Effects of management regimes and harvesting date on herbage quality in rangelands: relationships with traits.

Permanent grasslands supply ecological and agronomical services, among which herbage quality for herbivores is of prime importance. Environmental factors as the climate and management regimes (fertilization or grazing) affect herbage quality, as assessed by its digestibility and nutritive value content (crude protein, fibre content). Most previous studies have focused on digestibility only, and for a limited number of species. In the present study, we used plant functional traits in order to understand the effects of management regimes on herbage quality of rangelands located on Larzac Causse in the South of France. 16 abundant species from rangelands submitted to grazing associated or not with fertilization were studied. Herbage quality (digestibility, crude protein content and fibres content), leaf/stem ratio, dry matter content (DMC), N and C content of all aboveground organs were measured at two dates: the growth beginning and the peak of biomass. Our results showed at interspecific level that (i) fertilization improved herbage quality via an increase in crude protein content and a decrease in fibre content, which resulted in an increase in digestibility; in addition, the dry matter content of stems and leaves decreased and N content increased; (ii) there was a decrease in herbage quality between the beginning of growth and the peak of biomass produced, associated to an increase in DMC and a decline in N concentration; (iii) herbage quality and functional traits differed depending on the organ considered, leaves showing a better quality as compared to others. Functional traits can thus be used to predict differences in herbage quality in relation with management regimes, dates of sampling and the organ considered. (Résumé d'auteur