I 
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• 
, 
r~=====::Ciii¡;¡qs~!l<fU"C'l:lJRE AND COIfi'OSITlON IN LEGllIfE SELECTIVITY 
j ~1 CSOffiv¡ r, Carlos E. r.:scano 
~ Tropical Pastures Program 
, '411 ,: nt o Internacional de Agricultura Tropical 
\;P.I =A.l partado Aéreo 6713. Cali. Colombia 
COlfCClON HISTO~~ ABSTRACT 
To explore possible relationships between canopy structure-and 
co~osition with forage selectivity, the proportion of legume in the diet 
of grazing animals (y) and in the available forage (x) were anaIyzed for 
contrasting grass-legume pastures. There is evidence that in some 
temperate and tropical grass-legume pastures, the relative distribution of 
grass and legume in the sward canopy affects legume selectivity by the 
grazing animal. Results presented here also suggest that legume 
selectivity is influenced by growth habit of the grass as it affects sward 
structure, by relative palatability of che legume and by season of the 
year. With prostrate grasses in association with a palatable legume, 
animals selected throughout the year a slightly higher proportion of 
legume than that avaiIable in the total biomass. In contrast, in a 
prostrate grass in associationwith an unpalatable 1egume, animals 
selected against the 1egume regardlass of stock1ng rate applied. In the 
case of associations with a tall-bunch grass, legume selectivity was 
highly variable, but a tendency existed for seleetion to oecur mainIy in 
the dry season, as has been observed i~ a number of studies. 
These results may have important implications on pasture management 
requirements of tropical grass-legume pastures exposed to a range of 
ecosystems with varying climatie eonditions. 
Introduction 
The preferential selection of green leaves by grazing animals has 
been widely demonstrated in temperate and tropical pastures (Chacon and 
5tobbs, 1976; B6hnert ee al., 1985). Leaf seleetion and ingestive 
behavior of animals grazing tropical grasses are gene rally affected by 
sward structure. Major factors are leaf bu1k density (kgj[ha'cmJ), total 
herbage mass (kg/ha) and sward height (Allden and Whittaker, 1970; Stobbs, 
1973; Moore et al., 1985). In the case of grass-legume associations, the 
legume proportion in the diet of the grazing animal could be influenced 
by: (1) proportion of the legume in the sward, (2) relative distribution 
of the morpho1ogieal components of grass and legume in the sward canopy, 
and (3) herbage mass, height and density of the pasture (Mi1ne et al., 
1982). With temperate grass-legume mixtures, e.g., ryegrass-white elover, 
a larger proportion of the variatíon of 1egume in the diet of sheep was 
aecounted for by eonsidering the legume content in the upper horlzons of 
the sward (linear, r 2 - .83) compared with che total sward (linear, r 2 -
.57; Milne et al., 1982). Also in a tropical grasa-legume pasture of 
Hemarthria altissima ev. Floralta - Aeschynomene ameriCana, the proportion 
of the legume in the diet of catele exhibited a quadratic relationship,(R2 
.79) with che legume proportion in che upper layer of ehe pasture ,(Moore 
71 
. i 
• 
US ,M ES 
et al., 1985). In both studies, the proportion of the l,egume in the diet 
was greater than the proportion in the grazed horizon of the svard, 
possibly as a result of canopy modification by the grazing animal prior to 
biting (Moore et al., 1985). More studíes involving measurements of the 
depth of the grazed horizon and che relationship between lesume proportion 
in different strata of the pasture and in che diet selected by grazing 
animals are needad to improve understanding of factors controlling legume 
selectlon in tropical pastures under different climatic condicions. With 
this objective, the proportions of leguree in the diet and in the total 
biomass were analyzed for different types of grass-legume mixtures in,two 
,eeosystems. The possible Lmplications cf these results on pasture 
management are explored. 
Experimental Pastures 
Past:\1res with prastrate grasses. Measurements of proportion of. 
legume selected by grazing animals vere carried out at the Carimagua 
Research Station for one year in mixtures of pros trate and semi-erect 
speeies of Brachiaria (~. humidicola [CIAT 679], 'R. dictyoneura (CIAT 
6133], ~. brizantha [CIAT 644], ~. ruziziensis [CIAT 629l]) vith Arachis 
pintoi (CIAT 17434), a stoloniferous legume. The site represented a vell 
drained savanna ecosystem at latitude 4'37' North with an average rainfall 
of 2100 mm (April-November) and a vell dafined dry season (December-
Karch). Each assoeiation was established in 1227 m2 plots with two 
replications. Grezing was aeross the four mixtures using a rotational 
sche=e of 7/21 days (grazing/rest) with ~.o esophageal fistulated steers 
(200 kg), resulting in an eq~ivalent stocking rate of 4 A/ha. Measure· 
ments of the proportion of legume in the total available biomass and in 
the diet selected by grazing animals vere made on a monthly basis, during 
the first, third and last day of grazing of each association. A wide 
range of 1egume proportion occurred in the available forage because of 
spittlebug damage to ~. ;uziziensis and~. brizantha. 
Measurements vere also conducted in a different ecosystem at 
Quiliehao, Cauea Valley, Colombia in a Brachiaria dictyoneura (CIAT 
- Desmodium ovalifolium (CIAT 350, a stoloniferous legume) pasture. 
site is located at latitude 39 6' North, vith an average rainfall of 
6133) 
lhe 
1772 
mm with a bimadel distribution (March-June and September-December). 
Pastures were agatn rotationally grezed by Zebu steers (200 kg) at 
stocking rates of 3.0, 4.0 and 5.5 A/ha, with 7 deys of grazing and 21 
days oí resto Dietary forage samples from esophageal fistulated steers 
were obtained every 28 deys in the paddocks composing the rotation. 
During the-study (2.5 years), large differences in botanical composition 
of the avai1able forage have been generated as a result of stocking rate 
treatments and season of the year. 
Past:\1res with bunch grasses. A grazing trial 1s in progre ss at 
Carimagua_involving =lxtures of Andropogon gaYanUs ev. Carimagua 1 (an 
erect, -tall bunch grass) vith Centrosema aeutifolium (CIAT 5277 - 5568, a 
stoloniferous-twining legume). Legume selectivity ls belng measured on a 
monthly basis through esophageal co11ections from steers in pastures 
continuously grazed at .15, 1.0 and 1.5 A/ha and rotationally grazed at 
1.5 A/ha. The proportions of legume in the diet and in the available 
72 
,l. 
iSSL!1dM. SUSLUi&kZiLML 3d hU :: 2a( !1 j ¡¿ dU lL 
, , 
¡ 
forage obtained over one rainy season and one dry season were used for the 
analysis. 
Analysis of Results 
!he proportion of legume in the diet (y) and in che total available 
forage (x) obtained in che three grazing trials were analyzed using a 
model whieh allows the description of animal selectivity with a single 
parameter. !he basic equation used was (Chesson, 1983): 
el/ql 
"i - -c::"e l-¡T:q"'1-"7+-'-e'é-2::-¡r=q'""2-
where: ai selection index 
el - legume amount in the diet 
e2 - grass amount in the diet 
q¡ - legume amoune in the available forage 
q2 grass amount in the availab1e forage 
To relate the proportion of 1egume in the diet to proportion in che 
pasture the above equation was transformed as fol1ows: 
el _ [(ai ql )/(1 - ai)]/[l + (ai ql /(1 - ai)] 
el + e2 - q1 1 - ql 
Ibe parameter estimated with this equation ("i) representa seleetion 
of the legume eomponent snd implies a curvilinear relationship between the 
proportion in the diet and in che available forage. Estimates of ai can 
vary from <0.5 (seleetion against the legume) to >0.5 (se1eetion in favor 
of the 1egume) in the range O to l. Ibe validity of ai is dependent on 
two assumptions: (1) that lt does not vary with time oro in other words • 
• it is an instantaneous measur~ óf animal behavior, and (2) that it is not 
dependent on abso1ute quantities ot biomass available. 
Ibe funetion was fitted to the ~ata by minimizing the sum of squared 
deviations. Because the error distrE'utic ",,- "oq ov"-: tne ''1ge of O 
1 (error at these poines tends to O) t ,e ( re transforme,' i" "'he y 
axis using the arcsine transformation. eh ,¡ges brought about by th" 
transformation were small. but it was (O·' ;!eptual.l.Y desirable to proceed 
with this step. A eonfidence interval at the .01 significance level was 
ca1culated for each of the established re1ationships. assuming a normal 
error distribution. 
Legume Se1ection and Pasture Type 
Ibe grazing trials from which the data for this analysis were derived 
were not speeifically designed to study relationships between 1egume 
proportion in the diet and in the available forage. However. the range of 
data obtained in eontrasting grass-legume pastures was thought to be 
useful for exploring general relationships between legume selected by 
grazing animals and legume available in the pasture. Extremely limited 
information pf this type is available in the literature for tropical 
grass-legume pasturas. 
Legume selection in pastures with prostrate grasses. Ibe re1ation. 
ship between lagume proportion in the diet and in the avai1able foragé in 
73 
i 
ehe Brachiaria spp - a. pintoi pastures (figure 1) wes described by an ai 
- .523. This indicetes that the legume was selected in slightly higher 
proportion than what was present in the total available forage. In the ~. 
dietyoneura - Q. ovalifolium pasture (figure 2), the selection indices 
(ai) were .316, .318 and .295 for ehe high, medium and low stocking rates, 
respectively, indicating that in the"three stocking rates animals selected 
against the legume in a similar manner, as has been observed in other 
grazing trials involving Q. ovalifolium (CrAT, unpublished results). 
!hese results are interesting since they suggest that in this association 
legume selectivity was not related to forage availability resulting from 
ehe three stocking rates imposed. at least with the range of data 
analyzed. However, it is conceivable that at higher levels of legume 
availability in the pasture, legume proportion in the diet might be higher 
ehao ehat available in the total biomass, as selection for grass becomes 
increasingly difficult in a strongly legume dominant pasture. rn this 
case the response function would be described by a curve with e sigmoid 
shape (Chesson. 1983). 
lt is postulated that the differenee in the selection indiees of the 
legumes in the two associations with pros trate grasaes la primarily 
related to the palatability of the two legumes. !he low palatability of 
U. ovalifolium (CIAT 350) has been associated with high tanuin eoncentra-
tion, parti.ularly when grown on sulfur deficient sons (Lascano and 
Salinas, 19&2; Salinas and Lascano, 1982). 
Legume selectlon in pastures with blJll&h grasses. !he proportions of 
legumes in the diet and available in the forage in the a. gayanus - ~. 
@cutifqlium pasture Were analyzedfor the whole year, wet season and dry 
season (figure 3). !he selection indices were .244, .088 and .479 for the 
entire year, ehe wet season and ehe dry season, respectively. Even though 
the data were extremely variable, there ls a tendency for animals to 
select against the legume in the wet season but in favor of the legume in 
ehe dry season. Results from other experimenta support ehis interpreta-
tion. In pastures of A. gaYanus with Puerarla phaseoloides (CIAT 9900, a 
twining legume) and Stylosanthes capitata (CrAT 1405, 1019 + 1315, a 
semierect, sub-shrub legume) , animals preferentially selected ehe legume 
in ehe dry season in Carimagua (Sohnert et al., 1985). Work condueted in 
dry tropical areas of Australia has a150 indicated higher legume, e.g., ~. 
humilis and~. hamata, selectivity in ehe dry season than in the wet 
season (Hunter et al., 1976; Cardener, 1980). In addition, seasonal 
changes In legume selectivity have also been observed in subtroplcal areas 
of Australia (Stobbs, 1977). 
lt is suggested that with bunch-type grasses, animals can be highly 
selective wieh a clear preferenee for the legume in the dry season. Such 
does not appear to be the case for legumes grown in association with 
prostrate grasses. 
Legume Selectlvity and Grazing Kanag~ment 
A clear understanding of the relationship between legume proportion 
in ehe diet of grazing animal s and in ehe available forage, as affected by 
pasture structure (within or between different sward types) , is 
fundamental for ehe development of grazing management strategies for 
different grass-legume mixtures in the tropics. 
7.4 
, , 
, . 
¡ 
1.00 
I¡j 
O 0.80 ~ 
w O 
~ 
::¡ 
t7 0.60 o w 
-1 
lA. 
O 
z 0.40 O 
.t: '" .523 ¡::: 
a: 
O 
n. 0.20 
."0 O 
a: 
a.. 
0.00 
0.00 0.20 OAO 0.60 0.60 1.00 
PROPORTION OF AVAlLABlE lEGUME 
Figure 1. Reialíonship belween legume proportion in Iha diet and in total ava/labie foraga in 
8rachiaria spp. A. pínroi pastures. 
• 
75 
1.00....-------------" 
.... 
w 
i5 
¡¡,:¡ 0.80 
w 
:lE 
:::J fil 0.60 
.... 
... 
O 
z 0.40 
O 
~ ¡r 0.20 
O 
a: 
a. 
HSR (5.5 A ha") 
o 
o 
o 
.(=.316 
0.00'..¡..<::::.,...--.-..2.,.~,--...__.__.__._........,.__I 
0.00 0.20 0.40 0.60 0.80 1.00 
PORPORTION OF AVAILABLE LEGUME 
1.00 
1.00~------------.. 
ti 
i5 
¡¡,:¡ 0.80 
w 
:lE 
:::J 
~ 0.60 
.... 
... 
O 
z 0.40 
O 
~ ¡r 0.20 
O 
/. 
,/ , MSR (4.0 A ha") 
,,,,,,,,, 
"O o 
,,/ 
"o ~ . .c:>.; &,/0 .n-",,-
09-'0 
, , 
, 
, 
o 
,,/ 
,," , 
.t =.318 
ex: 
a. ,,' O.oo~:..._-r---r__r-r__,___.__r__.-I 
0.00 0.20 0.40 0.60 0.80 1.00 
PROPORTION OF AVAlLABLE LEGUME 
l' ti 
i5 LSR 13.0 A ha") , 
" 
, 
¡¡,:¡ 0.80 , , 
w 
" :lE ", 
:::J 
",,' el 0.60 .... , 
w , 
.... 
, 
... '-'0 O 0.40 
, 
o , 
z , o 
O ,," ~ , 
" 
a: , 
O 0.20 , 
.r ".295 a. / O .-a: /'. 
a. 
0.00 
0.00 0.20 0.40 0.60 0.80 1.00 
PROPORTION OF AVAILABLE LEGUME 
Figure 2. Relatlonship between legume proportlon in the diet and in total 8vallable forage in a Brachiaria 
dictyoneura ' O: oVlf/ifolium pasture at three stocking rates (SRI. 
76 
.. 
¡ 
li! 
; a 
!?: 
W 
::¡¡ 
:;) 
CJ 
w 
..... 
.... 
o 
z 
o ¡:: 
a:: 
o 
a. 
o 
a:: 
a.. 
'., 
, 
1.00 1.00 ~ 
,1' 1- / 
< WET + ORY SEASON / w WET SEASON 
, 
/ a ,1' , , , 
0.80 /' ~ 0.80 , /' , 
o / W .1' , 
.1' 
;' :2 .1' 
,/ :;) .1' o ~ 
0.60 , CJ 0.60 o ~ o /' W ~ 
/ ..... 
, 
" 
u. ,/ 
" 
o , 
0.40 , z 0.40 , /0 
/~o o " Ii: ,/ / 
-< = .244 o , 0.20 , 0.20 , , a.. , 
/ o " /0 o a:: 
"" °0 <9 a. , 0.00 0.00 
0.00 0.20 0.40 0.60 0.80 1.00 0.00 0.20 0.40 0.60 0.80 1.00 
PROPORnON OFAVAlLA8LE LEGUME PROPORTION OF AVAILABLE LEGUME 
t-
1.00 
w DRYSEASON a 
!?: 0.80 
w o A A ::¡¡ 
" :;) 
" o 
" CJ 0.60 
" w 
" ..... 
" u.. 
o 
z 0.40 
.0 ¡:: 
..(= .479 a:: 
o 0.20 o a. 
o o 
a:: ~ 
a. 
0.00 
0.00 0.20 0.40 0.60 0.80 1.00 
PROPORTION OF AVAlLA8LE lEGUME 
Figure 3. Relationship between legume proportíon in the díel and In total available forage in an 
Andropogon gayanus - Centrosema acutifolium pasture ín dífferent seasons of the year. 
77 
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Based on the different selectlon indices found for the three grass-
legume associations considered above, one would expect that management 
requirements for an adequate grass-le~~e balance over time would diffar 
-between ecosystems. In araas with defined dry season, e.g., savanna 
ecosystem, associations of palatable le~~es> e.g., A. pintoi, with 
prostrate grasaes, e.g., Brachiaria,spp., could probably be managed under 
eontinuous grazing with seasonal adjustments of stocking rate, sinee the 
legume is selected in a similar proportion to what is available. In 
contrast, in the same environment associations of less palatable and 
aggressive legumes, e.g., D. ovaljfolium, with relatively palatable 
grasses, e.g., Brachiaria spp., would require some form of deferred 
grazing to prevent legume dominance or alternatively continuous grazing 
but vith a lower stocking rate. Ibis was partially demonstrated in a 
grazing trial in the Cerrados of Brazil, characterized by a long dry 
season (5 to 6 months). In a~. ruziziensis - Calopogonium mucungides 
(low palatabil~ty) pasture that was -legume dominant (30\ grass and 70% 
legume) under continuous grazing, an adequate grass-legume balance (75\ 
grasa and 25% legume) was aehieved by reducing the stocking rate from 2.5 
to 1.5 A/ha (CIAT, unpublished results). In areas with undefined dry 
season, e.g., tropical rain forest, pastures with prostrate grasses and 
unpalatable legumes viII certainly require rest perioda to maintain 
acceptable grass-legume balance. 
Grazing management to keep an adequate balance of grass and legumes 
in pastutes of bunch grass-twining legume associations, e.g., A. gayanus -
Centrosema spp., appears to depend on the aggressiveness and relative 
palatability of the legume as well as on grazing pressure and climate. 
Pastures based on well adapted and aggressive legumes in are as with short 
dry seasons are likely to require rotational grazing to prevent legume 
dominance. Frequency of grazing is al 50 likely'to be very critical since 
animals select against che legume during most of the year, more so if it 
is of low palatability. Yith les s aggressive legumes. more frequent or 
continuous grazing may be required to reduce competition from the grass 
and favor the legume. 
Recent experience indica tes that rotacional grazing is required in 
some areas. Current available evidence from the tropics indicates that 
animal performance 15 as good under continuous grazing with set stocking 
as with any other grazing management ('c Mannetje et al., 1976). However, 
pasture management should be viewed not only in terms of animal 
performance but also in terma of total animal produetion and pasture 
stability. One example of this 1s given by Stobbs (1969) ln Uganda wlth 
Panlcum maximum - Macroptilium atropurpureum grazed continuously and 
rotationally. Animal gains at the same stocking rate were similar for the 
conttnuous and three paddock rotational grazing system but lower for a six 
paddock rotation. However, afcer three years of grazing. the proportion 
of l. maximum was much lower under continuous grazing than under 
rotational grazing. Pastures under continuous grazing also had more weeds 
than pastures rotationally grazed. In the long run, it is likely that: ~' 
animal production would have been greater in che more stable pastures 
under rotational grazing. 
If it is accepted that different grass-legume associations have 
different grazing management requirements for long term persistence, then 
the question ls: ·~at strategies should be followed to define these 
78 
: 
requirements7" Cne approach i5 che traditional sma11-p10t grazing 
experiment with a combination of grazing frequencies and grazing pre5sures 
in a factorial arrangement (Paladines and Lascano, 1983) or in a central 
composite design (Mott, 1983). Another alternative is the flexible 
management approach proposed by Spain et al. (1985). Yith this strategy, 
no fixed stocking rates or grazing frequencies are employed. Rather, 
stocking rate and grazing frequency are adjusted depending on two pasture 
. parameters: (1) stocking rate is adjusted when grazing pressure reaches 
. selected limits, and (2) grazing frequency ls adjusted when legume 
proportion reaches se1ected 1imits. 
Conc1usions 
Grass-legume pastures are an important a1ternative for increasing 
1ivestock production in tropical areas. However, to assure suceess with 
thesa pastures grown under different environments it is necessary to 
develop through we11 designed experiments appropriate grazing strategies. 
!bis in turn requires an understanding of not only how sward structure and 
eomposltion interact with animal selectivity, but also of how other 
factors such as competition for soil nutrients, density of growing points, 
and residual leaf area affect pasture productivity. 
Literatura Cited 
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intake by grazing sheep: rhe interre1ationship of factors influencing 
herbage intake and availability. Austra1ian J. Agr. Res. 21:755. 
B6hnert, E., C. Lascano and J.H. Yeniger. 1985. Botanica1 and chemical 
composition of the diét selected by fistulated steers under grazing 
on improved grass-legume pasture in the tropical savannas of 
Colombia. l. Botanical composition of forage avaiIable and se1ected. 
Sonderdruck aus Zeitschnirt fur Tierzuchtung und Zuchtungsbiologie 
Bd. 102, H.S. 5:385. 
ehacon, E. and T.R. Stobbs. 1976. Influence of progressive defoliation 
of a graes sward on tbe eating behaviour of catt1e. Austra1ian J. 
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Chesson, J. 1983. !be est~tion and analysis of preferenee and its 
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Gardener, C.J. 1980. Diet selection and liveweight performance of steers 
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Hunter, R.A., B.O. Siebert and M.J. Breen. 1976. !he boranical and 
chemica1 composition of tbe diet selected by steers grazing 
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Mannetje, L. 't, R.J. Jones and T.R. Stobbs. 1976. Pasture evaluation by 
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79 
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Milne, J .A., J. Hodgson, R. TIlompson. 11. C. Soucer and G". T. Barchram. 
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Mott, C.O. 1983. Evaluación del ger.noplasma forrajero bajo diferentes 
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Paladines, O. and C. Lascano. 1983. Recomendaciónes para evaluar 
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Parcelas. pp 165-183. CIAT, Cali. Colombia. 
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la calidad de Desmodiym ovalífolíU;. Pastos Tropicales. Boletín 
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Cong., Kyoto, Japan. p 1153. 
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80