agriculture
Article
A High Plant Density and the Split Application of Chemical
Fertilizer Increased the Grain and Protein Content of Cowpea
(Vigna unguiculata) in Burkina Faso, West Africa
Haruki Ishikawa 1,* , Benoît Joseph Batieno 2, Christian Fatokun 1 and Ousmane Boukar 1
1 International Institute of Tropical Agriculture (IITA), PMB 5320, Oyo Road, Ibadan 200001, Oyo, Nigeria;
c.fatokun@cgiar.org (C.F.); o.boukar@cgiar.org (O.B.)
2 Institut de l’Environnement et de Recherches Agricoles (INERA), Ouagadougou 04 BP 8645, Burkina Faso;
bbjersi2003@yahoo.fr
* Correspondence: h.ishikawa@cgiar.org; Tel.: +234-70-0800
Abstract: In West Africa, cowpea (Vigna unguiculata) is an important source of protein for many
people. To meet the protein demands, the cowpea yields per unit area and its seed protein content
must be increased. We evaluated the effects of the planting density and topdressing (fertilisation)
timing on the cowpea yield and its protein content. High density (HD: 40 × 40 cm) and super
high density (SHD: 40 × 20 cm) plantings were the most efficient approaches for enhancing cowpea
yields. Across different regions in Burkina Faso, under such approaches, the yield significantly
increased by as much as 214.5%, with an average value of 88.9%. Fertilisation was not required



 to achieve the significant increases in cowpea production following dense planting. Although the


yield increased, the seed weight per plant decreased with the increase in planting density. Applying
Citation: Ishikawa, H.; Batieno, B.J.;
topdressing when the plants started flowering increased the seed protein content significantly by
Fatokun, C.; Boukar, O. A High Plant
up to 24.4%. Simple and robust technologies, such as high-density planting and topdressing, can be
Density and the Split Application of
Chemical Fertilizer Increased the rapidly disseminated for increased cowpea yields and protein content improvement for Burkina Faso
Grain and Protein Content of Cowpea and for other countries.
(Vigna unguiculata) in Burkina Faso,
West Africa. Agriculture 2022, 12, 199. Keywords: legumes; cowpea; fertilization; seed yield; planting density; seed protein content
https://doi.org/10.3390/agriculture
12020199
Academic Editors: Jose
1. Introduction
C. Jimenez-Lopez and
Alfonso Clemente Legumes are important sources of protein and other nutrients, and they are vital
for satisfying the increasing demands for food and feed. Legume crops can provide
Received: 27 October 2021 multiple services, contributing to sustainable food production. In addition to serving as
Accepted: 14 January 2022 a fundamental and worldwide source of high-quality food and feed, legumes contribute
Published: 31 January 2022
to reducing greenhouse gas emissions, as they release 5–7 times less greenhouse gases
Publisher’s Note: MDPI stays neutral per unit area compared with other crops [1]. Legume farming can also result in carbon
with regard to jurisdictional claims in sequestration in soils, with values estimated at 7.21 g kg−1 of dry matter, with 23.6 C kg−1
published maps and institutional affil- versus 21.8 g C kg−1 per year, and can reduce fossil fuel energy inputs compared with
iations. cereal crops due to their lower requirement for N fertilizer, corresponding to 277 kg ha−1
of CO2 per year [1]. Cowpea (Vigna unguiculata) is an annual legume grown extensively
in Africa, the Americas, Asia, and Europe. It is of vital importance to the livelihoods of
millions of people in West Africa.
Copyright: © 2022 by the authors.
Cowpea is a starchy legume that complements cereal-rich diets. West Africa accounts
Licensee MDPI, Basel, Switzerland.
for more than 80% of its annual world production (7.23 million metric tons [2]), and the
This article is an open access article
harvested cowpea is primarily consumed locally. Nutritionally, dry cowpea grains contain,
distributed under the terms and
conditions of the Creative Commons on average, 23–32% proteins and 50–60% carbohydrates [3]. It is a nutritious and inex-
Attribution (CC BY) license (https:// pensive source of protein for both rural and urban consumers [4]. Additionally, cowpea
creativecommons.org/licenses/by/ fodder is a valuable source of feed for livestock [5], making it attractive to small-scale
4.0/). farmers [6]. Burkina Faso is an important cowpea producer (third largest in production)
Agriculture 2022, 12, 199. https://doi.org/10.3390/agriculture12020199 https://www.mdpi.com/journal/agriculture
Agriculture 2022, 12, 199 2 of 10
in West Africa [7]. Although some cowpea farmers in Burkina Faso adapted recent man-
agement practices, which are not yet common due to limited budgets [8]. In addition, the
prevalence of chronic malnutrition in Burkina Faso remains of concern. According to a
nutritional survey conducted in the country in 2016, the national prevalence of chronic
malnutrition was estimated to be 7.6% for all men, women and children (Ministry of Health
and National Institute of Statistics and Demography (Burkina Faso) [9]). Therefore, there is
an urgent need to establish sustainable agriculture strategies in Burkina Faso, along with
simple and easy-to-apply technologies that allow local farmers to easily implement these
strategies and/or techniques. Such farming strategies for the use of farmers’ preferred
crops are indispensable for Africa’s farming future. The high-protein content in cowpea
can help combat malnutrition; thus, cowpea has an important role in the food security
for millions of people [10], including the most economically depressed communities in
developing countries [11].
International and national cowpea breeding programs have developed and released
various improved varieties for their adoption in cowpea production in Africa. High-
protein cowpea breeding has been conducted in West Africa since 1975 [12]. However,
high-protein cowpea is yet to be released. Grain proteins are considered a polygenic
trait influenced by environmental and management factors [13]. The main genetic loci
identified for protein content and seed size are highly heritable; therefore, these traits can
be improved in cowpea (with other favourable agronomic traits) through breeding [14,15].
In cowpea, the single-seed weight and grain crude protein content range from 0.13 to
0.30 g and 23% to 33%, respectively [16]. Within the same plant, the single-seed weight
and protein content can vary by up to 0.1 g and 10%, respectively [13]. Therefore, breeding
programs that could simultaneously address genotypic averages and reduce intra-plant
heterogeneity are required. However, any analysis of polygenes and their application in the
breeding or control of environmental factors in farmers’ fields is challenging. Conversely,
if management practices, such as topdressing, can influence the grain protein content,
it would indicate that the protein content can be manipulated in the field. Topdressing
(applying additional fertilizer while the crop is growing) when rice, soybean, or wheat
are at grain maturity can increase the seed nitrogen content [17–19]. However, it remains
unknown if this is also the case for cowpea.
Although the potential yield of improved cowpea varieties is between 3000 and
4000 kg/ha, the current average yield in West Africa is only 450 kg/ha [20]. Therefore,
the most persistent challenge to expanding the production and consumption of cowpea
is through closing the ‘yield gap’. For example, cowpea production in northern Nigeria
generally uses wide rows (75 cm apart). This may be because, in Nigeria, the equipment
used for ridging is the same as that used for other grain crops, such as maize, soybean,
sorghum, and millet. Additionally, spacing and density can vary depending on whether
cowpea is the sole crop, or whether it is intercropped. The low density resulting from wide
row spacing usually leads to low yields in grain legume crops such as cowpea in West
Africa [21]. Kamara et al. [22] reported that the plant density is an important component of
the yield of some grain crops (such as cowpea and soybean), and it is important to determine
the optimum plant densities for different areas and varieties as they have different growth
rates. Therefore, adopting the high-density planting method for cowpea can improve the
yield and protein production per unit area.
This study aimed to establish a simple method for increasing the cowpea yield and
protein content of seeds. The effects of the planting density and topdressing timing on cow-
pea yield and the seed nitrogen content were investigated. Our findings could contribute
to the achievement of stable and sustainable food production in West Africa.
2. Materials and Methods
2.1. Plant Material and Growth Conditions
The varieties, IT99K-573-2-1 and IT98K-205-8, were obtained from the International
Institute of Tropical Agriculture (IITA) cowpea breeding unit and KVx442-3-25 was obtained
Agriculture 2022, 12, 199 3 of 10
from the Institut de l’Environnement et de Recherches Agricoles (INERA) cowpea breeding
program. Specimens were grown in an experimental field at the INERA, Saria station,
Koudougou, Burkina Faso (12◦17′ N, 2◦09′ W). The three varieties were grown under
different planting densities and fertilizer treatments, as described in Section 2.2. Each plot
measured 3.2 × 3.2 m and was established in a complete randomised block design with
three replications. Weed control was manually undertaken two times. Insecticides were
sprayed for insect control at approximately 30 days after sowing (‘Pacha 25EC’: 15 g/mL
lambda cyhalothrin and 10 g/L acetamiprid) and at approximately 45 days after sowing
(‘K-Optimal 250 mL’: 15 g/L lambda cyhalothrin, 20 g/L acetamiprid). No symptoms of
water stress or nutrient deficiency were evident during the growing period. Cultivation
was repeated for three years (2018–2020). The annual effective rainfall (June to October)
was 743.7 mm, 632.0 mm, and 914.2 mm for 2018, 2019, and 2020, respectively.
2.2. The Effect of Planting Density and Fertilization on Seed Yield and Weight per Plant
The following four planting density treatments were established: (1) normal density
(ND, 40 cm between plants and 60 cm between rows); (2) high density (HD, 40 cm between
plants and rows); (3) super high density (SHD, 40 cm spacing between plants and 20 cm
spacing between rows); and (4) hyper high density (HHD, 20 cm between plants and
rows). Chemical fertilizer (a ratio of nitrogen, to phosphorous, to potassium = 15:15:15)
was applied four weeks after sowing (4WAS) for the HD and SHD plots (‘HD + fertilizer’,
‘SHD + fertilizer’). The amount of fertilizer applied was indicated in kilograms per hectare.
The sampling and analyses were repeated over three years (2018–2020).
Cowpea seeds were harvested after full maturity, and harvested pods were dried under
sunlight for a week. After threshing, the seed weight was measured. Differences among
treatments were determined using the Dunnett’s test using JMP 5.0.1a (SAS Institute, Cary,
NC, USA). In all analyses, a probability value of less than 0.05 was considered statistically
significant (p < 0.05).
2.3. The Effect of Planting Density at Different Locations
The effect of the planting density on the cowpea yield in farmers’ fields were also
investigated in three different regions of Burkina Faso for two years (2019 to 2020). The
plot sizes and treatments were similar to those in Section 2.1. Three planting density plots
(normal, high density, and super high density) were established, as in Section 2.2. The
regions were characterised by their annual rainfall. The north region was semi-arid with
an annual rainfall of 520 mm, the central region was semi-arid with an annual rainfall of
780 mm, and the south region was sub-humid with an annual rainfall of 1,175 mm. The
fertilizer was added at a rate of 100 kg/ha at 4WAS, except in the control plot.
Around the experimental field, the project staff randomly visited cowpea fields and
measured the planting density. After that, we contacted 25 cowpea farmers in each village.
At the harvest period, a part of cowpea seeds in the farmers’ fields were harvested by
project staff (1 m × 1 m2), and the seed weight was recorded. The cowpea yield of farmers’
fields was estimated using the seed weight. Differences among treatments were determined
using the Dunnett’s test using JMP 5.0.1a (SAS Institute, Cary, NC, USA). In all analyses, a
probability value less than 0.05 was considered statistically significant (p < 0.05).
2.4. The Effect of the Topdressing Timing and Evaluation on the Seed Nitrogen/Protein Content
Plant materials, the experimental location, and the treatments were similar to those
in Sections 2.1 and 2.2. The following seven timings of the fertilization treatments were
established: (1) control (no chemical fertilizer applied); (2) fertilizer applied four weeks
after sowing (4WAS); (3) fertilizer applied eight weeks after sowing (8WAS); (4) fertil-
izer applied immediately after the first flower bloomed in the treatment plot (fertilizer
after first flower bloomed); (5) fertilizer applied when 20% of the flowers had bloomed
(20% flowering); (6) fertilizer applied when 50% of the flowers had bloomed (50% flower-
ing); and (7) fertilizer applied twice, four weeks after sowing, when 50% of the flowers had
Agriculture 2022, 12, x FOR PEER REVIEW  4 of 10 
 
established: (1) control (no chemical fertilizer applied); (2) fertilizer applied four weeks 
after sowing (4WAS); (3) fertilizer applied eight weeks after sowing (8WAS); (4) fertilizer 
applied immediately after the first flower bloomed in the treatment plot (fertilizer after 
first flower bloomed); (5) fertilizer applied when 20% of the flowers had bloomed (20% 
Agriculture 2022, 12, 199 flowering); (6) fertilizer applied when 50% of the flowers had bloomed (50% flower4inofg1)0; 
and (7)  fertilizer applied  twice,  four weeks after sowing, when 50% of  the flowers had 
bloomed (fertilizer 4WAS + 50% flowering). In all treatments, the fertilizer used was read‐
ibllyo oamvaeidla(bfleer tiinli zBeurr4kWinAa SF+as5o0 %(Nfl:Po:wKe =ri n15g:)1.5I:n15a)ll. tArella ttmreeantmts,etnhtes freercteiliivzeedr  u1s0e0d kwga fserretialidzielyr 
pavera ihlaab; lien itnreBautmrkeinnta 7F, ahsaolf( Nof: Pth:Kis =w1a5s: 1a5p:p15li)e.dA altl 4trWeaAtmS eanntds trhecee oivtheedr 1h0a0lfk wgafes ratiplipzleierdp eart 
5h0a%; i nflotwreeartimngen. t 7, half of this was applied at 4WAS and the other half was applied at
50% flCoowepreina gs.eeds were harvested after their full maturity in 2019 and 2020. After thresh‐
ing, tChoew sepeedass eweedrsew oevreenh‐darrvieeds taetd 8a0f te°Cr  thfoeri r2f4u lhl,m aantdu r2i0ty siened20s1 w9 earned r2a0n2d0o.mAlfyte sretlhercetsehd- 
finrogm, t ehaecshe terdeastwmernet.o Evaecnh- dserliecdteadt s8e0ed◦C lofto wr a2s4 shca, nanedd2 w0 istehe ad Fsowuerirer rtaranndsofmorlmy  isneflreacrted 
sfrpoemctreoamchetterera (tFmTe‐InRt. 6E1a0c0h; JsAelSeCctOed Csoerepdolroattiwona,s Tsocaknynoe, dJawpaitnh) aeqFuoiuprpierdt wrainthsf oar rmefliencftranrecde 
uspneitc t(rNomRFe tPeRr O(F4T1-0IR‐N6)1, 0b0r;oJaAdSbCaOndC KoBrpr obreaatmion s,pTloitkteyro (,KJaBpRaBnB) ‐e6q0u0i0pBpSe)d, aw hiathloagerenfl leacmtapn aces 
tuhnei tli(gNhRt sFoPuRrcOe,4 a1n0-dN a)n,  bInroGaadAbsa nddetKecBtorrb. eTahme spelicttrear a(KcqBuRiBsiBti-o6n0 0(400B0S0) ,tao h10a,l0o0g0e ncmla−m1) wp as 
cthoendliughctesdo urscien,ga nthdea dniIfnfuGsaeA rsefdlectetactnocre.  Tmhethspoedc tarta 8a ccqmu−is1 irtieosnol(u4t0i0o0n two 1it0h,0 3020 camcc−u1m) uwlas‐
tciondsu. Tctoe deutseirnmgitnhee tdhief fsueseedr neflitercotgaennc ecomnettehnot,d thate 8caclmib−r1atrieosno lmutoidonelw foitlhlo3w2inacgc tuhme umlaetihoonds. 
Toof Idsheitkearmwain eet tahl.e [2s3e]e wd ansi tarpopgleinedc.o Tnhte nste,etdh percoatleiibnr acotinotnenmt owdaesl cfaollcluolwatiendg bthy emmulettihpolydinogf 
tIhshei nkiatwroagetna clo. n[2t3e]nwt bays tahpep Nlie:Pd .coTnhveesreseiodnp fraoctteoirn (c5o.4n5t)e nant dw eaxspcraelcsusilnagte idt absy am puelrtciepnlytaingge 
othf ednryit rwogeeignhcto, nfotellnotwbiyngth Me Nur:Pancaoknav ert saiol.n [2f4a]c.t or (5.45) and expressing it as a percentage of
dry wTehieg hdti,ffoerlleonwceins gaMmounragn atrkea temt aeln.t[s2 4w].ith different  timings of  fertilizer  applications 
wereT dheetedrimffeinrendc ues ianmg oDnugntnreeatttm’s etnestst iwni JtMh dPi f5f.e0r.e1nat (tSimASin Ignssotiftufetret)i.l izer applications were
determined using Dunnett’s test in JMP 5.0.1a (SAS Institute).
3. Results 
3. Results
3.1. The Effects of Planting Density on Cowpea Cultivation 
3.1. The Effects of Planting Density on Cowpea Cultivation
Figure 1 shows the yield of the three improved cowpea varieties grown under differ‐
ent pFlaingutirneg 1dsehnosiwtiesst hanedy ifeelrdtiloizfetrh terethatrmeeenimtsp. Uronvdeedr cNoDw,p tehae vyaierlidet oief sITg9r9oKw‐n57u3n‐2d‐e1r wdaifs- 
5fe5rent plantwa2s.55 k5g2./5hak.
in
g C
g
/ohm
depnasritia. Coamt
eivs anpealy
d, tfheer tyiliiezledr otrfe IaTtm99eKn‐t5s7.3U‐2n‐d1 eurnNdDer, HthDe  yainedld SoHfDIT 9w9K-5ratively, the yield of IT99K-573-2-1 under HD and aSsH s
73-2
Dignwif
-1
ais‐
csaignntliyfi chaingthleyr h(9ig7h9e.6r k(9g7/h9a.6, akngd/ h10a1, 4a.n4d kg1/0h1a4,. 4rekspg/echtaiv,erleys)p. eAclttihvoeulyg)h.  tAhlet hyoieulgdh utnhdeery iHelDd 
+u nfedretirliHzeDr +anfder StiHlizDe r+ afnerdtiSliHzeDr w+ afesr htiilgizheerr wthaasnh tihgeh eyrietlhda nunthdeery HielDd aunndd eSrHHDD, tahned dSifHfeDr‐,
ethnecedsi fwfeerreen ncoest swtaetriestincoatllyst asitgisntiicfiaclalynts. iTghneifi yciaenldt.s oTfh IeTy98ieKld‐2s0o5f‐8I Tex9h8Kib-i2te0d5- a8 seixmhiilbairt epdata‐
tseimrni.l aInr  paadtdteirtino.nI,n thade dKitVioxn4,4t2h‐3e‐K25V yxi4e4l2d- 3e-x2h5ibyiiteeldd eax shimibiitlaedr paastitmerinla; rhpoawtteevrenr;,h iot wweavs esri,gi‐t
wniafiscasingtnlyifi hciagnhtelyr uhnigdheerr SuHnDde +r fSeHrtDiliz+efre trhtialniz eurntdhearn aunnyd oetrhaern ytreoathtmeretnrte a(Ftmigeunrte (1F)i.g Uunred1er). 
HUnHdDer, tHheH lDea,vthees loefa evaecsho pf leaancth opvlearnltapovpeerdla, ppplaendt, gprloawntthgr wowast hstwunastesdtu, anntedd t,haenrde twhaerse pwooasr 
pdoevoerldopevmeelonpt mofe pnotdosf apnodd sseaendds.s Tehederse. fTorhee,r tehfeo rree,stuhlets rfeosru tlhtse fHorHtDhe trHeaHtmDetnret aatrme ennott arree‐
pnoorttreedp.o  rted.
 
Figure 1. Effect of planting density and fertilizer treatments on cowpea seed production. Bars indicate
standard deviation. Asterisks indicate significant differences using the Dunnett’s test (* = p < 0.05,
** = p < 0.01).
 
The seed weight per plant gradually decreased with the increasing planting density
for all varieties (Table 1). For example, the total seed weight of IT99K-573-2-1 under ND
was 11.23 g/plant, whereas it was 8.38 and 4.44 g/plant under HD and SHD, respectively.
The reduction was approximately 30–40%.
Agriculture 2022, 12, 199 5 of 10
Table 1. Total seed weight per plant with different planting densities.
Variety/Planting Density Total Seed WT No. of Plants per Estimated Seed(g/plant) Plot (3.2 × 3.2 m) WT (kg/ha)
IT99K-573-2-1 normal density 11.23 70 786.0
High density 8.38 144 1206.5 *
Super high density 4.44 266 1180.2 *
KVx442-3-25 normal density 11.24 70 786.9
High density 7.77 144 1118.9 *
Super high density 4.17 266 1108.6 *
IT98K-205-8 normal density 8.07 70 564.9
High density 5.21 144 750.5 *
Super high density 2.78 266 738.8 *
Asterisks indicate significant difference vs. control value through Dunnett’s test p < 0.05.
3.2. The Effects of the Planting Density on Cowpea Yield in Different Agro-Ecological Zones in
Farmers’ Fields
Table 2 shows the locations of the multi-location trials, the annual rainfall, the esti-
mated yield, and the planting density of farmers’ fields. Farmers used a wider planting
density than the experimental fields and the yield range was 400–600 kg/ha. In addition,
the farmers used local varieties and they did not use the improved variety.
Table 2. Location of base station and target village/region, basic productivity, and planting density
at farmers’ fields.
Name of Village North West Annual Estimated Yield PlantingLatitude Longitude Rainfall (mm) (kg/ha) Density (cm)
Tougouri (north) 13◦31′ 0◦52′ 520 594.7 75.7 × 62.0
Pouni (central) 11◦95′ −2◦54′ 780 402.7 64.6 × 52.9
Tiefora (south) 10◦63′ −4◦55′ 1175 415.2 68.9 × 49.1
Saria station ◦ ′ ◦ ′
(INERA, BF) 12 28 −2 15 790 - 60.0 × 40.0
In the northern region, the yield of the three improved varieties varied widely with
ND. KVx442-3-25 had a higher yield than IT99K573-2-1 and IT98K-205-8; the yields were
724.2, 460.4, and 355.5 kg/ha, respectively (Table 3). The HD and SHD treatments resulted
in an increase in yields of 123.3% and 214.5% compared to ND, respectively. Yields with
HD and SHD were 113.8% and 120.2% higher than that of ND in the central region, and
were 61.9% and 77.9% higher than that of ND in the southern region, respectively.
Table 3. Effect of high-, super high-density planting for the yield.
Name of Village Variety andPlanting Density Yield (kg/ha) Standard Error
Rate of Yield
Increase (%)
IT99K573-2-1 ND 460.4 47.5 -
IT99K573-2-1 HD 876.3 * 63.9 90.3
IT99K573-2-1 SHD 1167.6 * 83.6 153.6
KVx442-3-25 ND 724.2 60.8 -
Tougouri (north) KVx442-3-25 HD 1097.1 * 126.7 51.5
KVx442-3-25 SHD 1295.9 * 179.2 78.9
IT98K-205-8 ND 355.5 39.3 -
IT98K-205-8 HD 794.2 * 63.9 123.3
IT98K-205-8 SHD 1118.2 * 95.9 214.5
Agriculture 2022, 12, 199 6 of 10
Table 3. Cont.
Name of Village Variety and Rate of YieldPlanting Density Yield (kg/ha) Standard Error Increase (%)
IT99K573-2-1 ND 1031.6 143.1 -
IT99K573-2-1 HD 1916.4 * 176.6 85.8
IT99K573-2-1 SHD 1889.3 * 204.6 83.1
KVx442-3-25 ND 656.5 197.3 -
Pouni (central) KVx442-3-25 HD 1279.0 * 144.3 94.8
KVx442-3-25 SHD 1154.1 * 153.0 75.8
IT98K-205-8 ND 611.9 39.5 -
IT98K-205-8 HD 1308.2 * 116.3 113.8
IT98K-205-8 SHD 1347.6 * 114.9 120.2
IT99K573-2-1 ND 774.6 127.9 -
IT99K573-2-1 HD 1159.0 * 89.6 49.6
IT99K573-2-1 SHD 1378.1 * 103.8 77.9
KVx442-3-25 ND 1079.3 39.0 -
Tiefora (south) KVx442-3-25 HD 1499.0 * 86.1 38.9
KVx442-3-25 SHD 1375.9 * 110.3 27.5
IT98K-205-8 ND 665.5 127.3 -
IT98K-205-8 HD 1077.7 * 100.3 61.9
IT98K-205-8 SHD 1056.9 * 130.7 58.8
ND: normal density, HD: high density, SHD: super high density. Asterisks indicate significant difference vs.
control (ND) value using Dunnett’s test p < 0.05.
3.3. The Effects of Topdressing Timing on Cowpea Protein Content and Yield
Compared with the control, fertilizer treatments increased the cowpea yield (Table 4).
However, this increase was not statistically significant under 8WAS for both IT99K-573-2-1
and IT98K-205-8. The ‘after first flower bloomed’ treatment resulted in a significantly
higher yield compared with the controls for all varieties. For KVx442-3-25, all fertilizer
treatments significantly increased the yield compared with the control. Conversely, the seed
protein content was only significantly higher than that of the control under the ‘fertilizer
after first flower bloomed’ treatment.
Table 4. Productivity and protein content under different timings of fertilizer application.
IT99K-573-2-1 KVx442-3-25 IT98K-205-8
Fertilizer Treatment Productivity SE Protein Productivity SE Protein Productivitykg/ha Content (%) kg/ha Content (%) kg/ha SE
Protein
Content (%)
Control 552.5 73.2 16.2 644.0 76.3 12.8 488.0 66.5 15.6
Fertilizer 4WAS 1238.7 * 125.1 17.0 1165.9 * 80.1 13.1 835.0 * 68.1 15.6
Fertilizer 4WAS +
50% flowering 1109.1 * 130.4 14.9 1116.3 * 77.2 13.0 505.4 38.7 15.0
Fertilizer after first
flower bloomed 1150.5 * 90.5 18.1 * 1064.2 * 90.6 15.6 * 837.8 * 69.5 17.2 *
Fertilizer 20%
flowering 1180.9 * 94.7 16.7 1113.4 * 63.0 13.4 720.3 89.1 15.3
Fertilizer 50%
flowering 1054.2 * 82.9 16.7 934.0 * 61.8 13.5 826.3 * 83.9 15.2
Fertilizer 8WAS 935.6 114.0 15.4 976.5 * 87.3 13.4 629.0 49.4 15.9
SE indicates the standard error. Asterisks indicate significant difference vs. control value using Dunnett’s test p < 0.05.
3.4. The Effects of Topdressing at the Early Flowering Period on Cowpea Protein Content
Table 5 shows the effects of topdressing after the first flowers bloomed on the cowpea
seed protein content. Under the control treatment, the protein contents of IT99K-573-2-1,
KVx442-3-25, and IT98K-205-8 were 16.84%, 13.08%, and 16.81%, respectively. The protein
content was significantly higher with topdressing treatment, compared to the control,
for all varieties in both years. The protein contents were 20.1% (IT99K-573-2-1), 16.35%
Agriculture 2022, 12, 199 7 of 10
(KVx442-3-25), and 19.82% (IT98K-205-8). The highest increase in protein was 24.4% in
2019 (Table 5). The protein content varied with the variety and the year.
Table 5. Effects of topdressing (additional fertilizer) treatment at flowering period for protein content
(%) in cowpea seeds.
2019 2020
Topdressing Rate of Topdressing Rate of
Variety
Control after First Increasing after First IncreasingFlowers Protein Control Flowers Protein
Bloomed (%) Bloomed (%)
IT99K573-2-1 16.84 20.11 * 19.4 16.19 17.77 * 10.0
KVx442-3-25 13.08 16.35 * 24.4 12.81 14.99 * 17.1
IT98K-205-8 16.81 19.82 * 17.9 15.59 17.11 * 9.1
Asterisks indicate significant differences vs. control value using Dunnett’s test p < 0.05.
4. Discussion
Plants often compete with each other after their emergence; therefore, the planting
density is often adjusted to maximize the yield [25–30]. The planting density is particularly
important for the growth of grain legumes, such as cowpea and soybean [22]. Jallow and
Fergusson [31] and Kwapata and Hall [32] reported that the effects of plant density on the
cowpea seed yield were consistent with those for soybean. Generally, yields do not increase
linearly with an increase in plant density due to interplant competition [22,28,33]. Our
results support these previous findings. Therefore, the cowpea seed yield was limited by
the seed weight per plant. The seed weight per plant is an easy-to-use indicator for the yield.
The seed weight and/or yield per plant can be easily determined through observation.
Therefore, this indicator can be used to evaluate and disseminate results at local institutes
or farmers’ fields.
The cowpea varieties responded differently to planting density in the different areas.
The improved varieties, IT99K573-2-1 and IT98K-205-8, are extra-early maturing varieties.
They are recommended for areas of the Sahel, to semi-arid regions, to avoid the adverse
effects of terminal drought [34]. The extra-early maturing characteristics may have con-
tributed to the higher yield of these varieties under HD and SHD, as their yield was higher
in the north and central regions. In addition, KVx442-3-25 with the SHD treatment recorded
higher yields using the fertilizer application compared to other varieties. These results pro-
vided important information on the suitability of these varieties under different conditions
and their different responses to fertilizer applications. Furthermore, the findings suggest
that the cowpea yield can be easily improved, even in the case of farmers with no access to
fertilizer. The results demonstrate the effectiveness of HD and SHD planting in different
agro-ecological zones in Burkina Faso. Even if the cowpea yields for any individual farmers
do not improve dramatically, the effects of HD and SHD planting could have a large impact
on national or regional production.
The fertilizer application is primarily used to increase the grain yield, but it can also
be used to increase the seed protein content. The grain nitrogen content can be increased
using nitrogen topdressing during the ripening period in rice [17,35,36], soybean [18,37,38],
and wheat [19,39,40]. In cowpea, the seed protein content drastically increases during the
pod elongation period, especially after the full pod elongation [41]. This suggests that the
best time for fertilizer application is before and after the flowering period. As we expected,
the seed protein content was most affected by topdressing during the flowering period.
Topdressing increased the seed protein content for all three cowpea varieties; however,
varietal differences were observed. When farmers purchase and apply fertilizer, they expect
higher yields, and they may not consider the potential increase in grain protein content.
Therefore, fertilizer application could be a strategy to achieve an increased yield and an
increased protein content simultaneously. On the other hand, NPK fertilizer is easy to buy
for small-scale farmers and is always available in Burkina Faso. However, the fertilizer
Agriculture 2022, 12, 199 8 of 10
applied is a compound and is not appropriate for topdressing because the efficiency of
the less soluble nutrients, especially phosphate, is greatly reduced. Topdressing during
the flowering period is a simple and robust method of increasing the cowpea seed protein
content and yield. Moreover, due to the limitation of the research budget, soil analyses
were not performed in this study. The soil fertility and the soil type of the target filed
is also important for cowpea yield and/or fertilizer application [42]. However, the soil
fertility and soil types are distributed in “mosaic” patterns in Burkina Faso and the Sudan
Savanna area [42]. Therefore, it is also important that the selection of genotypes improve
the long-term stable yields across diverse soil types in the country.
Although a high protein content in grains is desirable from a nutritional perspective,
excessive N topdressing may lead to several problems, such as lodging, a reduced yield, a
lower quality, and altered taste, as well as changes in the grain cooking properties [9,40,43].
Furthermore, the optimal rates of N for the quality of the grain proteins across the different
cowpea-growing agroecologies in the region need to be determined. Therefore, it may
be necessary to conduct a sensory evaluation of the taste of high-protein cowpea with
consumers.
5. Conclusions
In Burkina Faso, the utilization of cowpea, which is a familiar and traditional crop, is
important for technology dissemination and sustainable agricultural practices. Moreover,
it is important that the management practices can be easily disseminated and implemented.
In this study, we demonstrated that high-density planting and topdressing during flowering
can increase the yield and grain protein content of cowpea. High-density planting increased
the cowpea yield by 88.9% and topdressing during flowering increased the seed protein
content by 24.4%. Of course, it is necessary to evaluate the soil type, the soil fertility, and the
varieties of cowpea the farmers use in their locations to adapt to the developed techniques
widely. Even so, these simple/easy management practices could improve the income and
nutritional status of populations, via increased protein intake, in developing countries.
Author Contributions: Conceptualization, H.I.; methodology and formal analysis, H.I.; investigation
and field work, H.I. and B.J.B.; writing-original draft preparation, H.I.; writing-review and editing,
H.I., B.J.B., C.F. and O.B.; project administration, H.I., C.F., B.J.B. and O.B.; funding acquisition, H.I.
All authors have read and agreed to the published version of the manuscript.
Funding: This work was conducted as part of an international collaborative research project funded
by the Japanese Ministry of Agriculture, Forestry and Fisheries (MAFF, Japan).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Acknowledgments: We thank Diane Kiendrebeogo, Salo Ferdina, Harouna Simpor, and Hamidou
Sidibe for technical support during field evaluations in Burkina Faso, and Nurudeen Salaudeen for
technical support during seed nitrogen analyses in IITA Nigeria.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Stagnari, F.; Maggio, A.; Galieni, A.; Pisante, M. Multiple benefits of legumes for agriculture sustainability: An overview. Chem.
Biol. Technol. Agric. 2017, 4, 2. [CrossRef]
2. FAOSTAT. Available online: http://www.fao.org/faostat/en (accessed on 1 October 2019).
3. Cruz, F.J.R.; de Almeida, H.J.; dos Santos, D.M.M. Growth, nutritional status and nitrogen metabolism in Vigna unguiculata (L.)
Walp. is affected by aluminum. Aust. J. Crop Sci. 2014, 8, 1132–1139.
4. Inaizumi, H.; Singh, B.B.; Sanginga, P.C.; Manyong, V.M.; Adesina, A.A.; Tarawali, S. Adoption and Impact of Dry-Season
Dual-Purpose Cowpea in the SemiArid Zone of Nigeria; IITA: Ibadan, Nigeria, 1999; p. 14.
5. Singh, B.B.; Ajeigbe, H.A.; Tarawali, S.A.; Fernandez-Rivera, S.; Abubakar, M. Improving the production and utilization of
cowpea as food and fodder. Field Crops Res. 2003, 84, 169–177. [CrossRef]
Agriculture 2022, 12, 199 9 of 10
6. Kamara, A.Y.; Ewansiha, S.U.; Ajeigbe, H.A.; Okechukwu, R.; Tefera, H.; Boukar, O.; Omoigui, L.O. Improvements in Grain
and Fodder Yield of Cowpea (Vigna unguiculata) Varieties Developed in the Sudan Savannas of Nigeria over the Past Four
Decades. In Innovative Research along the Cowpea Value Chain; Boukar, O., Coulibaly, O., Fatokun, C.A., Lopez, K., Tamo, M., Eds.;
International Institute of Tropical Agriculture (IITA): Ibadan, Nigeria, 2012; pp. 179–188.
7. Jones, A.; Breuning-Madsen, H.; Brossard, M.; Dampha, A.; Deckers, J.; Dewitte, O.; Gallali, T.; Hallett, S.; Jones, R.; Kilasara, M.
Soil Atlas of Africa; European Commission: Luxembourg, 2013.
8. Ishikawa, H.; Drabo, I.; Batieno, B.J.; Muranaka, S.; Fatokun, C.; Boukar, O. Characteristics of famers’ selection criteria for cowpea
(Vigna unguiculata) varieties differ between north and south regions of Burkina Faso. Exp. Agric. 2019, 56, 94–103. [CrossRef]
9. Ministry of Health (Burkina Faso); National Institute of Statistics and Demography (Burkina Faso). Burkina Faso National
Nutrition Survey 2016. Available online: http://ghdx.healthdata.org/record/burkina-faso-national-nutrition-survey-2016
(accessed on 1 December 2021).
10. Tarawali, S.A.; Singh, B.B.; Peters, M.; Blade, S.F. Cowpea Haulms as Fodder. In Advances in Cowpea Research; Singh, B.B.,
Mohan Raj, D.R., Dashiell, K.E., Jackai, L.E.N., Eds.; International Institute of Tropical Agriculture (IIA): Ibadan, Nigeria; Japan
International Research Center for Agricultural Sciences (JIRCAS): Tsukuba, Japan, 1997; pp. 313–325.
11. Jayathilake, C.; Visvanathan, R.; Deen, A.; Bangamuwage, R.; Jayawardana, B.C.; Nammi, S.; Liyanage, R. Cowpea: An overview
on its nutritional facts and health benefits. J. Sci. Food Agric. 2018, 98, 4793–4806. [CrossRef] [PubMed]
12. International Institute of Tropical Agriculture (IITA). IITA Annual Report; IITA: Ibadan, Nigeria, 1976.
13. Iseki, K.; Olaleye, O.; Ishikawa, H. Intra-plant variation in seed weight and seed protein content of cowpea. Plant Prod. Sci. 2020,
23, 103–113. [CrossRef]
14. Kelly, J.D.; Gepts, P.; Miklas, P.N.; Coyne, D.P. Tagging and mapping of genes and QTL and molecular marker-assisted selection
for traits of economic importance in bean and cowpea. Field Crops Res. 2003, 82, 135–154. [CrossRef]
15. Lo, S.; Muñoz-Amatriaín, M.; Boukar, O.; Herniter, I.; Cisse, N.; Guo, Y.N.; Roberts, P.A.; Xu, S.; Fatokun, C.; Close, T.J.
Identification of QTL controlling domestication-related traits in cowpea (Vigna unguiculata L. Walp). Sci. Rep. 2017, 8, 6261.
[CrossRef]
16. Ehlers, J.D.; Hall, A.E. Cowpea (Vigna unguiculata L. Walp.). Field Crops Res. 1997, 53, 187–204. [CrossRef]
17. Patrick, R.M.; Hoskins, F.H.; Wilson, E.; Peterson, F.J. Protein and amino acid content of rice as affected by application of nitrogen
fertilizer. Cereal Chem. 1974, 51, 84–95.
18. Jayapaul, P.; Ganesaraja, V. Studies on response of soybean varieties to nitrogen and phosphorus. Indan J. Agron. 1990, 35, 329–330.
19. Gauer, L.E.; Grant, C.A.; Bailey, L.D.; Gehl, D.T. Effects of nitrogen fertilization on grain protein content, nitrogen uptake, and
nitrogen use efficiency of six spring wheat (Triticum aestivum L.) cultivars, in relation to estimated moisture supply. Can. J. Plant
Sci. 1992, 72, 235–241. [CrossRef]
20. Abate, T.; Alene, A.D.; Bergvinson, D.; Shiferaw, B.; Silim, S.; Orr, A.; Asfaw, S. Cowpea. In Tropical Grain Legumes in Africa and
South Asia: Knowledge and Opportunities; ICRISAT: Nairobi, Kenya, 2012; pp. 12–13.
21. Kamara, A.Y.; Ewansiha, S.U.; Boahen, S.; Tofa, A.I. Agronomic response of soybean varieties to plant population in the Guinea
Savannas of Nigeria. Agron. J. 2014, 106, 1051–1059. [CrossRef]
22. Kamara, A.Y.; Tofa, A.I.; Kyei-Boahen, S.; Solomon, R.; Ajeigbe, H.A.; Kamai, N. Effects of plant density on the performance of
cowpea in Nigerian savannas. Exp. Agric. 2018, 54, 120–132. [CrossRef]
23. Ishikawa, H.; Boukar, O.; Fatokun, C.; Shono, M.; Muranaka, S. Development of calibration model to predict nitrogen content in
single seeds of cowpea (Vigna unguiculata) using near infrared spectroscopy. J. Near Infrared Spectrosc. 2017, 25, 211–214. [CrossRef]
24. Muranaka, S.; Shono, M.; Myoda, T.; Takeuchi, J.; Franco, J.; Nakazawa, Y.; Boukar, O.; Takagi, H. Genetic diversity of physical,
nutritional and functional properties of cowpea grain and relationships among the traits. Plant Genet. Resour. 2016, 14, 67–76.
[CrossRef]
25. Firbank, L.G.; Watkinson, A.R. On the analysis of competition within two-species mixtures of plants. J. Appl. Ecol. 1985, 22,
503–517. [CrossRef]
26. Yahuza, I. Yield-density equations and their application for agronomic research: A review. Int. J. Biol. Sci. 2011, 1, 1–17.
27. Holliday, R. Plant population and crop yield: Part II: Yield and plant population in British crops. Field Crop Admin. 1960, 13,
247–254.
28. Willey, R.W.; Heath, S.B. The quantitative relationships between plant population and crop yield. Adv. Agron. 1969, 21, 281–321.
29. Harper, J.L. Population Biology of Plants; Academic Press: London, UK, 1977.
30. Bulson, H.A.J.; Snaydon, R.W.; Stopes, C.E. Effect of plant density on intercropped wheat and field beans in an organic farming
system. J. Agric. Sci. 1997, 128, 59–71. [CrossRef]
31. Jallow, A.T.; Fergusson, T.U. Effects of plant densities and cultivar on seed yield of cowpeas (Vigna unguiculata (L.)) in Trinidad.
Trop. Agric. 1985, 62, 121–124.
32. Kwapata, M.B.; Hall, A.E. Determinants of cowpea (Vigna unguiculata) seed yield at extremely high plant density. Field Crops Res.
1990, 24, 23–32. [CrossRef]
33. Egli, D.B. Plant density and soybean yield. Crop Sci. 1988, 28, 977–981. [CrossRef]
34. Ishikawa, H.; Drabo, I.; Batieno, B.J.; Muranaka, S.; Fatokun, C.; Boukar, O. Comparative analysis of farmers’ selection criteria for
cowpea (Vigna unguiculata) varieties in Niger and Burkina Faso. Jpn. Agric. Res. Q. JARQ 2019, 53, 159–167. [CrossRef]
Agriculture 2022, 12, 199 10 of 10
35. Islam, N.; Kamal, A.M.A.; Islam, M.R. Effect of cultivar and time of N application on grain yield and grain protein content of rice.
Bangladesh Rice J. 1990, 1, 11–16.
36. Souza, S.R.; Stark, E.M.L.M.; Fernandes, M.S. Effects of supplemental-nitrogen on the quality of rice proteins. J. Plant Nutr. 1993,
16, 1739–1751. [CrossRef]
37. Bhangu, R.; Virk, H.K. Nitrogen management in soybean: A review. Agric. Rev. 2019, 40, 129–135. [CrossRef]
38. Gan, Y.; Stulen, I.; Keulen, H.; Kuiper, P.J.C. Effect of N fertilizer top-dressing at various reproductive stages on growth, N2
fixation and yield of three soybean (Glycine max (L.) Merr.) genotypes. Filed Crops Res. 2003, 80, 142–155. [CrossRef]
39. Rawluk, C.D.L.; Racz, G.J.; Grant, C.A. Uptake of foliar or soil application of 15N-labelled urea solution at anthesis and its affect
on wheat grain yield and protein. Can. J. Plant Sci. 2000, 80, 331–334. [CrossRef]
40. Brown, B.; Westcott, M.; Christensen, N.; Pan, B.; Stark, J. Nitrogen Management for Hard Wheat Protein Enhancement; PNW578;
Oregon State University: Corvallis, OR, USA, 2005.
41. Ishikawa, H.; Ikazaki, K.; Iseki, K. Visual observation of cowpea pod elongation to predict nitrogen accumulation in immature
seeds. Plant Prod. Sci. 2021, 24, 224–229. [CrossRef]
42. Iseki, K.; Ikazaki, K.; Batieno, J.B. Cowpea yield variation in three dominant soil types in the Sudan Savanna of West Africa. Filed
Crops Res. 2021, 261, 108012. [CrossRef]
43. Saio, K.; Nikkuni, I.; Ando, Y.; Otsuru, M.; Terauchi, Y.; Kito, M. Soybean quality changes during model storage studies. Cereal
Chem. 1980, 57, 77.