# Plant density and nitrogen fertilization optimization on sorghum grain yield in Mali

Joseph Sékou B Dembele, Boubacar Gano, Mamoutou Kouressy, Léonce Lamine Dembele, Mohamed Doumbia, Komla Kyky Ganyo, Sékouba Sanogo, Adama Togola, Karim Traore, Michel Vaksman, et al.

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## To cite this version:

Joseph Sékou B Dembele, Boubacar Gano, Mamoutou Kouressy, Léonce Lamine Dembele, Mohamed Doumbia, et al. Plant density and nitrogen fertilization optimization on sorghum grain yield in Mali. Agronomy Journal, 2021, 10.1002/agj2.20850. hal-03443070

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# Plant density and nitrogen fertilization optimization on sorghum grain yield in Mali

Joseph Sékou B Dembele<fnref n="1" />,<fnref n="2" />,<fnref n="3" />

Boubacar Gano<fnref n="1" />,<fnref n="3" />

Mamoutou Kouressy<fnref n="2" />

Léonce Lamine Dembele<fnref n="2" />

Mohamed Doumbia<fnref n="2" />

Komla Kyky Ganyo<fnref n="4" />

SékouBa Sanogo<fnref n="2" />

Adama Togola<fnref n="5" />

Karim Traore<fnref n="2" />

Michel Vaksman<fnref n="2" />,<fnref n="6" />,<fnref n="7" />

Niaba Teme<fnref n="2" />

Diaga Diouf<fnref n="3" />

Alain Audebert<fnref n="1" />,<fnref n="6" />,<fnref n="7" />

## Correspondence

Joseph S.B. Dembélé, Centre d'Etude Régional pour l'Amélioration de l'Adaptation à la Sécheresse (CERAAS), Institut Sénégalais de Recherches Agricoles (ISRA), Route de Khombole, BP 3320, Thiès, Sénégal.

Email: josephsekou.dembele@ucad.edu.sn

Assigned to Associate Editor David Clay.

## Abstract

Sorghum [Sorghum bicolor (L.) Moench], a staple food crop in Mali, has low yields for several reasons including that many farmers do not have the financial resources to purchase state-of-the-art genetics and fertilizers and information is not available on how to optimize yields for heirloom variety. To improve their economic and environmental sustainability, Mali farmers need to understand how to invest their limited resources. In many situations this involves the use of open-pollinated varieties that have a range of tillering capabilities. This study determined the best population density and nitrogen (N) fertilization combinations for eight locally available sorghum varieties. The research was conducted in 2018 and 2019 and the experimental design was split-split-plot randomized block, the treatments were two plant densities (26,600 and 53,300 plants ha⁻<fnref n="1" />), three N levels (0 kg ha⁻<fnref n="1" />, 89 kg ha⁻<fnref n="1" />, and 178 kg ha⁻<fnref n="1" />), and eight varieties that had a range of tillering characteristics. Each treatment was replicated three times and six of the varieties were open pollinated. Results showed that each variety had a different yield response to plant density and N rate. For example, the tall guinea (hybrids FADDA and PABLO) and the short durra-caudatum A12-79 cultivar produced higher yields when planted at 53,300 plants ha⁻<fnref n="1" /> and

**Abbreviations:** D1, 26,600 plants ha⁻<fnref n="1" />; D2, 53,300 plants ha⁻<fnref n="1" />; MRR, marginal rate of return; N0, nitrogen rate of 0 kg ha⁻<fnref n="1" />; N1, nitrogen rate of 89 kg ha⁻<fnref n="1" />; N2, nitrogen rate of 178 kg ha⁻<fnref n="1" />.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

© 2021 The Authors. *Agronomy Journal* published by Wiley Periodicals LLC on behalf of American Society of Agronomy

<footnote n="1">¹ Centre d'Etude Régional pour l'Amélioration de l'Adaptation à la Sécheresse (CERAAS), Institut Sénégalais de Recherches Agricoles (ISRA), Route de Khombole, Thiès BP 3320, Sénégal</footnote>

<footnote n="2">² Institut d'Economie Rurale (IER), LABOSEP de Sotuba, Bamako BP 262, Mali</footnote>

<footnote n="3">³ Laboratoire Campus de Biotechnologies Végétales, Dép. de Biologie Végétale, Faculté des Sciences et Techniques, Univ. Cheikh Anta Diop (UCAD), BP 5005, Code postal 10700, Dakar-Fann, Dakar, Sénégal</footnote>

<footnote n="4">⁴ Institut Togolais de Recherche Agronomique (ITRA), Lomé BP 1163, Togo</footnote>

<footnote n="5">⁵ Institut Polytechnique Rural de Formation et de Recherche Appliquée de Katibougou IPR/IFRA - BP 06, Koulikoro, Mali</footnote>

<footnote n="6">⁶ CIRAD, UMR AGAP Institut, Montpellier F-34398, France</footnote>

<footnote n="7">⁷ UMR AGAP Institut, Univ Montpellier, CIRAD, INRAE, Institut Agro, Montpellier F-34398, France</footnote>
fertilized with 178 kg N ha⁻¹ N2D2 treatment. For the short *guinea-caudatum*
improved varieties C2_007-03 and C2_075-15 and *caudatum* GRINKAN and
SOUMBA varieties the optimum seeding rate was 53,300 plants ha⁻¹ that were fertil-
ized with 89 kg N ha⁻¹ (N1D2). For the tall local *guinea* TIEBILE variety the highest
yields were observed when it was seeded at a rate of 26,600 plants ha⁻¹ and fertil-
ized with 89 kg N ha⁻¹. Grain yield increase was associated with yield components
and growth traits for eight varieties studied. Panicle numbers per square meter and
chlorophyll index were associated with grain yield in *guinea* hybrid. Grain number
per panicle and 1,000 grain weight were involved in increasing grain yield in *cau-
datum* varieties. Nitrogen rate and planting density combination in terms of grain
yield varied with different sorghum [*Sorghum bicolor* (L.) Moench] varieties stud-
ied. Knowledge of optimum combinations will help producers decide which option
to promote to booster sorghum production in Mali.

# 1 | INTRODUCTION

Sorghum [Sorghum bicolor (L.) Moench] is a major staple
cereal crop in the Sahelian and savannah zones of West Africa,
where it constitutes the main food of millions of rural popu-
lations (Belton & Taylor, 2004; Kante et al., 2017). In Mali,
the amount of cultivated grain sorghum increased 51% from
1990 to 2020 (FAOSTAT, 2020). This increase is attributed
to the increased cultivation of marginal soils (Gigou et al.,
2004; Kante et al., 2019). Grain yields are very low and aver-
age 930 kg ha⁻¹ (FAOSTAT, 2020). The low yields are the
result of low use of improved varieties (13–33% nationwide),
highly variable rainfall, and the low use of chemical inputs
(Ibrahim et al., 2014; Macauley & Ramadjita, 2015). For rural
communities there is a critical need to increase grain yields to
meet the growing demand (Iqbal et al., 2012).

Increasing sorghum yields requires a combination of fac-
tors including the adoption of improved hybrids and matching
the plant population and nitrogen (N) rate to the climate (Sal-
lah et al., 2007). Information on how to optimize the cultivar
selection, seeding rate, and N rate in semi and arid regions is
lacking (Fernandez et al., 2012; Sallah et al., 2009). Exten-
sive research on the relationship between seeding rates and
N rates have been conducted in humid climate worldwide.
However, this research cannot be directly applied to African
steppe because complex interactions between cultivar, seed-
ing rate, soil, and N influence yields. This problem is further
confounded by producers that are growing crops on marginal
soils are often limited by access to commercial fertilizers and
current varieties. For example, many of the locally available
varieties are open pollinated and have a range of tillering capa-
bilities. Seeding rate recommendations will be very different
in varieties with high and low tillering capacity. In addition,
due to financial and cultural habits, farmers are hesitant to dis-

continue historical farming techniques. This paper provides
information needed to improve the food and economic secu-
rity of Mali farms.

Numerous studies have demonstrated the importance of plant density and N fertilization on sorghum grain yield (Arunakumari & Rekha, 2016). Previous studies reported that optimum plant density depends on each crop (Biswas et al., 2014), beyond which the competition between plants for light, water, and nutrients becomes important and can lead to decreased crop yields (Li et al., 2016). Nitrogen is also one of the most important nutrients which must be used in an optimal quantity depending on plant density as its lack or excess can reduce crop productions (Ferraris & Charles, 1986; Sher et al., 2016).

During the last decades, to maintain soil fertility, Malian
producers relied on organic manure, low quantities of mineral
fertilizers, long-term fallows, and low seeding rate (10,000
plants ha⁻¹) of a local open-pollinated sorghum cultivar
(Coulibaly et al., 2019; Dumont, 1966) to produce sorghum
(Ibrahim et al., 2014; Macauley & Ramadjita, 2015). The cur-
rent recommendation of sorghum grown in Mali is a seeding
rate of 26,600 plants ha⁻¹ of an open-pollinated variety and
the application of 89 kg ha⁻¹ of N (Chantereau et al., 2013;
Kouyate & Wendt, 1991). What is unknown is will recom-
mendations designed for local open-pollinated sorghum cul-
tivars be appropriate for modern hybrid and improved open-
pollinated sorghum varieties. To our knowledge, this is the
first study that highlights optimum combinations of plant den-
sities and N rates on different sorghum varieties responses
on grain yields in the Sudan–Sahelian area of Mali for grain
yield increase. Therefore, it is relevant to define an opti-
mum combination of plant density, N, and sorghum varieties
(hydrids, opened-pollinated improved and local varieties) on
grain yields grown in different Mali production zones. The
objectives of this study were to: (a) determine the optimum combination of plant density and N fertilization responses of eight sorghum varieties grain yield and (b) identify the grain yield contributing parameters from each variety under the two combinations.

## 2 | MATERIALS AND METHODS

### 2.1 | Site characteristics

Two consecutive experiments were conducted during 2018 and 2019 rainy seasons at the Sotuba Agronomic Research Station (12°39' N, 07°56' W) of the Malian Institute of Rural Economy (IER). The Sotuba site (Bamako) was located in the Sahelo–Sudanian zone with a rainy season between May and October, characterized by poorly distributed rainfall, an average of 866 mm on the period 1981–2010 (Coulibaly et al., 2019). The total annual rainfall of 840 and 1,158 mm were recorded in 2018 and 2019, respectively (Figure 1). Mean monthly temperatures during the growing season ranged between 27 and 28 °C in 2018 and 2019 (Figure 1). The soil of the site was a sandy-loam soil characterized by low levels of soil organic carbon (C), total N, and exchangeable potassium (K), are presented in Table 1. Soil particle size analysis was done by the hydrometer method (Anderson & Ingram, 1993) and pH was determined by the electrometric method in a soil solution with a soil/water ratio of 1/2.5 (Jackson, 1967). Soil organic C was determined using the modified Walkley–Black wet oxidation method (Nelson & Sommers, 1982) and total N was determined by the Kjeldahl digestion method (Bremner &, Mulvaney, 1982). Soil available P was determined by the Bray II method and K was extracted with a neutral ammonium acetate solution and was determined by flame photometry (Benton & Jones, 2001). Experimental soil samples were collected before the trial installation at 0-to-30-cm depth and analyzed by the Soil-Water-Plant Laboratory of the IER in Sotuba. The soils are typical upland soils used for sorghum

### Core Ideas

* Hybrid and durra varieties produced better yield under high N rate and plant density.

* *Caudatum* varieties produced better grain yield with N and plant density moderate use.

* Local *guinea* variety produced better grain yield with moderate use of N and low plant density.

* Panicle number and SPAD value were traits explaining grain yield increase of hybrid varieties.

* Grain number and grain weight were traits explaining grain yield increase of *caudatum* varieties.

production in the Sahelo–Sudanian zone of West Africa (Buah & Mwinkaara, 2009).

### 2.2 | Plant materials

Plant material was composed of eight contrasted varieties representing the diversity of cultivated sorghums in Mali. It includes two tall hybrids (FADDA and PABLO), five short open-pollinated improved varieties (GRINKAN, SOUMBA, C2_007-03, C2_075-15, A12-79) and the tall local variety TIEBILE (open-pollinated) (Table 2). These varieties are widely cultivated by the farmers in West Africa countries due to their adaptability and important agronomic characteristics (CEDEAO-UEMOA-CILSS, 2016; Gano et al., 2021).

### 2.3 | Experimental design and crop management

A split-split-plot randomized block was the experimental design. Each block was replicated three times and the treatments were two seeding densities as the main factor,

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>FIGURE 1 Rainfall and temperature mean recorded in (a) 2018 and (b) 2019 rainy seasons at Sotuba in Mali</figcaption></figure>
<table><caption>TABLE 1 Soil properties of the experimental site in Sotuba at soil depth of 0–30 cm in 2018 and 2019 rainy seasons at Sotuba in Mali</caption><thead><tr><th>Property</th><th>Soil depth<br>cm</th><th>Experiment (Sotuba)</th></tr></thead><tbody><tr><td>Sand, %</td><td>0–30</td><td>75.83</td></tr><tr><td>Silt, %</td><td>0–30</td><td>18.98</td></tr><tr><td>Clay, %</td><td>0–30</td><td>5.19</td></tr><tr><td>pH (water)</td><td>0–30</td><td>5.75</td></tr><tr><td>Soil organic C, g kg<sup>-1</sup></td><td>0–30</td><td>0.47</td></tr><tr><td>Total N, g kg<sup>-1</sup></td><td>0–30</td><td>0.12</td></tr><tr><td>Available P, mg kg<sup>-1</sup></td><td>0–30</td><td>15.80</td></tr><tr><td>Exchangeable K, mg kg<sup>-1</sup></td><td>0–30</td><td>20.00</td></tr></tbody></table>

<table><caption>TABLE 2 Characteristics of eight sorghum varieties and hybrids used during 2018 and 2019 rainy seasons at Sotuba, in Mali</caption><thead><tr><th>Variety</th><th>Origin</th><th>Race</th><th>Plant height<br>cm</th><th>Panicle type</th><th>Grain color</th><th>Tillering</th><th>Grain yield<br>Mg ha<sup>-1</sup></th><th>Photoperiod<br>sensitivity</th></tr></thead><tbody><tr><td>FADDA</td><td>Mali</td><td>Guinea (hybrid)</td><td>300</td><td>loose</td><td>light brown</td><td>high</td><td>4</td><td>slightly sensitive</td></tr><tr><td>PABLO</td><td>Mali</td><td>Guinea (hybrid)</td><td>400</td><td>loose</td><td>white-yellowish</td><td>medium</td><td>4</td><td>sensitive</td></tr><tr><td>GRINKAN</td><td>Mali</td><td>Caudatum (OP)</td><td>200</td><td>semi-compact</td><td>white</td><td>low</td><td>2.8</td><td>slightly sensitive</td></tr><tr><td>SOUMBA</td><td>Mali</td><td>Caudatum (OP)</td><td>240</td><td>semi-compact</td><td>yellowish</td><td>low</td><td>2.8</td><td>slightly sensitive</td></tr><tr><td>TIEBILE</td><td>Mali</td><td>Guinea local (OP)</td><td>360</td><td>loose</td><td>white</td><td>high</td><td>2.5</td><td>sensitive</td></tr><tr><td>C2_007-03</td><td>Mali</td><td>Guinea-Caudatum (OP)</td><td>210</td><td>loose</td><td>white</td><td>low</td><td>3.3</td><td>slightly sensitive</td></tr><tr><td>C2_075-15</td><td>Mali</td><td>Guinea-Caudatum (OP)</td><td>170</td><td>loose</td><td>white</td><td>high</td><td>3.3</td><td>slightly sensitive</td></tr><tr><td>A12-79</td><td>Mali</td><td>Durra-Caudatum (OP)</td><td>175</td><td>semi-compact</td><td>white</td><td>low</td><td>4</td><td>slightly sensitive</td></tr></tbody></table>

Note. OP, open-pollinated.

three N levels as the subplot, and eight varieties as the sub-subplot. Plant densities were 26,600 (D1) and 53,300 plants ha⁻¹ (D2) and N levels were 0 (N0), 89 (N1), and 178 kg ha⁻¹ (N2).

The seeding distances were 0.75 m between rows and either 0.25 (high) or 0.50 m (low) between the plants that were seeded into hills. Experimental unit area was 18 m² with six rows length of 4.5 m long and 4 m wide each. The experimental soil was plowed to approximately 30-cm depth. Seedings were done on 18 June 2018 and 5 July 2019 after a rainfall of about 30 mm at a rate of five to six seeds per hill (i.e., 8–10 kg of seed per hectare). Thinning was performed 15 d after sowing to one plant per hill. Nitrogen was applied in the form of urea (46% N) in two fractions, 3 wk after thinning (50%) and 50% before panicle initiation. Basal application of phosphorus (P) (46 kg ha⁻¹ P₂O₅) was homogeneously made in all plots before sowing as “phosphate naturel de Tilemsi” (PNT) granulated (31% P₂O₅). Two manual weeding were carried out in each experiment and crop ridging was performed after the second N application. Dikes were realized between each N level and between replications to reduce N exchange and water runoff from plot to plot. All plots were treated with EMACOT 050 WG insecticide (Emamectine benzoate 50 g

kg⁻¹) to control legionary caterpillar (*Spodoptera exempta*) attacks.

## 2.4 | Sampling and measurement

Contiguous plants in the middle of each experimental plot were sampled for measurements towards the end of flowering. Leaf area index was measured on a plot (1 m²) according to Wilhelm et al. (2000) using Sunscan Septometer (DeltaT Device Ltd) equipped with an external BF5 sensor. SPAD value (chlorophyll index) measurements were done using the portable SPAD-502 (SPAD 502-Minolta) device on the main stem of the third ligulated leaf from top to bottom on three plants taken in each elementary plot. Plant height was measured with a graduated ruler at physiological maturity on the same plants used for the chlorophyll estimation. Grain, straw, and yields components were recorded on an area of 2.25 m². They were taken on six hills for low density and on 12 hills for high density. Panicle number per square meter was counted manually before the harvest. After harvest, panicles and aboveground biomass were sun dried for 4 wk. The total biomass (dry biomass and panicles) obtained in each
elementary plot was extrapolated in kg ha⁻¹. After threshing panicles, grains were winnowed, weighed, and then extrap-olated to kg ha⁻¹. Thousand (1,000) grain weights were obtained by counting grains with an electric counter (NUMI-GRAL, Chopin technologies). Number of grains per panicle was based on an average panicle grain weight to its 1,000 grain weight.

## 2.5 | Statistical analysis

The combined analysis was performed across environments (Venables & Smith et al., 2019) according to the model developed by Carmer et al. (1989). The “Shapiro–Wilk” normality and “Bartlett” residue homogeneity tests were performed to identify and exclude aberrant data for each measured variable. Least Significant Difference (LSD) at the 5% threshold of probability was used to discriminate factors studied (plant density, N, and variety) and the interactions between plant density and N fertilization for each trait. Correlations and linear regression analyses were performed in R software, version 3.6.2.

The partial budget analysis was calculated according to the method proposed by Buah and Mwinkaara (2009) by taking into account cost benefit and marginal rate of returns (MRR %). To estimate the economic parameters, the average price of a kilogram of sorghum grain was evaluated on information basis collected from local prevailing markets in Bamako during 2018 and 2019. Variables cost included N and seed (hydrids and open-pollinated) and labor N application. All cost and benefits were calculated per hectare in US dollars (US$). The following concepts used in the partial budget analysis are defined as follows:

- Average grain yield (kg ha⁻¹) for each treatment excluding treatment N0D2 (0 kg N ha⁻¹ and 53,333 plants ha⁻¹).

- Gross benefit per hectare is the product of the sorghum price on the local market and grain yield per treatment.

- Variable costs (VC) is the sum of fertilizer costs, cost of fertilizer (N) application and seed costs.

- Net benefit (NB) for each treatment is the difference between the gross benefit and the variable costs.

- Marginal rate of returns (MRR in percent) is the ratio of the difference between the net benefit of treatments to the difference between the variable costs of the same treatments. It is calculated according to the following expression:

MRR (Between level 0 and n)

$$ = \frac{\text{Change in net benefit (NBn - NB0)}}{\text{Change in variable costs (VCn - VC0)}} \times 100 $$

where 0 represents without nitrogen (0 kg ha⁻¹) application at low plant density and n represents nitrogen levels N1 (89 kg ha⁻¹) and N2 (178 kg ha⁻¹) at low and high plant density. NB and VC represent respectively net benefit and variable costs calculated at each N level and plant density combinations.

# 3 | RESULTS

## 3.1 | Grain yield and yield components

The analysis of variance showed significant effects of the density, N rate and variety on grain yield, panicle number and weight, 1,000 grain weight (except N), and grain number per panicle. For grain yield, there were significant interactions between N x variety (P < .001) and between density x N x variety (P < .01), while density x variety was not significant. For panicle number, there were also significant interactions between density x variety, N x variety, and for panicle number and weight there were significant interactions between N and varieties. For panicle weight there was also a significant interaction between density and variety (Table 3).

In terms of grain yield, different varieties responded differently to N rate and planting density. Under different plant density and N fertilizer combinations, the tall *guinea* hybrid varieties FADDA (6,611 kg ha⁻¹), PABLO (4,850 kg ha⁻¹), and short *durra-caudatum* variety A12-79 (5,342 kg ha⁻¹) produced a higher grain yield in N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹) treatment when compared to other treatments (Table 4). However, the short *guinea-caudatum* varieties C2_007-03 and C2_075-15 produced 4,498 kg ha⁻¹ and 4,188 kg ha⁻¹ under N1D2 treatment. There was no significant difference between N2D2 and N1D2 treatments (Table 4). The short *caudatum* varieties GRINKAN (4,056 kg ha⁻¹) and SOUMBA (3,884 kg ha⁻¹) recorded the best grain yields in N1D2. The tall local variety TIEBILE of *guinea* race produced a statistically identical grain yield in N2D1 (178 kg N ha⁻¹ and 26,600 plants ha⁻¹), N1D2, N2D2, and N1D1 (89 kg N ha⁻¹ and 26,600 plants ha⁻¹) with an average of 3,634 kg ha⁻¹. All varieties produced lowest grain yields in N0D1 (0 kg N ha⁻¹ and 26,600 plants ha⁻¹) compared to the N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹), N1D2 (89 kg N ha⁻¹ and 53,300 plants ha⁻¹), N2D1 (178 kg N ha⁻¹ and 26,600 plants ha⁻¹) and N1D1 (89 kg N ha⁻¹ and 26,600 plants ha⁻¹) treatments (Table 4).

Panicle number per square meter produced by A12-79, C2_007-03, C2_075-15, GRINKAN, SOUMBA, and TIEBILE was identical in the N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹), N1D2 (89 kg N ha⁻¹ and 53,300 plants ha⁻¹), and N0D2 (0 kg N ha⁻¹ and 26,600 plants ha⁻¹) treatments. The panicle number per square meter increased significantly with increasing plant densities and N fertilizer rates and the N2D2 and N2D1 treatments produced the highest values
<table><caption>TABLE 3 Analysis of variance of plant density, N fertilization, variety and their interaction effects on grain yield and its components in 2018 and 2019 at Sotuba in Mali</caption><thead><tr><th>Source of variation</th><th>Grain yield</th><th>Panicle no. per m²</th><th>Panicle weight per m²</th><th>1,000-Grain weight</th><th>Grain no. per panicle</th></tr></thead><tbody><tr><td>Density (D)</td><td>***</td><td>***</td><td>***</td><td>*</td><td>***</td></tr><tr><td>Nitrogen (N)</td><td>***</td><td>*</td><td>***</td><td>ns</td><td>*</td></tr><tr><td>Variety (V)</td><td>***</td><td>***</td><td>***</td><td>***</td><td>***</td></tr><tr><td>D × V</td><td>ns</td><td>**</td><td>*</td><td>ns</td><td>ns</td></tr><tr><td>N × V</td><td>***</td><td>**</td><td>**</td><td>ns</td><td>ns</td></tr><tr><td>D × N × V</td><td>**</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr></tbody></table>

Note. ns, nonsignificant at the .05 probability level.

*Significant at the .05 probability level.

**Significant at the .01 probability level.

***Significant at the .001 probability level.

for the FADDA and PABLO varieties, respectively (Table 4). Panicle weight per square meter for all varieties increased with increasing plant densities and N rate. The C2_007-03, C2_075-15, GRINKAN, TIEBILE, and SOUMBA varieties produced highest panicle weight per square meter in N1D2 and N2D2 treatments. For A12-79, PABLO, and FADDA varieties, the highest numeric panicle weight was produced in the N2D2 treatment (but was statistically identical to the N2D1 treatment for FADDA). C2_007-03, C2_075-15, FADDA, GRINKAN, and PABLO varieties produced higher 1,000 grain weights in N1D1 and N2D1 treatments. However, 1,000 grain weight for A12-79 and TIEBILE increased in N1D1 treatment and for SOUMBA variety, it was important in N1D2 treatment. Grains number per panicle increased with increasing plant densities. In each plant density, the grains number per panicle was statistically identical in N1 and N2 levels but it increased from N0 to N1 levels. Grains number per panicle varied among varieties under different treatments. A12-79, C2_007-03, GRINKAN, and SOUMBA varieties produced higher grains per panicle in N2D1 and N1D1 treatments and C2_075-15 in N2D1 treatment. No significant difference was observed among N2D1, N1D1, and N1D2 treatments for grain number per panicle in GRINKAN. In addition, FADDA produced higher grains per panicle in N1D1 and PABLO and TIEBILE varieties performed in the N1D1 and N1D2 treatments. Grain number per panicle obtained by TIEBILE in both treatments (N1D1 and N1D2) was statistically identical to that of the N2D2 treatment (Table 4).

## 3.2 | Growth and development parameters

Analysis of variance showed significant effects of the density, N, and variety (P < .05) on SPAD values, leaf area index, straw yield, and plant height. Interactions effects between density x variety and N x variety were significant for plant height. The N and variety interactive effect was significant on SPAD

values, straw yield, and plant height as was between density x N x variety on straw yield (Table 5).

For chlorophyll index estimation (SPAD values), results showed an increase in the SPAD values with increasing N fertilizer and decreased with increasing plant density (Table 6). C2_007-03, FADDA, and PABLO varieties accumulated maximum chlorophyll in N1D1 (89 kg N ha⁻¹) and N2D1 (178 kg N ha⁻¹) treatments at 26,600 plants ha⁻¹. A12-79 and GRINKAN varieties recorded more chlorophyll index in N2D1 and N2D2 (178 kg N ha⁻¹) at 26,600 ha⁻¹ and 53,300 plants ha⁻¹. The best SPAD values were obtained in N1D1 and N2D2 with C2_075-15, in N2D2, N1D1, and N2D1 with SOUMBA and in N2D1 with TIEBILE. For all varieties, the leaf area index and straw yield increased significantly with increasing plant densities and N fertilizer rates, with N2D2 treatment demonstrating the highest values. The leaf area index produced by SOUMBA and GRINKAN in N2D2 and N1D2 (89 kg N ha⁻¹ and 53,300 plants ha⁻¹) treatments, and straw yield produced by C2_007-03 in N2D2 and N1D2 treatments, were statistically similar. Plant height increased with N2D2 treatment for A12-79, C2_007-03, C2_075-15, and TIEBILE varieties. The PABLO variety was arithmetically higher in N2D2, N2D1, and N1D2 treatments (but was statistically identical). However, FADDA and SOUMBA varieties were taller in N2D2 and N1D2 treatments. In addition to, GRINKAN variety was taller under N2D2 and N2D1 (Table 6).

## 3.3 | Economic analysis

Partial budget analysis was achieved on data from the 2018 and 2019 rainy seasons. Results showed that the net benefits and marginal rate of return (MRR, percentage) for each variety varied between treatments (combination of plant density and N fertilization) except for the N0D2 treatment (0 kg N ha⁻¹ and 53,300 plants ha⁻¹) which was not included in this
<table><caption>TABLE 4 Grain yield and yield components of eight sorghum varieties under plant density and N fertilization in 2018 and 2019 at Sotuba in Mali</caption><thead><tr><th>Variety</th><th>Treatments</th><th>Grain yield<br>kg ha<sup>-1</sup></th><th>Panicle no. per m<sup>2</sup></th><th>Panicle weight per<br>m<sup>2</sup></th><th>1,000 Grain<br>weight</th><th>Grain no. per<br>panicle</th></tr></thead><tbody><tr><td rowspan="7">A12-79</td><td>N0D1<sup>a,b</sup></td><td>1,852d<sup>c</sup></td><td>2.67b</td><td>267d</td><td>20.2ab</td><td>2,948cd</td></tr><tr><td>N1D1<sup>d</sup></td><td>3,138c</td><td>2.82b</td><td>471c</td><td>22.4a</td><td>4,365ab</td></tr><tr><td>N2D1<sup>e</sup></td><td>3,127c</td><td>2.89b</td><td>478c</td><td>21.8ab</td><td>4,720a</td></tr><tr><td>N0D2<sup>f</sup></td><td>2,896cd</td><td>5.33a</td><td>464c</td><td>20.1b</td><td>2,531d</td></tr><tr><td>N1D2</td><td>4,004b</td><td>5.55a</td><td>570b</td><td>20.8ab</td><td>3,545c</td></tr><tr><td>N2D2</td><td>5,342a</td><td>6.81a</td><td>715a</td><td>21.3ab</td><td>3,216cd</td></tr><tr><td>Avg</td><td>3,393BC</td><td>4.34C</td><td>494B</td><td>21.1C</td><td>3,554A</td></tr><tr><td rowspan="7">C2_007-03</td><td>N0D1</td><td>2,520c</td><td>3.11b</td><td>324c</td><td>21.1ab</td><td>2,678bc</td></tr><tr><td>N1D1</td><td>3,237b</td><td>3.41b</td><td>415b</td><td>23.2a</td><td>4,020a</td></tr><tr><td>N2D1</td><td>3,390b</td><td>3.85b</td><td>444b</td><td>23.1a</td><td>4,007a</td></tr><tr><td>N0D2</td><td>3,486b</td><td>5.55a</td><td>428b</td><td>19.9b</td><td>2,418c</td></tr><tr><td>N1D2</td><td>4,498a</td><td>5.92a</td><td>541a</td><td>21.1ab</td><td>3,168b</td></tr><tr><td>N2D2</td><td>4,577a</td><td>6.22a</td><td>548a</td><td>21.4ab</td><td>3,103b</td></tr><tr><td>Avg</td><td>3,618B</td><td>4.67C</td><td>450BC</td><td>21.6C</td><td>3,232AB</td></tr><tr><td rowspan="7">C2_075-15</td><td>N0D1</td><td>2,574c</td><td>3.59c</td><td>420d</td><td>19.6ab</td><td>2,810c</td></tr><tr><td>N1D1</td><td>3,264b</td><td>4.64bc</td><td>513b</td><td>20.3a</td><td>3,319b</td></tr><tr><td>N2D1</td><td>3,231b</td><td>4.52bc</td><td>470c</td><td>20.3a</td><td>3,824a</td></tr><tr><td>N0D2</td><td>2,274cd</td><td>5.89ab</td><td>393e</td><td>16.9b</td><td>2,323d</td></tr><tr><td>N1D2</td><td>4,188a</td><td>6.15ab</td><td>585ab</td><td>19.4ab</td><td>3,497b</td></tr><tr><td>N2D2</td><td>4,294a</td><td>6.66a</td><td>609a</td><td>18.0ab</td><td>3,391b</td></tr><tr><td>Avg</td><td>3,304BC</td><td>5.24B</td><td>498B</td><td>19.1D</td><td>3,194AB</td></tr><tr><td rowspan="7">FADDA</td><td>N0D1</td><td>2,139e</td><td>4.00c</td><td>348d</td><td>21.5ab</td><td>2,335d</td></tr><tr><td>N1D1</td><td>4,308c</td><td>4.74c</td><td>604bc</td><td>22.9a</td><td>4,308a</td></tr><tr><td>N2D1</td><td>5,948b</td><td>7.96ab</td><td>830ab</td><td>22.7a</td><td>3,492b</td></tr><tr><td>N0D2</td><td>3,154d</td><td>6.15b</td><td>585c</td><td>19.8b</td><td>2,901c</td></tr><tr><td>N1D2</td><td>4,883c</td><td>6.50b</td><td>711b</td><td>20.5ab</td><td>3,682b</td></tr><tr><td>N2D2</td><td>6,611a</td><td>8.03a</td><td>878a</td><td>21.6ab</td><td>3,133bc</td></tr><tr><td>Avg</td><td>4,507A</td><td>6.23A</td><td>659A</td><td>21.5C</td><td>3,309AB</td></tr><tr><td rowspan="7">GRINKAN</td><td>N0D1</td><td>2,957cd</td><td>2.95bc</td><td>420bc</td><td>22.1ab</td><td>3,063b</td></tr><tr><td>N1D1</td><td>3,109c</td><td>3.52b</td><td>448bc</td><td>23.4a</td><td>3,337ab</td></tr><tr><td>N2D1</td><td>3,469bc</td><td>3.86b</td><td>476ab</td><td>23.1a</td><td>3,512a</td></tr><tr><td>N0D2</td><td>2,865cd</td><td>5.33a</td><td>381c</td><td>19.3b</td><td>2,147c</td></tr><tr><td>N1D2</td><td>4,056a</td><td>5.78a</td><td>531a</td><td>22.3ab</td><td>3,295ab</td></tr><tr><td>N2D2</td><td>3,757b</td><td>5.93a</td><td>533a</td><td>22.3ab</td><td>3,150b</td></tr><tr><td>Avg</td><td>3,369BC</td><td>4.56C</td><td>465BC</td><td>22.1BC</td><td>3,084B</td></tr><tr><td rowspan="7">PABLO</td><td>N0D1</td><td>2,041d</td><td>3.16d</td><td>341d</td><td>24.6ab</td><td>2,017c</td></tr><tr><td>N1D1</td><td>3,242c</td><td>4.67c</td><td>491c</td><td>25.7a</td><td>3,386a</td></tr><tr><td>N2D1</td><td>3,941b</td><td>5.55b</td><td>526b</td><td>25.8a</td><td>3,136b</td></tr><tr><td>N0D2</td><td>3,001c</td><td>5.96b</td><td>476c</td><td>22.3b</td><td>2,159c</td></tr><tr><td>N1D2</td><td>3,770b</td><td>6.55a</td><td>539b</td><td>24.8ab</td><td>3,249ab</td></tr><tr><td>N2D2</td><td>4,850a</td><td>6.42a</td><td>715a</td><td>24.0ab</td><td>3,148b</td></tr><tr><td>Avg</td><td>3,474BC</td><td>5.38B</td><td>515B</td><td>24.5A</td><td>2,849C</td></tr></tbody></table>

(Continues)
<table><caption>TABLE 4 (Continued)</caption><thead><tr><th>Variety</th><th>Treatments</th><th>Grain yield</th><th>Panicle no. per m²</th><th>Panicle weight per m²</th><th>1,000 Grain weight</th><th>Grain no. per panicle</th></tr></thead><tbody><tr><td rowspan="7">SOUMBA</td><td>N0D1</td><td>2,310d</td><td>2.67c</td><td>302d</td><td>21.1b</td><td>2,497bc</td></tr><tr><td>N1D1</td><td>2,758c</td><td>3.45b</td><td>419b</td><td>22.3ab</td><td>3,307a</td></tr><tr><td>N2D1</td><td>3,116c</td><td>4.13b</td><td>424ab</td><td>22.1ab</td><td>3,136ab</td></tr><tr><td>N0D2</td><td>2,778c</td><td>5.52ab</td><td>354c</td><td>21.0b</td><td>2,150c</td></tr><tr><td>N1D2</td><td>3,884a</td><td>5.55ab</td><td>435ab</td><td>25.0a</td><td>2,610b</td></tr><tr><td>N2D2</td><td>3,404b</td><td>5.70a</td><td>443a</td><td>23.0ab</td><td>2,526b</td></tr><tr><td>Avg</td><td>3,041C</td><td>4.50C</td><td>396C</td><td>22.4BC</td><td>2,704C</td></tr><tr><td rowspan="7">TIEBILE</td><td>N0D1</td><td>2,678b</td><td>3.36c</td><td>318d</td><td>23.2ab</td><td>2,128c</td></tr><tr><td>N1D1</td><td>3,650a</td><td>4.89bc</td><td>464b</td><td>24.8a</td><td>2,914a</td></tr><tr><td>N2D1</td><td>3,668a</td><td>5.26b</td><td>469b</td><td>24.0ab</td><td>2,703b</td></tr><tr><td>N0D2</td><td>2,620b</td><td>6.13ab</td><td>343c</td><td>20.5b</td><td>1,882cd</td></tr><tr><td>N1D2</td><td>3,676a</td><td>7.19a</td><td>613a</td><td>23.4ab</td><td>2,764ab</td></tr><tr><td>N2D2</td><td>3,618a</td><td>6.30ab</td><td>604a</td><td>23.5ab</td><td>2,724ab</td></tr><tr><td>Avg</td><td>3,318BC</td><td>5.52B</td><td>469BC</td><td>23.0B</td><td>2,519C</td></tr></tbody></table>

<table><caption>TABLE 5 Analysis of variance of the effects of plant density, N fertilization, variety and their interactions on development and growth parameters in 2018 and 2019 at Sotuba in Mali</caption><thead><tr><th>Source of variation</th><th>SPAD value</th><th>Leaf area index</th><th>Straw yield</th><th>Plant height</th></tr></thead><tbody><tr><td>Density (D)</td><td>*</td><td>***</td><td>***</td><td>***</td></tr><tr><td>Nitrogen (N)</td><td>***</td><td>**</td><td>***</td><td>***</td></tr><tr><td>Variety (V)</td><td>***</td><td>**</td><td>***</td><td>***</td></tr><tr><td>D × N</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr><tr><td>D × V</td><td>ns</td><td>ns</td><td>ns</td><td>***</td></tr><tr><td>N × V</td><td>*</td><td>ns</td><td>**</td><td>***</td></tr><tr><td>D × N × V</td><td>ns</td><td>ns</td><td>**</td><td>ns</td></tr></tbody></table>

Note. ns, nonsignificant at the .05 probability level.

*Significant at the .05 probability level.

**Significant at the .01 probability level.

***Significant at the .001 probability level.

study (Table 7). Net benefit for the eight sorghum varieties increased with the use of agricultural inputs. For the *guinea* hydrid varieties (FADDA, PABLO) and *caudatum-durra* variety A12-79, the N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹) treatment increased net benefit with respective values of 2007, US$1,435 and $1,607 ha⁻¹ and MRR of 1,151, 686, and 877%, respectively. Results indicate that the N2D2 treatment is the most efficient in terms of net benefit and MRR for FADDA, PABLO, and A12-79 varieties than the N1D1 (89 kg N ha⁻¹ and 26,600 plants ha⁻¹), N1D2 (89 kg N ha⁻¹ and 53,300 plants ha⁻¹), N2D1 (178 kg N ha⁻¹ and 26,600

plants ha⁻¹) treatments. On the other hand, the N1D2 treatment was economically more profitable for the *caudatum-guinea* C2_007-03 and C2_075-15 and *caudatum* GRINKAN and SOUMBA varieties with net benefit of $1,263, $1,275, $1,231 and $1,175 ha⁻¹, respectively. The MRR for these varieties in N1D2 were 717, 602, 378, and 584%, respectively, compared to the N2D2, N2D1, and N1D1 treatments. The N1D2 and N1D1 treatments provided the highest net benefits for the TIEBILE *guinea* type variety with amounts of $1,107 and $1,099 ha⁻¹ and MRR values of 334 and 323%, respectively, compared to the N2D1 and N2D2 treatments (Table 7).

<footnote>ᵃN0, nitrogen rate of 0 kg ha⁻¹.</footnote>

<footnote>ᵇD1, 26,600 plants ha⁻¹.</footnote>

<footnote>ᶜValues in a column followed by different letters are significantly different at P < .05. Lower-case and upper-case letters indicate comparisons within treatments of each variety and among varieties, respectively.</footnote>

<footnote>ᵈN1, nitrogen rate of 89 kg ha⁻¹.</footnote>

<footnote>ᵉN2, nitrogen rates of 178 kg ha⁻¹.</footnote>

<footnote>ᵉD2, 53,300 plants ha⁻¹.</footnote>
<table><caption>TABLE 6 Physiological and growth parameters of eight sorghum varieties under plant density and N fertilization in 2018 and 2019 at Sotuba in Mali</caption><thead><tr><th>Variety</th><th>Treatments</th><th>SPAD values</th><th>Leaf area index</th><th>Straw yield<br>kg ha<sup>-1</sup></th><th>Plant height<br>cm</th></tr></thead><tbody><tr><td rowspan="7">A12-79</td><td>N0D1<sup>a,b</sup></td><td>38c<sup>c</sup></td><td>1.4c</td><td>7,704d</td><td>124c</td></tr><tr><td>N1D1<sup>d</sup></td><td>45b</td><td>2.04bc</td><td>11,324c</td><td>174b</td></tr><tr><td>N2D1<sup>e</sup></td><td>47a</td><td>2.15bc</td><td>12,426c</td><td>186b</td></tr><tr><td>N0D2<sup>f</sup></td><td>38c</td><td>2.23bc</td><td>12,150c</td><td>158bc</td></tr><tr><td>N1D2</td><td>44b</td><td>2.55b</td><td>15,389b</td><td>184b</td></tr><tr><td>N2D2</td><td>47a</td><td>3.75a</td><td>20,167a</td><td>217a</td></tr><tr><td>Avg</td><td>43BC</td><td>2.35BC</td><td>13,193C</td><td>174D</td></tr><tr><td rowspan="7">C2_007-03</td><td>N0D1</td><td>44c</td><td>1.92d</td><td>12,444c</td><td>205c</td></tr><tr><td>N1D1</td><td>50a</td><td>2.53c</td><td>14,537b</td><td>249b</td></tr><tr><td>N2D1</td><td>50a</td><td>2.82b</td><td>15,037b</td><td>258b</td></tr><tr><td>N0D2</td><td>40d</td><td>2.44c</td><td>12,533c</td><td>229bc</td></tr><tr><td>N1D2</td><td>48b</td><td>3.03b</td><td>18,556a</td><td>257b</td></tr><tr><td>N2D2</td><td>48b</td><td>3.49a</td><td>18,593a</td><td>295a</td></tr><tr><td>Avg</td><td>46AB</td><td>2.71AB</td><td>15,283B</td><td>249C</td></tr><tr><td rowspan="7">C2_075-15</td><td>N0D1</td><td>41b</td><td>1.98c</td><td>8,778e</td><td>156c</td></tr><tr><td>N1D1</td><td>44a</td><td>2.67b</td><td>13,546c</td><td>196b</td></tr><tr><td>N2D1</td><td>41b</td><td>2.59b</td><td>13,713c</td><td>202b</td></tr><tr><td>N0D2</td><td>36c</td><td>2.08c</td><td>10,333d</td><td>159c</td></tr><tr><td>N1D2</td><td>41b</td><td>2.69b</td><td>16,296b</td><td>197b</td></tr><tr><td>N2D2</td><td>44a</td><td>3.16a</td><td>17,454a</td><td>224a</td></tr><tr><td>Avg</td><td>41CD</td><td>2.53B</td><td>13,353C</td><td>189D</td></tr><tr><td rowspan="7">FADDA</td><td>N0D1</td><td>37c</td><td>1.75e</td><td>9,511e</td><td>299b</td></tr><tr><td>N1D1</td><td>49ab</td><td>2.63d</td><td>16,926d</td><td>300b</td></tr><tr><td>N2D1</td><td>51a</td><td>3.23c</td><td>21,000b</td><td>324ab</td></tr><tr><td>N0D2</td><td>36c</td><td>2.62d</td><td>16,481d</td><td>293b</td></tr><tr><td>N1D2</td><td>48b</td><td>3.65b</td><td>19,533c</td><td>333a</td></tr><tr><td>N2D2</td><td>48b</td><td>4.68a</td><td>23,511a</td><td>341a</td></tr><tr><td>Avg</td><td>44B</td><td>3.10A</td><td>17,827A</td><td>315B</td></tr><tr><td rowspan="7">GRINKAN</td><td>N0D1</td><td>38c</td><td>1.74d</td><td>11,644cd</td><td>181b</td></tr><tr><td>N1D1</td><td>44b</td><td>3.12b</td><td>12,210c</td><td>194ab</td></tr><tr><td>N2D1</td><td>47a</td><td>2.53c</td><td>16,356b</td><td>212a</td></tr><tr><td>N0D2</td><td>37c</td><td>2.51c</td><td>11,093d</td><td>191ab</td></tr><tr><td>N1D2</td><td>43b</td><td>3.26ab</td><td>16,259b</td><td>204ab</td></tr><tr><td>N2D2</td><td>45ab</td><td>3.58a</td><td>19,074a</td><td>216a</td></tr><tr><td>Avg</td><td>42C</td><td>2.79AB</td><td>14,439B</td><td>200D</td></tr><tr><td rowspan="7">PABLO</td><td>N0D1</td><td>42c</td><td>1.62e</td><td>8,648e</td><td>241d</td></tr><tr><td>N1D1</td><td>52a</td><td>3.4b</td><td>14,645bc</td><td>357c</td></tr><tr><td>N2D1</td><td>52a</td><td>2.67c</td><td>13,907c</td><td>414a</td></tr><tr><td>N0D2</td><td>42c</td><td>1.93d</td><td>10,667d</td><td>383b</td></tr><tr><td>N1D2</td><td>50b</td><td>3.22bc</td><td>15,259b</td><td>409ab</td></tr><tr><td>N2D2</td><td>50b</td><td>3.78a</td><td>19,600a</td><td>414a</td></tr><tr><td>Avg</td><td>48A</td><td>2.77AB</td><td>13,787BC</td><td>370A</td></tr></tbody></table>

(Continues)
<table><caption>TABLE 6 (Continued)</caption><thead><tr><th>Variety</th><th>Treatments</th><th>SPAD values</th><th>Leaf area index</th><th>Straw yield</th><th>Plant height</th></tr></thead><tbody><tr><td rowspan="7">SOUMBA</td><td>N0D1</td><td>42c</td><td>1.84c</td><td>10,111e</td><td>220c</td></tr><tr><td>N1D1</td><td>48a</td><td>2.37bc</td><td>14,822b</td><td>248b</td></tr><tr><td>N2D1</td><td>47ab</td><td>2.52b</td><td>14,167c</td><td>259b</td></tr><tr><td>N0D2</td><td>42c</td><td>2.58b</td><td>10,824d</td><td>258b</td></tr><tr><td>N1D2</td><td>47b</td><td>3.52a</td><td>14,556bc</td><td>285ab</td></tr><tr><td>N2D2</td><td>48a</td><td>3.62a</td><td>16,711a</td><td>297a</td></tr><tr><td>Avg</td><td>46AB</td><td>2.74AB</td><td>13,532BC</td><td>261C</td></tr><tr><td rowspan="7">TIEBILE</td><td>N0D1</td><td>35c</td><td>2.16d</td><td>8,056e</td><td>308e</td></tr><tr><td>N1D1</td><td>41b</td><td>2.58c</td><td>17,689c</td><td>370bc</td></tr><tr><td>N2D1</td><td>46a</td><td>3.08b</td><td>17,294c</td><td>379b</td></tr><tr><td>N0D2</td><td>34d</td><td>2.21d</td><td>11,389d</td><td>329d</td></tr><tr><td>N1D2</td><td>40b</td><td>3.02b</td><td>18,352b</td><td>353c</td></tr><tr><td>N2D2</td><td>40b</td><td>3.32a</td><td>20,944a</td><td>409a</td></tr><tr><td>Avg</td><td>39D</td><td>2.73AB</td><td>15,620B</td><td>358A</td></tr></tbody></table>

## 3.4 | Linear regression among grain yield, yield component, and growth parameters under plant density and N fertilizer

Linear regression analyses were performed on variables measured under plant density and N fertilization (Table 8). Grain yield was positively and significantly correlated to panicle weight per square meter, leaf area index, straw yield, and plant height (hybrids, open-pollinated improved and local). For tall *guinea* hybrid varieties FADDA and PABLO, grain yield was positively correlated with panicle number per square meter. No significant correlation was observed between grain yield and straw yield for TIEBILE, a local check. SOUMBA, an improved *caudatum* type, has been positive and significantly correlated between its 1,000 grain weight and grain yield. GRINKAN, an improved variety has been positive correlation between grain yield and number of grains per panicle. In addition, SPAD values (chlorophyll content) was positively correlated with grain yield in tall *guinea* hybrid varieties FADDA, PABLO, and tall *guinea* local TIEBILE.

## 4 | DISCUSSION

Plant density by N fertilization rate combinations supported better option for grain yield increase through its contributing parameters across two cropping seasons data collected in

2018 and 2019 in Sotuba, Mali. Results showed that grain yield varied greatly with different plant densities and N fertilization combinations for each variety (Table 4). Grain yield increased from N0D1 (0 kg N ha⁻¹ and 26,600 plants ha⁻¹) to N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹). In this study, the highest grain yield under N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹) was obtained with taller *guinea* hybrid FADDA and PABLO and the shorter *durra-caudatum* A12-79 improved varieties compared to the N0D1 treatment (Table 3). These results indicated that the increase in grain yield of these varieties relied on an increase in plant density when the N application rate increased. Similar studies conducted by Fan et al. (2019), Ullah et al. (2020), and Wei et al. (2019) showed the optimum advantages of plant density and N combinations on grain yield increased in improved cereal varieties. The authors indicated that this increase in grain yield was explained by an adaptation of varieties to agronomic practices such as plant density and N rate. In the N2D2 treatment FADDA, PABLO, and A12-79 (except panicle number per square meter) produced higher panicle number per square meter, panicle weight per square meter, leaf area index, and straw yield (Table 6). These varieties under N2D1 (178 kg N ha⁻¹ and 26,600 plants ha⁻¹) treatment recorded important SPAD values. This suggests that maintaining the optimum combination of plant density and N allows a better expression of these traits and leads to higher yields (Kaizzi et al., 2012; Kugbe et al., 2019; Sahu et al., 2018). Similar findings were reported by Golla and Chalchisa (2019) and Mahama

<footnote><sup>a</sup>N0, nitrogen rate of 0 kg ha⁻¹.</footnote>

<footnote><sup>b</sup>D1, 26,600 plants ha⁻¹.</footnote>

<footnote><sup>c</sup>Values in a column followed by different letters are significantly different at P < .05. Lower-case and upper-case letters indicate comparisons within treatments of each variety and among varieties, respectively.</footnote>

<footnote><sup>d</sup>N1, nitrogen rate of 89 kg ha⁻¹.</footnote>

<footnote><sup>e</sup>N2, nitrogen rates of 178 kg ha⁻¹.</footnote>

<footnote><sup>f</sup>D2, 53,300 plants ha⁻¹.</footnote>
<table><caption>TABLE 7 Economic analysis for the combination of plant density and nitrogen fertilization on eight sorghum varieties in 2018 and 2019 at Sotuba in Mali</caption><thead><tr><th>Variety</th><th>Treatments</th><th>Grain yield<br>kg ha<sup>-1</sup></th><th>Price of<br>sorghum<br>grain<br>US$ kg<sup>-1</sup></th><th>Gross<br>benefits</th><th>Total<br>variable cost<br>US$ ha<sup>-1</sup></th><th>Net benefits</th><th>Marginal<br>rate of<br>return<br>%</th></tr></thead><tbody><tr><td rowspan="5">A1279</td><td>N0D1<sup>a,b</sup></td><td>1,852</td><td>0.325</td><td>601.9</td><td>12.54</td><td>589.35</td><td></td></tr><tr><td>N1D1<sup>c</sup></td><td>3,138</td><td>0.325</td><td>1,019.85</td><td>87.37</td><td>932.48</td><td>459.13</td></tr><tr><td>N2D1<sup>d</sup></td><td>3,127</td><td>0.325</td><td>1,016.28</td><td>128.73</td><td>887.54</td><td>257</td></tr><tr><td>N1D2<sup>e</sup></td><td>4,004</td><td>0.325</td><td>1,301.3</td><td>87.37</td><td>1,213.93</td><td>835.65</td></tr><tr><td>N2D2</td><td>5,342</td><td>0.325</td><td>1,736.15</td><td>128.73</td><td>1,607.42</td><td>877.2</td></tr><tr><td rowspan="5">C2_007-03</td><td>N0D1</td><td>2,520</td><td>0.325</td><td>819</td><td>12.54</td><td>806.45</td><td></td></tr><tr><td>N1D1</td><td>3,437</td><td>0.325</td><td>1,117.03</td><td>87.37</td><td>1,029.65</td><td>211.73</td></tr><tr><td>N2D1</td><td>3,390</td><td>0.325</td><td>1,101.75</td><td>128.73</td><td>973.02</td><td>143.6</td></tr><tr><td>N1D2</td><td>4,156</td><td>0.325</td><td>1,350.7</td><td>87.37</td><td>1,263.33</td><td>716.52</td></tr><tr><td>N2D2</td><td>4,577</td><td>0.325</td><td>1,487.53</td><td>128.73</td><td>1,358.79</td><td>475.96</td></tr><tr><td rowspan="5">C2_075-15</td><td>N0D1</td><td>2,574</td><td>0.325</td><td>836.55</td><td>12.54</td><td>824</td><td></td></tr><tr><td>N1D1</td><td>3,264</td><td>0.325</td><td>1,060.8</td><td>87.37</td><td>973.43</td><td>200</td></tr><tr><td>N2D1</td><td>3,231</td><td>0.325</td><td>1,050.08</td><td>128.73</td><td>921.34</td><td>83.96</td></tr><tr><td>N1D2</td><td>4,188</td><td>0.325</td><td>1,361.1</td><td>87.37</td><td>1,273.73</td><td>601.73</td></tr><tr><td>N2D2</td><td>4,294</td><td>0.325</td><td>1,395.55</td><td>128.73</td><td>1,266.82</td><td>381.6</td></tr><tr><td rowspan="5">FADDA</td><td>N0D1</td><td>2,139</td><td>0.325</td><td>695.18</td><td>25.09</td><td>670.07</td><td></td></tr><tr><td>N1D1</td><td>4,308</td><td>0.325</td><td>1,400.1</td><td>99.92</td><td>1,300.17</td><td>842.09</td></tr><tr><td>N2D1</td><td>5,948</td><td>0.325</td><td>1,933.1</td><td>141.28</td><td>1,791.81</td><td>965.45</td></tr><tr><td>N1D2</td><td>4,883</td><td>0.325</td><td>1,586.98</td><td>99.92</td><td>1,487.05</td><td>1,091.84</td></tr><tr><td>N2D2</td><td>6,611</td><td>0.325</td><td>2,148.58</td><td>141.28</td><td>2,007.29</td><td>1,150.91</td></tr><tr><td rowspan="5">GRINKAN</td><td>N0D1</td><td>2,957</td><td>0.325</td><td>961.03</td><td>12.54</td><td>948.48</td><td></td></tr><tr><td>N1D1</td><td>3,109</td><td>0.325</td><td>1,010.43</td><td>87.37</td><td>923.05</td><td>-33.91</td></tr><tr><td>N2D1</td><td>3,469</td><td>0.325</td><td>1,127.43</td><td>128.73</td><td>998.69</td><td>43.36</td></tr><tr><td>N1D2</td><td>4,056</td><td>0.325</td><td>1,318.2</td><td>87.37</td><td>1,230.83</td><td>377.82</td></tr><tr><td>N2D2</td><td>3,757</td><td>0.325</td><td>1,221.03</td><td>128.73</td><td>1,092.29</td><td>124</td></tr><tr><td rowspan="5">PABLO</td><td>N0D1</td><td>2,041</td><td>0.325</td><td>663.33</td><td>25.09</td><td>638.22</td><td></td></tr><tr><td>N1D1</td><td>3,242</td><td>0.325</td><td>1,053.65</td><td>99.92</td><td>953.72</td><td>421.62</td></tr><tr><td>N2D1</td><td>3,941</td><td>0.325</td><td>1,280.83</td><td>141.28</td><td>1,139.54</td><td>431.46</td></tr><tr><td>N1D2</td><td>3,770</td><td>0.325</td><td>1,225.25</td><td>99.92</td><td>1,125.32</td><td>650.94</td></tr><tr><td>N2D2</td><td>4,850</td><td>0.325</td><td>1,576.25</td><td>141.28</td><td>1,434.96</td><td>685.72</td></tr><tr><td rowspan="5">SOUMBA</td><td>N0D1</td><td>2,310</td><td>0.325</td><td>750.75</td><td>12.54</td><td>738.2</td><td></td></tr><tr><td>N1D1</td><td>2,758</td><td>0.325</td><td>896.35</td><td>87.37</td><td>808.98</td><td>94.78</td></tr><tr><td>N2D1</td><td>3,116</td><td>0.325</td><td>1,012.7</td><td>128.73</td><td>883.97</td><td>125.68</td></tr><tr><td>N1D2</td><td>3,884</td><td>0.325</td><td>1,262.3</td><td>87.37</td><td>1,174.93</td><td>584.34</td></tr><tr><td>N2D2</td><td>3,404</td><td>0.325</td><td>1,106.3</td><td>128.73</td><td>977.57</td><td>206.32</td></tr><tr><td rowspan="5">TIEBILE</td><td>N0D1</td><td>2,678</td><td>0.325</td><td>870.35</td><td>12.54</td><td>857.8</td><td></td></tr><tr><td>N1D1</td><td>3,650</td><td>0.325</td><td>1,186.25</td><td>87.37</td><td>1,098.88</td><td>322.6</td></tr><tr><td>N2D1</td><td>3,668</td><td>0.325</td><td>1,192.1</td><td>128.73</td><td>1,063.37</td><td>177.2</td></tr><tr><td>N1D2</td><td>3,676</td><td>0.325</td><td>1,194.7</td><td>87.37</td><td>1,107.33</td><td>333.913</td></tr><tr><td>N2D2</td><td>3,618</td><td>0.325</td><td>1,175.85</td><td>128.73</td><td>1,047.12</td><td>163.2</td></tr></tbody></table>

<footnote><sup>a</sup>N0, nitrogen rate of 0 kg ha<sup>-1</sup>.</footnote>

<footnote><sup>b</sup>D1, 26,600 plants ha<sup>-1</sup>.</footnote>

<footnote><sup>c</sup>N1, nitrogen rate of 89 kg ha<sup>-1</sup>.</footnote>

<footnote><sup>d</sup>N2, nitrogen rates of 178 kg ha<sup>-1</sup>.</footnote>

<footnote><sup>e</sup>D2, 53,300 plants ha<sup>-1</sup>.</footnote>
<table><caption>TABLE 8 Relationship between grain yield and development, growth and agronomic parameters of each variety under plant density and N fertilization in 2018 and 2019</caption><thead><tr><th rowspan="3">Variety</th><th rowspan="3">Explained trait</th><th colspan="10">Explicative traits</th></tr><tr><th colspan="2">PNm<sup>-2</sup></th><th colspan="2">PWm<sup>-2</sup></th><th colspan="2">1,000GW</th><th colspan="2">GN/P</th><th colspan="2">SPAD values</th><th colspan="2">LAI</th><th colspan="2">STY</th><th colspan="2">PH</th></tr><tr><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>A12-79</td><td rowspan="3">Grain yield</td><td>0.45</td><td>ns</td><td>0.97</td><td>***</td><td>0.08</td><td>ns</td><td>0.12</td><td>ns</td><td>0.43</td><td>ns</td><td>0.96</td><td>***</td><td>0.98</td><td>***</td><td>0.87</td><td>**</td></tr><tr><td>C2_075-15</td><td>0.34</td><td>ns</td><td>0.97</td><td>***</td><td>0.02</td><td>ns</td><td>0.45</td><td>ns</td><td>0.45</td><td>ns</td><td>0.82</td><td>**</td><td>0.91</td><td>***</td><td>0.77</td><td>**</td></tr><tr><td>C2_007-03</td><td>0.43</td><td>ns</td><td>0.95</td><td>***</td><td>0.24</td><td>ns</td><td>0.11</td><td>ns</td><td>0.16</td><td>ns</td><td>0.91</td><td>***</td><td>0.77</td><td>**</td><td>0.81</td><td>**</td></tr><tr><td>FADDA</td><td></td><td>0.76</td><td>**</td><td>0.94</td><td>***</td><td>0.31</td><td>ns</td><td>0.22</td><td>ns</td><td>0.70</td><td>**</td><td>0.82</td><td>**</td><td>0.91</td><td>***</td><td>0.72</td><td>**</td></tr><tr><td>GRINKAN</td><td></td><td>0.37</td><td>ns</td><td>0.91</td><td>***</td><td>0.32</td><td>ns</td><td>0.59</td><td>*</td><td>0.39</td><td>ns</td><td>0.58</td><td>*</td><td>0.75</td><td>**</td><td>0.57</td><td>*</td></tr><tr><td>PABLO</td><td></td><td>0.61</td><td>*</td><td>0.89</td><td>**</td><td>0.03</td><td>ns</td><td>0.49</td><td>ns</td><td>0.55</td><td>*</td><td>0.69</td><td>*</td><td>0.86</td><td>**</td><td>0.80</td><td>**</td></tr><tr><td>SOUMBA</td><td></td><td>0.45</td><td>ns</td><td>0.65</td><td>*</td><td>0.81</td><td>**</td><td>0.31</td><td>ns</td><td>0.35</td><td>ns</td><td>0.84</td><td>**</td><td>0.58</td><td>*</td><td>0.79</td><td>**</td></tr><tr><td>TIEBILE</td><td></td><td>0.20</td><td>ns</td><td>0.75</td><td>**</td><td>0.05</td><td>ns</td><td>0.41</td><td>ns</td><td>0.74</td><td>**</td><td>0.78</td><td>**</td><td>0.39</td><td>ns</td><td>0.68</td><td>*</td></tr></tbody></table>

Note. PNm², panicle number per m²; PWm², panicle weight per m²; 1,000GW, 1,000 grain weight; GN/P, grain number per panicle; LAI, leaf area index; STY, straw yield; PH, plant height; R², coefficient of determination; ns, nonsignificant at the .05 probability level.

*Significant at the .05 probability level.

**Significant at the .01 probability level.

***Significant at the .001 probability level.

Significant at the .0001 probability level.

et al. (2014) on improved maize (Zea mays L.) and sorghum varieties (Table 4).

The short guinea-caudatum improved varieties C2_007-03 and C2_075-15 produced higher grain yield under treatments N2D2 (178 kg N ha⁻¹ and 53,300 plants ha⁻¹) and N1D2 (89 kg N ha⁻¹ and 53,300 plants ha⁻¹) statistically significant compared to N0D1 (0 kg N ha⁻¹ and 26,600 plants ha⁻¹). However, the N1D2 treatment is most beneficial than N2D2 treatment in terms of grain yield for these varieties. The N1D2 treatment increased grain yield over N0D1 treatment by 78% for C2_007-03 and 63% for C2_075-15 (Table 4). C2_007-03 and C2_075-15 had higher panicle weight per square meter in N1D2 and a higher leaf area index, straw yield, and plant height (Table 6) in N2D2 treatment. For caudatum races GRINKAN and SOUMBA, grain yield produced per variety under N1D2 treatment was 37 and 68% greater than its N0D1 treatment, respectively (Table 4). Under the N1D2 treatment, panicle weight per square meter, grain number per panicle, and 1,000 grain weight were better expressed in GRINKAN and SOUMBA varieties compared to the N0D1 treatment. The expression of these traits can contribute to increasing grain yield in GRINKAN and SOUMBA varieties. Our results showed that N application of 89 kg ha⁻¹ with 53,300 plants ha⁻¹ (N1D2) would be the optimum to improve grain yield of C2_007-03, C2_075-15, GRINKAN, and SOUMBA improved varieties. Sai Siddartha Naik et al. (2018) and Zhang et al. (2016) obtained similar results in sorghum crops. According to Ciampitti and Vyn (2011), although significant work has been done on optimizing plant density and N rate, but this remains a difficult issue to resolve due to the large variability of agricultural lands and the strong interaction between variety, density, and N. The local variety TIEBILE obtained statistically the same grain yield under treatments with N application at different plant densities (Table 4). This means that the tall guinea local variety TIEBILE does not need a significant amount of N (178 kg ha⁻¹) and a high plant density (53,300 plants ha⁻¹) to produce the more grains but had increased its height, leaf area index, and straw yield under N2D2 treatment. Nitrogen application of 89 kg ha⁻¹ with 26,600 plants ha⁻¹ (N1D1) would be beneficial to obtain a good grain yield for TIEBILE. According to Chantereau et al. (2013), Kouressy et al. (2020), and Vaksmann et al. (1996), the essential of local sorghum is characterized by a high straw biomass with low grain yield. However, it is difficult to propose this sorghum type for intensive agricultural practices because of its low N nutrient valorization for grain production. Their findings corroborate with our results where the N application of 178 kg ha⁻¹ with a density of 53,300 plants ha⁻¹ did not significantly increase TIEBILE grain yield. A high N application with high plant density would not improve grain yield of local variety but seems favorable to vegetative part increase (Table 6). Similar findings were observed by Dawadi and Sah (2012).
Current study results suggest that the tolerance or response to the N fertilizer and planting density combination in terms of grain yield varied with different sorghum varieties (hybrid, improved, and local). Similar results were reported by Ullah et al. (2020), Zhou et al. (2019), and Dwivedi et al. (2016) in rice (Oryza sativa L.) and pearl millet [Pennisetum americanum (L.) Leeke] varieties. Knowledge of agronomic practices for determining the optimum density and N combination for each variety is a research activity. The application of these combinations become a necessity by producers through extension agency for sorghum intensive production in Mali based on producer resources, inputs access, and availability.

Among the nutrients, the use of N is crucial in achieving high yield, but the response of cereals (sorghum) to N input is highly variable (Dwivedi et al., 2016). Technically, the N2D2 treatment for FADDA, PABLO, and A12-79 procured the best grain yield; economically, this treatment was the most productive with high net benefit and MRR. However, the N1D2 treatment will be the most interesting option in grain production for C2_007-03, C2_075-15, GRINKAN, and SOUMBA as they have higher net benefits and MRR. Under N1D2 and N1D1 treatments, the local variety TIEBILE obtained higher net benefit and a MRR of 333 and 322%, respectively. The MRR profitability threshold for adoption of new technologies is 200% (Coulibaly et al., 2008). These treatments can be considered as economically satisfactory for these respective varieties. Mankoussou et al. (2017) revealed that economic gain of fertilizers decreased with increasing doses and led to net benefit and MRR decrease. Our results indicated that economic gain depends on both fertilizer (N) and genetic material type. The results showed that in addition to agronomic practices, knowledge of the economic gain for determining the optimum for each variety would be a necessity to reduce expenses and increase the grain yield and net benefit of sorghum producers in Mali.

Grain yield depends on variety agronomic practices, in particular plant density and N fertilization. Its improvement relied on its components but also on growth traits. The regression analysis explained relationship among grain yield and yield components, and between grain yield and growth traits for each variety (hybrids, improved, and local) under all treatments (Table 8). This confirms strong dependences of grain yield on growth parameters and yield components under plant density and N in the botanical sorghum races used in our study (Sadeghi & Bahrani, 2002). According to Trachsel et al. (2016), a faster crop growth is indicative of assimilate availability potentially resulting in greater prolificacy and higher grain yield.

The panicle number per square meter and SPAD values are components that best explained grain yield in the *guinea* hybrid FADDA and PABLO. This result showed that the N

rate and planting density had greater effect on SPAD values and panicles number in tall *guinea* hybrid varieties FADDA and PABLO (Table 8). Nitrogen improves chlorophyll content (SPAD values) allowing plants to make their photosynthesis (Tajul et al., 2013); and panicle number promoted mainly by tillering, while planting density improves panicle number thus causing plant numbers increase per area unit (Manjeet et al., 2017; Zhou et al., 2019). These results are in agreement those of Srivastava et al. (2019) in maize. For *caudatum* improved varieties GRINKAN and SOUMBA, grain yield was explained specifically by grains number per panicle and 1,000 grain weight. According to Kondombo et al. (2017), panicle number per square meter and 1,000 grain weight are the major components associated in grain yield of *guinea* improved varieties. Therefore, grains number per panicle is the best grain yield predicator in *caudatum* improved varieties. Senior researchers on sorghum (Kouressy et al., 2008; Shamme et al., 2016; Vadez et al., 2012) reported that variation between sorghum varieties for different yield components and growth traits to improvement grain yield may be due to the plant genetic constitution, agronomic practices and to its capacity to exploit water.

## 5 | CONCLUSIONS

This current study showed that grain yield and its yield components, chlorophyll index and growth parameters varied depending on plant density, N fertilization, and variety. It can be concluded that 178 kg N ha⁻¹ and 53,300 plants ha⁻¹ combination produced better grain yield and high marginal rate of return for taller *guinea* hybrid varieties FADDA and PABLO and shorter *durra-caudatum* variety A12-79. The application of 89 kg N ha⁻¹ with 53,300 plants ha⁻¹ is the optimal combination to acquire the higher grain and marginal rate of return for short *guinea-caudatum* improved varieties C2_007-03 and C2_075-15 and short *caudatum* improved varieties GRINKAN and SOUMBA. These varieties can be used by producers wanting increased sorghum production with a moderate use of N. Rate 89 kg N ha⁻¹ with 26,600 plants ha⁻¹ is the economically beneficial treatment for taller local *guinea* variety TIEBILE. The local variety has a low response to agricultural improvement practices. Panicle weight per square meter, leaf area index, plant height, and straw yield were the main traits associated with varieties grain yield increase studied. Panicle number per square meter and SPAD value were the specific traits associated with grain yield in the *guinea* hybrid FADDA and PABLO. Grain number per panicle and 1,000 grain weight were the specific traits associated with grain yield in the *caudatum* GRINKAN and SOUMBA. These findings will help producers which option to promote to booster sorghum production in Mali.
## ACKNOWLEDGMENTS

We are thankful to undergraduate all students, technicians and workers of Sotuba agroclimatology unity that helped us in achievement this work. This work constitutes part of doctoral research studies funded by DAAD, the German exchange service program. This research activity was also funded in part by Bill and Melinda Gates foundation under Sorghum Genotyping Tools project and benefit the facilities of the Centre d'Etude Régional pour l'Amélioration de l'Adaptation à la Sécheresse (CERAAS).

## AUTHOR CONTRIBUTIONS

Joseph Sékou B DEMBELE: Conceptualization; Data curation; Formal analysis; Funding acquisition; Methodology; Writing-original draft. Boubacar GANO: Conceptualization; Formal analysis; Methodology; Software; Writing-review & editing. Mamoutou Kouressy: Funding acquisition; Project administration; Supervision; Writing-original draft. Léonce Lamine DEMBELE: Data curation; Formal analysis; Software. Mohamed DOUMBIA: Data curation; Formal analysis. Komla Kyky Ganyo: Writing-review & editing. SékouBa SANOGO: Data curation; Formal analysis. Adama TOGOLA: Writing-review & editing. Karim TRAORE: Writing-review & editing. Michel VAKSMAN: Conceptualization; Methodology; Project administration; Supervision; Writing-review & editing. Niaba TEME: Conceptualization; Methodology; Project administration; Resources; Writing-review & editing. Diaga Diouf: Supervision; Writing-review & editing. Alain AUDEBERT: Conceptualization; Funding acquisition; Methodology; Project administration; Supervision; Writing-review & editing

## CONFLICT OF INTEREST

The authors declare no conflict of interest.

## ORCID

Joseph Sékou B Dembele https://orcid.org/0000-0003-2889-7457

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**How to cite this article:** Dembele, J. S. B., Gano, B., Kouressy, M., Dembele, L. L., Doumbia, M., Ganyo, K. K., Sanogo, SBa, Togola, A., Traore, K., Vaksman, M., Teme, N., Diouf, D., & Audebert, A. Plant density and nitrogen fertilization optimization on sorghum grain yield in Mali. *Agronomy Journal*. 2021;1–16. https://doi.org/10.1002/agj2.20850