  Growing mixtures of annual arable crop species or genotypes is a promising
way to improve crop production without increasing agricultural inputs. To
design optimal crop mixtures, choices of species, genotypes, sowing proportion,
plant arrangement, and sowing date need to be made but field experiments alone
are not sufficient to explore such a large range of factors. Crop modeling
allows to study, understand and ultimately design cropping systems and is an
established method for sole crops. Recently, modeling started to be applied to
annual crop mixtures as well. Here, we review to what extent crop simulation
models and individual-based models are suitable to capture and predict the
specificities of annual crop mixtures. We argued that: 1) The crop mixture
spatio-temporal heterogeneity (influencing the occurrence of ecological
processes) determines the choice of the modeling approach (plant or crop
centered). 2) Only few crop models (adapted from sole crop models) and
individual-based models currently exist to simulate annual crop mixtures. 3)
Crop models are mainly used to address issues related to crop mixtures
management and to the integration of crop mixtures into larger scales such as
the rotation, whereas individual-based models are mainly used to identify plant
traits involved in crop mixture performance and to quantify the relative
contribution of the different ecological processes (niche complementarity,
facilitation, competition, plasticity) to crop mixture functioning. This review
highlights that modeling of annual crop mixtures is in its infancy and gives to
model users some important keys to choose the model based on the questions they
want to answer, with awareness of the strengths and weaknesses of each of the
modeling approaches.
