Development of a lucerne model in APSIM next generation: 2 canopy expansion and light interception of genotypes with different fall dormancy ratings

Lucerne (Medicago sativa L.) canopy expansion, as quantified by leaf area index (LAI), is the crop process that determines the amount of intercepted total radiation during each regrowth cycle. A challenge is to capture seasonal changes of canopy expansion rate in response to the environment. This research integrates parameters and functions of lucerne canopy expansion into the Agricultural Production Systems sIMulator (APSIM) next generation (APSIM NextGen) model (LeafArea module) to simulate canopy expansion and light interception. Over 20 years of detailed field experimental datasets, with multiple treatments, from Lincoln University were used for model development. Functions derived from a fall dormancy (FD) 5 rated genotype were grown under an industry standard defoliation treatment to parametrize the model. These functions were tested further using genotypes with an FD2 or FD10 rating under longer and shorter defoliation regimes, all under irrigated conditions. The APSIM NextGen lucerne model predicted the LAI expansion pattern in each growth cycle as a double sigmoid curve requiring functions that define the lag phase, basal bud initiation, the linear leaf area expansion rate (LAER; m2 m-2 degrees Cd), and canopy senescence which represents the loss of LAI over time. LAI was well predicted for experiments under the standard (42-day) and long (84 day) defoliation treatments for FD5, with NashSutcliffe efficiency (NSE) of 0.61 and 0.55. However, the derived parameters and functions overestimated LAI under an extreme short defoliation treatment (28-day), NSE values ranged from 0.38 to 0.78. LAER was lower for the short-defoliation intervals (28-day), probably due to a depletion of carbon and nitrogen reverses in perennial organs. For FD2 and FD10, different LAER functions were generated from field observed data and used to improve simulation agreement. There was fair agreement for the 84-day treatment (NSE of 0.32) and the 42-day treatment (NSE of 0.38), but poor agreement for the 28-day treatment for FD10 (NSE = -0.88). The estimated extinction coefficient (k) was the same for seedling and regrowth crops, and consistent across defoliation treatments and FD classes. With the LeafArea module and k value, the APSIM NextGen lucerne model can now estimate daily LAI and intercepted radiation. Future model development includes validating the LeafArea module in different environments. However, a more mechanistic model approach is required to link canopy expansion to carbon and nitrogen reserves in lucerne plants that experience intense defoliation.