Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Field studies were conducted to document seedling johnsongrass Sorghum halepense (L.) Pers. growth as influenced by date of emergence in sugarcane (Saccharum interspecific hybrids, cultivar 'CP 65-357'). The crop was in the plant cane year of growth in 1982 and 1983, and the first stubble year in 1984. The experiments were randomized complete block designs. Johnsongrass seedling emergence days were approximately three weeks apart, from April or early May through mid-July for a total of five emergence days each year. There were 36, 60, and 72 replicates for emergence days numbered 1 to 3, 4, and 5, respectively, over three years. Individual johnsongrass plants were established in the center of the plot and plants were a minimum of 2 m apart. Seedling johnsongrass physical characteristics were measured and recorded weekly until mid-July. After this time, sampling was at 2 to 3 wk intervals. Johnsongrass survival, culm height, canopy diameter, stems/plant, seedheads/plant were measured across time. In the fall prior to first frost, standing biomass was harvested and recorded after drying for 96 hr at 80(DEGREES)C. Survival analysis gave a X('2) = 1.85 (3 df, P = 0.6) and described a negative linear relationship with later emergence in sugarcane. Plants from emergence days 1, 2, 3, 4, and 5 survived until end-of-season 83, 64, 47, 20, and 10% of the time, respectively. These surviving plants overwintered to produce rhizome johnsongrass plants the next spring 47, 25, 17, 3, and 0% of the time, respectively. Negative linear relationships with later emergences were observed for height of tallest johnsongrass culms, canopy diameter, and seedheads/plant at end-of-season (Sum of Squares Regression/Sum of Squares Treatments, or Q('2), = 0.97, 0.91, and 0.76, respectively). Stems/plant showed a negative exponential relationship with time of emergence at end-of-season (Q('2) = 0.99). Johnsongrass height and canopy diameter growth rates also declined linearly with time of emergence in sugarcane (Q('2) = 0.83 and 0.76, respectively). Surviving johnsongrass plants from emergence day 1 attained a mean height and canopy diameter of 209 and 176 cm, respectively, at end-of-season. Plants from emergence day 5 produced less than 1% of the biomass produced by emergence day 1 plants. Cumulative distribution analysis indicated that a sugarcane canopy radius of 75 to 80 cm would control subsequent johnsongrass emergences and as such, marked the end of the period for required control of seedling johnsongrass in sugarcane.