Citation: Diatta, O.; Ngom, D.; Ndiaye, O.; Diatta, S.; Taugourdeau, S. Structure and Phenology of Herbaceous Stratum in the Sahelian Rangelands of Senegal. Grasses 2023, 2, 98–111. https:// doi.org/10.3390/grasses2020009 Academic Editor: Huakun Zhou Received: 24 February 2023 Revised: 13 April 2023 Accepted: 9 May 2023 Published: 17 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Structure and Phenology of Herbaceous Stratum in the Sahelian Rangelands of Senegal Ousmane Diatta 1,2 , Daouda Ngom 2 , Ousmane Ndiaye 1 ,Sékouna Diatta 2 and Simon Taugourdeau 3,4, * 1 Senegalese Agricultural Research Institute, CRZ Dahra, Dahra Djioloff BP 01, Senegal 2 Department of Plant Biology, Cheikh Anta Diop University, FST, Dakar BP 5005, Senegal 3 CIRAD DP PPZS, UMR SELMET, Dakar BP 2057, Senegal 4 UMR SELMET, Univ Montpellier, CIRAD, INRAE, Institut Agro, CEDEX 5, 34398 Montpellier, France * Correspondence: simon.taugourdeau@cirad.fr Abstract: This study describes the structure and phenology of Sahelian rangelands during the rainy season. It was undertaken on a grazed plot and a fenced plot. Measurements were taken every 10 days over two rainy seasons between July and October 2018 and between August and October 2019. The measurements included the phenological stage, the vegetative and flowering height, the coverage, and the phytomass. The results showed that phytomass did not systematically differ between grazed and fenced plots. Flowering started as early as mid-July, but most of the plants flowered in early October. Flowering and fruiting occurred earlier (based on the number of days after the first rain) in the late rainy season (2019) than in the early one (2018). These stages reached their peak in October; senescence began in October. Vegetative height, reproductive height, and coverage were similar between the two years. Height peaks were similar, but they were reached earlier (based on the number of days after the first rain) in the late rainy season than in the early one. Coverage peaks were similar (59.8% in the early rainy season vs. 65.8% in the late one). Vegetative height (around 30 cm), reproductive height (around 36 cm), and coverage (around 60%) reached their maximum in October, but reproductive height was greater than vegetative height. These parameters were lower at the grazed site compared to the fenced site, and flowering started earlier at the grazed site. The pasture had a negative impact on coverage, height, and early flowering. Structure and phenology were more sensitive than to changes in rainfall between years. Phenology results will help determine the best time to harvest the phytomass (phytomass stock), as the nutritional quality of the forage is known to decrease after fruiting. Keywords: annual herbaceous plants; grazing; pastoralism; phenology; rainy season; vegeta- tion structure 1. Introduction The Sahel is a bio-geographic entity defined primarily by its arid-to-semi-arid tropical climate, controlled by the Gulf of Guinea monsoon and the Saharan Harmattan [1]. Its average rainfall varies from 100 mm at the desert border to 600 mm at the southern limit of the Sahel in contact with the Sudanian zone [2]. It is probably the largest ecological zone in the world with the highest average annual temperature (28–30 ◦ C), very close to that of the southern Sahara [1]. The minimum annual average is between 18 and 20 ◦ C and the maximum average between 35 and 38 ◦ C[3]. Pastures receiving between 200 and 450mm of annual rainfall are traditionally exploited by transhumance [4]. Savannah, bush, and steppe formations are characteristic of regions influenced by Sahelian and Sudanian climates. These climates, characterized by their long dry seasons, lead to formations that have adapted to the severity of climatic conditions. The Sahel is the domain of thorny steppes. Sahelian vegetation is therefore characterized by a high degree of heterogeneity. On the one hand, the factors that determine this heterogeneity are essentially related to water and, on the other hand, to the factors linked to the action of herds (due to the Grasses 2023, 2, 98–111. https://doi.org/10.3390/grasses2020009 https://www.mdpi.com/journal/grasses