Douglas D. Buhler’s research while affiliated with Kansas State University and other places

First- and second-year seedbank emergence of 23 summer annual weed species common to U.S. corn production systems was studied. Field experiments were conducted between 1996 and 1999 at the Iowa State University Johnson Farm in Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for most species were planted in plastic crates. Seedling emergence was counted weekly for a 2-yr period following seed burial (starting in early spring). Soil temperature at 2 cm depth was estimated using soil temperature and moisture model software (STM2). The Weibull function was fit to cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time (HTT), and day of year (DOY). To identify optimum base temperature (Tbase) and base matric potential (ψbase) for calculating TT or HTT, Tbase and ψbase values ranging from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each species. The search for the optimal model for each species was based on the Akaike's Information Criterion (AIC), whereas an extra penalty cost was added to HTT models. In general, fewer seedlings emerged during the first year of the first experimental run (approximately 18% across all species) than during the second experimental run (approximately 30%). However, second-year seedbank emergence was similar for both experimental runs (approximately 6%). Environmental effects may be the cause of differences in total seedling emergence among years. Based on the AIC criterion, for 17 species, the best fit of the model occurred using Tbase ranging from 2 to 15 C with four species also responding to ψbase = −750 kPa. For six species, a simple model using DOY resulted in the best fit. Adding penalty costs to AIC calculation allowed us to compare TT and HTT when both models behaved similarly. Using a constant Tbase, species were plotted and classified as early-, middle-, and late-emerging species, resulting in a practical tool for forecasting time of emergence. The results of this research provide robust information on the prediction of the time of summer annual weed emergence, which can be used to schedule weed and crop management.

Selecting effective weed management options requires biological, ecological, and economic information. This study compared model-based to standard-herbicide weed control in a corn (Zea mays L.)/soybean [Glycine max (L.) Merr.] rotation that had a long-term history of different tillage and weed management practices. The model integrates weed population dynamics, herbicide efficacies, and economic information to evaluate preplant incorporated (PPI) or preemergence (PRE) weed control options based on weed seed bank size and postem-ergence (POST) strategies based on weed seedling densities. There were fewer weeds in standard-herbicide compared with model-based treatments. No-tillage had the greatest numbers of weeds 3 out of 4 years. Soybean yield was reduced the first year of the study in several treatments receiving model-based weed control and the third year in all model-based treatments. Corn yields were greatest in reduced tillage. Resuits of using model recommendations to control weeds were mixed, with PRE recommendations being insensitive to a common cocklebur (Xan-thium strumarium L.) infestation. Our conclusions agree with those of others that the nature of the weed pressure may be a prevailing influence on the outcome of using weed control recommendations of bioeconomic models.

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.

Composted manure produced in deep-bedded hoop structures constitutes a source of nutrients and organic materials that can influence crop and weed emergence, growth, competitive interactions, and soil physical properties. The impact of composted swine manure on corn, soybean, winter wheat, velvetleaf, giant foxtail, and common waterhemp emergence and early growth were compared at compost rates of 0, 8, 16, or 24 Mg C/ha. Compost amendment had no effect on crop emergence but did reduce weed emergence. Inhibition of seedling emergence ranged between 15 and 57% for giant foxtail, 0 and 23% for velvetleaf, and 16 and 76% for common waterhemp. Soil amendment enhanced weed growth but not crop growth. A response surface regression analysis indicated that, while large-seeded crops have constant relative growth rates, small-seeded weed species increase their relative growth rates with compost amendment (P < 0.0001, r(2) = 0.5252). Nomenclature: Common waterhemp, Amaranthus rudis Sauer #(3) AMATA; corn, Zea mays L. 'Pioneer 3563'; giant foxtail, Setaria faberi SETFA Herm.; soybean, Glycine max (L.) Merr. 'Pioneer 92B84'; velvetleaf, Abutilon theophrasti ABUTH Medicus; winter wheat, Triticum aestivum L. 'Arapahoe'. Additional index words: Compost amendment, relative growth rate, germination inhibition, organic amendment, seedling emergence, soil quality. Abbreviations: EI%, percentage emergence inhibition; MTE, mean time of emergence; RGR, relative growth rate; VOA, volatile organic acids.

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.

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.

Previous research has shown that maple (Acer spp.) leaf litter resulted in fewer common dandelions (Taraxacum officinale) when mulched into established turfgrass. However, the leaves used in that research may have contained herbicide residues and were separated by genus, not species. Our research compared the effects of pesticide-free mulched maple and oak (Quercus spp.) leaves on dandelion populations in an established kentucky bluegrass (Poa pratensis) stand maintained as a residential lawn on sandy loam soil. The objectives of this study were to quantify the effectiveness of maple or oak leaf mulches as an organic common dandelion control method and to identify which maple species and rates (particle size and rate per unit area) provided the most effective control. The experimental design was a randomized complete block with treatments arranged as a 5 × 2 × 2 + 1 factorial, with tree leaf species, leaf particle size, leaf application rate, and control as main factors. Leaf species were red maple (Acer rubrum), silver maple (A. saccharinum), sugar maple (A. saccharum), high sugar content sugar maple, and red oak (Quercus rubra). Particle sizes were coarse (0.4-1.0 inch 2) and fine (≤0.2 inch 2), and application rates were low (0.5 kg.m -2) and high (1.5 kg.m -2). Mulch applications were made in Fall 2003 and 2004 and data were collected beginning in Spring 2004 on kentucky bluegrass spring green-up, and common dandelion plant counts. The high application rate, regardless of tree genus or species, resulted in the highest green-up ratings. Common dandelion plant counts after one (2003) and two (2003 and 2004) mulch applications at the high rate showed that up to 80% and 53% reduction was achieved, respectively. Results indicate thatmulching leaves regardless of genus (oak or maple) or maple species into established turfgrass as a leaf litter disposal method will increase spring green-up and contribute to a reduction in common dandelion population.

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.

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.