Douglas D. Buhler

50th Anniversary—Invited Article: Challenges and opportunities for integrated weed management

Abstract Despite several decades of modern weed control practices, weeds continue to be a constant threat to agricultural productivity. Herbicide-resistant weeds and weed population shifts continue to generate new challenges for agriculture. Because of weed community complexity, integrated approaches to weed management may help reduce economic effects and improve weed control practices. Integrated weed management emphasizes the combination of management techniques and scientific knowledge in a manner that considers the causes of weed problems rather than reacts to existing weed populations. The goal of weed management is the integration of the best options and tools to make cropping systems unfavorable for weeds and to minimize the effect of weeds that survive. No single practice should be considered as more than a portion of an integrated weed management strategy. The best approach may be to integrate cropping system design and weed control strategies into a comprehensive system that is environmentally and economically viable. Management decisions must also be made on a site- and time-specific basis. Considering weeds in a broader ecological and management context may lead to the use of a wider range of cultural and management practices to regulate weed communities and prevent the buildup of adapted species. This will help producers manage herbicides and other inputs in a manner that preserves their effectiveness and move weed scientists toward the development of more diverse and integrated approaches to weed management.

Theoretical and practical challenges to an IPM approach to weed management

Abstract Modern weed control tactics have played a major role in the productivity of cropping systems. Herbicides have been an effective component of weed control for major crops, greatly reducing yield losses and facilitating reduced tillage systems. Although these benefits are important, weed problems, soil erosion, and environmental concerns persist. Herbicides will continue to play a key role in most production systems, but weed species will continue to evolve and weed communities shift in response to selection pressures. Weed science must develop and incorporate additional practices to create integrated management systems that diversify selection pressures and reduce environmental degradation. Integrated pest management (IPM) may provide a useful framework for the development of integrated weed management systems. The basic principles of IPM are well established and have been successfully applied to many agricultural pests. However, the application of IPM to weed management has lagged behind other pest management disciplines. Many of the concepts and approaches of IPM are relevant to weed management, but these were not developed specifically for weed management and are not sufficient to address it adequately. Principles of IPM unique to weed management need to be delineated, developed, and put into practice. Although IPM for other pests provides an excellent framework, weed science must develop its own theory, management tactics, and monitoring procedures based on the unique characteristics of weed communities.

Emergence and persistence of seed of velvetleaf, common waterhemp, woolly cupgrass, and giant foxtail

Abstract Annual emergence and seed persistence of common waterhemp, velvetleaf, woolly cupgrass, and giant foxtail were characterized in central Iowa for 4 yr following burial of seeds collected and buried in autumn 1994. First-year emergence as a percentage of the original seed bank ranged from 5 to 40%, and the relative order was common waterhemp < velvetleaf < giant foxtail < woolly cupgrass. During the second and third years, there were no differences in percent emergence among species, with emergence percentages ranging from 1 to 9% of the original seed bank. During the fourth year, seedlings continued to emerge from only the velvetleaf and common waterhemp seed banks. A greater percentage of common waterhemp seed persisted each year and 12% of the original seed was recovered after 4 yr of burial. Five percent of the velvetleaf was recovered at the end of the fourth year. No woolly cupgrass and giant foxtail seed was recovered after the third and fourth years. The proportion of the seed that was accounted for from year to year through emergence and seed recovery varied by species and year. Total recovery of velvetleaf ranged from 61 to 87%, common waterhemp from 50 to 81%, woolly cupgrass from 29 to 79%, and giant foxtail from 23 to 79%. Based on the results of this research, velvetleaf and common waterhemp form more persistent seed banks than woolly cupgrass and giant foxtail. Therefore, woolly cupgrass and giant foxtail should be more amenable to management through seed bank depletion than velvetleaf and common waterhemp. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medik. ABUTH; woolly cupgrass, Eriochloa villosa (Thunb.) Kunth ERBVI.

Weed Seed Bank Dynamics During a Five-Year Crop Rotation1

Abstract: Cultural practices used for crop production influence the composition of the weed seed bank in the soil. This paper reports the results of a 5-yr experiment to characterize the weed seed bank conducted on a farmer-managed field in central Iowa. The number of weed seeds in the soil and their vertical distribution were examined each October. At the initial sampling in October 1994, the field had been in hay production and about 80% of the weed seeds were common waterhemp and foxtail species. The cropping sequence over the next 3 yr was corn/soybean/corn using a ridge tillage system. Over this period, the density of common waterhemp seeds declined each year. The density of foxtail seeds declined by almost 90% during the first year of corn and did not change during the following years of soybean and corn production. Prior to moldboard plowing of the hay sward in 1994, weed seeds were concentrated in the upper 10 cm of soil. Moldboard plowing resulted in a more uniform distribution of the weed seeds over the upper 20 cm of soil, and the distribution across depths remained relatively constant during the 3 yr of corn and soybean production. During the final year of the experiment, the field was rotated to oat and reseeded with hay species. The number of common waterhemp and foxtail seeds in the soil greatly increased following oat/hay production and seeds were concentrated in the upper 10 cm of the soil profile. Results indicated that the processes affecting the weed seed bank in production fields are complex and will vary greatly based on the production practices used and the timing of their application. Nomenclature: Common waterhemp, Amaranthus rudis Sauer #3 AMATA; corn, Zea mays L.; foxtail species, Setaria spp.; oat, Avena sativa L.; soybean, Glycine max (L.) Merr. Additional index words: Cropping systems, ridge tillage, seed bank, seed distribution.

Utilizing Sorghum as a functional model of crop-weed competition. I. Establishing a competitive hierarchy

Abstract Application of nitrogen (N) fertilizer to sorghum at planting is a common practice that could confound competitive relationships of the crop with weeds. We studied the competitiveness of grain sorghum (Pioneer Brand 8333) relative to that of the annual weed shattercane and the perennial weed johnsongrass. The taxa are closely related, so survival requirements should be similar thus increasing the likelihood of finding differences associated with traits of the crop vs. weediness. Objectives of this research were to establish a competitive hierarchy for this crop–weed complex and to determine if relative competitiveness was affected by added N. A replacement design experiment was used in which plants were grown for 31 d in soil-filled pots placed outdoors. Taxa were planted in monocultures and 50:50 mixtures, representing all possible combinations of taxa, at a total density of 16 plants pot−1. Soil moisture was maintained at field capacity by daily additions of water or 30 μg ml−1 N in the form of an inorganic salt solution (KNO3). There was no response to the solution containing exogenous N likely because the amount of N in soil was greater than demand. Actual shoot and root dry weights in mixtures were compared with the expected dry weights, which were calculated as 50% of the root and shoot dry weights in monoculture. For grain sorghum, actual dry weights in mixture were often better than expected. Replacement series indices calculated from dry weight data described grain sorghum as competitively superior to its weedy relatives. These results indicate that further research on N management for cultivated sorghum, as a means of increasing crop competitiveness relative to that of weeds, may be unwarranted. However, a better understanding of other competition mechanisms inherent in grain sorghum might suggest management alternatives to enhance crop competitiveness with weeds. Nomenclature: Sorghum, Sorghum bicolor (L.) Moench ‘Pioneer Brand 8333’; shattercane, Sorghum bicolor (L.) Moench SORVU; johnsongrass, Sorghum halepense (L.) Pers. SORHA.

Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas

Abstract Interest in the population dynamics and geographic distribution of common milkweed (Asclepias syriaca L.) has recently increased due to the importance of common milkweed in the life cycle of the monarch butterfly (Danaus plexippus). A survey of common milkweed occurrence in various habitats was conducted in Iowa in June and July of 1999. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn (Zea mays L.) and soybean (Glycine max L. Merr.) fields. Corn and soybean fields had 85% fewer patches than roadsides. Conservation reserve program fields had the greatest average area infested. While common milkweed was frequently found in corn and soybean fields, average frequency and patch sizes were much greater in noncrop areas.

Impacts of composted swine manure on weed and corn nutrient uptake, growth, and seed production

Abstract Hoop structures bedded with crop residues are becoming increasingly popular for swine production in the northcentral United States. Compost made from bedding materials and swine manure can be used as a soil amendment. A 3-yr field experiment was conducted in Boone, IA, to determine how composted swine manure affected selected soil characteristics and nutrient uptake, growth, and seed production of corn and three weed species (giant foxtail, velvetleaf, and common waterhemp) grown in mixture with corn. Two soil management systems, designed to provide equivalent amounts of N to corn, were compared: one that received composted manure and an average of 118 kg N ha−1 as synthetic fertilizer and another that received no composted manure and an average of 143 kg N ha−1 as synthetic fertilizer. Soil organic matter, P, K, and early-season NO3-N levels were greater in the (+) compost system. The N concentration of velvetleaf shoots, the P concentration of giant foxtail and common waterhemp shoots, and the K concentration of shoots of all three weed species also were greater in the (+) compost system. Compost application consistently increased common waterhemp height, common waterhemp biomass, and velvetleaf height, but increased velvetleaf biomass in only 1 yr and had no effect on giant foxtail height or biomass. Measurements of weed seed production, conducted in the final year of the study, showed that compost increased velvetleaf and common waterhemp seed production but had no effect on giant foxtail seed production. Compost consistently increased corn height and leaf K concentration but generally had no effect on corn yield. Results of this study indicate that large differences can exist among crop and weed species in their response to soil amendments. Depending on the weed species present, use of composted swine manure may increase requirements for weed management in corn production systems. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medicus ABUTH; corn, Zea mays L.

Impact of composted swine manure and tillage on common waterhemp (Amaranthus rudis) competition with soybean

Abstract Use of composted swine manure produced in deep-bedded hoop structures is a promising approach for recycling farm waste products and improving soil fertility, but little is known about its effects on crop–weed interactions. A 2-yr study was conducted to evaluate the effect of compost amendments and tillage on soybean–common waterhemp competition. Experiments were conducted in no-tillage and chisel plow main plots with compost applied to one of two types of subplots. Common waterhemp and soybean growth was measured in sub-subplots accommodating weed-free soybean and soybean with common waterhemp sown at soybean planting, soybean emergence (VE), soybean second-node stage (V2), and soybean sixth-node stage (V6). Soybean heights were not influenced by compost or common waterhemp sowing time. Soybean stem diameters were influenced by year, tillage regime, and an interaction between compost and common waterhemp sowing time. In contrast, common waterhemp heights and basal diameters were greater when sown at planting and VE in compost-amended subplots than in compost-free subplots. Overall, there was a negative quadratic relationship between common waterhemp biomass and soybean yield (r2 = 0.746). The extremely low common waterhemp emergence in V2 and V6 treatments suggested that early-season weed suppression was sufficient to protect soybean from common waterhemp competition. The sex determination of 2,557 common waterhemp plants showed a marginally higher male to female ratio in compost-amended treatments than in compost-free treatments (P = 0.0611). A linear-slope regression indicated that common waterhemp fecundity was positively related to individual plant biomass, with a change in slope occurring at 118.7 g. Under the conditions present in this experiment, compost did not enhance soybean yield but increased the competitive ability of waterhemp. Because composted swine manure can have a major influence on competition of common waterhemp with soybean, effective weed management practices should be in place when this soil amendment is used. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; soybean, Glycine max (L.) Merr.

Corn, soybean, and weed responses to spring-seeded smother plants

ABSTRACT Field and greenhouse research was conducted to better define the characteristics of a spring-seeded smother plant system for weed control in corn (Zea mays L.) and soybean (Glycine max [L.] Merr.). In the greenhouse, emergence and early growth of two medic (Medicago) species was greatest when planted 1.5 cm deep. In the field, Berseem clover (Trifolium alexandrinum L.), four medic species, and yellow mustard (Brassica hirta Moench) were planted immediately after corn or soybean planting in a 25-cm-wide band centered over the crop row. All species achieved 45% or more ground cover by 72 days after planting and yellow mustard often generated ground cover more quickly than other species. Weed suppression varied, with yellow mustard and Sava medic (Medicago scutellata L.) often providing greater weed suppression than other species. However, weed suppression was highly variable and crop yields were usually reduced compared with weed-free plots. Killing Sava medic 30 days after planting reduced weed suppression and did not increase corn yield compared with allowing the medic to survive until maturity. Delaying seeding of Sava medic until rotary hoeing increased weed suppression compared with rotary hoeing alone.

Utilizing Sorghum as a functional model of crop–weed competition. II. Effects of manipulating emergence time or rate

Abstract Interest in using crop competitiveness as an integrated weed management tool is increasing. Our objective was to describe traits that could be sources of the competitiveness we previously observed in grain sorghum grown in association with shattercane, which is a common annual weed and a close relative of the crop. Such information could aid in developing management practices for cultivated sorghum to improve its competitiveness with weeds. A bioassay was conducted to compare emergence of the crop and the weed in the greenhouse, and vegetative growth was monitored for 31 d in a within-row competition study. Results described a crop that competed well with the weed and other crop plants and agreed with studies showing that relative time of emergence influenced competitiveness. The mechanism by which grain sorghum emerged before the weed was a by-product of domestication that reduced glumes surrounding the wild-type seeds. This could be shown experimentally by hulling shattercane seeds, which then emerged almost as quickly as the grain sorghum. When planted in the grain sorghum row, shattercane plants from hulled seeds decreased the number of leaves and the root mass of the crop. Similarly, the time between emergence of the crop and emergence of shattercane was lessened by planting shattercane seeds early, and this increased the leaf number of the weed and shoot mass of the crop. It might be possible to increase weed suppression in grain sorghum by using management practices, such as more equidistant crop planting patterns that exploit the competitiveness already present, but which is being lost to interactions among crop plants. Nomenclature: Shattercane, Sorghum bicolor (L.) Moench SORVU; sorghum, Sorghum bicolor (L.) Moench ‘Pioneer 8310’, ‘Pioneer 8771’.