General inputs for computing yield are the desired planting date (PD), the length of the growing season (IGROW) and the last day of the year to plant without yield loss (JLAST).  Planting date delays are characterized on a specified planting period for optimum yields.  The model considers the number of days to prepare the seed bed and plant (REQWRK) as prerequisite for planting.  Using the first work period window specified in the Trafficability tab of the Crop input screen, DRAINMOD keeps track of work days with REQWRK as the minimum needed for planting.  When enough working days have occurred to complete the operation, the planting date is fixed, the length of planting date delay is determined, and the relative yield is computed.  Two yield reduction factors are used, PDRF and PDRF2.  If planting occurs after the optimum period but prior to the end of the first reduction period (with length DELAY1), then yields are reduced by PDRF.  When planting is delayed after the first reduction period, yield is adjusted using PDRF2.

 

The relative yield considering planting date delays independent of other stresses can be written as

 

YRp = 100 PDRF * PDELAY,        if PDELAY < DELAY1,

 

or

 

YRp = 100 PDRF * DELAY1 PDRF2 * (PDELAY DELAY1) ,         if  PDELAY > DELAY1

 

where PDELAY is the number of days since the last day of the year to plant without yield reduction.  Parameters for several locations were summarized by Seymour (1986) and are presented in Table 13.  A consideration with yield calculations is that the spring trafficability window should be large since the planting delay factor will account for the working days.

 

Table 13.  Corn yield response factors for planting date delays (adapted from Seymour, 1986).

 

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Location                Optimum        DELAY1        PDRF                PDRF2

Planting 

Date

----------------------------------------------------------------------------------------------------------------------

England                4-30                  30                 0.95                  3.6

Illinois                4-30                  31                 0.81                  ----

Iowa                        5-15                  30                 0.76                  2.1

Kansas                5-5                  20                 0.62                  ----

Louisiana                3-15                  45                 0.72                  ----

New York                5-1                  13                 0.89                  1.32

North Carolina        4-10                  42                 0.88                  1.62

 

North Dakota        5-17                  38                 0.79                  ----

Ohio                        5-1                  22                 0.60                  1.8

 

MEAN VALUES

(Excludes England)                   30                 0.76                  1.71