A short-term effect of multi-species pastures and the plant’s physiological response on pasture growth

Multi-species pastures that include species with dissimilar physiological responses to water restriction (i.e., dehydration avoidance) and species that have a greater soil water access (i.e., deep-rooted species), can buffer the reduction in summer growth experienced by more shallow-rooted species and can out-yield pastures with less capacity to access soil water. The aim of the present study was to evaluate the performance of multi-species pastures comprising both deep-rooted and shallow-rooted species, under grazing by sheep, in relation to pasture growth, the seasonal changes in pasture botanical composition, and the plants’ physiological responses to field conditions across the year. The evaluated pastures were: Bromus valdivianus (Bv); Lolium perenne + Trifolium repens (Lp); Plantago lanceolata (Pl); B. valdivianus + L. perenne + T. repens (grass mixture pasture, GM); and B. valdivianus + L. perenne + T. repens + P. lanceolata (complex mixture pasture, CM). Herbage mass, net photosynthesis, leaf water relations and soil water content (SWC) at 10, 40 and 70 cm soil depths were evaluated. The SWC fluctuation through the year was related to rainfall within each season, and differences between the treatments were taken as an indirect indicator of soil water access and soil water uptake by the roots. During the soil water restriction period, mixed pastures based on species that differed in their soil water access buffered the reduced growth rate of the sensitive species, due to a decrease in competition for water uptake within the deeper soil layer (CM pasture). A shift of dominance between species, as measured in the CM pasture, buffered the reduction in pasture growth rate seen in the monocultures. The Bv pasture, which is tolerant of water restriction, had a similar annual yield to the multi-species pastures (both GM and CM). The present study highlighted the capacity of the deep-rooted species (B. valdivianus and P. lanceolata) to avoid periods of soil water restriction by extracting water from deeper soil layers.