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Effect of duration of jointed goatgrass interference on winter wheat grain yield

R. L. Anderson
1992 Res. Prog. Rep. West. Soc. Weed Sci.
Researchers are evaluating herbicides for control of jointed goatgrass in winter wheat. Postemergence herbicides are preferred to soil-applied herbicides because producers could treat only the infested areas of their fields and thereby reduce herbicide costs. If postemergence herbicides are developed for jointed goatgrass control, knowledge of the “critical” interference period will aid in deciding when to apply these herbicides.

This 2-year study examined duration of jointed goatgrass interference in winter wheat. Jointed goatgrass was established at 18 plants/m2 0, 2, 4, and 6 weeks after ‘Vona’ winter wheat planting (Sep. 22, 1987 and Sep 19, 1988), and also on March 1, 1988 and 1989, to measure effect of time of emergence on wheat yield loss. In an adjacent study, jointed goatgrass established at 18 plants/m2 at wheat planting was removed near March 1, April 1, May 1, June 1, and June 15. A full-season interference treatment also was included. Winter wheat grain yield loss was determined by comparing interference treatments to a weed-free control. The experimental design for both studies was a randomized complete experimental design for both studies was a randomized complete block with four replications. Plot size was 2 m2.

The duration of jointed goatgrass interference effect on winter wheat grain yield is shown in the adjacent figure. The yield loss relationship for the time of emergence study was Y = 30.6 – 0.29X (X = days after Sep. 15), and for the removal time study, Y = 8.7 + 0.15X (X = days after March 1). Wheat yields of the weed-free plots were 2200 kg/ha in 1988 and 1250 kg/ha in 1989. Jointed goatgrass at 18 plants/m2 reduced grain yield by 30% when it emerged with winter wheat. As jointed goatgrass emergence was delayed, yield loss decreased; however, yield loss was still greater than 15% when jointed goatgrass emerged on November 1, approximately 40 days after planting. Yield loss from the march 1 emergence was 8%, demonstrating that jointed goatgrass emergence in the spring also is detrimental to winter wheat yields. The time of removal study indicated that jointed goatgrass caused 10% yield loss when removed on march 1. Based on these relationships, early spring (before March 1) would be the most effective time for a foliar-applied herbicide to control jointed goatgrass. A postemergence herbicide applied during early spring would minimize winter wheat yield loss due to interference of fall-established plants as well as minimize yield loss due to plants emerging after the herbicide application. (USDA-ARS, Central Great Plains Research Station, Akron, CO 80720).