SPATIAL AND TEMPORAL STABILITY OF WEED POPULATIONS OVER FIVE YEARS

Authors: COLBACH, NATHALIE - INRA, FRANCE; Forcella, Frank; JOHNSON, GREGG - UNIVERSITY OF MINNESOTA

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Weeds often occur in patches in agricultural fields. Management techniques, such as herbicides, need to be used to control weeds in these patches to avoid crop yield losses. These same management techniques need not be used between the patches of weeds. Consequently, the idea of patch spraying in precision farming is very exciting and holds considerable promise for increasing profits and decreasing herbicide use. However, an important, but largely unknown, aspect of weed patches is their stability through time. If patches are mapped in one year but occur elsewhere in the following year, then maps of weed patches may not be especially useful. We mapped annual and perennial weeds annually between 1993 and 1997 in a 3.5 acre no-till field sown to row crops each year. We found that weed densities were more variable across crop rows and patches were exaggerated along rows, probably due to planters and combines spreading weed seeds and rhizomes. Patches were most distinct for annual weeds and, more generally, for species whose seeds were dispersed by combines. On the other hand, patches were more persistent for perennial weeds and, more generally, for species whose seeds dispersed prior to combine harvesting. For an annual grass weed, green foxtail, locations of patches in one year could only be used to predict patch locations in the following year, not longer time intervals. These results indicate that weed maps of this important species, and others like it, are valid only for one year. In terms of precision farming, the practical implication of this research is that a field map of weed infestations cannot be used reliably for patch-spraying of herbicides more than one year after the field was mapped.

Technical Abstract: Weed mapping and spatially variable herbicide applications would be facilitated if the size and location of weed patches within arable fields were maintained through time. However, the temporal and spatial stability of weed populations is largely unknown. For this reason, annual and perennial weeds were sampled annually (1993 to 1997) at 410 permanent grid points in a 1.3 ha no-till soybean field. Geostatistical techniques were used to examine (a) spatial structure within years; (b) relationships of spatial structure to literature-derived population parameters such as seed production, seed longevity, etc.; and (c) stability of relationships across years. Within years, densities were more variable across crop rows and patches were exaggerated along rows. Spatial relations were strongest for annuals and, more generally, for species whose seeds were dispersed by combine harvesting. Patches were most persistent for perennials and, more generally, for species whose seeds dispersed prior to combine harvesting. For species such as green foxtail, locations of patches in year i could be used to predict patch locations in year i + 1, but not years 3 i + 2. Such short term stability suggests that weed maps developed in one year may not be useful beyond the following year.