Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/22/2005
Publication Date: 4/6/2006
Citation: Clay, S.A., Kreutner, B., Clay, D.E., Reese, C., Kleinjan, J., Forcella, F. 2006. Spatial distribution, temporal stability, and yield loss estimates for annual grasses and common ragweed (Ambrosia artimisiifolia) in a corn/soybean production field over nine years. Weed Science. 54:380-390. Interpretive Summary: Field maps of weed infestations help producers and consultants better understand the variability of needed levels of weed control within a field. However, whether such maps can be used from one year to the next is not known. At any particular location within a field, the presence or absence of a weed may differ each year. Equally interesting, the density of this same weed also may change yearly from below crop-damaging levels to above such levels. Changes in the presence/absence or densities of annual weedy grasses and common ragweed were studied in a quarter-section field rotated with corn and soybean in eastern South Dakota. Weeds were evaluated annually for nine years at each point in a 50 x 100 foot grid system throughout this field. Furthermore, the densities of the weeds were used to project corn and soybean yield losses across the whole field as well as at each point within the grid. This permitted assessment of whether or not weed control was required. Annual grass weeds (foxtail and barnyardgrass) were relatively stable over time. For example, 40% of the grid points contained annual grasses in six of nine years. In contrast, only 24% of grid points contained common ragweed in three or more years. Thus, presence/absence maps would be more reliable for annual grasses than for common ragweed. Average weed densities over the entire field were high enough that control would have been recommended for both weed species. When site-specific variability was taken into account, about 80% of the field was recommended for herbicide application when soybean was the crop, but as little as 15% of the field was recommended for spraying when corn was the crop. Weed scientists, agronomists and agrichemical personnel will benefit from these results through better understanding of how the spatial variability of weeds can be used to fine-tune weed management recommendations.
Technical Abstract: Spatial and temporal stability of annual grass and common ragweed infestations were examined and management strategies were developed based on estimated weed density data collected in a 65-ha production field over a nine-year period (1995–2004) in a no-till/strip-till corn/soybean rotation. Weeds were counted in 0.1 m2 areas on a 15 by 30 m grid. The impacts of each weed infestation on treatment recommendations were estimated each year using field means, medians, and site-specific densities through kriging. Impacts were calculated using low and high incremental yield loss values (I) reported in the literature and 5 and 10% yield loss limits. The annual grass infestation was stable in time and space with 40% of the same grid cells having a grass infestation for at least six of nine years. Common ragweed infestation was relatively unstable with only 24% of the grid cells having an infestation for three or more years. Yearly field average densities of each weed, which ranged from 12 to 131 grass plants m-2 and <1 to 37 common ragweed plants m-2, were generally high enough so that estimated yield losses exceeded 5% and a field-wide treatment would have been recommended regardless of crop or chosen incremental yield loss values. When management decisions were based on the yearly field median density, which ranged from 0 to 40 grass plants m-2 and was 0 for common ragweed plants, field-wide grass treatment would have been recommended in some corn and all soybean years, whereas common ragweed would not have been treated in any year. Estimating yield losses based on weed density per grid cell resulted in management maps that ranged from > 80% field treatment for grass in 1998, 2000, and 2003 (soybean) to less than 15% of the field recommended for common ragweed treatment in 2002 and 2004 (corn). In most cases, choosing a high or low I yield loss value had minimal impact on treatment boundary lines because if the weed was present its density was generally high enough to result in >5% yield loss. Weed scientists, agronomists and agrichemical personnel will benefit from these results through better understanding of how the spatial variability of weeds can be used to fine-tune weed management recommendations.