Subjective evaluation of weed-crop competition - is it good enough?
by Bob Hartzler

January 28, 1997 -- Economic thresholds are one of the foundations of integrated pest management, yet these tools are rarely used in Midwest agriculture. Decisions on whether to control a specific infestation usually are based on subjective evaluations of the farmer, ag chem dealer or crop consultant. However, it is unknown whether these people making management decisions have a realistic impression of the relationship between weed populations and crop losses.

A survey was conducted at an ISU field day to evaluate farmers' and industry personnel's understanding of the weed density - crop yield loss relationship. Mature velvetleaf plants were placed between 30" soybean rows at a density of one velvetleaf per 20 feet of row. The plot size was approximately 1/4 acre. Farmers and ag-industry employees to predict the yield loss caused by this infestation. Persons taking the survey were told to assume that the velvetleaf emerged at the same time as the crop and that the soybeans had a yield potential of 50 bushels per acre. The average yield loss estimate by 90 farmers was 5.7%, whereas 45 ag-business employees predicted a 5.3% yield loss (Figure 1). NebraskaHERB, an economic threshold/ postemergence decision making tool developed at the University of Nebraska, predicts a 4.3% yield loss from this weed infestation. Approximately 50% of the farmers were within 2% of the predicted yield loss compared to 36% of industry persons. The maximum yield loss estimate by farmers was 30% (2 responses) compared to a maximum of 22% by persons in ag-industry.

Participants were also asked what would be there management recommendation if the velvetleaf population was present in soybeans three weeks after planting. Recommendations for managing the velvetleaf were influenced by the person’s yield loss estimate, with persons predicting low yield losses (< 4%) more likely to recommend cultivation or no action (Table 1). The NebraskaHERB recommendation for this infestation was cultivation. More industry persons were in agreement with the computer recommendation than farmers.

Table 1. Action recommended for managing velvetleaf density of one plant per 20 row ft in soybeans. Ames, IA.

The results of this survey suggest that the subjects had a fairly good understanding of the competitiveness of velvetleaf in soybean. It should be recognized that a relatively small number of persons participated in the survey and that the participants may not be representative of the population of farmers and industry persons. However, the survey provides evidence that persons involved in weed management decisions should be able to make sound recommendations based on subjective evaluations if properly trained.

The full results of this survey will be published in an upcoming issue of Crop Protection.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University