The GOSSYM-COMAX model for simulating cotton growth and production was evaluated during the 1988 crop season on two field experiments at Stoneville, MS. The objective of this evaluation was to determine the feasibility of using this crop model as part of an overall crop production system model and to develop a better understanding of the capability of GOSSYM-COMAX to simulate cotton production on selected soil types. The evaluation consisted of comparisons between simulated values and field plot measurements for factors such as timing of fruit production, plant height, and crop yield.

The field experiments used for the evaluation were installed on two soil types, Tunica clay and Bosket very fine sandy loam. The study on the Tunica soil included two levels of each of the following production factors: irrigation, primary tillage, nitrogen fertilization, and plant population. The study on the Bosket soil included two levels of these factors: row spacing, variety, and growth regulator. The study located on Tunica soil was installed in a wide-bed production system with two rows 1 m apart per bed, and the other study was solid planted on 0.75 m and 1 m row spacings. Standard production practices were used for weed control, and an aggressive insect control program was maintained throughout the season to minimize damage by insects. Precipitation during the growing season was well below normal and resulted in non-uniform growth patterns, especially on non-irrigated treatments on the Tunica soil. All plots on the Bosket soil were irrigated as needed.

Field observations were made to establish the dates of emergence, first square, and first open boll. In addition, plants from the various treatments were mapped three times at intervals of approximately 2 weeks beginning 1 week after the first bloom. Mapping measurements included plant height, number of main stem nodes, leaf area, nodal position of each fruit form, and the dry weight of leaf, stem, square, and boll tissue.

First year results indicated that the model predicted earlier fruiting and maturity than was observed in the field. This trend was more pronounced on the Tunica soil than on the Bosket soil. Plant heights on the Tunica soil were less than the simulated heights but followed a pattern similar to the simulation. Plant height on the Bosket soil followed the simulation very closely up until the crop started to mature. The simulated crop then experienced stresses which caused height to remain at about the same value for the rest of the growing season and allowed actual growth to exceed predicted growth. Lint yield comparisons were mixed with respect to the nature and magnitude of differences between simulated and measured values. For some treatments, predicted yields exceeded measured yields, for others the reverse was true, and yet others agreed closely.

The overall results were not surprising in light of the production complications experienced during the growing season. Chemical burn from herbicide drift occurred on the 34th day after emergence of the crop on the Tunica soil. No squares were visible at this time, but the first 6 true leaves exhibited yellowing with the margins of some leaves exhibiting senescence. Very few plants died as a result of the burn, but growth was retarded for 5 or 6 days. Therefore, delayed fruit initiation and stunted growth were not surprising for treatments on the Tunica soil.

Soils data used in the model were obtained from previously developed GOSSYM-COMAX soils files developed for soils of the same type as that on the test sites.

Representative field measurements of plant factors associated with individual treatments were difficult to obtain due to the non-uniform growth patterns which occurred. The non-uniform growth was more pronounced on non-irrigated treatments, but was present to some extent on all treatments.

In general, simulated values for the plant parameters compared in this evaluation did not agree closely with the measured values, but values for plant height did tend to parallel the simulated values during most of growing season. It is felt that the models capabilities have not been adequately evaluated at this location; therefore, this effort will be continued.