Back to Contents6.2.4 Distribution and Impact of Perennial Dicot Weeds in Row Crops
Investigators: Bill Witt, Richard Barnhisel, J.D. Green and Jim Martin
Annual loss due to interference from weeds in corn, soybean, and cotton in the southern region is about $815 million. Over the past few years, the appearance of perennial weedy species has become the greatest challenge to weed management. The two primary reasons are because of an increase in conservation tillage practices adopted by growers and because control of numerous annual weedy species is easily attained by herbicide treatments. Conservation tillage is practiced widely across the southern region. The soil disturbance associated with conventional tillage provided control of perennial reproductive structures, especially species with deep growing tap roots. As growers have adopted production practices with less tillage, several perennial species have become more troublesome to manage.
Data on perennial weed management in southern cropping systems is limited and the data void takes many forms. First, the assessed impact of perennial dicots on yield of is limited to silverleaf nightshade and hogpotato in cotton, honeyvine milkweed in corn, and pokeweed in corn and soybean. The rest of the United States has a similar data void. Common milkweed, Jerusalem artichoke and perennial sowthistle in soybean and common milkweed in corn have been studied to determine yield loss from these perennials. Secondly, the impact of crop manipulation through planting date, row spacing and crop seeding rate on perennial dicot growth is unknown. Thirdly, an appropriate method to survey fields for perennial species has not been developed adequately. Perennials initiate new-shoot growth from crowns and root buds which results in several stems per plant. The dilemma facing a weed manager is to determine whether to count stems or attempt to count the number of plants (crowns). Due to the nature of their growth habit, some perennial dicots, such as honeyvine milkweed and trumpetcreeper, exist as distinct communities within a field. Others, such as common pokeweed, occur more uniformly across a field. Gerhards et al. (1997) in Nebraska has shown the utility of mapping weed populations on a field scale. Large differences existed in the distribution of several species. Hemp dogbane, a perennial dicot of the midwest, was found to occupy only 2% of a field. Annual species, generally, are more widespread throughout a field, but they too do not infest an entire field.
The introduction of global positioning system (GPS) technology has allowed for major advances in site-specific crop management. Mapping of fields for differences in soil pH, organic matter content and nutrient status has allowed growers to apply nutrients or lime in selected portions of the field rather than the entire field. As portions of fields have been identified as more productive than others, growers have rapidly adapted this technology for crop management by increasing crop populations in those areas of high production potential and reducing populations in those areas of low production potential.
GPS technology would allow growers to target specific weed management strategies at those areas of the field where the perennial weeds occur. This will reduce broadcast spraying of entire fields with herbicides, and therefore, reduce the amount of herbicides used and minimize environmental impacts.
The sub-objectives for this element of the project include: 1) to develop a method to monitor the distribution of perennial weed species in crops and accurately map the location of these species in fields and correlate the presence of these species with the following: elevation, north or south facing slope, and the soil properties pH, organic matter content, clay content, and nutrient status; 2) to develop a method to accurately determine crop yield loss, if any, from perennial weed species; and 3) to distribute the information from the research component through IPM specialists and consultants, extension personnel, and growers via IPM training programs and regular extension education programs.
Farm sites that have been mapped previously by the PIs will be used in this project. Fields at these sites have been mapped with GPS technology on a 23-m to 66-m grid spacing and the following data are available for each grid section: soil pH, organic matter content, texture, and nutrient status. The farmers at these sites have cooperated with the PIs previously and yield monitors and GPS receivers are on the farmer’s harvesting equipment. These fields are in a soybean and corn rotation and these crops will be used as model crops (corn for tall, upright crops, soybeans for shorter, dense canopy crops) for this project. In Sub-Objectives 1 and 2 (described below), large plots will be established across each field. The width of the plot will be two widths of the harvesting equipment (12 to 16 rows) and the length will be the distance of the entire field. This large plot will go across grids that have been mapped previously and will allow for crop harvesting that will utilize yield monitors and allow for determining crop yield at numerous points during the harvesting process. Perennial dicots known to infest fields at these sites are common pokeweed, honeyvine milkweed, trumpetcreeper, common milkweed, and hemp dogbane.
Sub-Objective 1 - Perennial dicot weeds in each grid will be documented 4 weeks after crop planting. These data will be entered into the GPS/GIS system to produce maps of each field indicating the location of these species. The occurrence and frequency of each species will be correlated with the soil factors described above.
Sub-Objective 2 - After fields have been mapped (Sub-Objective 1), plots will be established that contain either weeds or no weeds for a paired-plot analysis of crop yield. In those plots containing weeds, the following treatments will be evaluated: 1) a weed management treatment based on WeedMak (a UK extension developed computerized decision aid for herbicide selection); 2) a weed management treatment based on the recommendation of the local herbicide dealer utilized by the cooperating farmers; and 3) removal of perennial weeds. The following data will be collected: weed population by species 4 weeks after planting and at harvest, estimate of weed propagules formed immediately before crop harvest, and crop yield. Crop yield data from weedy and non-weedy treatments will be compared to determine the impact of the perennial weed species on crop yield. Regression analysis will be used in an attempt to determine a percent yield loss based on weed population on an area basis.
Sub-Objective 3 - Information obtained in Sub-Objectives 1 and 2 will be distributed to IPM specialists, herbicide dealers and crop consultants, and extension personnel. Information will be posted in the Precision Ag WebPage with links to the IPM WebPage. The major dissemination will be via the University of Kentucky Precision Ag and the IPM programs. This program has an established record of offering "scouting schools" for IPM scouts and IPM practitioners..
The farmers at the two sites are agreeable to having this research conducted on their farms. In addition, these farmers routinely use yield monitors on their harvesting equipment and these harvesters have GPS receivers installed. The farmer at each location will select the dealer or crop consultant to make a weed management strategy in Sub-Objective 2. In addition to the PIs, extension personnel in Integrated Pest Management, Weed Science, Soils, and Biosystems and Ag. Engineering will be involved in the educational programs under Sub-Objective 3. Extension agents for agriculture in each county will participate in the project.
This project is expected to benefit producers of row crops in Kentucky as well as the southern region of the US. While we will investigate perennial weed distribution and yield effect in corn and soybean, the methods employed should be effective for other agronomic crops such as cotton, peanuts, and grain sorghum and other crops grown in rows or in solid stands. Currently, growers are treating entire fields in an attempt to reduce perennial dicot populations. These fields may not need to be treated at all because the yield reductions caused by these perennial species is not known and because it is difficult to locate the perennial weeds in growing crops for spot treatments. Herbicide applications for perennial weeds should be reduced substantially. For example, if a certain perennial dicot occupies 20 percent of the field and is reducing crop yield, then only 20% of the field would be treated using GPS technology to accurately locate the sites in which the weeds are growing.