Feed selection by goats on tropical semi-humid rangeland*
K. Becker and J. L o h r m a n n Institute for Animal Production in the Tropics and Subtropics, Hohenheim University, Stuttgart,
Germany
(Accepted 15 May 1992)
ABSTRACT
Becker, K. and Lohrmann, J., 1992. Feed selection by goats on tropical semi-humid rangeland. Small Rumin. Res., 8: 285-298.
Small East African goats ( 191 ) were observed for over 3 months while foraging in their natural environment. The vegetation composition of 48 plots measuring 10 × 10 m was assessed quantita- tively. From recordings of feed selection and vegetation availability, preference indices were calcu- lated for groups and individual plant species. Green vegetation covered 79% of the total sample area in the wet and 40% in the dry season. Goats spent 33 and 37% of their grazing time actively eating in the wet and dry period, respectively. The average preference index for browse was stable, whereas it declined considerably for grass with progressive dryness. Animals gained weight throughout the ex- periment. Highest variability was found in acid detergent ligain (ADL) of foraged browse species. Digestibility in vitro of organic matter (IVOMD) varied from 38 to 65% and content of metaboliza- ble energy (ME) from 5.2 to 9.7 MJ kg -~ DM. Goats selected a ration containing 13.5% crude pro- tein and 7.24 MJ ME in DM. No significant correlations were found between IVOMD, ME and pref- erence index. Multiple correlation for crude fat and hemicellulose with preference index was highly significant (r 2 = 0.67, n = 21 ). The ranking order for browse emerging from in vitro results did not match with animals' choice.
INTRODUCTION
Q u a n t i t a t i v e i n f o r m a t i o n on the phys io log ica l a n d nu t r i t i ona l a spec t s o f goa ts fo rag ing f reely o n o p e n r a n g e l a n d is l ack ing for t r op i ca l s e m i - h u m i d regions . I t is e x p e c t e d t h a t h e r b i v o r o u s a n i m a l s and , in pa r t i cu la r , se lec t ive feeders such as goa ts a re h ighly a d a p t a b l e to chang ing e n v i r o n m e n t a l a n d feed ing c o n d i t i o n s ( S i l a n i k o v e a n d Brosh , 1989; H o f m a n n , 1989; Dis te l a n d P r o v e n z a , 1991 ).
L ignin inc rus t a t ion o f p lan t cell walls toge the r wi th assoc ia ted pheno l i c acids
*Dedicated to Professor Dr. Christian Gall on his 65th birthday. Correspondence to: K. Becker, Institute for Animal Production in the Tropics and Subtropics, Hohenheim University, Posffach 70 05 62, D-7000, Stuttgart 70, Germany.
286 K. BECKER AND J. LOHRMANN
are probably the most important factors which can potentially limit forage digestibility and, thus, intake in ruminants. In particular, at the most critical time of the year when forage availability is at its lowest and consists predom- inantly of browse, the flexibility of morphophysiological adaptations, i.e., of the digestive tract and intermediate metabolism, initiated by the change in macro and micro chemical compounds of the biomass, is an advantage to indigenous animals (Hofmann, 1989; Malechek, 1990).
Among farm animals the goat is well-known for utilizing a wide spectrum of native forage plants including foliage from trees and shrubs. At pasture, in contrast to stall-feeding, the animals are free to choose their diet from a vari- ety of different plant species as man has only limited influence on the goats' feeding.
The objectives of the present study were to assess the feeding behaviour - in particular the relationships between diet selection, forage availability and palatability of local Malawian goats sustained on a deciduous bush commu- nity in both wet and dry seasons.
M A T E R I A L S A N D M E T H O D S
Site of investigation The study was carded out in 1985 (Malawi German Livestock Develop-
ment Programme (GTZ), Ministry of Agriculture) in the Central Region of Malawi which is situated at 13 ° 50'S, 34 ° 30'W. The mean annual tempera- ture at the Central Region's lake-shore plain was 23 ° C with a mean daily sun- shine duration of 8 h. During the 1984-85 rainy season, the site recorded 883 m m of rain. One of the main climatic characteristics is the combination of high temperature and humidity in March and dry air in September/October. The experimental site has ferralitic soils and lithosols (Brown and Young, 1965 ). Vegetation is characterized as savanna woodland. Most of the trees and shrubs shed their leaves during the dry season.
Animals The goats were of the local Malawi type which belong to the 'Bantu' species
of Small East African goats (Mason, 1981 ). A total of 191 adult female goats weighing 24.2 _ 1.3 kg were included in the observations conducted in the wet and dry season. Twenty-nine representative non-lactating non-pregnant in- dividuals were tatooed and ear tagged for easy identification in the growth trial. Goats were weighed over a period of 81 days. The animals were dipped and drenched twice a month.
Goats were taken for grazing by shepherds for about 7 h: 07:00 to 11:00 and 13:00 to 16:00. Goats were watered before leaving the enclosure in the morn- ing and after their mid-day rest.
FEED SELECTION BY GOATS ON TROPICAL SEMI-HUMID RANGELAND 287
Experimental procedures The study was carried out towards the end of the wet (March 23 to June 3 )
and during the dry season (September 30 to October 22 ). Sites where the herders frequently take their goats were selected. For plant
species identification, a herbarium of 80 different species was established. Botanical identification was carried out by an expert (A.J. Salubeni, National Herbarium, Chancellar College, Zomba, Malawi).
Vegetation was assessed at four sites in the grazing area. Sample areas were divided into plots which were randomly chosen along projected transects, i.e., long rectangular strips as defined by Pratt and Gwynne (1977). Transects were 10 m wide and 150-300 m long, subdivided into plots of 10 × 10 m (Ta- ble 1 ). Vegetation composition was assessed quantitatively in every third plot. At the end of the wet season, vegetation composition in the four sites was assessed by recording the cover. Cover of different plant groups (grass, browse, herbs) was defined as the percentage of area of ground surface covered or shaded by a vertical projection of aerial parts of a plant onto the plane surface of the ground (Risser, 1984; Cain and Castro, 1959 ). During the dry season, the plant cover could be recorded for plant groups only and not for individual browse species. Consequently, vegetation composition was estimated by plant frequency on 30 plots of site 2. The total area examined at the end of the wet and during the dry season was 10 000 m 2 and 3000 m 2, respectively (Table 1 ) - considerably more than the minimal area of 200-2000 m 2 examined by Whittaker ( 1978 ) for similar vegetation types.
Diet composition was estimated by direct observation of goats while feed- ing (Holechek et al., 1984 ). Total observation time was 50 h for the wet sea- son and 20 h for the dry. In order to ensure uniform procedures on different sites, grazing animals were observed for the same length of time in the morn- ing and in the afternoon. As it was not possible to count the number of goat bites on individual plants, diet composition was estimated by recording feed-
TABLE 1
Site, sample area, number of plots and transects of sites investigated
Site Site area Sample area Number of No. (m 2) (m 2)
ing station intervals (FSI) (Schwarz, 1981; Schwarz et al., 1985; Scheuer- mann et al., 1980). Goats were followed for 15 to 30 rain. The plants, plant parts and the time (rounded to 5 s) spent at the plant by the goat without taking more than three Steps were recorded. It was assumed that the time spent at a plant reflects the proportion of that plant in the diet. However, this procedure was not valid for individual browse species during the dry season when goats fed on litter. Therefore, diet composition during the dry season was estimated by the frequency at which species were counted during all FSI (sum of all observations).
From these recordings, the following were calculated for plant groups and browse species: average feeding time, proportion of individual plants in goat diet, frequency of feeding on plant groups and individual species, feeding time on respective vegetation.
From the recordings of feed selection and vegetation data, preference in- dices were calculated for plant groups and browse species covering more than 0.25% of the ground surface. On the basis of the preference indices, plant groups and browse species were ranked in palatability classes.
Preference factor (PF) or index (Table 2 ) was calculated as:
P F - plant consumed plants available
(1)
where plant consumed is taken to be the feeding time on respective vegetation as a percentage of the total feeding time and plant availability as the cover of plant material not exceeding a height of 2 m (%). In the dry season, the pref- erence factor was derived as follows:
TABLE2
Cover, feeding t ime and preference index ~ for groups o f natural pasture plants in wet and dry season
Wet season Dry season
cover feeding t ime preference cover feeding t ime preference 2 (% of total) index (% of total) index
P F - frequency at which browse species were counted during all FSI ( 2 ) frequency in all examined plots
Based on PF, the palatability was categorized into four classes (Table 3 ).
Chemical methods Leaves of browse species and the most common grasses which were of the
same size and appearance as those ingested by the goats were plucked by hand, sun-dried and then ground to pass through a 1-mm sieve. Contents of dry matter and organic matter were determined by standard procedures and crude fat, nitrogen and ash according to Weende Analysis. The neutral detergent fibre (NDF) , acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined by the procedures of Goering and Van Soest (1970). Macro mineral elements (Ca, Mg, K, Na, P) were determined according to Nau- mann and Bassler (1976). In vitro DM digestibility (IVOMD) and metabol- izable energy (ME) content were determined according to Menke et al. (1979).
RESULTS
Vegetation characteristics and animal behaviour Green and very dense vegetation covered 79% of the total sample area which
was 61.9% grass, 12.6% browse and 4.5% herbs. Dry season vegetation cov- ered only 40% of the sample area with grass and browse making up 15 and 25%, respectively. Herbs were present in negligible amounts (Table 2). The habitus of browse plants was diverse. Some had all their leaves in the lower strata up to a height of 2 m, whereas in others, leaves could hardly be found below 2 m.
Andropogon schirensis was the most frequent grass species. Common herbs were Polygala erioptera, Triumfetta pentandra, Kohantica coccinea and Ver- nonia cinerea. Altogether 58 browse species were found in the different sites. The most frequent were Markhamia acuminata, Bauhinia petersiana, Com- bretum fragans and Friesodielsia obovata. Tables 4 and 5 summarize cover,
290 K. BECKER AND J. LOHRMANN
TABLE4
Cover of vegetation, relative feeding time, plant preference index and palatability for end of wet sea- son period
Cover up to Relative Preference Feeding station Palata- 2 m feeding time index intervals bility t (%) (%) (s)
~h.p., highly palatable; v.p., very palatable; p., palatable; n.p., not palatable.
preference index, palatability and feeding time on the respective biomass in the wet and dry season. Dry season observations were restricted to the two parameters, preference index and palatability.
All animals remained in a healthy condition throughout the experimental periods. In both grazing periods (wet and dry season), animals spent 33% (980 of 3000 observation min) and 37°/0 (440 of 1200 observation min) of their time foraging and browsing on pasture. Animals were seldom seen to rest on the ground during the 7-h grazing period. There was a clear tendency of decreased total feeding time and average duration of FSI with less palata- bility of plants (Table 4 ).
In the wet season, average preference indices were 3.68 for browse, 0.75 for grass and 1.47 for herbs. The palatability of grass decreased considerably in the dry season (0.22), but the palatability of browse remained unchanged (Table 2). Thorn-bearing acacias (A. polyacantha, A. nilotica and A. nigres- cens) were 'very palatable'.
During the dry season, the palatability of leaves of some species was un- changed (F. obovata, C. apiculatum) but that of others decreased to a greater
FEED SELECTION BY GOATS ON TROPICAL SEMI-HUMID RANGELAND
TABLE 5
Preference index and palatability of individual plants in the dry season
Holarrhena pubescens 6.0 p. Stereosperum kunthianum 6.0 p. Combretum collinum
leaves 5.8 p. fruits 2.5 p.
Xeromphis obovata 5.5 p. Combretum fragans
leaves 5.3 p. fruits 3.2 p.
Hoslundia opposita 4.6 p. Lannea discolor 2.0 p. Bauhinia petersiana 0.6 p. Diplorhyndius condylocarpon 1.3 p. Bauhinia thoningii 0.4 n.p. Markhamia accuminata 0.0 n.p.
~h.p., highly palatable; v.p., very palatable; p., palatable; n.p., not palatable.
extent (D. condylocarpon, B. petersiana) compared to those in the wet sea- son. In addition, a number of blossoms (C. africana, L. bussei) and fruits (A. polyacantha) were highly accepted.
Grewiaflavescens had the lowest crude protein with 7.8% and A. polyacan- tha the highest with 19.1% in DM. For NDF, ADF and ADL the highest val- ues were 2.6-, 3.0- and 5.9-times the respective lowest values. The IVOMD and derived ME contents of browse varied from 38 to 65% and 5.2 to 9.7 M J, respectively (Table 6).
2 9 2 K. BECKER AND J. LOHRMANN
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FEED SELECTION BY GOATS ON TROPICAL SEMI-HUMID RANGELAND 293
TABLE 7
Coefficients of determination of simple and multiple linear regressions (r 2 ) between palatability and various biochemical plant characteristics
Independent variables
CP Crude fat NDF ADF ADL Cellulose Hemi- Gas produc- ME cellulose tion (Gp)
phorus + M E + hemi- cellulose + N a cellulose + sodium + P
Preference 0.25 0.34 0.45* 0.67** 0.68** 0.71"* index
*P< 0.05. **P< 0.01.
TABLE8
Chemical composition of diet selected by goats grazing shrubland in Malawi derived from direct observation
Diet component % DM
CP 13.5 Ether extract 3.2 NDF 57.8 ADF 37.2 ADL 11.4 Cellulose 25.8 Hemicellulose 20.6 Ash 8.7 Ca 1.16 P 0.20 Na (gkg -~ DM) 0.20 DE (MJ kg -l DM) 8.83 ME (MJ kg -I DM) 7.24
Macro mineral content was high for calcium, potassium and magnesium, low for phosphorus and very low for sodium.
In the rainy season, variations in calculated hemicellulose were quite high, followed by lignin and crude protein. Hemicellulose almost disappeared in browse leaves and seemed to be compensated for by neutral detergent solu- bles (NDS) and lignin. Between the two seasons, most of the biochemical plant constituents were significantly different (P< 0.05 ), whereas IVOMD and ME content did not change.
294 K. BECKER AND J. LOHRMANN
The correlations of preference index with crude fat (r 2 = 0.42, n = 21 ) and hemi-ceUulose (r 2 = 0.44, n = 21 ) were significant. Multiple correlation coefficients of preference index with crude fat and hemi-cellulose (r 2 = 0.67, n = 21 ) were highly significant (P< 0.01 ). This correlation increased to 0.71 by including sodium and phosphorus (Table 7 ). Correlations between crude protein, NDF, ADF, ADL and preference index were non-significant. In vitro gas production and derived ME contents were also non-significant with pref- erence index.
Recording of feeding time and frequency on individual browse species and ground cover enabled the derivation of biochemical composition of the in- gested feed (Table 8 ). In order to simplify calculations and to limit these to an appropriate degree of accuracy, it was assumed that the diet was composed of equal parts of leaves of browse species and grasses. The small proportion of herbs was not considered. It appears from Table 8 that free grazing goats select a diet deficient only in inorganic elements, in particular sodium. The average daily gain recorded was 20 + 17 g. Following the factorial approach to calculate total energy requirement, 4.85, 0.73 and 0.54 MJ ME were cal- culated for maintenance, activity (2 kJ ME km - 1 kg- 1 ) and growth per an- imal and day, respectively. It was assumed that selected feed contained 7.24 MJ ME kg-1 DM (Table 7 ). Based on the derived requirement of 6.12 MJ ME animal - 1 and d - 1, a daily intake of 6.12/7.24 = 0.845 kg natural forage would be needed to meet the animals' demands. Feed consumption per live body mass was 3.3% and 75.5 g D M / W °75
D I S C U S S I O N
Hitherto, one of the major deficiencies in feed habitat studies has been the failure to assess voluntary intake in relation to feed availability (Merrill and Taylor, 1981 ). In the experiments reported here, both vegetation composi- tion and palatability of the plants were assessed. The arrangement of obser- vation plots along transects for studying browse vegetation cover proved use- ful under the prevailing conditions. Feeding times recorded by direct observation together with the frequency of feeding for individual goats led to realistic estimations of forage intake (Table 8 ). These estimations are consid- erably higher than those reported by Ramirez et al. (1990), Nunez-Hernan- dez et al. (1989), Boutouba et al. (1990) and Holechek et al. (1990) but comparable, on a metabolic body mass basis, to estimates by Vulink and Drost ( 1991 ) on free ranging cattle. The optimal opportunity for feed selection is a reasonable explanation for the higher voluntary intake of forage and shrubs in our studies, which is also reflected in good animal performance (20 g av- erage daily gain).
Use of in vitro techniques for estimating feed energy content (Nastis and Malechek, 1988) may have biased values for DM intake. Inoculum source
FEED SELECTION BY GOATS ON TROPICAL SEMI-HUMID RANGELAND 2 9 5
used was from cattle fed a standardized ration (Menke et al., 1979 ), so that a possible negative effect of the donor's diet (due to intake of defended plants and thus presence of inhibitory factors in the rumen) on estimates for in vitro digestibility of hand-plucked browse plants is not to be expected (Nastis and Malechek, 1988 ). High proportions of dry browse leaves in goats' diets were also observed by Schacht and Malechek (1990) for goats in Brazil. Goats selected heterogeneous diet comprising 32 browse species at the end of the wet season and 23 during the dry season when fruits and flowers of herba- ceous plants were also of particular importance. In spite of the large appetite the goats displayed, it is suggested that a daily feeding time of 7 h was suffi- cient and had no negative effect on voluntary intake. Cell wall consumption per animal d - 1 was estimated to be about 490 g ( 19 g kg- l body mass) which compared well with figures given by Van Soest ( 1983 ) when effective rumi- nation of fibre is expected. Thus, limitations in forage availability of requisite cell wall quality were obviously non-existant. On the other hand, browsers are capable of adapting to low quality fibre by increasing the passage rate of for- age. They digest mainly the soluble cell contents and easily digestible cell wall tissues (Hofmann, 1989; Lechner-Doll and Von Engelhardt, 1991 ). This il- lustrates the effective ability of goats to increase the intake of plants with high palatability but low digestibility (Hofmann, 1989 ) as is the case in this study with Combretum collinum.
Goats may choose browse species of low value, e.g., Combretum collinum not only because of palatability but also instinctively for pharmacological rea- sons due to the presence of biologically active molecules which can play a positive role in inhibiting the growth of pathogenic microbes in the gastro- intestinal tract (Schr~igle, 1990). However, the same anti-herbivore com- pounds may also cause inhibition of microbial growth in the rumen if oppor- tunities for diet selection do not permit the animal to utilize the entire range of plants or plant parts to which it is adapted. A mixed diet might, therefore, be obligatory for the browsing animal as it not only provides a ration which is better balanced but also prevents the animal's detoxification mechanism from being confronted with a large dose of one single toxin (Moss, 1991 ). There is evidence that it is easier for the animal to cope with a comparable amount of toxins from many different plants rather than from one single source (Goldstein and Spencer, 1985; Freeland et al., 1985; Brendan and Thomson, 1991 ).
As there is a difference between efficiency in the ecological context and efficiency in agricultural production (Moss, 1991 ), it is not surprising that goats chose plants in a different order than what one might expect from ana- lytical parameters (Table 9 ). Some plant species at the bottom of the prefer- ence list (Table 4 ) contained over 70°/o cell solubles but had quite low in vitro digestibilities presumably because of the presence of substantial amounts of secondary plant compounds (Van Soest, 1983; Nastis and Malechek, 1988;
296 K. BECKER AND J. LOHRMANN
TABLE 9
First and last five browse plants according to goats' preference and in vitro OM digestibility
Top five Bottom five
in vitro results/goats' choice in vitro results/goats' choice 1 st Holarrhena No. 14 21 st Cornbretum No. 4
pubescens collinum 2 r i d Xeromphis No. 13 20th Cassia No. 20
obovata petersiana 3rd Acacia No. 10 1 9 t h Cornmiphora No. 17
nilotica marlothii 4th Grewia No. 8 1 8 t h Diplorhynchus No. 3
Bayer , 1990) . In take o f these p lants was low which can be a t t r i bu ted to the p resence o f such c o m p o u n d s , e.g., t annins . Rank in g o rde r emerg ing f ro m in v i t ro tests d id no t m a t c h an imals ' choice (Tab le 9) . F u r t h e r sys temat ic re- search mus t look for a conc lus ive exp l ana t i on for this n o n -co n fo rm i ty .
T h e s ignif icant cor re la t ions o f feed p re fe rence wi th e the r ex t rac t an d hem- icellulose (Tab le 7 ) is in teres t ing, bu t the reasons are no t clear. Bet te r corre- la t ions o f p lan t cons t i tuen t s wi th p re fe rence index t h an the obse rved I V O M D in the p resen t s tudy suggest tha t c o m p o s i t i o n o f p lants has grea ter po ten t ia l for p red ic t ing ra t ion c o m p o s i t i o n f r o m shrub land for grazing goats.
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