Drought impact on the nutrients of forage plants in a semi-arid rangeland and its potential implications for sustaining herbivores

Abstract

The impact of drought on the nutrient content of important natural forage plants in semi-arid rangelands is poorly understood. Most studies to date have shown that drought induced stress resulted in lower plant nutrient concentrations. The consequential declines in forage quality may be an important limiting factor in sustaining wild herbivore populations through periods of drought. In the semi-arid southern Kalahari of South Africa, the most severe drought recorded in the last 60 years resulted in high plant and animal mortalities. The aim of this study was to determine if the severe drought impacted the nutritional value of plants in this region and resulted in nutrient deficient forage. For this, the nutrient content of the most abundant woody and grass species was determined during the drought and the first year of above-average rainfall following the drought. The impact of drought differed markedly between dicotyledonous woody species and monocotyledon grasses. In the former, it resulted in decreased crude protein concentrations but increased macronutrient concentrations. In the latter, crude protein contents were higher and the content of most macronutrients lower. Despite these differences, the forage on offer to herbivores during the drought was not nutrient deficient. During the drought, the dry seasons when plant nutrients were naturally at their lowest, persisted for longer than normal. The prolonged dry seasons also resulted in shortened wet seasons, when plants were most nutritious. It was concluded that the prolonging of the period when forage quantity and quality is most limiting, is likely the main limitation in sustaining herbivores during periods of drought. More studies in semi-arid rangelands are however needed to better understand the impact of drought on plant nutrients in these systems.