T he EPIC plant growth model was developed to estimate soil productivity as affected by erosion throughout the U.S. Since soil productivity is expressed in terms of crop yield, the model must be capable of simulating crop yields realistically for soils with a wide range of erosion damage. Also, simulation of many crops is required because of the wide variety grown in the U.S. EPIC simulates all crops with one crop growth model using unique parameter values for each crop. The processes simualted include leaf interception of solar radiation; conversion to biomass; division of biomass into roots, above ground mass, and economic yield; root growth; water use; and nutrient uptake. The model has been tested throughout the U.S. and in several foreign countries.

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Weight per seed (SDWT) of maize (Zea mays L.) has been rcported to show no response when seed number was manually decreased. This field experiment was designed to test the generality of this lack of response across techniques of reducing seeds per ear and across genotypes. Bagging ears of a dent hybrid 2 to 4 d after silking reduced seed number by 15 to 45% and increased SDWT by 19 to 25%. The same treatment applied to a popcorn hybrid reducvd seed number by 12 to 36% with no significant increase in SDWC. Open pollinated varieties representing different seed sizes showed similar variability in SDWT response to reduced seed number. When SDWT of the dent hybrid and one of the varieties increased in response to reduced reed number, seed volume increased by a similar fraction. Removal of the apical half of the ear of the dent hybrid caused a different SDWT response than ear bagging. Such removal 5 to 15 d after silking decreased SDWT while removal 20 to 25 d after silking produced no significant change. Thus, it appeared that the response of SDWT to reduced seed number depended on genotype, how seed number was reduced, and when the reduction occurred. The reduction in SDWT due to early ear cutting was not due to a reduction in number of endosperm cells or volume of the developing seeds. The possibility of a physical restraint on SDWT by the developing pericarp of some cultivars was discussed.

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