Seasonal effects of drought on the productivity and fodder quality of temperate grassland species

During a two-year field experiment we tested the effects of seasonal drought events on the growth rates, physiological activity and fodder quality of several perennial C3 grasses that are commonly used for agricultural production. All tested grasses were subject to at least ten weeks of rainfall exclusion, either in spring, summer or fall of the years 2014 and 2015. The resistance of growth rates varied between seasons. While growth losses were lowest due to drought in spring, losses during summer and fall drought were much higher. Due to strong compensational effects after the release from drought annual net primary productivity was hardly affected by drought in either season. Against our expectations, growth rates of reproductive developmental stages were less affected by drought than those of vegetative stages. Moreover, measurements of leaf water potential and stomatal conductance, as well as carbon discrimination, showed that generative spring growth was least affected by drought. Plants during reproductive stages, thus, may invest more in drought resistance than during vegetative growth. Forage quality patterns, including nitrogen content, crude ash, digestible organic matter and net energy for lactation, were less straightforward in their response to seasonal drought. While quality parameters were reduced under drought in general, the release from drought enhanced forage quality, but seasonal differences could not be perceived.