Cropping Systems Control Winter Annual Grass Weeds in Winter Wheat

Downy brome (Bromus tectorum L.), jointed goatgrass (Aegilops cylindrica Host), and volunteer cereal rye (Secale cereale L.) are winter annual grass weeds that are increasingly troublesome in the winter wheat (Triticum aestivum L. emend. Thell.)-fallow rotation areas of the western USA. Six dryland cropping systems—continuous no-till winter wheat, winter wheat-fallow with fall tillage, winter wheat-fallow with fall applied herbicide, winter wheat-fallow-fallow, winter wheat-sunflower-fallow, and winter wheat-proso millet-fallow—were compared for their effect on winter annual grass densities in winter wheat. Winter annual grass densities averaged 145, 4.4, and 0.4 plants/sq yard for the 1-, 2-, and 3-yr systems, respectively. Eradication of the winter annual grasses was not achieved with any of the systems. Dockage and foreign material levels in wheat grain were lower in 3-yr than in 2-yr cropping systems. Jointed goatgrass was the most persistent annual grass investigated. The ProblemDowny brome, jointed goatgrass, and volunteer cereal rye are increasingly troublesome winter annual grass weeds in the winter wheat-fallow rotation areas of the USA. A 1989 mail survey of Colorado farmers indicated that these three weeds infested over 1.3 million acres of cropland in the state. The estimated cost of these infestations to Colorado wheat growers was over $20 million annually. Herbicides are not available to provide selective control of these grasses in winter wheat. Chemicals and tillage control these weeds during the fallow period, but sufficient seed usually remains to reinfest the following winter wheat crop. Crop rotation may be an effective way to control these weeds in winter wheat, but few reports exist of the effects of cropping systems on these weeds. Literature SummaryDowny brome at densities of 20, 33, and 54 plants/sq yard reduced winter wheat yields by 10, 15, and 20070, respectively, when plants emerged within 14 d of winter wheat. Jointed goatgrass at 15 plantslsq yard reduced winter wheat yield in Colorado by 27 and 17% when emerging 0 and 6 wk after winter wheat, respectively. In addition to reducing winter wheat yield, jointed goatgrass also lowered grain quality by increasing dockage or foreign matter. In Oregon, 162 rye plantslsq yard reduced winter wheat yield by 33% when rye was removed in February compared with 69% when not removed. Jointed goatgrass spikelets and rye seed are serious contaminants of winter wheat grain. The longevity of downy brome seed varies widely. Freshly harvested downy brome seed exhibit a rapid and high germination percentage, suggesting that all viable seed will germinate when conditions are suitable. Viable seed were reported 5 yr after burial, however. Jointed goatgrass seed persists in undisturbed soil from 3 to 5 yr depending on site conditions. By the third year, less than 8% of seed buried 2 in (5 cm) deep survived. All seed were nondormant 3 yr after burial. The period of greatest jointed goatgrass seed loss from soil coincided with peak emergence in the fall. Less than 1% of rye seed was viable 3 yr after burial. Study DescriptionField experiments were conducted at the High Plains Agricultural Lab northwest of Sidney, NE, at an elevation of 4270 ft above sea level. Soil type: Duroc loam Cropping systems: Continuous winter wheat (no till) Winter wheat-fallow with fall sweep tillage Winter wheat-fallow with fall applied atrazine at 1 lb ai/acre Winter wheat-fallow-fallow Winter wheat-sunflower-fallow Winter wheat-proso millet-fallow Check—winter wheat-fallow with fall sweep tillage and no weeds Each of the above main plot treatments was divided into three sub-plot treatments: Downy brome Jointed goatgrass Volunteer cereal rye Herbicides used: Landmaster BW, a mixture of glyphosate (Roundup) and 2,4-D amine, after wheat harvest except in cropping system 2 Atrazine in late August after wheat harvest in cropping systems 4 and 6 Glyphosate prior to wheat seeding in cropping system 1 and in first year of fallow in system 4 Pendimethalin (Prowl) preplant incorporated before sunflower in system 5 2,4-D low volatile ester in wheat for all cropping systems Cultivars planted: Winter wheat—‘Arapahoe’ Proso millet—‘Sunup’ Sunflower—‘Dahlgren 855’ Applied QuestionTo what extent can crop rotation reduce winter annual grass weed pressure in dryland winter wheat production? None of the three winter annual grasses were eradicated with the cropping systems studied in this research. In 1993, when environmental conditions did not promote winter annual grass germination in the winter wheat crop, no difference between 2- and 3-yr systems were observed. The winter annual grass weeds thrived in the continuous winter wheat system in both 1993 (data not shown) and 1994 and made wheat production impossible (Table 1). Higher weed densities in 1994 resulted in more treatment differences. Weed densities in the 2-yr systems were about 3% of those in the continuous wheat system. Weed densities in the 3-yr systems were just 10% of the levels in the 2-yr systems. Jointed goatgrass was the most persistent of the three weeds studied, and crop rotations may have to be longer than 3 yr if a heavy infestation of jointed goatgrass is present. No difference in winter annual weed densities was observed between the two 2-yr systems or the three 3-yr systems. Winter wheat yields were not affected by the cropping system. RecommendationThe introduction of a late spring-planted crop into the winter wheat-fallow system can reduce winter annual grass weed densities by over 90%. While no 3-yr system eliminated winter annual grass weeds, grade reduction and dockage of grain due to jointed goatgrass and rye contamination were reduced. Timing of fallow tillage and chemical treatments is critical to all systems. Prevention of annual grass seed production during fallow is critical to the success of these systems. The 3-yr systems allow more opportunities to recover from mistakes and environmental conditions that permit winter annual grass weeds to produce seed. Table 1Effect of dryland cropping systems on winter annual grass densities in winter wheat at Sidney, NE, in April 1993 and 1994. Weed DooulationCropping system†Downy bromeJointed goatgrassRYeAll weeds-------------------plantslsq yard----------------CW176 109 148 145 WW-Ft0.18.62.23.6WW-Fh0.211.83.75.3WW-F-F0.11.40.10.5WW-SF-F0.20.70.20.3WW-PM-F0.30.90.10.4†Cropping system abbreviations: CW, continuous no-till winter wheat; WW-Ft. winter wheat-fallow with fall tillage; WW-Fh, winter wheat-fallow with fall herbicide; WW-F-F, winter wheat-fallow-fallow; WW-SF-F, winter wheat-sunflower-fallow; and WW-PM-F, winter wheat-proso millet-fallow.