Johnsongrass competition and control in soybeans

Field studies were conducted at two locations in West Tennessee to determine the competitive effect of johnsongrass [Sorghum halepense (L.)Pers.] on soybeans [Glycine max(L.)Merr.]. The objectives of these studies were to determine: (1) the most critical parameters in predicting the economic threshold of johnsongrass in two common soybean cropping systems in the Midsouth, (2) the critical johnsongrass-free requirement of soybeans, (3) the critical duration of johnsongrass in soybeans, (4) the optimum time for application of sethoxydim [[2-[-1- (ethoxyimino) butyl-]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen- 1-one]] at 1.12 kg/ha plus oil concentrate at 1 percent v/v to johnsongrass, and (5) the degree of competition for available soil water between johnsongrass and soybeans.

All experiments were conducted as randomized complete block designs with a minimum of four replications. The threshold studies consisted of either eight rows or three rows by 9.1 m long for the .25 m and 1.0 m row spacings, respectively. The competition study consisted of three rows one m wide by 7.6 m long. Critical population and measurements were taken from the center row of each plot or its equivalent.

Johnsongrass dry matter and the number of johnsongrass culms produced per hectare were more highly correlated with soybean yield reduction than the number of johnsongrass plants per hectare. The number of johnsongrass plants per hectare was determined to be the most practical means of predicting soybean yield reduction. Regression analysis indicated that each johnsongrass plant reduced soybean yields 30 to 45 g in the two row spacings.

The critical johnsongrass free requirement of soybeans was determined to be four weeks after planting. If soybeans were not kept free of johnsongrass for at least four weeks, johnsongrass would be able to reestablish and reduce soybean yields.

The critical duration of johnsongrass was determined to be seven weeks after planting for mechanical removal and five weeks after planting for chemical control with sethoxydim. The critical duration of johnsongrass in soybeans spans a wider range depending on control measures employed, since mechanical removal is a physical process occurring within a short period of time while the chemical control is a physiological process occurring over a period of several hours to several days depending upon johnsongrass stage of growth and environmental conditions.

Optimum johnsongrass control was obtained when sethoxydim was applied to johnsongrass at a height range of 25 to 50 cm.

Soybean yield per cm of available soil water was highest where johnsongrass was controlled season-long or by applying sethoxydim at four weeks after planting. This observation indicates that when water is critically limiting, soybeans are better able to utilize available soil water when johnsongrass is not present.