In our group we use crop models mainly to i) gain an insight into the crop growth dynamics in complex agricultural systems and fluctuating environments, ii) characterize the main constraints of production environments, iii) assess potential of new technologies (e.g. success of new cultivars, optimization and construction of specific “cultivar x management packages”) in given environment and, iv) “what if analysis” and future foresight (e.g. in context of climatic changes). APSIM model requires several inputs to simulate the crop within the system: i) daily weather information (Tmax, Tmin, rainfall, solar radiation), ii) soil information (soil type, water &nutrient status), iii) crop and genotype specific coefficient.Calculation of genotypic coefficients for APSIM is generally the most effective if done in this particular order;

- Phenology (~60% fit with observed data)
- Photoperiod sensitivity (wherever relevant)
- Canopy (~30% fit with observed data)
- Grain number/grain size (~10% fit with observed data)

Detailed description of particular crop parameters can be found at www.apsim.com. In the case of sorghum, reasonable minimum parameterization requirement can be achieved by:

**Add 1**

Calculating the thermal time requirement from “end of juvenile” to “panicle initiation” phase (tt_endjuv_to_init, APSIM sorghum module annotation) which is generally the most variable phenological phase determining the whole crop duration.To determine the crop duration we need to observe thermal time to flowering in the field at minimum. Then tt_endjuv_to_initpheno-phase can be calculated as;

tt_endjuv_to_init= [tt to flowering (observed) – tt_germ_to_emerg (conserved) –tt_emerg_endjuv (conserved) – tt_init to flag (conserved) - tt_flag_to_flower (conserved)];

(Note: We can improve the phenological coefficients estimation if we record tt to panicle initiation, tt to flag leaf,and tt to maturity.)

**Add 2**

Duration of the crop is sensitive to photoperiod (calculated as a function of day of year and latitude; can be outputted from APSIM) if the cultivar is photoperiod sensitive. The fotoperiod affects sorghum tt_endjuv_to_init phase and is generally stable [pp (11.5;13.5h)]. The most variable coefficient is the slope of extension in duration of tt_endjuv_to_init phase (pp_slope [tt_endjuv_to_init extension per h of pp) which has to be calculated. The minimum observation requirement is tt to flowering of the crops sown in different time of the year within photoperiod sensitivewindow11.5-13.5h. For these tt_endjuv_to_init (above)and consequently the slope of extension in duration of tt_endjuv_to_init phase can be calculated.

**Add 3**

Released APSIM version canopy module simulates the canopy based on the total plant leaf area (TPLA) approach which requires calculation of i) TPLAmax, ii)TPLA inflection ratio, iii) TPLA production cf.

TPLA approach of canopy prediction integrates the number of fully expanded leaves, their individual size, and tiller number, and includes an adjustment for the area of expanding leaves (Hammer et al., 1993). To parameterize the TPLA coefficients, we need several observed data points on LA and corresponding TT, tiller number, thermal time to flag leaf (can be generalized by TT_to_flower-100oCdays) during the plant growth. Then TPLAmax defines the maximum total plant leaf area as:

tplaMax = (pow(FTN + 1.0, tillerCoef) * pow(finalLeafNo,mainStemCoef)) * scm2smm; TillerCoef = 0.66 (is generally considered constant); FTN- fertile tiller No, We optimize the mainStemCoef (using solver add-in in excel [hyperlink to xls]) TPLAinflection ratio, TPLAproductioncf define skewness and breath of the TPLAmax function (“early vigour” related parameters); then every day TPLA is calculated:

TPLA = TPLAmax / (1 + exp(-tpla_prod_coef * (TTemerg_to_now - tpla_inflection))); TPLA inflection = (TTemerg_to_flag * tpla_inflection_ratio) TTemerg_to_flag = Flowering time - 100 TT Here we optimizec_tpla_inflection_ratio ; -p_tpla_prod_coef (using solver add-in in excel [hyperlink to xls])

**Note:** leaf_app_rate1 andleaf_app_rate2 [tt requirement per leaf-1and tt requirement per leaf-1of last 4 leaves] is considered rather constant, but can be calculated and parameters changed;

**Add 4**

For most of the cultivated genotypes grain number [dm_per_seed] and grain size [maxGFRate] coefficients are rather common. However, these coefficients can be calculated as: Maximum grain number is a function of the plant growth rate between panicle initiation and the start of grain filling;

[maxGFRate = (deltaDMPlant)/nDays]

and the final grain number is calculated by dividing the growth rate by a genotypic value.

Grain number = growthRate/dm_per_seed

Grain size is determined by grain growth rate, the effective grain-filling period, and the redistribution of assimilates post-anthesis. Grain size is determined by calculating the potential daily grain filling rate which should not exceed the set maximum grain filling rate (maxGFRate, genotype specific value). Grain demand or potential grain filling rate (maxGFRate) is a function of the increase in plant biomass per grain per degree day (totDMCaryopsis, above). Then:

maxGFRate= (0.0000319 + 0.4026 * totDMCaryopsis) * 1000; // in mg/grain/oCd

The parameterization sheets to ease calculation of particular crop coefficients for released APSIM model can be found here:

- Download Excel
- Download Excel
- Download Excel

**Photoperiod sensitivity (xls allowing calculation of pp_slope)**

**Canopy parameters (xls allowing optimization for TPLAmax, TPLAinflection ratio, TPLAproduction cf, leaf_app_rate1 and leaf_app_rate2 using solver application)**

**Grain number/grain size (xls allowing calculation of these coefficients)**