Modelling responses of forages to climate change with a focus on nutritive value

Process-based models (PBMs) are important tools for predicting and understanding the impacts of climate change on grassland systems. The models should be able to simulate changes in sward nutritional value (NV) over time in order to better understand the interactions between grasslands and ruminant nutrition. Changes in the NV of forage may not only alter animal performance but also the need for other feeds, the management and productivity of the system, the quality of final products and the environmental impacts of production.Climate change is expected to affect the NV of grasslands by affecting plant physiological processes, and viaeffects on species composition. An increase in temperature may cause an increase in NDF and lignin content of forages, thus reducing digestibility by domestic herbivores. On the other hand, under high CO2, analyses on both temporary and permanent grasslands indicate a strong increase in soluble sugar content, which increases the energetic value and the aptitude for ensiling of grass. Conversely, a decrease of the CP content in grass dry matter up to 30% is observed. The reduction in the forage protein and energy content lowers the rumen microbial synthesis and availability of microbial proteins for ruminant growth and production, and may also lead to increased production of methane (a greenhouse gas) by methanogens in the rumen. Ruminants kept in extensive systems that are based on low protein forages may be sensitive to these negative effects.On the contrary, an increase in soluble sugar and decrease in CP content would rather be positive for ruminants in intensive high protein forage systems. Modelling grassland NV is often based on variables describing the energy and protein content of forage. However, a wide range of variables are used to define forage NV in experimental data, presenting challenges for modellers. Further complication arises from the dissimilar feeding regimes used across the different production systems in countries and regions. The aim of this work was to review the extent to which current grassland PBMs are capable of characterising the NV of forage species in grassland swards in relation to projected climate change. This includes the identification of the modelling approaches used, the key characteristics of the forages represented and the production systems these models have been developed for.