Predicted yield and nutritive value of an alfalfa–timothy mixture under climate change and elevated atmospheric carbon dioxide

Climate change studies have often focused on individual forage species although legume-grass mixtures are predominant on dairy farms in northern areas of North America. We assessed the effect of (i) future climate conditions (temperature and precipitation) and elevated atmospheric CO₂ concentration ([CO₂]), separately and together, on yield of alfalfa (Medicago sativa L.) and timothy (Phleum pratense L.), grown alone or in mixture, and (ii) an adaptation strategy (timing and number of harvests) on future yield and nutritive value of an alfalfa–timothy mixture. Forage dry matter (DM) yield and nutritive value for two contrasting climate areas in eastern Canada were simulated with the Integrated Farm System Model over two future periods (2020–2049 and 2050–2079) using three climate models and two representative concentration pathways (RCP 4.5 and 8.5) of greenhouse gas emissions. Under projected future climate and without adaptation, annual forage yield of both species and the mixture increased in the colder area and decreased in the warmer area. In both areas, first-cut yield increased due to faster growing degree-day accumulation, while regrowth yield decreased due to greater water and temperature stresses. Under elevated [CO₂], annual yield and the alfalfa percentage in the mixture increased. When combining climate change and elevated [CO₂], yield increased, except with the more drastic scenario (RCP 8.5, 2050–2079) in the warmer area, and forage nutritive value was reduced. With adaptation, the mixture yield was increased from 5 to 35%, while nutritive value was generally maintained under all future scenarios, mostly because of additional cuts.