RESEARCH ARTICLE

Early generation selection for high yielding cowpea genotypesin additive series intercropping systems with sorghumF.K. Padi

Cowpea Improvement Programme, CSIR, Savanna Agricultural Research Institute, Tamale, Ghana

Keywords

Additive series intercropping; cowpea; early

generation selection; parent–offspring

regression.

Correspondence

F.K. Padi, CSIR, Savanna Agricultural Research

Institute, PO Box 52, Tamale, Ghana.

Email: padifrancis@yahoo.co.uk

Received: 27 April 2007; revised version

accepted: 21 July 2007.

doi:10.1111/j.1744-7348.2007.00179.x

Abstract

Defining appropriate selection strategies for developing cowpea varieties adap-

ted to additive series intercropping systems is an important requirement for

cowpea breeders and producers in sub-Saharan Africa. One hundred and

forty-three F2:3 cowpea families and their subsequent 99 F3:4 families derived

from a cross between a sole bred cultivar, Apagbaala, and a traditional variety,

SARC-L02, were evaluated under additive series intercropping with sorghum.

Intercropping imposed a strong selection pressure for days to flowering such

that 31% of F2:3 families that flowered after 50 days produced too few grains

to permit their subsequent evaluation in the F3:4 generation. Narrow-sense

heritabilities estimated by parent–offspring regression were high for 100 seed

weight and days to flowering, moderate for biomass, low for grain yield and

insignificant for branches/plant and pods/plant. Retrospective selection at

40% intensity based on F3 grain yield recovered 5 of the 10 top yielding fami-

lies in the F4. No significant difference was observed between mean grain

yield of selected and rejected families (at the 40% selection intensity) as esti-

mated by a t-test. Sole and intercrop yields produced by six advanced breeding

lines included as controls showed poor correlation, and suggests selecting cul-

tivars under the target cropping system will produce better selection response.

Introduction

In field crop improvement for yield, targeting cultivar

development for adaptation to specific cropping systems

may be exploited to increase genetic gains from selection

(Ceccarelli, 1989; Annicchiarico, 1992). The need to

emphasise specific adaptation is supported by evidence

from genetics and agronomic experiments that suggest

that tolerance to different stress factors are under control

of different genetic systems (Hall et al., 1997; Wamatu &

Thomas, 2002). In such breeding schemes, selection is

targeted at traits that maximise yield based on the pre-

dictable environmental constraints that characterise the

cropping system. Cowpea [Vigna unguiculata (L.) Walp.]

production in the semi-arid tropics is under sole, or more

commonly intercropping systems with upland cereals. The

thrust of most cowpea breeding programmes is on select-

ing for sole cropping systems. Advanced breeding lines are

then tested under intercropping for selecting lines to rec-

ommend for intercropping. Useful variation for intercrop

adaptation in the breeding populations is therefore lost

before the evaluation of potential cultivars is conducted

under the target production environment. The low experi-

mental error and high precision with which genotypes

are assessed under sole cropping and for which higher

genetic gains are expected relative to evaluations under

intercropping is therefore of little help in improving yield

under intercropping. If yield potential is the major deter-

minant of realised yield under both sole and intercropping

systems, and yield potential is controlled by the same set

of plant characters under sole and intercropping systems,

then genotype � cropping system interaction should be

minimal and selection under sole cropping should eff-

ectively identify superior genotypes for intercropping

systems. The available evidence suggests, however, that

genotype � cropping system interaction is common in

Annals of Applied Biology ISSN 0003-4746

Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

391

field crops (Hauggaard-Nielsen & Jensen, 2001; Atuahene-

Amankwa et al., 2004; Gebeyehu et al., 2006).

The dominant intercropping system for cowpea in the

semi-arid tropics is the additive series in which sorghum,

millet or maize is planted at the typical population density

for sole cropping, and the cowpea is planted between rows

of the cereal after the cereal is well established. Intercrop-

ped cowpea is thus under high stress and grain yields are

low. Indeterminate, creeping type, long-duration varieties

that are highly sensitive to photoperiod predominate in

these cropping systems. In spite of the low yield potential

of these traditional varieties, the system provides large

quantities of cowpea biomass that is critically needed as

fodder particularly during the post-rainy season. The

determinate, day-neutral varieties developed by most

cowpea breeding programmes for sole cropping systems

do not adapt well to additive series intercropping systems.

Under intercropping, such genotypes are unable to

develop adequate foliage, and branching is reduced with

consequent reduction in grain and biomass yields (N’tare

et al., 1993). N’tare & Williams (1992) speculated that

improving cowpea for intercropping should focus on a

well-branched, prostrate growth habit, a well-developed

root system, and with high transpiration efficiency.

Agronomic traits with high heritability and displaying

high genetic correlation with grain yield under competi-

tive field conditions are useful as selection criteria to

improve cowpea yield under intercropping.

Early generation selection carried out under additive

series intercropping in segregating populations developed

from crosses between traditional cowpea varieties and

elite lines with high yield potential should be effective in

identifying superior lines under intercropping, and pro-

vide opportunities for developing cultivars specifically

adapted to intercropping. Early generation selection

under the target production system enables discarding

inferior lines early in the breeding programme to allow

expenditure of resources on the testing and selection of

fewermore promising lines. In self-pollinating crops, early

generation selection has frequently been found to be

effective (N’tare & Aken’Ova, 1985; Sharma, 1994; St

Martin & Geraldi, 2002). As observed by Bernado (2003),

from a purely genetic standpoint, early generation selec-

tion should be effective because the minimum genetic

correlation between an early generation line and its

descendant homozygous line is high.

The present study was conducted to examine the herit-

abilities of grain and biomass yields and of agronomic traits

of cowpea under intercropping with sorghum. Correla-

tions among agronomic traitswere also determined to pro-

vide information for designing strategies for selecting

superior cowpea genotypes under additive series inter-

cropping with sorghum.

Materials and methods

Parents and population development

A traditional cowpea variety, SARC-L02, commonly grown

in Ghana under intercropping was crossed with the com-

mercially most important improved cultivar, Apagbaala,

to generate the segregating population. SARC-L02 is an

indeterminate, creeping, photosensitive variety with

capacity to produce large quantities of biomass. Under

short photoperiods, it flowers in approximately 55 days

after sowing (DAS) and is classified as a long-duration

variety. Individual seed weight is typically 190 mg. Apag-

baala was developed by the Savanna Agricultural

Research Institute for tolerance to heat during reproduc-

tive development (Padi et al., 2004) and recommended

for sole cropping. It is erect and day neutral, taking

between 42 and 44 days from sowing to 50% flowering

depending on night temperatures. Apagbaala matures

65 DAS and is considered an early maturing variety. It

has low capacity for biomass production, and individual

seed weight is approximately 130 mg.

The F1 plants were raised in a screenhouse facility under

short photoperiods (12.2 reducing to 11.9 h) to produce

the F2 segregating population. Two hundred F2 plants

were raised in the field under short photoperiods at a spac-

ing of 100 � 75 cm between plants. Of the 200 plants,

165 F3 lines were obtained (each from an F2 plant) repre-

senting the number that flowered and produced seeds.

Field experiments

In 2004, 143 F2:3 lines were tested in the field under

intercropping with sorghum at Nyankpala (9�25#N,0�58#W) during the rainy season of July to October.

Twenty-two of the 165 F2 lines produced too few seeds

to warrant further testing. Environmental characteristics

during the period of the experiment are indicated in

Table 1. Three cowpea varieties including the parents of

the population and SARC-L01, a traditional Ghanaian

variety grown mainly under intercropping in Ghana,

were included in the experiments as controls. The exper-

iment was planted in a randomised complete block

design (with three replications on 31 July 2004. A 110-

day sorghum variety, Kadaga, was planted at a spacing

of 80 � 20 cm, one plant per stand in five-row plots

measuring 4 � 4 m. Test cowpea lines were planted

between rows of the sorghum crop by 15 DAS the sor-

ghum crop. Cowpea lines were planted in the first four

rows, leaving the fifth row space as a border between

test cowpea lines. The intrarow spacing of the cowpea

lines was 20 cm with two plants per stand giving a spac-

ing of 80 � 20 cm for the cowpea lines. The sorghum

Early generation selection in cowpea under intercropping F.K. Padi

392 Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

plants received inorganic fertiliser at the rate of 60 kg

N ha21, 60 kg P2O5 ha21 and 60 kg K2O ha21 in split

applications of 75% at 14 DAS and 25% at 56 DAS. The

experiment was weeded twice during the experiment

using hand hoes. The cowpea plants were sprayed three

times at 35, 46 and 60 DAS to control flower thrips (Meg-

alurothrips sjostedti) and a complex of pod sucking bugs.

The insecticide used was lambda cyhalothrin (Product

Karate�, Zeneca Agricultural Products, Wilmington, DE,

USA) at the rate of 20 g active ingredient per hectare.

Datawere collected on three of the four rows of cowpea,

ignoring the first row of each plot because of edge (or bor-

der) effects, leaving a net plot of 2.4 � 4 m2. The experi-

ment was monitored regularly to record days to 50%

flowering, number of pods and number of branches per

plant (average of five plants). At maturity, pods from

each plot were bulk harvested. Cowpea plants were cut

at ground surface and weighed immediately after final

harvest of pods. Samples of the foliage were dried in

a hot air oven at 60�C for 72 h and weighed to obtain

estimates of cowpea biomass production. Pods were

threshed manually and the grains were weighed. Seed

size was estimated as the weight of 100 seeds.

In 2005, 99 F3:4 lines that had adequate seeds to permit

evaluation in three replications were further tested.

Seeds were planted on 23 July 2005. In addition to the

three check varieties included in the 2004 trial, six

advanced breeding lines received from the International

Institute of Tropical Agriculture Ibadan, were also

included in the experiment. These lines were selected

from a set of 60 lines because in 2 years of evaluation

under sole cropping, they recorded the highest grain

yields. The field operations and data collection were

essentially the same as for the 2004 experiment.

Statistical analyses

One-way analyses of variance (ANOVA) were used to

examine differences among lines in each year for each trait

measured in the field using the Statistix computer package

(version 7; Analytical Software, Tallahassee, FL, USA).

From the ANOVA, the genotypic and error variance com-

ponents were estimated by equatingmean squares to their

expectation and solving for the component. The standard

error (SE) of a variance component was calculated

(Becker, 1975) as:

SE�r2�¼

�2=k2

�RiMS2i =fi þ 2

��1=2

where MSi is the mean square of effect i used to estimate

the variance component; fi is the degrees of freedom for

MSi and k is the coefficient for r2 in the expectation of

MSi.

Heritability in the broad sense (H) was estimated on

entry mean basis as the ratio of the genotypic variance

component to phenotypic variance component (Nyquist,

1991) as:

H ¼ r2G=�r2G þ r2e=r

�

where r2G is the genotypic component of variance, r2e is

the error component of variance and r is the number of

replications.

Two-sided confidence intervals were calculated to

determine the precision of heritability estimates as des-

cribed by Knapp et al. (1985). The lower 95% confidence

limit for H was estimated as:

1 2�Fa=2:df1;df2=F

�

and the upper confidence limit was defined as:

1 2�Fa=2:df2;df1

�F�� 2 1

where F = (1 2 h2)21.

Heritability in the narrow sense was estimated by re-

gressing F3:4 family (entry) means on F2:3 family means.

Genotypic and phenotypic correlations (Holland, 2003)

were calculated to examine relationships among physio-

logical and agronomic traits.

The effectiveness of early generation selectionwas evalu-

ated empirically by comparing the grain yield of F4 lines

derived from the top 30%, 40% or 50% yielding F2:3 in-

dividuals with F4 lines derived from those rejected at

each selection intensity. A t-test procedure was used to

determine the statistical significance of the selected and

rejected groups. This is referred to as retrospective selec-

tion. Also, grain yields of the check varieties were com-

pared with the top 10% of F4 families in the unselected

population.

Results

Early generation evaluation of cowpea families was con-

ducted under intercropping to select families adapted to

this cropping system. The consequences of retrospective

Table 1 Rainfall and photoperiod during the period of the experi-

ments, and soil chemical characteristics of the fields

Trait

Year (Generation)

2004 (F2:3) 2005 (F3:4)

Rainfall during trial (mm) 441 397

Photoperiod (h) 12.7–12.2 12.7–12.2

Total nitrogen (%) 0.021 0.032

Available P (mg kg21) 1.13 2.01

Organic carbon (g kg21) 5.8 6.6

pH 5.1 5.7

F.K. Padi Early generation selection in cowpea under intercropping

Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

393

selection in the F3 based on 30%, 40% or 50% selection

intensity for grain yield were compared with the ob-

servations under the experimental condition for which

lines were advanced without selection except that F2:3plants that produced too few seeds to permit planting

a minimum of three rows in the F4 were eliminated. This

selection protocol reflects what most practical plant

breeders would do during development of lines, that is,

eliminate obviously poor performing families from the

population. The aim was to define the key adaptive traits

for cowpea under intercropping. The sorghum crop was

established at the stand density recommended for sole

cropping, and because of the low soil fertility, fertiliser

application to the sorghum was crucial to give satisfac-

tory growth of the sorghum to create the competitive

conditions necessary to select cowpea genotypes suitable

to the additive series intercropping system. Cowpea

genotype had no effect on sorghum grain yield which

varied from 0.94 to 1.36 kg ha21. Other performance

characteristics of the sorghum plants are not reported.

Mean performance and variance components

Thewide range in days to flowering observed among cow-

pea families (Table 2) in each generation is expected

because one of the parents (SARC-L02) shows sensitivity

to photoperiod for days to flowering. The average photo-

period during the first 3 weeks after sowing in each year

(2004 for F3 and 2005 for F4 lines) was approximately

12.6 h, and for photosensitive families, the critical pho-

toperiod for flowering is expected to be 12.3 h based on

reactions to photoperiod observed in earlier years by the

parent SARC-L02. The mean and range for days to flow-

ering were lower in the F3:4 generation compared with

the F2:3 because a large proportion of late flowering fam-

ilies in the F2:3 did not produce adequate number of

seeds to permit their evaluation in the F3:4 generation.

In general, families that flowered later than the late

flowering parent did not set adequate number of seeds

to allow further evaluation in the F3:4 generation. Delays

in time to flowering because of extreme sensitivity to

photoperiod and/or late flowering genes restrict a geno-

type’s adaptation to intercropping under the experimental

conditions. The genotypic variance component for days to

flowering was highly significant in each generation, and

was much larger than the error variance (Table 3).

The plant growth conditionswere generally poor in F2:3,

mainly because of poor soil conditions (Table 1). This

affected F2:3 family performance for pod production

compared with the mean number and range for number

of pods among the F3:4 families. Although significant

genotypic variance components were observed for

the number of pods per plant, the experimental error

components were much larger particularly in the F3:4generation.

A number of highly branched families (with more than

4.6 branches per plant) in the F2:3 generation were asso-

ciated with lateness to flower and poor seed set. This

reduced the range observed for the number of branches

per plant in the F3:4 compared with the F2:3 generation.

The mean number of branches per plant was however

similar between F2:3 and F3:4 families. Genotypic vari-

ance for number of branches per plant was low in each

generation and much lower than the error component of

variance. In the F3:4 generation, the genotypic variance

component was just marginally significant (Table 3).

The mean seed size estimated as the weight of 100 seeds

shows that a large proportion of families were similar to

that of Apagbaala, the parent with the smaller seed size,

suggesting a dominance of small seed size to large seed

size.Much of the total variation in seed sizewas accounted

for by the genotypic component of variance relative to the

error component of variance.

The mean biomass production was greater in the F3:4compared with F2:3. This is so in spite of the fact that

a large proportion of families that could not set adequate

seeds in the F2:3 generation were associated with high

biomass production. In each generation, a significant pro-

portion of families produced a lot more biomass compared

with the better parent. The mean in each generation was

Table 2 Generation means ± SE and ranges (in parentheses) for agronomic traits of cowpea families grown under intercropping with sorghum

Trait

Generation Parentsa

F2:3 F3:4 Apagbaala SARC-L02

Days to 50% flowering 45.2 � 0.07 (39.7–64.7) 42.2 � 0.05 (39.3–49.3) 44 54

Pods/plant 7.9 � 0.09 (3.7–14.9) 14.4 � 0.22 (5.6–23.3) 12.4 10.5

Branches/plant 3.7 � 0.02 (2.3–5.1) 3.6 � 0.03 (2.9–4.6) 3.4 4.3

Hundred seed weight (g) 13.3 � 0.03 (9.0–20.2) 13.7 � 0.04 (10.1–18.9) 12.9 18.9

Biomass (t ha21) 1.67 � 0.022 (0.48–5.13) 1.77 � 0.028 (0.40–3.14) 1.47 2.34

Grain yield (t ha21) 0.23 � 0.004 (0.01–0.97) 0.69 � 0.009 (0.06–1.21) 0.63 0.45

aMean for 2004 and 2005 evaluations.

Early generation selection in cowpea under intercropping F.K. Padi

394 Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

higher than that of the parent with lower biomass pro-

duction capacity. Significant genotypic components of

variance for biomass production, larger than the error

components of variance were obtained in each generation

indicating potential for selecting families with large bio-

mass in this cross.

The evaluation of F3:4 families on soil of better fertility

and removal of families with low grain yield potential

improved mean grain yield by threefold in the F3:4 rela-

tive to the F2:3 generation. The size of the error compon-

ent of variance for grain yield was similar to that of the

genotypic component of variance (but with larger SE) in

the F2:3 generation. Compared with the F3:4 that was

evaluated under better conditions, the genotypic vari-

ance component was larger than the error component,

in concurrence with earlier observations that increasing

stress reduces the efficiency with which superior genotypes

in terms of grain yield are selected (Padi, 2004). Interest-

ingly, in these evaluations, the parent recommended for

sole cropping systems (Apagbaala) outperformed the

parent generally used for intercropping systems.

Heritabilities and trait correlations

In general, heritabilities determined on family mean basis

were high, reflecting the high genotypic to error compon-

ents of variance (Tables 4 and 5). For narrow-sense

heritability estimates, parent–offspring regression and

parent–offspring correlation produced similar values for

the majority of the traits. Also, the genotypic correlation

coefficients were higher than their corresponding pheno-

typic correlation coefficients.

Of all the traits studied, number of branches per plant

recorded the lowest heritability values in the F2:3 and F3:4generations. The lower limit for the broad-sense herita-

bility for this trait in the F3:4 was zero (Table 5). The nar-

row-sense heritability estimates were also not significant

as determined by any of the two methods (Table 6). The

correlation coefficients between branches per plant and

grain yield were not significant in both generations.

With biomass, significant genetic correlations were ob-

tained with a trend of highly branched families pro-

ducing large quantities of biomass compared with poorly

branched families.

Broad-sense heritability estimates for biomass produc-

tion were high in both generations, with moderately high

narrow-sense heritability as well. Significant correlations

were not observed between biomass and grain yield in

either generation. Families producing large biomass in

each generation were associated with lateness to flower

as evidenced by the positive genetic correlations between

days to flowering and biomass production.

Days to flowering displayed high broad-sense heritabil-

ity estimates in both generations, and the narrow-sense

heritability was high indicating progress can bemade from

early generation selection for days to flowering, consistent

with observations made by Hall et al. (1997). In the F2:3generation, families that recorded more than 50 DAS to

flower had reduced pod and seed set compared with ear-

lier flowering families, such that, significant negative

Table 3 Variance components ± SE from the analyses of variance of agronomic traits of F3 and F4 families of cowpea grown under intercropping

with sorghum

Trait

Genotypic Variance Error Variance

F3 Families F4 Families F3 Families F4 Families

Days to flowering 13.77 � 1.688 6.17 � 0.901 2.07 � 0.170 0.87 � 0.086

Pods/plant 3.46 � 0.594 4.55 � 1.42 4.54 � 0.374 14.92 � 1.470

Branches/plant 0.17 � 0.037 0.04 � 0.021 0.41 � 0.033 0.34 � 0.034

Hundred seed weight 6.94 � 0.826 5.93 � 0.85 0.41 � 0.034 0.41 � 0.040

Biomass 0.46 � 0.070 0.39 � 0.066 0.39 � 0.032 0.25 � 0.025

Grain yield 0.02 � 0.003 0.05 � 0.008 0.02 � 0.001 0.03 � 0.003

Table 4 Broad-sense heritabilities, phenotypic and genotypic correlations of F2:3 families of cowpea grown under intercropping with sorghuma

Days to Flower Pods/Plant Branches/Plant 100 Seed Weight Biomass Grain Yield

Days to flowering 0.95 (0.910–0.975) 20.34 � 0.057 0.23 � 0.060 20.01 � 0.078 0.43 � 0.054 20.35 � 0.060

Pods/plant 20.46 � 0.081 0.70 (0.428–0.828) 0.13 � 0.057 0.14 � 0.065 0.05 � 0.060 0.52 � 0.044

Branches/plant 0.41 � 0.089 20.04 � 0.117 0.55 (0.162–0.763) 0.17 � 0.064 0.50 � 0.043 0.07 � 0.060

Hundred seed

weight

0.01 � 0.086 0.14 � 0.096 0.18 � 0.096 0.98 (0.963–0.989) 0.21 � 0.067 0.18 � 0.068

Biomass 0.60 � 0.065 20.11 � 0.108 0.66 � 0.069 0.23 � 0.088 0.78 (0.593–0.885) 0.08 � 0.063

Grain yield 20.43 � 0.079 0.56 � 0.076 20.08 � 0.111 0.20 � 0.089 20.08 � 0.103 0.80 (0.616–0.819)

aValues above the diagonal are phenotypic correlation coefficients ± SE. Values below the diagonal are genotypic correlation coefficients ± SE. Values

in the diagonal (bold) are heritabilities; values in parenthesis are the lower and upper limits for heritabilities.

F.K. Padi Early generation selection in cowpea under intercropping

Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

395

correlations were observed between days to flowering

and pods per plant and grain yield. The strong selection

pressure on days to flowering imposed by the intercrop-

ping system on F2:3 families changed the relationship

between days to flowering and pods per plant to a signifi-

cant positive genetic correlation in the F3:4 generation.

Environmental effects on these traits reduced this corre-

lation to a small positive phenotypic correlation value

among the F3:4 families (Table 5). This significant corre-

lation between days to flowering and pods per plant,

however, did not translate into a significant correlation

between days to flowering and grain yield. Low esti-

mates for broad-sense heritability were recorded in each

generation for the number of pods per plant, with the

95% lower confidence limit being zero in the F3:4 gener-

ation. Similarly, the narrow-sense heritability estimates

are less than twice the magnitude of their respective

standard errors indicating a low level of significance

(Table 6).

Seed size, estimated by the weight of 100 seeds showed

the highest broad-sense heritability estimates in each gen-

eration among the traits studied. Similarly, narrow-sense

heritability estimates were high, and selection for this trait

in an early generation is expected to be effective. Geno-

typic correlation coefficients between seed size and the

two traits of primary interest to cowpea producers, grain

yield and biomass, were positive, significant, but weak in

each generation. The broad-sense heritability estimates

for grain yield were high in each generation. Its narrow-

sense heritability, however, was low but significantly dif-

ferent from zero.

Effectiveness of early generation selection and

comparison of elite F4 families with parental

lines

Retrospective selection in the F3 was based on selection

intensities of 30%, 40% or 50% for grain yield. Effec-

tiveness of this early generation selection was assessed

primarily on the expectation that the F4 mean perfor-

mance of selected lines will be significantly higher than

those not selected. Mean grain yield in the F4, however,

was not significantly different between selected and re-

jected families at all selection intensities tested (Table 7).

Selected families on average flowered earlier than rejected

families, the difference in days to flowering between the

two groups at the intensities tested do not have much

practical value though because it was typically less than

1 day. Hundred seed weight was significantly higher in

the selected families, but the reverse was true for biomass.

Of the top yielding 10% of the families in the unselected

population, selecting at 40% or 50% in the F3 yielded

only 5 of the 10 families and selection at 30% yielded 4 of

the 10 families (data not shown).

Grain yield differences obtained among the top 10 fam-

ilies were not statistically significant. Compared with the

better parent, only the highest yielding line (SARC 02-74)

produced significantly larger grain yield, although the

nine highest yielding genotypes were significantly better

yielding than the other parental line (Table 8). The most

significant gains were realised in terms of earliness

among the elite families when compared with the pa-

rents. Gains in biomass and seed size were variable;

however, the top 10 families offer opportunities for se-

lecting lines that combine larger seed size and biomass in

relation to the better parent for cultivar development. To

obtain an estimate of the reliability of the yields obtained

by the top 10% of F3:4 families, the top 10% of 60

advanced breeding lines selected under sole cropping

were included in the evaluation under intercropping.

Among the six lines, yield ranged from 0.33 to 0.87 t

ha21 (Table 8). Differences in grain yield under inter-

cropping was associated with differences in days to flow-

ering with a simple correlation of r = 20.83 (P = 0.041).

Table 5 Broad-sense heritabilities, phenotypic and genotypic correlations of F3:4 families of cowpea grown under intercropping with sorghuma

Days to Flower Pods/Plant Branches/Plant 100 Seed Weight Biomass Grain Yield

Days to flower 0.96 (0.915–0.976) 0.18 � 0.061 20.20 � 0.064 0.28 � 0.083 0.47 � 0.061 0.001 � 0.081

Pods/plant 0.57 � 0.194 0.48 (0.018–0.722) 0.02 � 0.056 20.01 � 0.069 20.12 � 0.065 0.19 � 0.001

Branches/plant 20.38 � 0.147 20.16 � 0.301 0.25 (0.00–0.599) 0.01 � 0.065 0.30 � 0.057 0.11 � 0.062

Hundred seed

weight

0.33 � 0.096 20.03 � 0.153 20.03 � 0.186 0.98 (0.958–0.988) 0.19 � 0.083 0.13 � 0.001

Biomass 0.68 � 0.071 20.37 � 0.160 0.89 � 0.202 0.23 � 0.106 0.82 (0.663–0.905) 0.06 � 0.003

Grain yield 0.001 � 0.117 0.24 � 0.002 0.04 � 0.207 0.18 � 0.001 0.01 � 0.001 0.83 (0.678–0.909)

aValues above the diagonal are phenotypic correlation coefficients ± SE. Values below the diagonal are genotypic correlation coefficients ± SE. Values

in the diagonal (bold) are heritabilities and values in parenthesis are the lower and upper limits for heritabilities.

Table 6 Narrow-sense heritabilities ± SE based on regression of F4

family means on F3 family means

Trait Heritability

Days to flowering 0.70 � 0.082

Pods/plant 0.15 � 0.144

Branches/plant 0.22 � 0.136

Hundred seed weight 0.77 � 0.033

Biomass 0.52 � 0.085

Grain yield 0.28 � 0.138

Early generation selection in cowpea under intercropping F.K. Padi

396 Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

Under the sole cropping, however, no significant corre-

lations were observed between grain yield and time to

flowering (r = 20.13, P = 0.800) among these advanced

breeding lines. The differences in adaptation to the dif-

ferent cropping systems shown by these lines was also

evident in the lack of correlation between grain yield

under sole and intercropping systems with r = 20.13

(P = 0.800).

Discussion

In semi-arid regions of West Africa where a considerable

quantity of cowpea is produced under intercropping with

cereals, farmers seek to maximise cereal yields while

benefiting from any additional grain and biomass harvest

that may accrue from intercrop cowpea (N’tare, 1990).

Thus, recommendations based on agronomic manipu-

lations for increasing yield of intercrop cowpea such as

replacement series intercropping (Terao et al., 1997) or

strip cropping (Cenpukdee & Fukai, 1992) have persisted

without a change in farmers’ practice of additive series

intercropping. The traditional approach in breeding cow-

pea for intercropping systems has involved early genera-

tion selection under sole cropping with only final

evaluation of advanced breeding lines under intercrop-

ping (N’tare, 1989; Ehlers, 1994). With the existence

of genotype � cropping system interaction (Nelson &

Robichaux, 1997), quantitative genetic theory (Falconer,

1989) suggests that such an approach will not efficiently

identify superior genotypes for intercropping. The grain

yield differences obtained from the six advanced breed-

ing lines under sole and intercropping observed in the

present study supports this theory, in that, the correla-

tion between sole and intercrop yield was not significant

(r = 20.13, P = 0.800). Selection pressure for grain yield

imposed in early generations during line development

under sole cropping potentially will eliminate lines that

are specifically adapted to intercropping.

Conceptually, yield of intercrop cowpea is determined

by tolerance to intercropping, appropriate phenology and

high yield potential. Of these three, tolerance to intercrop-

ping may be more difficult to select for, but is thought

to reflect tolerance to shade of the companion dominant

crop (N’tare & Williams, 1992). The effect of shade on

grain yield of intercrop cowpea is attributed mainly to

reduction in branching, and the number of branches

developed under intercropping has been suggested as

a selection criterion for grain yield. This explanation is

reasonable considering that the branches carry the pods.

The findings of the present study, however, do not sup-

port this contention. The lack of correlation between the

number of branches per plant and grain yield among F2:3

Table 7 Comparison of mean F3:4 performance of selected and rejected groups at various selection intensities in the F2:3 based on the whole

population, and that of only the elite families, and significance of the difference in mean performance determined by t-test

Trait

Retrospective Selection in the F2:3 based on Whole Population

Top 10% F3:4 lines compared

at 40% intensity in the F2:3

50% Intensity 40% Intensity 30% Intensity

Selected Rejected P Selected Rejected P Selected Rejected P Selected Rejected P

Grain yield

(t ha21)

0.69 0.68 0.848 0.70 0.68 0.667 0.72 0.68 0.213 1.08 1.05 0.715

Days to flower 41.7 42.1 0.066 41.5 42.1 0.005 41.4 42.1 <0.001 41.5 41.7 0.718

Hundred seed

weight (g)

14.1 13.2 0.002 14.2 13.4 0.006 14.1 13.5 0.034 15.1 12.6 0.004

Biomass

(t ha21)

1.63 1.90 0.003 1.60 1.88 0.004 1.59 1.84 0.014 1.98 1.91 0.729

Table 8 Performance of best yielding F3:4 families and checks grown

under intercropping with sorghum, and the proportion of elite families

recovered at 40% selection intensity in the F2:3a

Genotypes

Days to

Flower

Biomass

(t ha21)

Hundred seed

weight (g)

Grain Yield

(t ha21)

SARC 02-74 42.7 1.21 18.70 1.21

SARC 02-32 41.0 1.88 11.92 1.12

SARC 02-216 43.0 2.31 11.40 1.10

SARC 02-21 41.0 1.47 11.80 1.07

SARC 02-73 42.0 2.80 13.03 1.07

SARC 02-69 40.0 2.01 13.68 1.05

SARC 02-240 41.3 2.28 17.27 1.03

SARC 02-56 40.7 1.92 11.82 1.02

SARC 02-29 43.7 1.84 14.27 1.00

SARC 02-251 40.3 1.73 14.48 0.98

SARC-L01 56.3 2.29 13.87 0.14

SARC-L02 54.3 2.37 18.83 0.72

Apagbaala 45.3 1.69 13.37 0.92

IT 98K-503-1 51.0 3.10 17.23 0.33

IT 97K-818-35 46.0 1.28 16.83 0.34

IT 98K-128-3 43.3 2.89 15.93 0.62

IT 98K-506-1 43.7 1.78 16.08 0.78

IT 97K-499-35 41.0 1.36 15.97 0.87

IT 99K-718-6 44.7 2.79 17.97 0.47

SE (df = 202) 0.74 0.400 0.501 0.139

aGenotypes with prefix ‘IT’ are advanced breeding lines included as

checks in the 2005 experiment.

F.K. Padi Early generation selection in cowpea under intercropping

Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

397

families remained in the F3:4 generation even after the

elimination of highly branched families that were associ-

ated with late flowering. Moreover in both generations,

the correlation between number of branches per plant

and pods per plant were not significant. In addition, reli-

ance on the number of branches per plant as a selection

criterion for a target trait such as biomass with which it

showed highly significant correlations in the present

study is not warranted because of its low heritability

compared with the primary trait of interest (biomass).

The high error variances associated with establishing

genotypic differences for branching reduces the useful-

ness of this trait in selecting superior genotypes.

The early generation evaluation conducted under inter-

cropping has established a strong dependence of grain

yield on early phenology in both the F2:3 generation

(r = 20.43, P < 0.001) and among the six advanced

breeding lines (r = 20.83, P = 0.041). In the F2:3 genera-

tion, additive series intercropping imposed a strong

selection pressure on earliness to flower such that

families that flowered later than 50 DAS were not ade-

quately productive to permit their subsequent evalua-

tion in the F3:4 generation. The importance of early

phenological development to grain yield may be attri-

buted to two main reasons. First, high grain yield in

cowpea requires that competition for light during repro-

ductive development is not strong. Under intercropping,

therefore, families that are early to flower intercept

more photosynthetically active radiation for reproduc-

tive development relative to late flowering families

before the canopy of the cereal closes. Second, control of

flowering and post-flowering insect pests is critical to re-

alising high grain yield in cowpea. Under additive series

intercropping, insecticide application to the cowpea be-

comes impractical soon after the onset of reproductive

growth in the cereal, which coincides with rapid canopy

closure. Grain filling in pods formed in late flowering

genotypes is therefore affected. Shade tolerance that

may be expressed as increased branching in late flower-

ing genotypes (which are associated with a creeping

habit) is therefore of little practical value to grain yield

formation under additive series intercropping. This

explanation is supported by the observation that among

F3:4 families, the positive correlation between days to

flowering and pods per plant did not translate into a posi-

tive correlation between days to flowering and grain

yield. In late flowering genotypes, therefore, early phase

reproductive growth of flower development into pods

may not be as constrained as the later phase develop-

ment of grain filling. The higher grain yield recorded by

the earlier maturing, sole bred parent compared with the

traditional variety under intercropping is therefore not

surprising. Apagbaala is popular for its synchronous

flowering and high expression of determinacy that per-

mits farmers to undertake single harvesting. These traits

are also of relevance to grain yield under intercropping.

The significant correlation between earliness to flower

and grain yield among F2:3 families was not evident in

the F3:4 when families that were very late to flower were

eliminated. Within the range of maturity period for cow-

pea established by the intercropping system therefore,

yield potential becomes more important than days to

flowering in determining realised yield. This emphasises

the need to define a phenology appropriate to the partic-

ular cropping system and then identifying genotypes

with high yield potential. As stated earlier, this yield

potential needs to be established through evaluations con-

ducted under intercropping. High yield under sole crop-

ping has frequently been associated with semi-determinate

growth habit that produces two flushes of pods (Hall et al.,

1997). Under intercropping, the second flush of pods do

not contribute to grain yield because of the intense stress

imposed by the companion crop, and determinate geno-

types therefore become more productive. In addition to

the low heritability of pods per plant observed in the pres-

ent study, selection of superior families for grain yield

based on pod numbers will be ineffective.

The high investments in land and labour resources asso-

ciated with intercropping experiments makes it impera-

tive that strategies are used to reduce these requirements

early in a breeding programme targeted for intercropping.

In the present study, earliness to flower has been identi-

fied as a key adaptive trait to intercropping, andmay be an

important selection criterion to reduce the size of an early

generation population in breeding for intercrop adapta-

tion. The persistent negative correlation observed between

earliness to flower and biomass production capacity how-

ever implies that a low selection pressure is applied to

selection for earliness in the early generation population

to retain the chances of developing dual-purpose geno-

types for grain and fodder. The lack of correlation observed

between grain yield and biomass observed in the present

study suggests that developing dual-purpose types is fea-

sible if simultaneous selections are imposed for the

two traits in the subpopulation selected for appropriate

phenology. Families whose performance supports this

argument include SARC 02-240 and SARC 02-73 that

combine high grain and biomass yields under intercrop-

ping. Interestingly, both grain yield and biomass showed

positive correlations with seed size, a key trait that influ-

ences variety adoption in West Africa (Langyintuo et al.,

2003).

The often strong influence of the environment on

assessment of genotypes for grain yield and residual het-

erosis in the early generation (F3) was reflected in the

low narrow-sense heritability estimates for grain yield

Early generation selection in cowpea under intercropping F.K. Padi

398 Ann Appl Biol 151 (2007) 391–400 ª 2007 The Author

Journal compilation ª 2007 Association of Applied Biologists

observed in the present study. Consistent with this

observation, retrospective early generation selection for

grain yield did not increase the mean yield of the

selected families over the mean yield of rejected families

in the F4 evaluations. The lack of significant difference

per se between the mean of selected and rejected lines for

grain yield should lead to increased comparative effi-

ciency for early generation selection if early selection

was associated with recovery of a significant proportion

of elite families in the unselected population. In such

a situation, fewer lines are handled at reduced trans-

action costs compared with the approach reported in this

study where all progeny are advanced to the next gener-

ation. In the present study, only 50% of the elite fami-

lies were recovered by early generation selection at the

relaxed intensity of 40% or 50%. Practically, in a breed-

ing programme, only the top performing lines are

entered into extensive multilocation testing with farmers

so as to identify truly superior cultivars. The lack of sig-

nificant difference observed between the mean grain

yield of the five elite families recovered by early selec-

tion and the five elite families that were rejected in the

unselected population was associated with a similar lack

of significance for days to flowering and biomass pro-

duction. Grain sizes were however significantly larger in

the selected group compared with the rejected group of

elite families. These observations suggest that early gen-

eration selection at 40% intensity would have been

more efficient for the population used in the present

study. A caution with the early generation selection

approach is however that, strong G � E interaction for

grain yield have been documented in cowpea in semi-

arid regions (Padi, 2004), and the probability of obtain-

ing families with broad adaptation to the target region is

higher when a high proportion of elite families are tested

in multilocation evaluations. It remains to be tested

whether the genotypes best adapted to the target Sudan

and Guinea savannah ecoregions will be obtained from

the five selected or five rejected elite families based on

early generation selection.

The findings in the present study are valid for breeding

programmes targeting cowpea development for additive

series intercropping systems with upland cereal crops. In

breeding for other intercropping systems such as the re-

placement series or strip cropping, other plant characters

may be more important in selecting adapted cowpea

genotypes.

Acknowledgement

This research was supported by funds from Project Num-

ber 6 of the Challenge Programme for Water and Food

(CPWF).

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