Johnsongrass competition and control in corn

Field studies were conducted at two locations in East and Middle Tennessee to determine the competitive effects of johnsongrass [Sorghum halepense (L.) Pers.] on corn (Zea mays L.). The objective of these studies was to determine: (1) the competitive effects of johnsongrass on corn in relation to yield reduction and total biomass produced, (2) the critical johnsongrass-free requirement of corn, (3) the critical duration of johnsongrass competition in corn, (4) the percent johnsongrass control obtained from EPTC (S-ethyl dipropylthiocarbamate) and butylate (S-ethyl diisobutylthiocarbamate) plus the herbicide antidote R-25788 (N,N-diallyl-2,2-dichloroacetamide) and EPTC plus R-25788 and the herbicide extender R-33865 (0,0-dietyl-0-phenolphosphorothioate) and (5) the economic threshold of johnsongrass in corn.

All experiments were conducted as randomized complete block designs with four replications. Individual plots in both the threshold and the competition studies consisted of four rows, spaced 91 cm apart by 6.1 m long. Critical data were obtained from the two center rows.

Spring Hill

The critical johnsongrass-free requirement of corn was determined to be between two and four weeks after planting. A johnsongrass-free period of less than four weeks after planting allows sufficient johnsongrass regrowth resulting in significant yield reductions.

The critical duration of johnsongrass competition in corn was determined to be between four and six weeks after planting. Significant stover yield reductions occurred when johnsongrass was allowed to grow with corn for four or more weeks after planting, while six or more weeks of johnsongrass competition were required to significantly reduce grain yields. Season-long johnsongrass competition in corn reduced grain yields by 50% and stover yields by 67% and resulted in 20% lodging.

EPTC or butylate, plus the herbicide antidote did not provide acceptable johnsongrass control.

Johnsongrass plant populations of 16/6.1 meters of row or higher significantly reduced corn grain yield.

Knoxville

From evaluation of both the corn grain yield and the corn stover yields it was determined that no critical johnsongrass-free requirement existed for corn under these growing conditions.

The critical duration of johnsongrass competition in corn was determined to be between six and eight weeks after planting. Seasonlong johnsongrass competition in corn resulted in an 18% reduction in grain yields and a 37% reduction in stover yields.

EPTC plus the herbicide antidote and the herbicide extender resulted in good end-of-season johnsongrass control; approximately 80%. Yields from corn treated with the same herbicide were not significantly different from the weed-free check.