Diet selection in dairy goats grazing woody Mediterranean rangeland

Ž .Applied Animal Behaviour Science 57 1998 117–131

Diet selection in dairy goats grazing woodyMediterranean rangeland

A. Perevolotsky a,), S. Landau a, D. Kababia b, E.D. Ungar a

a Department of Agronomy and Natural Resources, Agricultural Research Organization, P.O. Box 6,Bet Dagan 50250, Israel

b Sheep and Goats Department, Extension SerÕice, Ministry of Agriculture, Tel AÕiÕ, Israel

Accepted 26 August 1997

Abstract

We studied species selection, diet quality and voluntary intake of Anglo-Nubian X Damascusdairy goats grazing woody Mediterranean rangeland in the Jerusalem Hills, Israel. Measurementscomprised direct observation of grazing behaviour, with the recording of all species encounteredalong the search path of a foraging goat, and the time spent grazing each species selected. Thefraction of observation time spent grazing was 0.84, of which woody and herbaceous vegetation

Ž .accounted for 0.68 and 0.15, respectively. Twenty plant types species or species groupings wereencountered along the foraging paths of the goats observed. Six plant types accounted for nearly80% of encounters. The overall probability of an encounter resulting in grazing was 0.37. Analysisby plant type showed that the conditional probability of being grazed increased with decliningprobability of encounter, especially for the rarest species encountered. Nevertheless, the relationbetween the proportion of grazing encounters and the probability of encounter, by plant type,showed quite small deviations from the 1:1 line. The mean duration of a grazing encounter was 35s. Encounter duration increased with the conditional probability of being grazed and with the sizegroup of the plant type. The relation between the proportion of grazing time for a plant type andthe probability of encounter with that plant type showed a wide scatter about the 1:1 line, thoughsize group explained some of this variation. The greatest proportion of grazing time was devoted

Ž .to annual herbaceous vegetation 16% . This vegetation group together with Sarcopoteriumspinosum, Calicotome Õillosa, Cupressus semperÕirens and Quercus calliprinos, accounted for64% of grazing time. On the basis of hand-collected bite-like samples, overall mean dietdigestibility and crude protein content were 48.4 and 11.1%, respectively, and foraging rate wasestimated to be 6.3 g DMrmin or 1350 g DM per day. q 1998 Elsevier Science B.V.

Keywords: Goat; Feeding and nutrition; Selectivity; Mediterranean ecosystems

) Corresponding author. Tel.: q972-3-9683389; fax: q972-3-9669642; e-mail: [email protected]

0168-1591r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0168-1591 97 00120-2

( )A. PereÕolotsky et al.rApplied Animal BehaÕiour Science 57 1998 117–131118

1. Introduction

Traditional livestock raising in the eastern Mediterranean region has been veryextensive since biblical times, with natural rangelands being the main source of forageŽ .Perevolotsky and Landau, 1992 . These rangelands contain a significant woody compo-

Ž . Ž .nent and comprise shrubland garrigue , woodland and scrubland maquis . In Israel, therangelands have assumed marginal importance in modern times as sheep and goatraising have become very intensive because of economic, cultural and political changes.The reputation of the goat as a prime cause of overgrazing and landscape degradation

Žhas also contributed to its disappearance from the rangelands Seligman and Perevolot-.sky, 1994 .

Small ruminant production is currently based on purchased feeds and high capitalŽ .inputs Landau et al., 1995 . However, cuts in subsidies, increasing competition and

high input costs have renewed interest in rangeland utilization as a means of reducingcosts. Furthermore, there is growing interest in livestock grazing on woody rangelands

Žas a means of controlling shrub encroachment and reducing fire hazard Davis et al.,1975; Etienne, 1989; Tsiouvaras et al., 1989; Severson and Debano, 1991; Etienne et al.,

.1995 .Re-introduction of goats to the native Mediterranean rangelands within the context of

a modern animal production system requires knowledge of the contribution of thisforage source to the goat’s diet. The purpose of the present study was to determinespecies selection, diet quality and voluntary intake of goats grazing Mediterraneanrangelands. This information is required in order to assess the potential impact ofgrazing on the vegetative community and to determine supplementary feed requirements.

2. Materials and methods

2.1. Study area

Ž XThe study was conducted near the Sataf springs in the Jerusalem mountains 31847 N,X .35814 E, 580 m above sea level . Average annual precipitation is 600 mm, occurring

Ž .between October and April Ashbel, 1951 . The site has rocky, steep slopes of variousŽ .aspects, with pockets of typical Mediterranean red soil Terra Rossa .

Ž .The vegetation comprises planted pine Pinus halepensis and P. brutia or cypressŽ .Cupressus semperÕirens trees, and native Mediterranean shrubland dominated by

Ž . ŽQuercus calliprinos a tall, evergreen shrub or tree , Calicotome Õillosa a moderate-sized. Ž . Žthorny shrub , Cistus spp. low shrubs and Sarcopoterium spinosum a thorny dwarf

.shrub . Pine trees occur in well-defined groves, while cypress trees had been plantedindividually within the shrubby community. The site includes abandoned agriculturalterraces with olive trees. Herbaceous vegetation mostly develops in open patches,

Ž .germinating during the winter December–January , and remaining green until late Aprilto mid-May. Herbaceous vegetation accounts for 20–25% of the total vegetative coverin natural habitats. However, in an open landscape, such as the olive terraces, it mayoccupy 80% of the area. For the purposes of the present study, the site was divided into

( )A. PereÕolotsky et al.rApplied Animal BehaÕiour Science 57 1998 117–131 119

Žfive habitats: pine groves, olive terraces, oak scrubland, hill-top and slopes areas on the.slopes not dominated by pine or oak . Total area of the study site was about 75 ha.

2.2. Animals and their management

The study was conducted on a commercial flock of 24 Anglo-Nubian X Damascusdairy goats. Approximate body weight and lactation yield for this flock are 60 and 120

Ž .kg, respectively Landau et al., 1993 . Kidding occurred over a 2-week period inJanuary. The goats grazed the rangeland between the morning and afternoon milkings,for an average of 4 h daily, accompanied by a shepherd. From mid-May, the animalsforaged for an additional period of up to 2 h daily in the late afternoon. Animalsreceived 1000 g supplementary feed daily, comprising whole corn grain and soybean-based pelleted concentrate. The ration was split between the morning and afternoonmilkings.

Ž .The author that conducted the field observations DK acted as shepherd throughoutthe experiment period. In general, the flock was watched over rather than directed.During observation sessions the observer took care not to influence the foraging searchpath.

2.3. ObserÕations

Grazing behaviour was observed on 54 days over the period 13 February to 23 July1990. The numbers of observation days were 6, 15, 9, 11, 4 and 9 in the monthsFebruary to July, respectively. Animals spent all available time foraging; there were nosignificant periods spent resting, ruminating or watering. Foraging comprised two

Ž .activities: walking searching on the between feeding stations scale and active grazingŽ .biting, chewing at a feeding station. A feeding station might be a single tree, shrub or

Ž .herbaceous patch. Due to the low animal density imposed 0.3 goat per ha , significantresource depletion over the study period was not apparent. Therefore, changes inresource availability were assumed not to play an important role in diet selection.

A total of 116 observations of grazing behaviour was made, spread over the entireflock, there being between one and four observations on any observation day. Observa-tions were recorded on a portable tape-recorder, and were mostly of 8–10 min induration. For each observation, the observer selected a goat at random, and recorded thedate, animal number, habitat and start time. At the start of a walking bout, the observerrecorded ‘walk’ followed by the code of all plant species encountered by the animalalong its search path. The criterion for an encounter was for part of a plant to be within

Ž .0.5 m reach of the animal’s head Cooper et al., 1988 . When the animal stopped to eatŽ . Žan event we term a ‘grazing encounter’ , the observer recorded ‘eat’ and the species or

.species grouping grazed or browsed. Eating time terminated when the animal stoppedremoving bites from the plant. Grazing time comprised such eating bouts only. Encoun-ters not resulting in selection are termed ‘non-grazing encounters’. Not all plants wereidentified at the species level. Most notably, herbaceous species were defined only asannual or perennial, and species of Cistus, Pinus and SalÕia were identified only at the


genus level. Trees with foliage above the browsing reach of the animal were notrecorded.

2.4. Intake determination

Following the recording of grazing behaviour, the goat was observed for a timedperiod of about 5 min during which time bite-like samples matching the quantity andquality of forage in the bites taken by the animal were clipped or plucked, and collectedŽ .Meuret et al., 1985; Peinado-Lucena et al., 1992 . Samples were dried and weighed,and the rate of DM collection was computed by dividing the mass collected by thesampling duration. Samples were usually taken from the same tree, shrub or herbaceouspatch as the animal had grazed. The goats had been hand-raised and were hand-milked,therefore, the close proximity of the observer during the tape-recorded sessions andduring manual simulation of grazing had no detectable impact on their foraging

Ž .behaviour. Meuret et al. 1985 obtained similar intake estimates by this method or byusing chromium sesquioxide as an inert faecal marker.

2.5. Data analysis

The following variables were derived from the tape-recorded information and formedthe basis of the data analysis:

TOB Duration of the i-th observationi

NGE Number of grazing encounters with plant type s during the i-th observations, i

NNE Number of non-grazing encounters with plant type s during the i-th observa-s, i

tionNTE Total number of encounters with plant type s during the i-th observations, i

Ž .sNGE qNNEs, i s, i

TPT Time spent grazing plant type s during the i-th observations, i

In order to analyse our data according to plant type, we described the foragingŽ .process as follows Owen-Smith and Cooper, 1987 . A goat selects a search path along

which it encounters different plant types. The relative frequency with which a plant typeis encountered is our estimate of its probability of encounter, PE :s

i s i

PE s NTE r NTE 1Ž .Ý ÝÝs s , i s , i

s

PE s1 2Ž .Ý s

Note that this probability is different from true relative cover since the search path ofthe animal can not be regarded as random.

On encountering a plant type, there is a certain probability that the goat will graze it.One way of expressing this probability is in terms of the frequency of grazing


encounters of a plant type relative to all encounters of the same plant type. This is ourestimate of the conditional probability of grazing the plant type once encountered,PGC :s

i i

PGC s NGE r NTE 3Ž .Ý Ýs s , i s , i

Ž .By applying Eq. 3 globally, we obtain the probability that any encounter results ingrazing:

s i s i

PGCs NGE r NTE 4Ž .ÝÝ ÝÝs , i s , i

We can also define the frequency of grazing encounters of a plant type relative to allencounters of any plant type. This is the overall probability of grazing a plant type,PGO , which is the product of PE and PGC :s s s

i s i

PGO s NGE r NTE 5Ž .Ý ÝÝs s , i s , i

Hence:s

PGO sPGC 6Ž .Ý s

The simplest null hypothesis is that PE and PGC are independent. Differences ins sŽ .PGC are determined by inherent features of the plant type e.g., taste and not by thes

relative abundance of the plant type. A very different hypothesis postulates that theanimal completely compensates for declining PE by increasing PGC . In other words,s s

PGO is the same for all plant types. Since PGO sPE =PGC , we would expect thes s s s

different plant types to be distributed along a single PGO isocline in PE –PGC space.s s s

The final foraging decision concerns the duration of a grazing encounter, TGE :s

i i

TGE s TPT r NGE 7Ž .Ý Ýs s , i s , i

Ž .By applying Eq. 7 globally, we obtain the overall mean duration of a grazingencounter:

s i s i

TGEs TPT r NGE 8Ž .ÝÝ ÝÝs , i s , i

The null hypothesis is that TGE is independent of PE and PGC .s s s

The allocation of foraging effort between plant types can be defined on an encounterbasis or on a time basis. Thus, the proportion of grazing encounters devoted to a planttype is:

i s i

FGE s NGE r NGE 9Ž .Ý ÝÝs s , i s , i


s

FGE s1 10Ž .Ý s

and the proportion of grazing time devoted to a plant type is:

i s i

FGT s TPT r TPT 11Ž .Ý ÝÝs s , i s , i

s

FGT s1 12Ž .Ý s

Ž .Under the null hypothesis of no selection sensu Hodgson, 1979 for any plant type,both FGE and FGT should be equal to the probability of encounter, PE . Independences s s

between PGC and PE implies that the location of plant types in FGE –PE ands s s s

FGT –PE space is around the 1:1 line.s s

Note that the proportion of grazing encounters devoted to a plant type can be definedin terms of the overall probability of grazing a plant type as follows:

FGE sPGO rPGC 13Ž .s s

Finally, for each observation we computed the proportion of observation time spentŽ . Ž .grazing. This was done separately for woody FTW and herbaceous FTH planti i

Ž .types, and for all plant types together FTF . Thus:i

s

FTW s TPT rTOB 14Ž .Ýi s , i iwoody

s

FTH s TPT rTOB 15Ž .Ýi s , i iherbaceous

s

FTF s TPT rTOB sFTW qFTH 16Ž .Ýi s , i i i i

It was not feasible to estimate the size of each patch or feeding station encountered,for inclusion in the information recorded during a grazing observation. Nevertheless, theamount of forage offered by a patch may be related to the behavioural parametersdefined above. In order to give patch size some expression in the analysis, we groupedall plant types encountered into five broad categories that reflect the amount of foragetypically offered by each plant type. The size groups are 1 to 5, with 5 being the largestplant type.

2.6. Chemical analyses

Material from the bite-like samples was dried at 608C for 3 days and weighed.Ž .Digestibility was determined in vitro by the method of Tilley and Terry 1963 with the


Ž . Ž .improvements proposed by Goldman et al. 1987 . Crude protein content CP wasŽ . Ž .determined as Kjeldahl nitrogen=6.25 AOAC, 1980 . Acid detergent fiber ADF was

Ž .determined according to Goering and van Soest 1970 , and the content of ADF proteinŽ . Ž .residues ADFP was determined according to Hogan and Lindsay 1980 .

3. Results

3.1. Grazing time and diet composition

The fraction of observation time spent grazing, averaged over all observations, FTF,Ž . Ž .was 0.84 SDs0.14; ns116 . FTF was higher in spring March, April, May than in

Ž . Ž .winter February or summer June, July ; mean FTF for these seasons being 0.86, 0.79Ž .and 0.78, respectively P-0.02 for ANOVA F-test . FTF varied significantly with

Ž .habitat Ps0.005 , ranging from 0.74 in the pine groves to 0.92 on the olive terracesŽ .Table 1 . Since the olive terraces were exploited in the spring only, this may accountfor the apparent seasonal dependence of FTF.

The fraction of observation time spent browsing the woody plant type, averaged overŽ .all observations, FTW, was 0.68 SDs0.24; ns116 , and that spent grazing herba-

Ž .ceous vegetation, averaged over all observations, FTH, was 0.15 SDs0.22 . NeitherFTW nor FTH varied with season. The degree of utilization of the various habitatsvaried with season. For example, peak utilization of the oak scrubland and olive terraceswas in May, while the pine groves were grazed in July only. Habitat had a highly

Ž . Ž .significant P-0.001 effect on both FTW and FTH Table 1 , primarily because of ashift in favour of herbaceous vegetation on the olive terraces.

A total of 20 plant types was encountered along the foraging path of the goatsobserved, and a total of 4124 encounters was recorded. The plant types are listed in

Ž .Table 2 in descending order of probability of encounter PE . Sarcopoterium spinosum,s

annual herbaceous plants, Cistus spp., Calicotome Õillosa, Cupressus semperÕirens andPinus spp. accounted for nearly 80% of encounters.

Table 1The proportion of observed foraging time of dairy goats on hilly Mediterranean rangeland spent in active

Ž . Ž .grazing FTF , and its partitioning between browsing of woody plant type FTW , and grazing of herbaceousŽ . Ž .vegetation FTH , at each of five habitats. Significance ANOVA F-test between habitats is indicated by

P-value

Habitat n FTF FTW FTH

Pine groves 18 0.74 0.62 0.12Olive terraces 11 0.92 0.34 0.58Oak scrubland 18 0.86 0.74 0.12Hill top 10 0.80 0.78 0.03Slopes 59 0.85 0.74 0.11All 116 0.84 0.68 0.15P-value 0.005 0.000 0.000

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Table 2Encounter and selection parameters by plant type for dairy goats grazing woody Mediterranean rangeland

Plant Size Probability Probability Duration Proportion Proportionspecies group of of being of of grazing of grazingor encounter grazed once grazing encounters time FGTs

Ž .grouping PE encountered encounter FGE Eq. 11s sŽ . Ž . Ž .Eq. 1 PGS seconds Eq. 9s

Ž .Eq. 3 TGEsŽ .Eq. 7

1 Sarcopoterium spinosum 2 0.231 0.33 22 0.205 0.1282 Annual herbaceous 3 0.180 0.35 33 0.171 0.1593 Cistus spp. 2 0.134 0.23 19 0.084 0.0454 Calicotome Õillosa 3 0.128 0.42 26 0.144 0.1055 Cupressus semperÕirens 4 0.058 0.51 61 0.080 0.1386 Pinus spp. 5 0.056 0.27 58 0.041 0.0677 Helianthemum syriacum 2 0.047 0.38 16 0.048 0.0228 Perennial herbaceous 1 0.029 0.58 18 0.045 0.0239 Other dwarf shrubs 2 0.024 0.01 11 0.001 0.00010 Quercus calliprinos 4 0.023 0.80 77 0.049 0.10711 SalÕia spp. 2 0.023 0.00 y y y12 Vines 2 0.014 0.77 49 0.028 0.03913 Rhamnus lycioides 3 0.011 0.77 43 0.024 0.02914 Teucrium creticum 1 0.011 0.57 26 0.018 0.01315 Olea europaea 5 0.008 0.78 103 0.016 0.04816 Rare trees 5 0.006 0.72 73 0.012 0.02417 Unknown y 0.006 0.76 20 0.012 0.00718 Pistacia lentiscus 3 0.004 0.67 55 0.008 0.01219 Pistacia palaestina 5 0.004 1.00 91 0.012 0.03020 Asphodelus ramosus 2 0.003 0.38 17 0.003 0.002


Ž Ž ..The overall probability of an encounter resulting in grazing, PGC Eq. 7 , was 0.37.The conditional probability of being grazed, according to plant type showed wide

Ž .variation around this global average PGC in Table 2 ; it ranged from 0.23 to 0.51 fors

the six most commonly encountered plant types. SalÕia spp. were never observed to beŽ .grazed 94 encounters and Pistacia palaestina was always grazed when encountered

Ž .18 encounters .Fig. 1 shows the location of plant types in PE –PGC space. Two isocliness s

Ž .representing constant PGO as explained in Section 2.5 were added. The isoclines ares

an order of magnitude apart, and lie within the range of the data. There was a clear trendŽ .for PGC to increase with declining PE . Linear regression of PGC on log PE yieldeds s s s

a slope coefficient significantly different from zero. This relationship explained 30% ofthe variation in PGC . Nevertheless, the location of the different plant types relative tos

the PGO isoclines shows that the animal did not increase PGC sufficiently tos s

compensate fully for a decrease in PE . That is, plant types of low PE tend to clusters sŽ .about the lower PGO isocline 0.005 , while plant types of high PE lie closer to thes s

Ž .higher PGO isocline 0.05 .sŽ Ž ..The mean duration of a grazing encounter, TGE Eq. 11 was 35 s, with wide

Ž .variation among plant types TGE in Table 2 . TGE tended to increase with thes sŽ .conditional probability of being grazed, PGC Fig. 2 . PGC and the size group of thes s

Ž .plant type as a discrete categorical variable explained 94% of the variation in TGE ,s

both factors being positively correlated with TGE . The ranking of the coefficients fors

the size groups obtained in the model solution matched that of the size groups. Thus, the

Ž . Ž .Fig. 1. The conditional probability of being grazed PGC in relation to the probability of encounter PE . Planttypes are numbered according to Table 2. Isoclines show combinations of PGC and PE that yield a constant

Ž .overall probability of being grazed PGO of 0.05 and 0.005.


Ž .Fig. 2. The duration of grazing encounters TGE in relation to the conditional probability of being grazedŽ .PGC . Plant types are numbered according to Table 2.

high coefficient of determination obtained in this analysis does not appear to be anartifact of using categorical size groups.

The proportion of grazing encounters devoted to each plant type is given in Table 2Ž .FGE . Despite the lack of independence between PGC and PE seen in Fig. 1, thes s s

Ž . Ž .Fig. 3. The proportion of grazing encounters FGE in relation to the probability of encounter PE . Plant typesare numbered according to Table 2.


Ž . Ž .Fig. 4. The proportion of grazing time FGT in relation to the probability of encounter PE . Plant types arenumbered according to Table 2.

location of plant types in FGE –PE space shows quite small deviations from the 1:1s sŽ .line Fig. 3 . The trend for plant types of low probability of encounter to have a higher

Ž .probability of being grazed Fig. 1 is reflected in the number of plant types locatedabove the 1:1 line in Fig. 3 at low probability of encounter. Note that Fig. 1 and Fig. 3

Ž .are related as defined in Eq. 16 above.Ž .The proportion of grazing time devoted to each plant type is given in Table 2 FGT .s

This variable is presumably more closely related to relative intake than FGE . Similars

studies of goat grazing behaviour suggest that the duration of a grazing visit and theŽ .number of bites taken are highly correlated Ganskopp et al., 1997 , although intake perŽ .bite can vary considerably between species Peinado-Lucena et al., 1992 . The greatest

Ž .proportion of grazing time was devoted to annual herbaceous vegetation 16% . Thisvegetation group together with Sarcopoterium spinosum, Calicotome Õillosa, CupressussemperÕirens and Quercus calliprinos, accounted for 64% of grazing time.

Fig. 4 shows the location of plant types in FGT –PE space. The scatter about the 1:1s s

line is wider than in Fig. 3, although size group accounts for some of this variability.Ž . ŽThe probability of encounter PE and the size group of the plant type as a categoricals

.variable explained 92% of the variation in FGT . This high coefficient of determinations

may partly reflect the categorical nature of the size group. A further indication of theimportance of size group in determining FGT is the regression of FGT on PE , thes s s

coefficient of determination increased from 54% to 78% after FGT had been normal-sŽ .ized by dividing by size group using ranks from 1 to 5 for sizes .


Table 3Ž . Ž . Ž . Ž .Collection rate C , digestibility D , crude protein content CP , content of acid detergent fibre ADF and

Ž . ŽADF protein residues ADFP of bite-like samples for the period February to May. Significance ANOVA.F-test between months is indicated by P-value

y1 Ž . Ž . Ž . Ž .Period C g min D % CP % ADF % ADFP %

February 5.12 44.8 11.0 32.2 6.18March 7.74 52.4 11.3 28.9 6.23April 4.96 48.1 12.4 27.0 6.63

a aMay 6.27 46.0 10.3All 6.33 48.4 11.1 29.2 6.31

bRMSE 5.43 9.25 3.44 2.97 1.69P-value 0.31 0.02 0.29 0.00 0.76

a insufficient sample size.bRoot mean square error of ANOVA.

3.2. Intake rate and diet quality

The mean rate of DM collection for the bite-like samples that mimicked goat foragingy1 Ž .behaviour was 6.3 g min SDs5.45; ns86 for the period February to May. Since

the corresponding average total foraging time was 4.2 h per day, of which 85% wasspent in active grazing, this yields a rough estimate of daily intake of about 1350 g DM.We did not detect a significant effect of month on the rate of DM collection in the

Ž .bite-like samples Table 3 .Diet quality parameters for the bite-like samples are given in Table 3. Overall mean

Ž .in vitro DM digestibility and crude protein content were 48.4% SDs9.61; ns94 andŽ .11.1% SDs3.46; ns82 , respectively. However, almost half the crude protein

content was found to be ADF protein residue.

4. Discussion

Ž .Goats devoted 84% of their time on the range to active foraging Table 1 . OurŽ .figures are much higher than those reported by Askin and Turner 1972 for Angora

Ž . Ž . Ž .goats 31% , but close to the results of Mill 1990 for Tunisian goats 77% . Both thelatter study and our study restricted daily grazing time. This may relate to the high levelof grazing activity during the hours on the range.

In all habitats except the olive terraces, the vast majority of active grazing time wasdevoted to woody plant types. This was true even in spring when lush herbaceousvegetation was quite abundant. This behaviour is unexpected given the fact thatherbaceous vegetation is generally of higher nutritional value. This is, nevertheless,

Žconsistent with many reports of selection by goats for browse material e.g., Sidahmed et.al., 1981; Harrington, 1982; Lu, 1988 . We previously found that non-lactating goats

raised under extensive management without supplementation spent about 70% of theirgrazing time on woody plants, but preferred herbaceous vegetation over any single

Ž .woody plant type Perevolotsky et al., 1992 . It is noteworthy that similar results wereobtained in the present study despite substantial supplementation.


The goats in our present study selected a very diverse diet. Only two of the 20 planttypes or groupings that were encountered on the range were grazed with a probability ofless than 0.2, once encountered. Probability of being grazed once encountered andprobability of encounter were not independent. Less frequently encountered plant typeshad a higher conditional probability of being grazed, although there was not fullcompensation in terms of overall probability of grazing. This relationship resulted insome deviation from correspondence between the relative frequency of encounter and

Ž .the relative frequency of grazing Fig. 3 . Though they used somewhat differentŽ .definitions of selectivity, Barroso et al. 1995 also found a trend of higher selectivity

for species of lower availability.Identifying cause and effect in the relation between probability of being grazed and

probability of encounter is difficult. It could be argued that certain plant types arerelatively rare because they are the plant type that the goat selects. On the other hand,there may be an inherent tendency by generalist foragers towards selective grazing ofrelatively rare plant types, which are rare for reasons other than herbivory. Various

Ž .explanations can be suggested Newman et al., 1992 , but a discussion of their relativemerits is beyond the scope of this study. Whatever the causal pathway, carefulmonitoring of the vegetative community will be required in the light of the differentialforaging pressure on rare plant types.

Grazing time per grazing encounter tended to be longer for plant types with higherŽ .probability of being grazed and thus, of lower relative frequency of encounter . This is

one reason for the strong deviation from equivalence between relative grazing time andrelative frequency of encounter. However, the inclusion of size group in the analysis ofencounter duration and relative grazing time resulted in very high correlations. Thus, itseems that diet selection in our present study can be largely explained in terms ofrelative abundance.

Acknowledgements

The authors wish to thank: the flock owners, the Zeltser family, for cooperation andhospitality; Hagit Baram for helping with data analysis; Rafi Yonatan for field assis-tance; Avraham Genizi and two anonymous reviewers for comments on the manuscript.We are grateful for financial support provided by the Jewish National Fund and theRange Management Advisory Board.

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Diet selection in dairy goats grazing woody Mediterranean rangeland