Drought events: characterisation and effects on grassland agronomic services

The comparison of drought events requires common criteria reflecting the stress ‘experienced’ by plants. This study evaluates the relevance of different indices for the characterization of the intensity of drought stress. The reflections concerning the indices are based on the results of drought trials (simulations with rain-out shelters) conducted by Agroscope between 2012 and 2014 on intensively managed grasslands situated at an altitude of between 470 and 1200 m. The indices based solely on climatic parameters are inadequate for describing the intensity of a drought treatment. The balance between evapotranspiration and precipitation (P – ETP), for example, does not permit us to characterise the intensity of the stress ‘experienced’ by plants, since it does not take into account the relationships between vegetation and soil. The indices based on the ratio between the actual water reserve in soil and the available water capacity are more relevant, and allow a better explanation of the yield gap between the plots exposed to drought and those properly supplied with water. In our trials, the yield gaps varied between 13 % and 60 %, depending on the situation. The effects of drought on the nutritive value are more difficult to pinpoint than the effects of drought on yield. The aforementioned indices do not permit an explanation of the observed variations in nutritional value. Drought affects vegetation and/or the environment via different processes, which may influence in an antagonistic way the quality of the forage. Despite this, in the absence of a major change in botanic composition, drought has much less impact on the quality of forage than on yield.