Short-term dynamics of standing mass and digestibility of herbaceous forages under grazing or set-aside in the Sahel region of Senegal

Abstract

The effect of grazing on the evolution of the herbaceous mass and its digestibility during the year was studied at Centre de Recherches Zootechniques de Dahra (CRZ), in the Senegalese Sahel. It is a livestock production area where the herbaceous layer, basis of the livestock feed, has a short growth period during the rainy season (3 to 4 months) and therefore conditions a particular resource use strategy. The study was conducted during the rainy and dry seasons over two consecutive years: from July 2018 to June 2020. The measurements of phytomass and forage quality (in vitro digestibility of organic matter, DMO) started ten days after the first rainfall and were repeated every ten days in the rainy season and thirty days in the dry season. The dynamics of the annual herbaceous is in three stages: a first one of low growth, a second one of rapid growth, and a third one of senescence. The maximum amount of standing mass under grazing was 1,992 ± 755 kg DM.ha^-1 in 2018 and 1,801 ± 601 kg DM.ha^-1 in 2019 while in the fenced area it was 2022 ± 375 kg DM.ha^-1 in 2018 and 2357 ± 664 kg DM.ha^-1 in 2019. DMO decreased markedly over the growing season: from 70% at the beginning of the rainy season, it averaged 52% during the rainy season and 30% over the dry season. During the growing season, the organic matter digestibility decreased as the herbaceous mass increased. The forage remained more digestible under grazing than under fencing. Grazing did not systematically decrease the amount of available phytomass significantly but improved the forage digestibility.