Weeds of Wheat in the Great Plains
Winter annual grasses such as jointed goat grass, downy brome, and volunteer rye constitute the most serious weed threats to winter wheat production in the Great Plains. Winter annual grasses reduce wheat yields and cost Great Plains wheat producers millions of dollars each year. Widespread adoption of reduced tillage farming and continuous wheat or wheat fallow systems have aided establishment and spread of winter annual grasses. The life cycles of these grasses are similar to that of winter wheat, making the use of herbicides nearly impossible. Also, winter annual grasses typically shed their seed slightly before wheat harvest, thus ensuring their survival in the system. There are no herbicides available that provide selective control of all winter annual grasses in winter wheat, unless a herbicide-tolerant wheat variety is available. The areawide pest management program demonstrated that winter annual grasses are reduced in diversified wheat production systems when compared to “wheat only” cropping systems.
Figure 1.6 Kochia.
A variety of annual broadleaf weeds are important in wheat as well. Mustards and henbit are important winter annual broadleaf weeds in the Great Plains, while kochia (Figure 1.6), sunflower, and Russian thistle are important summer annuals. Kochia is the most common summer annual weed in winter wheat in the Great Plains and has rapidly developed herbicide resistance. In a recent survey, more than 50 percent of kochia plants in dryland sites were resistant to sulfonylurea herbicides.
Use of a second crop in a 3-year rotation allows for cheaper, less chemical intensive control of winter annual grasses and kochia. The rotation allows for the use of herbicides and for grass germination in a non-grass crop that is competitive with the winter annual grasses. Diversified wheat cropping systems tend to have lower annual broadleafweed densities in the AWIPM study (reference Chapter 8-"Managing Weeds in Winter Wheat").