Introduction The U.S. cotton yield per unit of area has increased steadily since the thirties of last century. The national average yield of Upland cotton fluctuated around a mean of 213 kg [ha.sup.-1] [yr.sup.-1] of lint with no upward or downward trends from 1866 to 1936 (Miller, 1977). Meredith and Bridge (1982) reported that the national cotton yield rose rapidly from 1936 through 1960, with an average increase of 10.4 kg [ha.sup.-1] yr-1. During the period of 1961 to 1988, the average rate of gain in the national cotton yield was 5.6 kg [ha.sup.-1] yr-1 (Culp and Green, 1992). Cotton yield continued to increase but the rate has decreased since 1988 (Meredith, 2000; Lewis, 2001). Yield increases can be attributed to technological advances in production, such as the use of higher yielding cultivars, commercial fertilizer, irrigation, effective pesticides, and mechanization (Culp and Green, 1992).