Page 1 of 12

European Journal of Applied Sciences – Vol. 9, No. 5

Publication Date: October 25, 2021

DOI:10.14738/aivp.95.11002. Mounkaila, Y., Moussa, B., & Garba, I. (2021). NDVI, a Proxy for the Associated Millet and Cowpea Growth Stages in the Sahelian

Zone. European Journal of Applied Sciences, 9(5). 299-310.

Services for Science and Education – United Kingdom

NDVI, a Proxy for the Associated Millet and Cowpea Growth

Stages in the Sahelian Zone

Yahaya Mounkaila

Department of Agronomy, Faculty of vegetable production

Abdou Moumouni University, Niamey, Niger

Baragé Moussa

Department of Agronomy, Faculty of vegetable production

Abdou Moumouni University, Niamey, Niger

Issa Garba

Agrhymet Regional Center, Niamey, Niger

ABSTRACT

The NDVI PROBA-V 1 kilometer resolution was used to apprehend the associated

millet and cowpea growth stages in the Sahelian zone characterized by the presence

of these two crops on the same cultivated plots. The ten-day NDVI images values of

associated millet and cowpea were used to establish seasonal NDVI profiles which

were then crossed at growth stages followed during the cycle. When millet or

cowpea sowing dates are spread over several dekads during the season, their

growth stages observed per dekad during the cycle remain heterogeneous. When

the growing season begins until the end of July, the NDVI index values oscillate

between 0.15 and 0.27 with vegetative development marked by tillering for millet

and vegetative growth for cowpea. In response to rainfall, the NDVI index value

increases to peak between 0.31 and 0.34 in early September when crops are in full

growing. During the peak of the NDVI index, the growth stages of millet vary from

Boot stage, flowering and grain-filling. The growth stages of cowpea vary from

branching, flowering and pod formation in the three locations of the study. In the

case of millet and cowpea association on the same cultivated areas, the fluctuation

of the NDVI is influenced by the predominant crop, which is millet. The low spatial

resolution at 1 kilometer would imply, in addition to millet and cowpea, that the

NDVI value also takes into account other crops present such as sorghum or

abundant woody species in some Sahelian cultivated areas. The seasonal profile of

low spatial resolution NDVI is a tool that can be used to understand the stages of

growth of millet and cowpea associated during the cycle over relatively large and

homogeneous spaces for these two crops.

Key words: millet, cowpea, growth stages, NDVI.

INTRODUCTION

The crops under rain in the context of the Sahel are subject to many hazards which often disrupt

their growth during the production season. They can suffer from delayed start of the season,

dry sequences, pest pressure, flooding or early stopping of rains before the end of the cycle, etc.

The period from sowing to harvest remains a time of monitoring crops in the fields. In view of

Page 2 of 12

300

European Journal of Applied Sciences (EJAS) Vol. 9, Issue 5, October-2021

Services for Science and Education – United Kingdom

the contribution of crops under rain to the food security of the populations, most of the Sahel

States have put in place early warning systems for monitoring crops during rainfed agricultural

seasons. This monitoring makes it possible to determine growth delays during the cycle and to

anticipate even before the harvest on the identification of potential areas of production deficit

in order to provide appropriate responses.

Traditionally, monitoring crop growth has involved many observers in the field. This requires

substantial financial resources which are sometimes difficult to mobilize. To facilitate rapid

decision-making thanks to available agricultural information, one of the alternatives is to use

satellite image data. Opportunities for monitoring plant cover in general over large areas and

at lower cost are available through the use of low-resolution satellite images in the context of

the Sahel [1]. One of the much used satellite data is the Normalized Difference Vegetation Index

(NDVI). The normalized difference vegetation index is appropriate for assessing the vigor of the

vegetation and observing stressful situations during the cycle [2]. Many studies, for example,

have used NDVI images to explain the dynamics of vegetation cover change in recent years in

the Sahel [ 3-11]. The production of seasonal NDVI profiles also made it possible to better

distinguish land use classes in the Sahelian environment and to differentiate between

rangelands, cultivated fields, and fallows [12-13]. In the field of crop monitoring in the Sahelian

environment, the case of the association of millet and cowpea on the same cultivated plot

remains an agricultural practice frequently encountered in relatively large areas. Therefore,

monitoring of the growth of associated millet and cowpea crops during the cycle should be

considered using low spatial resolution satellite images from PROBA V. Millet and cowpeas

present a great interest due to the use of the cereal as a staple food for many Sahelian

populations and the use of the market value regard to the legume.

MATERIAL AND METHODS

Study zone

The study area is located West of Niger between 11.89 ° - 15.35 ° North latitude and 0.99 ° -

3.066 ° East longitude. It corresponds to the old administrative subdivision before 2012 of the

three departments of Kollo, Say and Ouallam. Rainfed agriculture is the main activity of rural

populations. The rainy season lasts from June to September [14]. The average annual

precipitation varies around 400 to 500 mm. The dry season covers the period from October to

May. Temperatures vary between 24 ° C and 45 ° C during the dry season, between 28 ° C and

31 ° C during the rainy season. The classification in bioclimatic zones ranges Kollo and Say in

semi-arid zone and Ouallam in arid zone [15]. The cultivated land in the study area is shown in

the extract from the land use map in Figure 1 [16]. The study zone is characterized by 3 major

geomorphological units, namely the plateaus where forest resources are generally present, the

sandy skirts and the shallows which represent the areas of cultivated fields [17]. The soils are

predominantly sandy and of low fertility [18-19]. The association of millet and cowpea on the

same cultivated area represents more than 73% of the types of association of crops in this

environment [20-21]. On average, the areas cultivated in combination with millet and cowpea

are 1.32 ha per household [20]. This association of millet and cowpea on the same cultivated

plot presents various interests for the Sahelian producer. Agronomic research has always

proposed different models of association of millet and cowpea according to the alternation of

the lines of the two crops at the time of sowing on the same cultivated plot. In Niger farming

Page 3 of 12

301

Mounkaila, Y., Moussa, B., & Garba, I. (2021). NDVI, a Proxy for the Associated Millet and Cowpea Growth Stages in the Sahelian Zone. European

Journal of Applied Sciences, 9(5). 299-310.

URL: http://dx.doi.org/10.14738/aivp.95.11002

areas and in the association system of millet and cowpea on the same land, the recommended

seedling spacing for millet is 1m x 1m, or 10,000 pockets per hectare [22]. The recommended

sowing density of cowpea by the Vulgarization Services is 1.50 m x 1.50 m [23]. However, in

peasant practices, cowpea is sown between millet rows two to six weeks after millet [24].

Fig 1: Presentation of the study zone and extract of occupation cultivated areas [16]

Material

Millet crop (Pennisetum glaucum (L.) R. and cowpea crop (Vigna unguiculata (L.) Walp) are

cultivated in association during the rainy seasons in the three localities of Kollo, Say and

Oualllam. These two crops were monitored on cultivated plots. Ten-day NDVI images of

associated millet and cowpeas were collected during the rainfed agricultural campaigns of 2016

and 2017. These images come from the 1 km resolution PROBA V satellite and can be

downloaded on the VITO website. But in this study, these images were obtained at the

Agrhymet Regional Center in Niamey as part of the MESA (Monitoring for Environment and

Security in Africa) project. A layer of administrative boundaries at the department level of the

National Geographic Institute of Niger (IGNN) was used as well as a land use map which served

as a mask to distinguish between cultivated and non-cultivated areas [16]. The software SPIRIT

(Software for the Processing and Interpretation of Remotely sensed Image Time Series) was

used to extract the values of NDVI images by decade during the rainy seasons in each of the

three localities of Kollo, Say and Ouallam.

Methods

The growth of millet and cowpea was monitored on cultivated plots. These two crops are

present in association on the same cultivated plots. The choice of cultivated plots was based on

the sampling technique resulting from the Crop Forecasting and Estimation Survey conducted