Field experiments were established in 1989 and 1990 at the Southern Piedmont Conservation Research Center near Watkinsville, GA, to determine effects of coastal bermudagrass density on cotton in a conservation tillage system. Cotton height, canopy width, leaf area indices, and seed cotton yields were determined at coastal bermudagrass densities of 0 to 3600 kg ha−1 as achieved by herbicide inputs. Soil water measurements were recorded in cotton plots with a range of coastal bermudagrass densities using time domain reflectometry. Cotton growth and yields were reduced by coastal bermudagrass competition both years of the study. At the highest coastal bermudagrass density of 3600 kg ha−1, cotton height was reduced compared to cotton alone as early as 5 wk after planting. Seed cotton yields were reduced 25% at the highest coastal bermudagrass densities both years of the study. At the 15-cm soil depth, coastal bermudagrass significantly reduced soil water in cotton. Soil water in cotton at 30, 45, and 60 cm was not affected by coastal bermudagrass.

Cotton, spurred anoda, and velvetleaf were grown in controlled-environment chambers at day/night temperatures of 32/23 or 26/17 C and CO2 concentrations of 350 or 700 ppm. After 5 weeks, CO2 enrichment to 700 ppm increased dry matter accumulation by 38, 26, and 29% in cotton, spurred anoda, and velvetleaf, respectively, at 26/17 C and by 61, 41, and 29% at 32/23 C. Increases in leaf weight accounted for over 80% of the increase in total plant weight in cotton and spurred anoda in both temperature regimes. Leaf area was not increased by CO2 enrichment. The observed increases in dry matter production with CO2 enrichment were caused by increased net assimilation rate. In a second experiment, plants were grown at 350 ppm CO2 and 29/23 C day/night for 17 days before exposure to 700 ppm CO2 at 26/17 C for 1 week. Short-term exposure to high CO2 significantly increased net assimilation rate, dry matter production, total dry weight, leaf dry weight, and specific leaf weight in comparison with plants maintained at 350 ppm CO2 at 26/17 C. Increases in leaf weight in response to short-term CO2 enrichment accounted for 100, 87, and 68% of the observed increase in total plant dry weight of cotton, spurred anoda, and velvetleaf, respectively. Comparisons among the species showed that CO2 enrichment decreased the weed/crop ratio for total dry weight, possibly indicating a potential competitive advantage for cotton under elevated CO2, even at suboptimum temperatures.