North African knapweed (Centaurea diluta Aiton), cornflower (Centaurea cyanus L.), corn marigold [Glebionis segetum (L.) Fourr.], rigid ryegrass (Lolium rigidum Gaudin), and corn poppy (Papaver rhoeas L.) are weeds of economic importance in the Iberian Peninsula, particularly due to limited herbicide options for effective control. For this reason, information about their seedling emergence has become critical to develop effective integrated management strategies and better time control actions. The aims of this study were to evaluate the effect of seed burial depth and soil disturbance on the emergence of these weed species. Two pot experiments were carried out to (1) quantify seedling emergence at three burial depths (2, 5, and 9 cm) and (2) characterize seedling emergence in response to different frequencies and timings of soil disturbance. Burial depth limited the emergence of G. segetum and P. rhoeas at 5 and 9 cm, respectively; while seedling emergence of C. diluta, C. cyanus, and L. rigidum were reduced by 92%, 90%, and 67% at 9 cm compared with 2 cm, respectively. Two or more sequential soil disturbance events increased total seedling emergence of C. diluta, P. rhoeas, and G. segetum compared with single events, while L. rigidum did not respond to repeated soil disturbance. These results suggest that turning the soil to bury weed seeds down to 5 cm or deeper would be a very effective method to control G. segetum and P. rhoeas and moderately effective to control C. cyanus. Also, the use of a stale seedbed would have some efficacy to reduce P. rhoeas and C. diluta weed pressure within the crop. This study illustrates how differences among species in seedling emergence in response to soil depth and disturbance can determine distinct emergence patterns ultimately influencing the selection of weed control tools and timing.

Ethiopian mustard (Brassica carinata A. Braun) is a biofuel crop recently introduced in the southeastern United States. For this crop to be successful, integrated weed management strategies that complement its rotation with summer cash crops must be developed. The objectives of this research were to evaluate the effect of previous season summer crops on winter weed emergence patterns during Ethiopian mustard growing season and to assess the impact of planting Ethiopian mustard on the emergence patterns of summer weed species. Gompertz models were fit to winter and summer weed emergence patterns. All models represented more than 80% of the variation, with root mean-square error values less than 0.20. The emergence pattern for winter weed species was best described using growing degree-day accumulation, and this model can be utilized for implementing weed control strategies at the critical Ethiopian mustard growth stages. The results also showed that summer weeds can emerge during the winter in northern Florida but do not survive frost damage, which might create off-season seedbank reductions before the summer crop growing season.

Horseweed, also known as marestail, is a problematic weed for no-till soybean producers that can emerge from late summer through the following spring. Overwintering cover crops can reduce both the density and size of fall-emerged weeds such as horseweed and reduce further spring emergence, although typically cover crops do not provide complete control. Cover crops may be integrated with additional spring herbicide applications to control emerged horseweed, and selective herbicides such as 2,4-D may be used to target horseweed while maintaining small grain cover crop growth. However, cover crops may affect herbicide deposition, which could reduce their efficacy to control weeds. The objective of this study was to determine how the amount and variability of 2,4-D ester spray solution deposition, measured with water-sensitive paper, was affected by a cereal rye cover crop and fall-applied saflufenacil. We also examined deposition at the soil surface relative to the cereal rye row position. In a year with greater cereal rye biomass accumulation, there was 44% less coverage and average deposit size was 45% smaller immediately adjacent to cereal rye rows compared with between rows and areas without cereal rye. Greater variability in these measurements was also noted in this position. Percent spray solution coverage was also 22% greater in plots that received saflufenacil in the fall, and deposits were 28% larger. In a year with less cover crop and winter weed biomass, no differences in spray deposition were observed. This suggests that small horseweed plants and other weeds immediately adjacent to cereal rye cover crop rows may be more likely to survive early spring herbicide applications, though the suppressive effects of cover crops may mitigate this concern.

Blessed milkthistle is considered to be a noxious weed in irrigated and rainfed areas of Pakistan due to its strong allelopathic effects on food crops. For sustainable wheat production, it is necessary to know the critical time for weed removal (CTWR) for blessed milkthistle to allow wheat growers to get maximum benefit from control of this weed. A field study was conducted in 2014 and 2015 at the College of Agriculture, University of Sargodha, Pakistan, to investigate the CTWR of blessed milkthistle in wheat. The field experiments were designed with seven treatments; weed free (control); 2, 3, 4, 5, and 6 wk after emergence (WAE); and weedy check. At 6 WAE, a significant reduction was noted in plant height (8% and 17%), number of productive tillers per square meter (16% and 16%), spike length (23% and 54%), grains per spike (13% and 34%), 1,000-grain weight (14% and 37%), grain yield (20% and 21%), and biological yield (24% and 50%) compared with control (weed-free plots) during 2014 and 2015, respectively. The logistic model supports the field study results and suggests that blessed milkthistle’s CTWR for wheat is 1 to 5 WAE based on acceptable yield losses of 5% to 15% during both years. The experimental results and logistic model indicate that blessed milkthistle should be controlled within 1 to 5 WAE to get better wheat crop harvests without compromising farmers’ profits. To our knowledge, this is the first study ever in Pakistan regarding the CTWR in terms of WAE of blessed milkthistle and could help other scientists create weed control strategies for other areas of the country.

Critical period experiments were conducted in 1997 near Milton, WI, and in 1998 near Edgerton, WI, in which Eriochloa villosa (Thunb.) Kunth emergence was characterized and related to Zea mays L. yield loss and weed seed production. At Milton, 2.7% of the spring seedbank emerged, and at Edgerton, 14.5% of the spring seedbank emerged. Peak time of E. villosa emergence occurred early in the season at both locations, accounting for 84% of the total season emergence at Milton by 37 d after planting and 90% of the total season emergence at Edgerton by 27 d after planting. A secondary peak in emergence occurred at each site, accounting for 14% of the total season emergence at Milton between 38 and 52 d after planting and 8% of the total season emergence at Edgerton between 28 and 43 d after planting. Following the secondary peak in emergence at each site, approximately 2% of the total season emergence occurred. Zea mays grain yield was reduced when E. villosa interfered with Z. mays past the V11 stage at Milton and the V3 stage at Edgerton. Zea mays yield reductions at Edgerton were greater, occurred under shorter periods of weed interference, and were associated with greater E. villosa biomass than yield reductions at Milton. Eriochloa villosa that emerged after the V2 Z. mays stage at Milton and the V3 stage at Edgerton did not cause crop yield loss; however, these plants produced seed at 143 and 63% of the initial spring seedbanks, respectively.

Common cocklebur and sicklepod are troublesome weeds in soybean in the southern United States. A field experiment was conducted from 1991 through 1995 to determine (1) the influence of tillage (no-till and tilled after initial seed deposition) and intraspecific and interspecific interference on seed production potential, emergence pattern, and soil seedbank of common cocklebur and sicklepod, and (2) the dominant species after introduction into a weed-free field. Under intraspecific interference, 1,430 and 1,392 common cocklebur achenes m−2 and 1,827 and 5,435 sicklepod seed m−2 were deposited to the seedbank after 1 and 2 yr of seed production, respectively. For both species, approximately 11% of the initial seedbank emerged under tilled conditions the first year after deposition. Under no-till conditions, only 0.7% of common cocklebur and 1.6% of sicklepod emerged. The second year after deposition, common cocklebur emergence in no-till decreased to 0.25% of the initial seedbank, while sicklepod increased to 8% of the initial seedbank and remained higher than in tilled plots. Under tilled conditions, common cocklebur became the dominant species, and sicklepod became dominant under no-till conditions. Seedbank depletion was greater for both species under tillage. Three years after initial seed deposition, sicklepod seed was 100% viable but common cocklebur achenes were not viable. Under no-till conditions, common cocklebur was depleted in the seedbank but sicklepod was not. Thus, sicklepod poses a greater long-term weed problem than common cocklebur, especially under no-till conditions.

A methodological approach to determine the optimum time to control weeds that integrates aspects of weed biology, weed-crop competition, and economics is presented. The approach is based on the concept of Time Density Equivalent: this is defined as the density of weed plants that germinate with the crop and compete until harvest that causes the same yield loss caused by a group of weeds with a given density, time of emergence, and time of removal. A model was developed that accounts for pattern of weed emergence and permits determination of timing of weed control that minimizes economic loss due to weeds emerging both before and after treatments. The outcomes of the model are presented with two examples: corn in competition with velvetleaf and soybean in competition with Amaranthus cruentus. For both crops, six different weed control strategies involving preemergence, chemical, and mechanical postemergence treatments are considered. The results obtained with the model are compared with the calculation of net margin based on assumptions of simultaneous emergence of crop and weeds and no effect of different times of control. Different control strategies are compared considering not only maximum net margin but also its dependence on time of control, because a strategy with a lower value of maximum net margin, but a flatter net margin curve, allows more flexibility of time of control.

A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76-cm soybean row spacings on emergence pattern and spatial aggregation of giant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addition, we characterized the soil seedbank and determined the quantitative and spatial relationship between the seedbank and seedling populations. Viable seeds of about 10 weed species and twice as many species of seedlings were identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respectively, each accounting for 60 to 70% of the total weed species density. Emergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper 10 cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common lambsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 9 WAP was 98, 96, and 76% greater in 76- than in 18-cm row soybeans only when the population of 76-cm row soybeans was 57% lower than the 18-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD were similar in describing the spatial aggregation of giant foxtail and common lambsquarters but not some velvetleaf seedling cohorts. The spatial aggregation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence seed dispersal, germination, and seedling emergence. Within a 1.5-ha area, aggregation declined with decreasing density. Within a 24-m2 area, the level of aggregation of all weed species decreased as seedling densities increased. These results indicated that soybean row spacing influenced neither weed emergence pattern nor weed spatial aggregation; thus, several management decisions can be similar in 18- and 76-cm row soybeans.

Emergence periodicity of common purslane, hemp sesbania, horse purslane, prickly sida, spurred anoda, and velvetleaf was determined from March to October during 5 yr at Stoneville, MS, in the absence of reseeding. In early March of each year prior to emergence determinations, plots were not tilled or tilled to depths of 0, 5, 10, or 15 cm. Yearly emergence periodicity of the six weed species was not affected by tillage. In each year, weeds of all species emerged in identifiable flushes and very few emerged at other times during the growing season. Velvetleaf emerged mainly during the early growing season from March through May. Peak periods of emergence for the other species varied slightly over the 5 yr, but conformed generally to the early or the midseason peak period (June, July, and August) or both. In most years, common purslane, horse purslane, hemp sesbania, prickly sida, and spurred anoda emerged during the midseason period. Annual patterns of temperature fluctuations were similar during the 5 yr, but relatively warm soil temperatures during the early growing season of 1977 may have caused earlier emergence of velvetleaf, prickly sida, and hemp sesbania. Weekly water balance (rainfall minus open-pan evaporation) varied during March through May of each year but was negative during each June, July, and August. Apparently the main effect of a positive water balance during most of the year was to cause merger of the early and midseason peak emergence periods in 1979. Early and midseason emergence peaks were possibly due to different populations of seeds able to germinate in cooler soil temperatures of the early season and warmer soil temperatures of the midseason, respectively.

The purpose of this study was to investigate the role of sunlight perceived by weed seeds during and after soil cultivation with moldboard and chisel plows on induction of seed germination. Daytime cultivation with a moldboard plow increased weed seedling emergence by as much as 200% above the levels recorded following nighttime cultivation; however, the magnitude of this promotion varied depending on season and field history, and occasionally it was nil. In contrast, when a chisel plow was used in primary tillage, no differences in seedling emergence were observed between daytime and nighttime cultivation. The absolute germination level in plots cultivated with a chisel plow was at least 200% greater than in plots cultivated with a moldboard plow. This observation suggests that microenvironmental factors other than light played an important role in promoting seed germination in chisel-plowed plots. Covering moldboard-plowed plots with opaque plastic immediately after cultivation did not reduce weed seedling emergence compared to plots that were either covered with transparent film or left uncovered. Our results suggest that only the light stimulus perceived by the seeds during soil cultivation is effective in triggering germination.

Weed management can be a significant challenge in cropping systems, partly because the effects of tillage systems on weed seedbank and seedling population dynamics are not well understood. Field research was conducted from 1994 to 1996 in established tillage plots consisting of moldboard plow (MP), chisel plow (CP), and no-tillage (NT). The objectives were to determine the effects of long-term tillage systems on the timing and duration of Setaria spp. emergence and percentage cumulative emergence from the soil seedbank and to investigate the effect of tillage on Setaria spp. density and seed production following glyphosate application at Setaria spp. heights of 5, 10, and 15 cm. NT contained a greater number of Setaria spp. seed in the 0- to 1-, 1- to 3-, and 3- to 6-cm depths than MP or CP systems. There was little difference between the three tillage systems at depths greater than 6 cm. Setaria spp. emergence was greater in NT than in MP or CP in 1994 and 1996 and greater than in MP in 1995. There was a substantial increase in Setaria spp. emergence in NT between 3 and 4 weeks after planting (WAP) in 1994 and between 5 and 6 WAP in 1995 and 1996. Significant emergence did not occur past 5 to 6 WAP in 1994 and 1995 but continued over a longer period of time in 1996. Setaria spp. plants consistently reached targeted herbicide application heights 4 to 9 d earlier in NT than in CP and MP. In 1994, final Setaria spp. density was greater in NT compared to CP and MP at the 5- and 10-cm herbicide application timings. When glyphosate was applied to 15-cm-tall Setaria, very few weeds were present following application across all tillage systems. In 1995, NT resulted in greater Setaria spp. density than MP or CP across all application timings. There was no difference in final Setaria spp. density between MP and CP across all glyphosate timings in 1994 and 1995. Seed production was negligible in MP and CP, regardless of glyphosate timing. In NT, however, significant seed production occurred, especially with early application. Results indicate that the effectiveness of nonresidual herbicides for Setaria faberi Herrm. control is influenced by tillage system and the timing of application.

Removal and plant-back studies were conducted in North Carolina in 1991 and 1992 to determine the critical period of large crabgrass competition in transplanted triploid (seedless) watermelon. For every week that large crabgrass remained in watermelon, medium (3.6 to 7.3 kg) melon yield decreased 3,996 kg and 716 fruit ha−1. For every week that large crabgrass emergence was delayed, yield increased by 814 kg and 142 fruit ha−1. Likewise, for every week that large crabgrass remained in watermelon, marketable (3.6 kg and over) yield decreased 5,582 kg and 911 fruit ha−1. For every week that large crabgrass emergence was delayed, yield was increased 881 kg and 151 fruit ha−1. Large crabgrass emerging after 6 wk had no effect on marketable fruit or number of watermelon. To achieve the greatest quality or quantity of medium or marketable fruit, a large crabgrass-free period between 0 and 6 wk after transplanting was necessary.

Solar heating (SH) of wet soil by mulching it with transparent polyethylene (PE) during the hot season increased soil temperature in a typical daily course which varied with soil depth. Annual weed species responded to soil heating in the laboratory with the same pattern as under SH conditions in the field. Rhizomes of bermudagrass (Cynodon dactylon L. Pers. ♯3 CYNDA) and johnsongrass (Sorghum halepense L. Pers. ♯ SORHA) were very sensitive to heat treatment, but purple nutsedge (Cyperus rotundus L. ♯ CYPRO) tubers were able to survive temperatures as high as 80 C for 30 min. Species having big and heavy seeds or vegetative propagules were able to emerge from deep layers of soil, thus practically escaping the lethal temperature prevailing in the upper layer. Transparent and black PE mulching effectively prevented water loss from soil, as compared with perforated PE and nonmulched control. CO2 concentration in the soil atmosphere under transparent PE mulching increased rapidly during the first week and reached a maximal level which was 20-fold higher than that formed in nonmulched soil. Ethylene at 0.2 ppm was detected only in a mulched soil environment. No differences in levels of CH4 or CO were detected.

Field studies were conducted in 2002 and 2003 to determine whether canopy formation influences pitted morningglory, common cocklebur, and sicklepod emergence in surface-tilled soybean. Each weed species was broadcast seeded before planting soybean in 19- and 97-cm-wide rows. Weed emergence beneath soybean was monitored after soybean emergence and compared with weed emergence in the absence of soybean (bareground treatment). Magnitude of daily diurnal soil temperature fluctuations diminished after soybean canopy formation, and light interception by soybean was positively related to the reduction in soil temperature. Canopy formation (50% light interception) occurred 16 to 17 d later in wide compared with narrow rows in both years. The red/far-red ratio of light available to seed on or near the soil surface was reduced from as much as 1.2 in full sunlight to less than 0.1 in the presence of a dense soybean canopy. Pitted morningglory emergence was not influenced by soybean canopy formation, whereas common cocklebur and sicklepod emergence were reduced as much as 33 and 68%, respectively. Although common cocklebur and sicklepod emergence diminished after soybean canopy formation, a small portion of the seedbank of both species emerged beneath the canopy. This research indicates that in a tilled system, emergence of some weed species is diminished by presence of an overlying canopy, but emergence does not completely cease with canopy formation. Late-season emergence of sicklepod and common cocklebur beneath a soybean canopy may contribute to replenishment of the soil seedbank, especially if these late-emerging cohorts are capable of surviving until the light environment is favorable for seed production.

Row spacing and the relative time of velvetleaf emergence affects the time of soybean canopy closure relative to velvetleaf, influencing the growth and development of velvetleaf. Field studies were conducted in northeastern Nebraska in 2002 and 2003 to describe velvetleaf growth as influenced by soybean presence or absence (velvetleaf grown with soybean or in monoculture), soybean row spacing (19 and 76 cm), and relative time of velvetleaf emergence. Velvetleaf seed production, leaf area (LA), and total dry matter (TDM) were greater in 76-cm- than in 19-cm-wide soybean rows. LA, TDM, and seed production of velvetleaf were reduced with later emergence times in both monoculture and with soybean. Velvetleaf LA, TDM, and seed production decreased when grown with soybean compared with when grown in monoculture. Practical implications of this study suggest that narrowing crop row spacing and controlling early-emerging velvetleaf in soybean can be an effective part of an integrated weed management strategy.

The efficacy of metolachlor on rigid ryegrass was determined under a no-till seeding system in an experiment conducted in 2004 and 2005 in South Australia. Metolachlor at a rate of 0.48 and 0.96 kg ai/ha was applied at 40 or 46 d (very early preplant, VEPP) and 20 or 23 d before crop sowing (early preplant, EPP), and at sowing (preplant, PP) in 2004 and 2005; the herbicide was incorporated by sowing. The control of rigid ryegrass was greater than 80% when metolachlor was applied PP; however, that application resulted in phytotoxic effects on emergence and yield of wheat; metolachlor was more phytotoxic when applied at 0.96 kg/ha than at 0.48 kg/ha. Metolachlor applied EPP provided 71 to 83% rigid ryegrass control, whereas VEPP application provided only 33 to 49% control. Reduction in wheat grain yield was not observed at these application times. This study indicates that metolachlor at 0.48 kg/ha could be safely applied around 20 d before crop sowing to selectively control rigid ryegrass in wheat.

Direct-seeding of cover crops is often promoted to reduce potential soil loss during the winter, enhance soil fertility, and reduce energy use and equipment traffic on fields. The impact of direct-seeding of cover crops on wild-proso millet seedling emergence in subsequent crops is unknown. In this study, a pulse–chase experiment determined the effect of five fall soil management strategies, two spring primary tillage levels, and two herbicide programs on wild-proso millet emergence in a subsequent crop of snap beans. Wild-proso millet seeds were sown following sweet corn harvest in two commercial production fields to simulate seed rain at a density of 500 and 1,000 seeds m−2 in the fall of 2004 and 2005, respectively. The experimental design was a split–split plot. Five fall treatments were applied to main plots and included direct drilling of cover crops, conventionally tilled and drilled cover crops, a winter fallow treatment, and two additional treatments that were direct-drilled and conventionally tilled in the fall but that were not seeded with cover crops to separate the effects of tillage and cover crops on wild-proso millet emergence. Main plots were split in the spring and snap beans planted without primary tillage or conventionally planted after the soil was tilled. Tillage and cover crop treatments applied in the fall influenced emergence in the spring but with slightly different outcomes at the two sites. Direct drilling without a cover crop produced more wild-proso millet seedlings in snap beans than the winter fallow plot; tillage before cover crop planting in the fall produced recruitment levels less than or equal to densities in the winter fallow plots, with one exception. At one site, cover crops increased emergence compared to plots without cover crops. Spring tillage did not alter the affect of the five fall management treatments on wild-proso millet emergence. Direct-seeding of cover crops should be done with equipment or methods that minimize soil disturbance to prevent movement of wild-proso millet seeds into protected and favorable zones of emergence in the soil.

Differences in periodicity and depth of weed seedling recruitment due to agronomic management practices, such as reduced tillage, have implications for weed competitive ability and management strategies. Periodicity and depth of seedling recruitment of 10 different weed species was measured in the field in 2004 and 2005. The seedling recruitment of rigid ryegrass, threehorn bedstraw, and wild radish seedlings was higher under minimum tillage than under no-till scenarios. In contrast, the seedling recruitment of Oriental mustard, annual sowthistle, squirreltail fescue, little mallow, and turnipweed was higher under the no-till system. The seedling recruitment of wild oat and African mustard was not influenced by the tillage system. The mean seedling recruitment depth of wild oat, rigid ryegrass, threehorn bedstraw, wild radish, and turnipweed was greater under minimum tillage than under the no-till system. These weeds are able to emerge from deeper in the soil profile. In contrast, the seedling recruitment depth under minimum-tillage and no-till systems was similar for African mustard, Oriental mustard, annual sowthistle, little mallow, and squirreltail fescue. These are all small-seeded species, which failed to emerge from deeper depths under either tillage system. In addition, all of these species except African mustard showed higher total seedling recruitment under the no-till system. Results of this study will facilitate weed-control timing decisions and provide validation data for weed seedling recruitment models.

Field trials were initiated at three Missouri locations in the fall of 2004 and 2005 to compare the efficacy of fall and early spring herbicide applications on winter and summer annual weed populations in no-till soybean. All three experiments received applications of chlorimuron plus sulfentrazone plus 2,4-D; chlorimuron plus tribenuron plus 2,4-D; and glyphosate plus 2,4-D in the fall, 60 d prior to planting (60 PP), 30 days prior to planting (30 PP), and 7 d prior to planting (7 PP). Good control of all winter annual weeds, except for common chickweed, was observed at planting from fall applications of chlorimuron plus sulfentrazone plus 2,4-D and chlorimuron plus tribenuron plus 2,4-D. Fall, 60 PP, and 30 PP applications of chlorimuron plus sulfentrazone plus 2,4-D provided relatively poor control of common chickweed. Glyphosate plus 2,4-D applied in the fall and 60 PP provided adequate control of early fall germinating winter annuals, such as common chickweed and henbit. However, these treatments provided poor control of winter annual species that exhibited some degree of spring germination. Control of summer annual weed species 7 wk after planting (WAP) was highly variable. Relatively poor control of common waterhemp was obtained from all treatments except chlorimuron plus sulfentrazone plus 2,4-D applied at all three spring timings. Giant foxtail control was highly variable ranging from 0 to 95%. Measurements of weed biomass collected 7 WAP revealed larger amounts of biomass present in the fall application timing of each herbicide treatment. Emergence of winter and summer annual weed species from early March to 7 WAP varied by herbicide treatment. Emergence of total winter and summer annual weed species was reduced with both residual herbicide treatments when compared with glyphosate plus 2,4-D and the untreated control.

Research in irrigated and nonirrigated corn production systems was conducted to evaluate the effect of leaf architecture of corn hybrids on weed management. The corn hybrids used in each study were ‘Pioneer 3394’ (upright leaf) and ‘Pioneer 3260’ (horizontal leaf). In the irrigated study, residual weed control treatments included two rates of prepackaged mixtures of metolachlor plus atrazine, encapsulated acetochlor plus atrazine, nonencapsulated acetochlor plus atrazine, or a tank mixture of simazine and metolachlor plus atrazine. In the irrigated experiments, horizontal leaf architecture reduced weed density (all three sites in 1 of 2 yr), weed biomass (five of six sites), solar radiation reaching the ground (all six sites), and weed seed production (one site each year) compared with upright leaf architecture. Weed density and weed biomass did not differ between herbicide rates or acetochlor formulation at any site. Corn hybrid was significant for yield at only one site. Reduced weed biomass did not translate into yield differences. The nonirrigated study evaluated two factors at four sites over 2 yr: leaf architecture (upright or horizontal leaf) and weed management program (preemergence residual and postemergence no residual) at two application rates. Neither weed density nor weed biomass was reduced because of corn leaf architecture or herbicide rates in the nonirrigated study. No interaction was detected in either irrigated or nonirrigated studies between leaf architecture and herbicide treatments, indicating that these factors are independent of one another. On the basis of these studies, it appears that horizontal leaf architecture of corn hybrids can assist in integrated weed management in irrigated corn production but may not be beneficial when corn is grown under drought-prone conditions.