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Seasonal changes in livestock diets in a semi-arid environment in northern Kenya

T Rutagwenda and M M Wanyoike

Department of Animal Production
Faculty of Veterinary Medicine
University of Nairobi
PO Box 29053, Nairobi, Kenya

ABSTRACT

Dietary preference studies of indigenous animals in Isiolo District, northern Kenya, showed that cattle mainly depended on grass and ate fewer plant species than other livestock species; camels and goats preferred to browse in all seasons; and sheep selected more grasses in the wet season but consumed more browse in the dry season. There was a dietary overlap between camels and goats and between cattle and sheep; the lowest overlap was between cattle and camels. Dietary overlap between animals generally increased during the dry season. The different feeding behaviour and variable dietary overlap of different animal species suggest that a multi-species grazing system might help to stop further degradation of already degraded areas.

RESUME

Variations saisonnières de la composition des rations des animaux d'élevage dans la zone semi-aride dans le nord-Kenya

Il ressort d'études effectuées sur les préférences alimentaires des animaux de race locale dans le district d'Isiolo au nord-Kenya que les bovins préféraient les graminées et consommaient moins les autres espèces végétales que les autres espèces d'animaux; quant aux camélidés et aux caprins, leur préférence allait aux ligneux, quelle que soit la saison; enfin, les ovins préféraient les graminées pendant l'hivernage et les délaissaient pour les ligneux pendant la saison sèche. Les camélidés étaient proches des caprins par le goût tandis que les ânes se rapprochaient plutôt des bovins; les divergences les plus prononcées (moins de 1% du temps consacré à l'alimentation) avaient été observées entre ces derniers et les camélidés. D'une manière générale, les préférences des différentes espèces se rapprochaient au cours de la saison sèche.

Etant donné la diversité des comportements alimentaires et des convergences observées entre les diverses espèces animales, l'association de diverses espèces végétales sur les parcours dégradés devrait permettre de freiner ce phénomène de dégradation.

INTRODUCTION

In many arid and semi-arid areas of the world, uneven rainfall distribution leads to wide fluctuations in the quantity and quality of forage available to animals. In northern Kenya, for example, which has a bimodal rainfall pattern with the long rains in March to May and the short rains in November and December, forage is plentiful during and immediately after the rainy seasons, but becomes scarce during the dry season, when some annual forages may disappear altogether (Herlocker, 1979; Rutagwenda, 1989). Different animals have evolved various mechanisms for coping with fluctuations in feed quality and availability (Langer, 1988). Some select high-quality plants to feed on (Hofmann, 1973; Kay et al, 1980); others improve the digestion of poor forage by prolonging the retention time of feed particles in the forestomach (Van Soest, 1982; Van Soest et al, 1988); and some adopt both mechanisms.

This paper describes the diets selected by animals in northern Kenya, and how different animal species cope with dry seasons in this environment. It also suggests strategies for feeding animals during dry seasons.

MATERIALS AND METHODS

Study area

The study was carried out at Ngarendare research station in Isiolo District, Kenya, 310 km north of Nairobi, at an altitude of 1100 m; annual rainfall here averages 510 mm. The vegetation of the study area is a thorn-bush savanna dominated by various Comiphora, Grewia and Acacia species. There is also a sparse cover of annual grasses, herbs and shrubs. Acacia woodlands and dense bush dominated by Grewia species are found along seasonal water courses, and perennial grassland is found on the flood plains. Almost 400 different plant species have been identified in the grazing area of approximately 100 km² (W Schultka, Department of Botany, University of Giessen, Germany, personal communication).

Experimental animals and their management

The animals used in the study were two breeds of sheep, the Red Masai and the Somali Blackhead (body weight 19-55 kg); two breeds of goats, the Small East African and the Galla (body weight 23-47 kg); Small East African zebu cattle (body weight 185-375 kg); and camels (Camelus dromedarius body weight 350-620 kg). All animals were male castrates.

The animals were herded and watered according to traditional practices in the area, but were also treated against internal parasites three times a year and were provided with a mineral lick in the enclosure all the times.

Vegetation biomass estimation

Vegetation biomass in the grazing area was estimated once a month. An area was selected that appeared to represent the general grazing range, and three 100-m transects were laid out in three different directions starting from a central point. Along each transect 1-m² quadrats were placed at 10-m intervals and the plant material in each quadrat was clipped. The clipped vegetation samples were dried (first in the field and later at 70°C overnight in the laboratory) and the weights were recorded.

Dietary preference

Feed preferences were assessed by direct observations (Buechner 1950; Schwartz, 1988) made every two weeks from 1984 to 1988. Six sheep (three of each species), six goats (three of each species), six camels and six cattle were observed for 10 minutes each between 0900 and 1100 hours. The time spent by each animal feeding on one plant was recorded to the nearest five seconds; the plant species and the part of the plant eaten were also noted. On each observation day, samples of the three plants most preferred by the animal were collected for chemical analysis and nylon-bag degradation studies.

Dietary overlap between animal species was calculated according to Rutagwenda et al (1990).

Chemical analysis

The 42 plant species most preferred by the animals were chemically analysed. Altogether 202 samples (34 grasses and 168 dicotyledons) were analysed for acid detergent fibre (ADF) and acid detergent lignin (ADL) according to Van Soest (1963; 1982) and Van Soest and Wine (1967). The cellulose content of the plants was calculated as the difference between the ADF and the ADL contents. The crude-protein content of the plants was determined by the standard Kjeldahl procedure.

Degradation of the selected plants (nylon-bag technique)

In the degradation experiments, eight sheep and eight goats (four of each breed), fitted with rumen fistulae, were used to test 285 samples (76 grasses and 209 dicotyledons) of 53 plant species (11 grasses and 42 dicotyledons) using the nylon-bag technique.

Plant samples collected during the feeding observations were dried at 70°C for 24 hours and ground in a Wiley mill to pass a through a 2-mm screen; the samples were tested two weeks after collection from the field. The nylon bags used measured 15 x 7 cm and were made from a fine grade (pore size 35 m m) of nylon material (Linker KG, Kassel, Germany) according to the procedure described by Ørskov et al (1980). A 5-g sample of ground plant material was weighed into each bag, the bags were tied with a nylon string, and five bags were fixed to a nylon washing line. The five bags on one line were introduced into the rumen at intervals and removed simultaneously (Nocek, 1985) to give incubation times of 6, 12, 24, 48 and 72 hours for single bags. After the bags were removed from the rumen they were washed with tap water until the effluent was clear, dried first in the field and later at 70°C for 24 hours, and weighed, and the dry matter disappearance was then calculated.

RESULTS

Vegetation biomass

The vegetation biomass of the herb layer (excluding browse) varied according to the rainfall pattern, from 2000 kg/ha during the rainy season to as low as 700 kg/ha during the dry season.

Number of plants consumed

Animals spent different times feeding on individual plants. The number of plants which occupied 90% of the feeding time of the animals, and the proportion of time spent feeding on dicotyledons (browses), during the dry and wet seasons are shown in Table 1.

Cattle ate fewer (P<0.001) plants species in both seasons than other animal species, and fed on fewer (P<0.001) plants during the dry season than during the wet season. There were no differences in the number of plant species eaten by sheep, goats or camels.

Cattle spent less time feeding on dicotyledons than other animal species. Both cattle and sheep spent much more time feeding on dicotyledons in the dry season than in the wet season. Goats and camels spent most of their feeding time on these species in all seasons.

Dietary overlap

The time spent by different animal species feeding on similar plants during the dry and wet seasons is given in Table 2. Camels and cattle had the lowest dietary overlap in all seasons. Camels and goats, and cattle and sheep, spent about 50% of their feeding time on similar plants during the dry season. Dietary overlap tended to increase during the dry season.

Table 1. Number of plants occupied 90% of the feeding tune and tune spent feeding on dicotyledons

Animal species

Dry season

Wet season

Number of observation days

Number of plant species

% of total time feeding on dicotyledons

Number of observation days

Number of plant species

% of total time feeding on dicotyledons

Mean

SD

Mean

SD

Cattle

10

1.8

0.4

17

21

7.3

2.6

28

Sheep

15

26.5

5.1

35

18

26.0

3.2

82

Goats

15

25.8

4.2

86

18

27.0

2.1

84

Camels

6

28.0

5.1

90

28

29.0

3.0

92

Degradability of selected plants

There were no differences between sheep and goats in the dry-matter disappearance rate (DMDR) of plant samples in the rumen. There were, however, differences in DMDR between plant species. The 285 plant samples tested were divided into three groups according to their DMDR at 48 hours:

· Group 1: plants with DMDR less than 50% at 48 hours (low quality)
· Group 2: plants with DMDR of 50-70% at 48 hours (medium quality)
· Group 3: plants with DMDR greater than 70% at 48 hours (high quality)

Table 2. Dietary overlap between animal species



Dietary overlap (% of total feeding time)

Dry season

Wet season

Animal species

Mean

SD

Mean

SD

Camels - cattle

3.3

3.3

8.5

3.3

Camels - sheep

30.5

5.4

14.2

6.9

Camels - goats

47.5

5.4

12.4

4.6

Came - sheep

49.6

5.7

20.1

5.9

Cattle - goats

12.6

3.3

23.3

4.0

Sheep - goats

36.6

9.0

43.0

6.7

The number of observations days in the dry and wet seasons, respectively, were: came (10,21); sheep (15,18); goats (15,18); camels (6,28)

Table 3. Time spent feeding on plant species of the three quality groups



% of total feeding time

Dry season

Wet season

Animal species

Group 1

Group 2

Group 3

Group 1

Group 2

Group 3

Cattle

90

10

-

60

31

9

Sheep

40

25

35

25

30

45

Goats

15

35

50

7

35

58

Camels

1

49

50

-

45

55

The time spent feeding on plants of different quality groups is shown in Table 3. Cattle and sheep spent more time feeding on poor quality plants, and less time on medium and high quality plants, during the dry season than during the wet season. Cattle never seemed to feed on good quality forage during the dry season. Goats spent much more time feeding on medium and high quality plants than on low quality ones during both seasons. Camels never selected a diet of low quality during the wet season, and spent very little time on such diets in the dry season. Obviously goats and camels are better able to select higher quality plants than are cattle and sheep.

DMDR of individual plants

The DMDR of three monocotyledon species collected repeatedly during the dry and wet seasons were compared (Figure 1). The DMDR was significantly (P<0.001) higher during the wet season than during the dry season, indicating that the quality of monocotyledons was lower during the dry season than during the wet season.

The DMDR of dicotyledons did not differ greatly from season to season (Table 4).

Chemical analysis of the selected diet

The crude-protein (CP) content of the diet selected by the animals was higher during the wet season than during the dry season (Table 5). The CP content of the diets of cattle during the wet season was about twice that during the dry season. However, the diet of cattle had a lower CP content than that of other animals during all seasons. For the other animals, CP content of the diets was already high during the dry season.

The estimated cellulose content of the diets of cattle was high during the dry season and decreased during the wet season (Table 5). For the other animals cellulose content was low and did not change much between seasons.

Figure 1. Dry matter disappearance rate of three grass species which were selected repeatedly during wet and dry seasons

Source: Rutagwenda (1989)

Table 4. Dry-matter disappearance rate (DMDR) (mean of eight samples) of two representative dicotyledons Vernonia cinerascens and Indigofera spinosa collected during the dry and wet seasons



Dry-matter disappearance rate (% DM)

Wet season

Dry season

Incubation time (hours)

Mean

SD

Mean

SD

Vernonia cinerascens

6

59.2

2.7

52.9

5.5

12

69.3

9.9

70.3

4.6

24

77.7

11.7

76.8

11.7

48

82.1

11.7

81.5

11.7

72

76.9

6.5

76.7

6.6

Indigofera spinosa

6

42.0

3.9

37.7

3.1

12

51.4

5.1

49.7

3.8

24

61.4

8.5

61.4

3.2

48

66.5

6.1

68.5

1.7

72

68.8

4.8

68.3

1.5

Table 5. Crude-protein and cellulose contents of the diet

Animal species

Crude-protein content (% dry matter)

Cellulose content (% dry matter)

Dry season

Wet season

Dry season

Wet season

Cattle

4-5

10-12

37-40

32-36

Sheep

9-11

11-13

20-29

21-25

Goats

11-14

17-22

15-22

16-22

Camels

14-17

18-22

14-22

14-17

DISCUSSION

This study in northern Kenya agrees with reports from other areas (Hoppe et al, 1977; Coppock et al, 1986) that different livestock species select diets of different compositions. Cattle always prefer grasses (monocotyledons). Sheep are intermediate feeders; they prefer grasses during the wet season when these species are of good quality and are available in sufficient quantifies, but as the dry season starts they switch to feeding on dicotyledons. Goats and camels are mainly browsers, spending over 80% of their feeding time on dicotyledons.

Dietary overlap (potential competition between animal species for available forage) is low between cattle and camels at all times, but high between camels and goats and between cattle and sheep during the dry season. Dietary overlap may not be critical during the wet season when forage is abundan, but during the dry season competition may reach critical proportions if animals depend on few plant species. These findings suggest a means of improving range management and reducing pressure on vegetation in overgrazed arid and semiarid areas, namely adopting multi-species grazing systems whereby animals with different feeding strategies are kept together. For example, keeping cattle and camels together should lead to improved forage utilisation, as cattle would use grass not used by camels and camels would use browse not used by cattle.

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