Author
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DONALD, WILLIAM |
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Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/25/2004 Publication Date: 1/31/2006 Citation: Donald, W.W. 2006. Between-observer differences in relative corn yield versus rated weed control. Weed Technology. 20(1):41-51. Interpretive Summary: For over fifty years, weed scientists have relied on relatively few field measurements for screening and judging how well different weed control treatments work to prevent yield loss in field crops. Generally they use rated weed control and crop yield. This article reviews and addresses some of the advantages and limitations of rated weed control as a quantitative scientific measurement. Also, data from previously published research on field corn was used to examine two assumptions of rated weed control for screening weed control treatments: Assumption 1) rated weed control is related to crop yield loss and can be used to estimate crop yield loss, and Assumption 2) different trained observers rate the same treatments alike consistently. Unfortunately, the results show that different observers can rate weed control of the same treated plots very differently. This inconsistency limits the use of rated weed control to estimate crop yield loss. Some observers are better able to distinguish smaller differences in weed control between treatments than can other observers. In tall growing competitive crops, like field corn, observers' rated weed control also can be biased by the weed cover of weeds growing between crop rows and the height of weeds growing in rows. This research should interest weed scientists, extension agents, and herbicide industry manufacturers who now use rated weed control to evaluate treatments for controlling weeds. Technical Abstract: Crop yield and weed control rating have been used to measure weed and crop response to weed management treatments, eliminate unacceptable weed management treatments, and select “best” treatments for recommendation to farmers. However, the mathematical relationship between crop yield and rated weed control has not been reported before from such treated screening experiments. Likewise, differences have not been reported before in rated weed control among experienced observers (i.e., reliability) when rating the same experiments and for an experienced observer over time (i.e., repeatability). Data from published experiments on zone herbicide application (ZHA) in field corn in which weeds reduced yield to various amounts were re-analyzed to examine these issues. For this study, relative corn yield was calculated as a percentage of the 1X broadcast herbicide rate for 2 observers and either 3 experimental site-years or their average. For observer A, relative corn yield (%) increased linearly as rated total weed control (%) increased for all 3 site-years and their average. For observer B, equations were curvilinear in 2 of 3 site-years. For both observers, equations accounted for little data variability in relative corn yield (r2 = 0.25 and 0.25 in site-year 1, respectively, 0.38 and 0.36 in site-year 2, 0.58 and 0.57 in site-year 3, and 0.43 and 0.42 for their average). When rated total weed control by observer A was graphed versus that of observer B, the relationship was a nearly “ideal” 1:1 linear relationship in only 1 of 3 site-years. In 2 other site-years, equations were nonlinear, indicating that one observer distinguished smaller differences between treatments at lower rated control than the other observer. Between-row total weed cover and in-row total weed height influenced observer weed control rating. |
