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Species abundance, food preference and nutritive value of goat diets in the semi-arid lands of east-central Kenya

N.K. Osolo, J.N. Kinuthia, C.K. Gachuiri, A.M. Okeyo,
M.M. Wanyoike and M. Okomo

University of Nairobi, Faculty of Veterinary Medicine, P.O. Box 29053, Nairobi, Kenya

Abstract
Introduction
Materials and methods
Results and discussion
References

Abstract

The key forage species and nutrient composition of forage species preferred by free-ranging East African goats in rangelands of eastern-central Kenya were determined. One hundred and five forage species were identified including 30 tree, 29 shrub, 20 grass and 25 forte species. Shrubs and short trees were the most preferred forage species and thus formed the bulk of the goat diets. Ironically the forage species most preferred by the goats were not necessarily the most abundant on the ground, indicating that ease of accessibility and palatability may be important determinants for choice of a forage specie. That grasses and fortes were the least preferred could be largely explained by the fact that goats are naturally browsers.

Introduction

Raising of livestock by pastoralists, agro-pastoralists or commercial ranchers is the single most important economic activity in the arid and semi-arid lands (ASAL) of Kenya. Over two-thirds of Kenya (approximately 492,100 km2) is classified as ASAL. More than half of this area receives less than 370 mm of variable rainfall annually and is further characterised by variability in pasture and water availability. During the peak of the rainy season, feed resources available to animals are high in quality and quantity and animals gain weight fairly fast. Conversely, at the peak of dry spell, available forage resources are too low in quality and quantity and animals inevitably lose weight. Important domestic animals in ASAL include, cattle, sheep, goats and camels.

Recent increases in the human population due to better living standards, have been accompanied by increases in total livestock numbers in the ASAL, especially among pastoralists who, to a large extent, depend on these animals for their subsistence. Increase in human population in high potential areas has also resulted in migration of people to the ASAL in search of arable land. In addition, among the indigenous communities, there is a growing tendency towards a more cash-oriented economy as evidenced by subdivision of communal lands into small individual parcels and acquisition of title deeds. Thus, families are increasingly becoming sedentary, but on land units too small to meet their food needs. There is therefore overstocking resulting in range degradation especially around settlements and cultivated areas.

Even with individualisation of land ownership, diminishing household land units and the terminal breakdown of traditional communal grazing systems, livestock production remains the mainstream economic activity for these areas. In order to make meaningful recommendations to land owners on proper management of these lands, especially under the currently emerging land use/ownership paradigms, range managers ought to have a thorough knowledge of the most suitable kinds of animals, their nutrient requirements, forage preferences and quality of available forage.

Information on food habits of pastoral livestock in the East African ASAL is scarce. Livestock select their diet from a complex mixture of plant species and/or plant parts (Arnold 1960; Arnold 1962). Why an animal selects one plant and not another is still a puzzle. Choice varies widely and seems to depend on availability. Diet selection may modify the species composition of the pastures in the long run (Illius 1986) with the potential danger of lowering the quality of the diet. Continued grazing pressure increases the number of unpreferred plant species at the expense of the preferred species (Tadingar, personal communication, 1984). Due to wide temporal and spatial variation in forage preference and nutrient composition of diets selected by different kinds of ruminants, research information generated from a given area and kind or class of animals, has limited inference (space) and should not be applied to too wide an area. It is therefore, important to establish local food habits and nutrient composition data bases for the various types of domestic and wild animals utilising particular range types.

The main purpose of this study was to determine the key forage species and nutrient composition of diets selected by free-ranging East African goats in rangelands of eastern-central, Kenya.

Materials and methods

Study area and procedures

The study was conducted at the University of Nairobi's Machang'a Field Station situated approximately 150 km east of Nairobi and next to Kamburu Dam on the eastern bank of upper Tana river. The station is approximately 500 ha in area and lies at 1°S latitude, 37°E longitude and about 700 m altitude. However, the study was limited to a 20 ha area fronting the Embu-Kitui main road. The area gently rises from the Kamburu Dam to the top of the ridge.

The mean annual rainfall is 650 mm coming in two seasons - March through May, short rainy season and October through December, long rains. Soils on the station are developed from undifferentiated types of gneisses of Basement System. They are well drained, shallow, dark red to yellowish red, stony loamy sand to clay (Kenya Soil Survey, Misc. Soil Paper M 19, 2nd impr. Nairobi 1979). Machang'a vegetation is described as bushed shortgrass savannah with buffer grass (Cenchrus ciliaris) and horsetail grass (Chloris roxburghiana) predominant. Saltbush is a common palatable shrub suitable for re-establishing pasture on overgrazed and eroded places (East African Livestock Survey Atlas, 1967). Recommended stocking is about three hectares per livestock unit (LU).

Species composition and relative abundance

At the beginning of the November/December wet (green) season of 1993, botanical composition and relative abundance of both herbaceous and woody plant species layers were determined. This involved the use of line intercept method, where 10 transects, 100 m long by 200 m apart, were laid across the slope and all herbaceous plant species intercepted by the tape identified. Herbaceous layer determination involved the quadrat method, where ten 1 m2 quadrats were laid at equal intervals along each transect. Each quadrat was further subdivided into four quarters and different plant species occurring in each quarter identified, enumerated and recorded. For trees and shrubs above 2 m high, 10 m x 10 m quadrats were used and different species occuring within each quadrat identified, counted and recorded. The relative abundance for each plant species was calculated as follows:

Relative abundance of sp A= Number of times sp A occurred/No of times all sp occurred

To convert number of plants per m2 into number of plants per ha, the density value is multiplied by 10,000 m2.

Number of bites and time spent on individual species

The method used to study forage preference involved close observation of randomly selected animals (one at a time) as they grazed with the others. A base flock of 45 East African goats was used from which one animal was selected for the bite count study. The selected goat was closely followed and observed by two enumerators who recorded the number of bites and time spent on each plant species as the goat grazed or browsed. A bite was taken as the act of breaking off or picking up a piece of forage. The time spent by the animal feeding on one individual plant without walking one full step was estimated in seconds and recorded. The animal was observed for 30 minutes beginning at 0930 hours.

The flock was grazed in different areas and a different goat was randomly selected for observation so that different goats were used on the five grazing units. The five goats selected during the first round of grazing rotation qualified as experimental animals for subsequent replicates. This observation procedure was repeated for as long as there was no noticeable change in the vegetation, i.e. until the rains began if it was a dry-season grazing, or until the dry season set in if it was green-season grazing.

Forage digestibility and chemical composition

Plant material representative of that selected by the goats was hand-clipped, sun dried for three days and then oven dried at 40°C for four days. The dry forage samples were ground in a Wiley Mill using a I mm screen. The material was later chemically analysed for NDF, ADF (Van Soest and Wine 1967); N P. Ca, Mg, K, Na, Zn, Cu. and Fe (AOAC 1975). Digestibility estimates using the nylon-bag technique (Ørskov et al 1980) for 48 hours incubation was also conducted.

Results and discussion

Vegetation: Botanical composition

Tables la and b give an inventory of the plant species occurring in the study area, and their relative density (plants/ha). One hundred and five plant species were identified comprising, 30 trees, 29 shrubs, 20 grasses and 25 fortes. Since the survey was conducted soon after the onset of the short rains, a large number of tree and shrub seedlings was observed.

The most dominant tree species within the study area was Acacia brevispica. Others included Combretum zeheri, Cammiphora riparia, Terminalia brownii, Grewia villosa, Acacia nilotica, Boswelia sp, Lannea flocesca and Commiphora boviniana in that order. The most dominant shrub was Hermania alhiensis. Others, in order of importance, were Grewia bicolor, Gnidia latifolia, Premna hilderbrandtii, Dichrostachys cinerea, Acalypha fruticosa and Maytenus putherlickioides. Of the grasses, annuals dominated the site due to their opportunistic nature. The most dominant grass species was Aristida keniensis, followed by Eragrostis caespitosa and Latipes senegalense. Most of the perennial grasses had not come up. Among the fortes, Ocimum sp was most dominant, followed by Rhoicissus tridentata, Pentas sp, Stylosanthes aspera and Acanthospernum hispidum.

Table 1a. Average plant population density (plants/ha) of trees and shrubs in the study area.

Trees

Plants/ha

Shrubs

Plants/ha

Acacia etbaica

10

Withania somnifera

10

Acacia brevispica

65

Triumfetta s p

40

Lannea triphylla

35

Grewia bicolor

80

Combretum moue

35

Sida ovata

45

Walbergia sp

10

Combretum aculeatum

45

Grewia villosa

40

Premna hilderbrandtii

75

Acacia nilotica

40

Indogofera schimperii

15

Acacia mellifera

20

Indigofera lupatana

5

Commiphora riparia

50

Acalypha fruticosa

70

Boswelia sp

40

Grewia similis

35

Commiphora africana

35

Hibiscus calyphylla

15

Zanthoxylum chalybea

5

Aspangus racenosus

40

Terminalia brownii

50

Hermania alhiensis

220

Acacia tortilis

5

Ormocarpum trichocarpa

50

Combretum zeheri

55

Cassia singueana

15

Acacia hockii

25

Dichrostachys cinerea

75

Steganotaenia araliaceae

5

Thumbergia holstii

10

Ochna inermis

10

Tephrosia villosa

20

Albizia amara

10

Ipomoea kituensis

5

Lannea floccosa

40

Maytenus putherlickioides

70

Commiphora boviniana

40

Ormocarpum kirkii

5

Commiphora campestris

10

Gnidia latifolia

80

Terminalia spinosa

5

Hoslundia opposita

25

Dalbergia melanoxylon

10

Indigofera clifordiana

15

Euphobia nyikae

25

Crotolaria gooformis

40

Commiphora schimperii

5

Croton dichogamus

5

Makhamia sp

15

Rutya fruticosa

5

Entada abyssinia

5

Hibiscus micranthus

20

Lonchocarpus sp

10

Zimania americana

5

Cordia gharaf

5



Table 1b. Average plant population density (plants/ha) of grasses and forbs in the study area.

Grasses

Plants/ha

Forbs

Plants/ha

Setaria verticilata

t*

Rhoicissus trindentata

5

Dactyloctenium aegyptica

2

Ocimum sp

80

Brachiaria reptans

1

Pentas sp

5

Aristida keniensis

6

Stylosanthes fruiticosa

5

Enteropogon macrostachys

2

Achranthes aspera

3

Eragrostis superba

1

Solanum incanum

3

Chloris virgata

t

Alternthera pungens

2

Eragrostis caespitosa

3

Acanthospernum hispidum

4

Latipes senegalense

3

Commelina benglhelensis

t

Heteropogon controtus

t

Tribulus terestris

1

Michrochloa kunthii

2

Pupalia lappaceae

1

Themeda triandra

t

Convolvulus favinosus

1

Digitaria macroblephera

2

Oxygonum sinuatum

t

Chlorisroxburghiana

t

Indigofera spicata

t

Digitaria gazensis

1

Ocimum bacilicum

1

Latipes senegalenses

t

Tephrosia pumila

t

Aristida adoensis

1

Blepharis hildebrandtii

t

Digitaria falutina

1

Phyllanthus maderaspatens

t

Tragus berteronianus

1

Barleria proxima

t

Rhynchelitrum roseum

t

Asystasia schimperi

t

Aristida sp

t

Hibiscus micranthus

t



Rhoicissus triandenta

t



Zornia glachidiata

1



Bidens pillosa

1



Ipomoea oenothrae

t

* t = trace amounts (<0.05 plants/ha).

Forage preference

Shrubs and short trees were the most preferred forage plants and hence formed the bulk of the goat diets (Table 2). The five most preferred shrubs were Maytenus putterlickioides, Premna hilderbrandtii, Hoslundia opposita, Dichrostachys cinerea and Combretum aculeatum in that order. However, these plants were not the most abundant on the ground suggesting that the plants/grazable plant parts were possibly both more accessible and palatable and therefore more seriously sought after by the goats. Only Premna sp showed a close relationship between its relative preference and its abundance on the ground - third and second, respectively.

Table 2. Mean number of bites and time (in seconds) spent on various forage species within the study area.

Forage species

Mean number of bites

Mean time spent

Maytenus putterlickioides (s*)

31

44

Ochna inermist (t)

24

36

Premna hildebrandtii (s)

23

30

Hoslundia opposita (s)

21

31

Dichrostachyus cinereas (s)

18

24

Combretum aculeatums (s)

18

20

Acacia hockii (t)

15

19

Grewia bicolor (s)

14

22

Acacia mellifera (t)

14

17

Thumbergia holstii (s)

13

16

Acalypha fruticosa (s)

13

16

Barleria proxima (g) f

13

18

Combretum moue (t)

11

20

Ocimum spp (f)

10

13

Grass (bulked)

8

10

Boswelia hilderbrandtii (t)

7

10

Hermania alhiensis (s)

4

11

*s = shrubs,
t = tree,
g = grass,
f = forte.

Table 3. The correlation and regression coefficients between plant species preference and nutrient components.


Nutrient component

Correlation

Simple regression

Partial (multiple regression

coefficient

coefficient

coefficient)

ADF (%)*

0.3400

0.35 (0.27)

-0.23 (0.59)

NDF (%)

0.0001

0.16 (0.21)

0.25 (0.49)

IVDMD (%)

-0.2700

-0.12 (0.11)

-0.13 (0.21)

Nitrogen (N)

0.3000

0.30 (0.29)

0.55 (0.41)

Phosphorus (P)

-0.0500

-1.83 (1.83)

-3.16 (4.28)

Sodium (Na)

0.3400

0.34 (0.03)

0.06 (0.03)

* ADF = acid-detergent fibre;
NDF = neutral-detergent fibre;
IVDMD = in vitro dry-matter digestibility.

Among the trees, Ochna inermis and Acacia sp were the two most preferred plants. Ochna inermis ranked second to the most preferred shrub, Maytenus putterlickioides. Ochna sp, however, ranked low in availability on the ground. Grasses and fortes were least preferred which is because goats are naturally browsers, and these plants were least abundant on the study area.

Table 3 represents correlation and regression of the number of bites (preference) on nutrient components (ADF, NDF, IVDMD, N. P. and Na). There was a positive and significant correlation between ADF, N and Na with number of bites; P showed a significant negative correlation.

ADF, N and Na had a significant positive causal (regression coefficient) effect on preference. NDF, IVDMD and P had no significant causal relationship with preference although P and digestibility had insignificant negative regression coefficients (-1.83 and -0.12, respectively) on preference. It can therefore be concluded that the different plant species were more preferred due to relative contents of ADF, N or Na and less preferred due to relative P contents (Tables 3 and 4).

A multiple regression analysis showed that the most important components that influenced preference in this study were N (0.55), and P (-3.16). NDF (0.25) and ADF (0.23) were less important due to large standard errors associated with the estimates. Conversely, although the partial regression coefficients of digestibility (-0.13) and phosphorus (3.16) were negative the estimates were insignificant due to large standard errors. Work is continuing in this area and with more data, a clearer picture of these relationships will become evident since it is possible that other components of the diets such as saponins and tannins that were not considered in this study could have played a major role in determining the relative dietary preference.

Table 4. Nutritive value of some of the forage plants preferred by goats at Machang'a.


Plant species

ADF

ADF

Ca

Mg

K

Na

Zn


P


N

%

%

(g/kg)

%

(mg/kg)

(mg/kg)

(mg/kg)

Premna hildebrandtii

26.73

35.80

8.15

1.6

15.7

272

67

2.3

23.4

Grass (bulked)

27.96

35.58

4.5

1.8

17.1

116

34

2.5

14.6

Indigofera spicata

28.98

35.53

55.3

6.2

13.3

61

32

-

-

Acacia ataxanthe

23.63

36.90

20.0

3.8

7.6

75

54

-

-

Boswelia hilderbrandtii

30.43

42.97

1.7

6.8

7.6

95

54

1.0

10.8

Aspiilia mossambiscensis

30.43

42.97

32.1

4.3

24.9

48

64

1.6

29.8

Maytenus putterlickioides

38.97

46.89

15.5

4.6

7.5

163

31

0.8

18.2

Combretum sp (bulk)

30.93

36.92

8.2

3.1

10.4

48

26

3.0

21.0

Crotolaria goodformis

20.39

36.14

10.3

2.5

17.6

34

64

-

-

Acacia hockii

22.21

36.21

11.2

1.9

8.7

-

28

0.8

13.3

Tephrosia villosa

35.48

53.28

11.4

2.5

10.4

55

41

-

-

Uchna inermis

34.96

55.91

19.8

3.4

4.9

20

23

0.8

25.1

Dichrostachys cineria

44.24

55.10

12.2

2.8

7.2

109

25

-

-

Grewia bicolor

36.50

59.92

19.8

3.9

11.5

41

20

2.0

21.9

Stephania abyssinica

31.25

49.20

11.7

2.5

20.5

41

50

-

-

Lantana camara

24.62

56.49

19.7

6.7

18.8

48

38

-

-

Dalbergia melanoxylon

38.15

67.47

14.7

2.9

14.5

82

12

-

-

Acacia mellifera

28.06

51.71

17.6

3.6

12.6

129

28

2.8

27.6

Acacia tortilis

46.44

52.39

12.8

2.9

12.6

88

24

-

-

Combretum aculeatum

27.64

48.21

15.1

2.3

11.5

82

73

3.0

21.0

Ocimum sp

28.15

44.53

16.1

4.6

21.6

55

61

1.8

25.9

Barleria proxima

39.63

53.93

39.5

5.9

10.1

109

35

1.8

20.5

ADF = acid-detergent fibre.

References

AOAC (Association of Official Analytical Chemists). 1975. Official Methods of Analysis. 12th edition. AOAC, Washington, DC, USA.

Arnold G.W. 1960. Selective grazing by sheep of two forage species at different stages of growth. Australian Journal of Agricultural Research 11:1027-1033.

Arnold G.W. 1962. Factors within plant association affecting the behavior and performance of grazing animals. In: Crisp D.J. (ed), Grazing in Marine and Terrestrial Environments. pp. 133-184.

Forage behavior and diet selection. In: Gudmundsson O. (ed), Grazing Research at Northern Latitudes. Plenum Press, New York, USA.

Ørskov E.R., De Hovell F.D. and Mould F. 1980. The use of the nylon bag technique for the evaluation of feedstuffs Tropical Animal Production 5:195-213.


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