also be observed that the longest cutting intervals lead to greatest daily production at the beginning of the vegetative cycle.

With a cutting interval of 20 days, the average annual yield is 1,660 forage units (FU) and 152 kg of digestible nitrogenous matter (DNM); intervals of 30 and 40 days produce a yield of 1,850 FU and 133 kg and 116 kg of DNM, respectively.

At intervals of more than 40 days it is impossible, under grazing conditions, to avoid left-over forage. Thus, an interval of 30 days should be maintained. An interval of 20 days may be considered for animals requiring more protein (fattening, milk production).

The figure of 1,850 FU obtained from the plots with 30-day cutting intervals has already been calculated, on the basis of production factors relating to the herds that exploit this kind of pastureland. In fact, more than 1,600 FU/ha cannot reliably be obtained from the pasture.

Botanical evolution

At the beginning there was a savannah-type formation, with a predominance of Hyparrhenia filipendula (Hfi) and H. rufa (Hru). Hyparrhenia filipendula (Hfi) continues to dominate in plots with short cutting intervals (20 to 40 days), but it occurs less frequently in large quantities where there are longer rest periods.

Hyparrhenia rufa (Hru); diminished occurrence everywhere.

Hyparrhenia diplandra (Hall) predominant in the control plot with one harvest only. Its presence is greatly diminished at all cutting intervals.

Setaria sphacelata (Set); continues to occur and is expanding almost everywhere.

With regard to Panicum phragmitoides (Pap), the longer the cutting interval the more this species develops. This confirms observation that in lightly stocked rangeland this species becomes dominant.

Observation of these plots reveals in addition that after five years, at the end of the experiment, Hyparrhenia spp. still occurs, but Hyparrhenia rufa has regressed, particularly in the control plot, in favour of Andropogon gayanus (Aga). Samples of tufts of Aga charted on a graph are seen to increase: they represent 10.7 % of the basic surface area of cover in 1968, 16 % in 1969 and 23.3 % in 1970.

Table 4 - Dynamics of the major species at different cutting intervals

Intervals

20d

30 d

40 d

60 d

80 d

Control

65

70

65

70

65

70

65

70

65

70

65

70

Hfi

44


13.2


19.7


20.4


16.2


23




26.3


15.5


26.7


8.5


12.6


14.7

Hru

15.1


40.9


37.4


30.5


15


1




4.5


6


2.1


0.6


2.6


0

Hdi

8.2


14.7


11.8


7.4


19.2


61




1.3


7.1


2.4


3.8


8.4


43.2

Set

14.8


2


6.3


16.4


7.5


7.5




21.8


15.3


5.5


21.8


14.2


5.9

Pap

14.5


7.3


8.8


7.5


15.6


10.4




15.7


9.7


15.3


31.8


22.1


9.4

Bra

1.6


11.7


9.5


17.6


17.2


5.2




12.1


41.1


23.1


26


21


7.4


26.5


21.2


18


18.2


14.1


0.1


TB Surface *


16


14.9


12.6


13.2


8.5


4.5

(*) Total basic surface in % of the soil surface

Brachiaria brizantha (Bra) shows general expansion, particularly with the 30-day cutting interval. Together with Pap and Set it makes up more than half the total basic cover. This cover area increases as the frequency of the cutting interval increases. In addition, it has considerable growth during the first year, spreading from already existing stocks, and then it diminishes and maintains its level of growth at approximately 2/3 that of the first year.

Table 5 - Average daily yield of new growth per hectare

Cutting Interval

20 d kg DM/d/ha

30 d kg DM/d/ha

40 d kg DM/d/ha

60 d kg DM/d/ha

80 d kg DM/d/ha

1

12.4

17.2

23.5

34.2

20.9

2

17.5

18.6

16.6

16.2

12.9

3

18.1

11.3

13.8

10.2

2.5

4

16.2

11.5

12.3

3.8


5

7.4

11.5

8.8



6

11.3

8.8

3.8



7

9.3

3.5




8

11.3





9

10.0





10

6.0





11

4.7





12

2.9





Average

10.6

11.8

14.0

16.1

12.1

Table 6 - Average annual productivity of new growth per tree tare

Intervals

20 d

30 d

40 d

60 d

80 d

Kg DM

2,770

3,425

3,710

4,830

3,760

FU

1,540

1,870

1,970

2,640

1,700

DNM in kg

152

157

156

193

90

Table 7 - Average forage value of new growth

Intervals

20 d

30 d

40 d

60 d

80 d

FU for 100 kg of DM

55.6

54.5

53.0

54.7

45.2

DNM as % of DM

5.5

4.6

4.2

4.0

2.4

DNM/FU

99

84

80

74

53

DM as % of VM

24.1

24.5

23.6

27.6

29.5

Pastureland on granitic soils (G 16)

Experimental conditions

This experiment was carried out over five years on natural pastureland on low-lying granitic, and therefore relatively rich, soil.

Chemical analyses of the yields of different plots were carried out in 1965 and 1968.

Productivity and average forage value of new growth

Table 5, which gives the average daily yield, shows us that after a spurt of growth of the vegetation at the beginning of the rainy season, the level of daily growth increases rapidly for all cutting intervals, after which it maintains its level of growth at around its average of between 10 and 16 kg of dry matter per day per hectare.

Once again, a high level of daily yield is observed in this vegetation where the cutting intervals are long, in the period preceding the first harvest.

It follows that productivity per hectare in terms of dry matter and of forage units is greater for long cutting intervals (more than 40 days of rest).

The highest annual yield is obtained with an interval of 60 days (4,830 kg DM and 2,640 FU/ha), and it is also with this interval that the highest level of nitrogenous matter is obtained (193 kg of DNM). The results with the 60-day interval may be attributed to the considerable development of Setaria sphacelata and above all, of Panicum phragmitoides. With the use of scythes, thus precluding the possibility of left-over forage, this interval is also profitable for its silage potential.

For grazing, an interval of between 30 and 40 days must be adopted because it is the Panicum phragmitoides which causes considerable wastage of forage. In any case, this interval produces between 1,870 and 1,970 FU per hectare and 175 kg of DNM per hectare, which is excellent.

Table 8 - Dynamics of the principal species at different cutting intervals

Cutting interval

20d

30d

40d

60d

80d

Control

Year

65

69

65

69

65

69

65

69

65

69

65

69

Hyparrhenia spp.

51.5


32.7


28


13.2


19.8


18.5




16.3


15.1


17.3


3.2


13


4.5

Setaris sphacelata

18


16.7


27.4


32.5


8.7


8.4




37.3


29


38.6


42.8


29.8


10

Loudetia kagerensis

2.2


21


5


1


19.6


13




3.4


11.8


4


1.1


11.3


7.8

Panicum phragmitoides

4.1


5.5


12.2


12.9


14.4


34.8




4


8.6


13.5


21.9


12.2


15

Urelytrum fasciculatum

0.1


7.7


3.4


17


11.3


4.1




6.5


1.7


7


3.2


14


30.1

Andropogon schirensis

3.6


3.2


2.8


12


6.6


7.3




14.8


17.2


11.3


3.4


2.3


18.6

Various Graminaceae

18.7


4.1


10.7


8


18


11.5




14.1


3.4


0.3


1.9


6


13.6

TB Surface

19.2


20.6


16.9


17.5


10.9


10.9




14.8


16.3


19.5


19


12.6


15.8

The total basic surface area (TB Surface, cover) is greater for this formation than for the basalt series studied. This is on the whole due to the great variety of species present and to the important contribution of Setaria sphacelata (Set). The control plot particularly is at a level of approximately 16 %, which may be attributed to the abundance of Panicum phragmitoides (Pap).

Hyparrheniae (Hyp. spp.) are present to a lesser extent than on basalt control plots or low-lying ground.

Setaria sphacelata (Set), a very good forage species, very highly palatable, appears in substantial quantities at all the cutting intervals, but particularly with the short intervals because of its tuft-like development.

Loudetia kagerensis (Lka), a characteristic species of the vegetation of poor granitic control plots; it seems unaffected by experiments.

Panicum. phragmitoides (Pap); its most characteristic development occurs in the undisturbed control plots, and its standard highest yield is obtained in plots with a 60-day cutting interval.

Urelytrum fasciculatum (Ure) reacts markedly to all the cutting intervals. This is of interest, since this species is not very attractive to animals for grazing. It is present in quantity only in conditions of undergrazing.

Andropogon schirensis (Aschi) has the same manifestation as Hyparrheniae. It occurs in greater quantity with short cutting intervals.

Finally, attention should be drawn to the high yield of Andropogon gayanus in plots with long cutting intervals, and particularly in the control plot.

Pastureland on old basalt at the base of slopes with Hyparrhenia diplandra predominant

Experimental conditions

An unobstructed valley bottom, rich in Hyparrhenia diplandra, was chosen for this experiment. The soil, more or less tectonic, is on advanced old red basalt and is part of the good soil of the region.

The section not exploited was burned every year over a long period in advance of the beginning of the tests. Such conditions are in fact favourable to Hyparrhenia diplandra. The plot with a cutting interval of 80 days, which was of little interest, was replaced in the second year by a plot with an interval of 30 days " high level ".

Here we will offer only one series of chemical analyses carried out in the first year, on tests done between 1966 and 1970.

Table 9 - Average daily yield of new growth per hectare

Cutting interval

20 d

30 d

40 d

60 d

80 d

1

21.4

19.5

29.1

36.1

32.2

2

25.4

24.5

12.5

12.8

17.0

3

12.0

12.8

9.3

16.1


4

6.0

12.7

11.0

2.6


5

12.6

18.3

10.8



6

5.2

19.3

3.5



7

12.4

6.9




8

11.2

4.5




9

9.6





10

5.4





11

5.0





12

4.2





Average

10.9

14.8

12.7

16.9

24.6

Table 10 - Annual productivity of new growth per hectare

Intervals

20 d

30 d

40 d

60 d

80 d

Kg DM

4,010

4,460

4,430

5,090

5,872

FU

2,000

2,230

1,900

2,030

2,368

Kg DNM

172

183

124

127

40

Table 11 - Forage value of new growth

Intervals/Units

20 d

30 d

40 d

60 d

80 d

FU for 100 kg DM

50

50

43

40

40.3

DNM as % of DM

4.3

4.1

2.8

2.5

0.7

DNM/FU

85

82

66

62

67

DM as % of VM

25.1

25.1

26.5

27.2

29.0

Average forage productivity and value of new growth

Even though the 80-day interval shows a higher rate of daily and annual yield in dry matter and in forage units (FU), the 30-day interval has a distinctly higher yield in nitrogenous matter (DNM).

The 80-day interval and even the 60-day interval are of little interest for harvesting or for grazing, because the stems are too thick and produce abundant left-over forage.

The plot with a 20-day interval produced only 4 tons of dry matter per hectare, that is, 2,000 FU and 172 kg of DNM. Its average daily productivity was also distinctly lower than the 30-day interval plot (10.9 as against 14.8 kg DM/ha/day).

Thus the higher yield of the 30-day interval (proposed on the basis of an interpretation of the results of stocking studies and rotation) was confirmed, and the annual productivity level per hectare is equal to the productivity of the good formations of the Adamaoua savannah.

With the 30-day interval, approximately 4.5 tons of dry matter per hectare and 2,230 FU and 183 kg of digestible nitrogenous matter is obtained. However, it is a delicate matter to isolate the 1966 analysis from the total of the studies over five years, because the formation has changed considerably in its composition, as is revealed in a study of botanical evolution.

The productivity of the first year is not very significant, because the build-up of reserves in a cespitose such as Hyparrhenia diplandra makes possible a very much higher offtake at the time of the first exploitation of the plots.

Table 12 - Dynamics of the principal species at different cutting intervals

Interval

20 d

30 d

40 d

60 d

80 d

Control

66

70

66

70

66

70

66

70

66

70

66

70

Hdi

65.5


52.1


75


55.1


88.2


81




9.8


15.7


32.2


18.2


35.8


34.7

Hru

0.8


0.5


0.8


1.7


0.8


0.2




32.5


12.7


47.1


39.5


1.7


0

Bra

10


18.3


8.6


8.8


8.1


0.1




3.6


8


2.6


0.4


11


3.8

Shi

0.3


0.3


0.6


0.3


0.1


0.1




5.5


1.5


0.9


0.5


0


0

Pass

0.1


0.3


4.1


05


0.4


1.9




15.3


7.1


0.7


2.3


3.6


0

Aga

0.2


12.1


0.1


1.4


0.8


3.1




0.1


1.1


3.8


4.6


6


31.6

Set

6


1.6


2.9


4


1.2


6.8




25


16.7


12.4


9.6


11.8


4.9

Pap

3.5


1.9


0


10.9


0


1.1




6.1


3.2


0


7.3


3.1


6.6

Bec

5.4


7.4


11


12.2


3.3


5




0.1


1.6


0


1.7


12.9


15.8

TB Surf.

26.1


21


22


25


21.9


19.8




13.7


12.5


13


12.3


14.1


0

Botanical evolution between 1966 and 1970

In the first place it is striking that a large number of species are found in this low-ground formation.

Other important observations: Hyparrhenia diplandra is markedly hampered at all the cutting intervals including in the control plot, which was not burned after the experiment had begun. Fire seems to be a factor in the development of Hyparrhenia diplandra, which, without fire, has to compete more with Andropogon gayanus and Beckeropsis uniseta.

It has already been noted that in low-ground conditions where there is a general occurrence of this species, 2 or 3 consecutive years of intensive grazing in a 30-day rotation cycle caused it to disappear. However, although Paspalum scrobiculatum does not have the productivity of Hyparrhenia diplandra, it became established. Paspalum scrobiculatum is demonstrably suited to short cutting intervals.

Hyparrhenia rufa (Hru) benefits from exploitation; however, the 30-day "high " interval does not suit it. This is due to phenological differences between Hyparrhenia diplandra and Hyparrhenia rufa. The former has high-level branching and is not suited to the low-level cutting. Low-level cutting does benefit Hyparrhenia rufa, which has the ability to spread out and develop tillers.

Brachiaria brizantha (Bra) in this formation has a rather low level of yield when under exploitation except where the cutting interval is 30 days " high ". This may easily be explained by the fact that this cutting height protects the moderately tall species from competition with the tall species without damaging them too much.

Schizachyrium platyphyllum (Shi) is very clearly the most frequently occurring species at the end of the five-year period of exploitation, at short cutting intervals. In respect of basic cover this may not be evident, for this is a delicate plant, which grows at a low level and gains little height by the end of its growing cycle.

Paspalum orbiculare (Pass) becomes significant in the short cutting intervals (of 20 and 30 days) because of the manner in which it spreads. This species partly replaces Hdi, but does not reach that level of productivity.

Andropogon gayanus (Age) and Beckeropsis uniseta (Bee) reveal their obvious fragility at every cutting interval.

Setaria anceps (Set) reacts favourably to short cutting intervals, probably because of its ability to spread tillers, as is the case with Hyparrhenia rufa.

Finally, we note the importance of Hyparrhenia filipendula (9.5 %) and Hyparrhenia bracteata (20.6 %) at the end of their cycle, at a 30-day cutting interval.

These species are undoubtedly responsible for the lower quantities of Hru in these plots.

Conclusions

The study of productivity of these three types of vegetation at different intervals of exploitation shows that at the beginning of the vegetative process, the production of dry matter and Forage Units is considerable, because the first cut is delayed. The daily yield of DM and FU is greatest for the longest cutting intervals, particularly up to the first cutting.

However, we know that the lowest yields of digestible nitrogenous matter correspond to the longest cutting intervals, although even these yields are not negligible (between 2.6 and 3.5 % of dry matter).

The first cutting has a retardatory effect on the productivity of the period that follows, after which productivity fluctuates less sharply.

With the shorter cutting intervals it is noted that there is a depressed rate of growth between the 3rd and 7th harvests with a 20-day cutting interval, whereas for the 30-day interval productivity is maintained throughout the rainy season and decreases only when the rains cease.

It is thus at the beginning of the rainy season that areas of the rangeland should be protected from grazing, rather than in the middle of this season.

In assessing the cutting interval which is most appropriate for the pastureland, this study demonstrates that an interval of between 25 and 35 days for formations on basalt soils should be employed, whereas a slightly longer interval (30 to 40 days) is more suitable for pastureland on granitic soils.

In comparing the productivity of these two types of pasture one might be tempted to think that they are the same. In fact, the plots on basalt soil on the plateau are barely in average condition in comparison to pastureland of this type. On the other hand the higher granitic slopes, with soils which are subject to erosion and are somewhat skeletal, are much poorer than those which were studied. In addition, in pastureland on granitic soils herds begin to lose weight faster when the rainy season ends. Here it is also more difficult than in the basalt area to avoid left-over forage, and apparently the spurt of growth that occurs on the return of the rains is much more marked on these granitic soils.

We consider that the levels of offtake by grazing of 1,650 FU on basalt soils and 1,250 FU on granitic soils can be maintained, taking into consideration the fact that variability in productivity of these soils is very noticeable, and is related to the proportion of low-lying pastureland within the total area of pastureland. Under intensive exploitation conditions (rotation, elimination of left-over forage, etc.) considerably higher yield may be expected from the granitic areas.

The study of pasture on low-lying soils on old basalt involves all the formations found in low-lying drained land. The same holds for low-lying granitic land, with certain specific differences.

In the rational intensive exploitation of these formations it is definitely not possible to retain the original plant association. Where Hyparrhenia diplandra is replaced by Hyparrhenia rufa and Hyparrhenia bracteata, Setaria and Paspalum pansum, as is the case with the 20- and 30-day cutting intervals, we may say that the pasture is improved.

Under grazing conditions, it is a good idea to establish intervals of 20 days for the first one or two years where there is heavy grazing (500 kg/ha), at least for the first half of the rainy season (up to mid-July).

For the plots with 20-day intervals we had to set the time of the first offtake at between one and one and a half months after the return of the rains, in a similar manner to the pattern of exploitation of rangeland. If the pasture is exploited too soon, this will jeopardize the growth of the grass over the whole season, particularly in the case of 20-day to 30-day intervals. It is necessary to allow perennials time to reconstitute the reserves they use up in order to grow during the dry season and the beginning of the rainy season; annuals must be allowed time to become established, and this is particularly true for grazing condition where there is a major and systematic choice of species consumed.

Therefore the yield of the first cutting is generally comparatively high in relation to the total, for the above reasons and also because of a physiological factor of plant development, whereby the vegetation grows very rapidly at the beginning of its growth cycle. Variations are more noticeable in the shorter cutting intervals: this is demonstrated by the values of the first cuttings, which are lower than might be expected, whereas the dry matter content is greater.

Several observations may be made concerning the botanical studies connected with these experiments:

The establishment of the inventoried species allowed comparative diagrams to be drawn up, and determined their form. In the long run however, this method is not easily put into practice with fragile species, which are in general annuals, because in spite of precautions their roots are uncovered, and the plants are damaged and pulled at the actual moment of analysis.

The method is very well suited to cespitous species.

In addition, changes in personnel over a period of five or six years make comparative interpretations between years more difficult. However, from this point of view the competence of the worker should permit objective comparisons within extremes of results.

With regard to the basic surface cover it is also necessary to take into consideration the fact that the analyses are somewhat delayed in time. The high level of desiccation which occurs in February, for example, caused a systematic under-estimate of the measurements carried out at that time; we note also that with the short cutting intervals, diagrams have revealed the splitting and dispersal of dying cespitose species.

Finally, on natural rangeland, calculations generally reveal slightly higher levels than those obtained from the growth curves of foddered animals.

The difference is in the region of 10 % only; it is easily explained, and thus provides confirmation of the basic worth of the method of calculation adopted. On the one hand, the early grazing by animals always results in some residue of the vegetation, which has lost some of its value through not having been consumed at the optimum moment. On the other hand, at the end of the rotation period, towards December, there is always more forage left underfoot in grazing than in the harvest, and there is a certain amount of left-over forage.


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