Herbicide-resistant crops, such as glyphosate-resistant (GR) soybean, allow for broad-spectrum, flexible weed control with minimal crop injury; however, the development of GR weeds, such as horseweed, has forced reliance on alternative herbicides for control of these weeds. While preplant (PP) herbicides provide excellent control of GR-horseweed, there are currently no POST herbicide control options within soybean. The objective of this study was to evaluate the efficacy of dicamba for the control of GR-horseweed when applied PP, POST, and sequentially in dicamba-resistant soybean. Dicamba applied PP at 600 g a.e. ha−1 provided 90 to 100% control of GR-horseweed 8 wk after application (WAA) across three field trials conducted in Ontario in 2011 and 2012. Similarly, sequential applications provided 91 to 100% control. This technology provides a much-needed POST option of dicamba to be applied as a rescue treatment to control weed escapes caused by late emergence or poor initial control following a PP herbicide application.

Soybean producers of South Carolina were surveyed in 2000 through mail and on-farm visits to determine which production practices limit seed yields the most. Production systems have direct implications on weed management practices, difficulties, and future problems. When asked to describe two extension and research focus areas that would improve current soybean production, the number one response was improved weed control strategies. Fifty-seven percent of the respondents indicated that weeds were the pest that most limited soybean yield. Sicklepod, Palmer amaranth, morningglory species, common cocklebur, yellow nutsedge, and Texas panicum were considered the most problematic weeds. More than half the producers (68%) used glyphosate-resistant soybean, with one-half of these farmers producing soybean in row widths > 51 cm. Of the farmers who grew conventional soybean, 10% used narrow rows (≤ 51 cm), whereas 22% used wide rows (> 51 cm). Only 27% of the farmers who planted glyphosate-resistant soybean used a soil-applied herbicide. Forty-three percent of farmers based the need for a postemergence herbicide on weed size, 21% used no repeatable method, 21% relied on days after soybean emergence, 11% depended on soybean size or groundcover, and only 4% used a combination of weed density and size. Thirty-five percent of the farmers did not rotate soybean with another crop. In fields where soybean parasitic nematodes reduced soybean growth or caused plant death, late-season weed infestations were often exacerbated, especially in the absence of nonresidual herbicides. Specific reasons for difficulty in managing weeds in soybean may include a diverse weed spectrum, which is difficult to control with a single herbicide, the use of wide-row soybean, the lack of crop rotation, the prevalence of nematodes that directly affect crop development and, in turn, weed management, the lack of an appropriate criterion for timing postemergence herbicide applications, and the greater attention paid to production of more valuable crops. Educational efforts addressing weed management problems and association of such problems with other production practices are needed if weed management is to be improved and weed-induced yield losses minimized.

Greenhouse studies were initiated to evaluate glyphosate alone and tank-mixed with acifluorfen, CGA 277476, chlorimuron, cloransulam-methyl, fomesafen, imazaquin, or pyrithiobac on seedling johnsongrass, broadleaf signalgrass, pitted morningglory, and hemp sesbania. Johnsongrass and broadleaf signalgrass control by glyphosate was not affected by the selective herbicides applied in mixtures. Pitted morningglory control by glyphosate ranged from 0% with 280 g ai/ha to 67% with 840 g/ha. There was an additive effect when selective herbicides were added to 280 g/ha glyphosate 2 wk after treatment (WAT). When acifluorfen was added to 560 g/ha glyphosate, pitted morningglory control 2 WAT increased to 100% compared with 55% with glyphosate alone. Similarly, the addition of fomesafen or acifluorfen to 840 g/ha glyphosate controlled pitted morningglory 2 WAT by 90 and 98%, respectively, compared with 67% with glyphosate alone. Only tank mixtures of acifluorfen, CGA 277476, or fomesafen, and 840 g/ha glyphosate reduced fresh weight compared with glyphosate alone 4 WAT. Acifluorfen, CGA 277476, and fomesafen controlled pitted morningglory by 85 to 100% when added to 1,120 g/ha glyphosate. Both acifluorfen and fomesafen effectively controlled hemp sesbania without the addition of glyphosate 2 WAT. Chlorimuron and pyrithiobac added to 1,120 g/ha glyphosate increased hemp sesbania control to 88 and 99%, respectively, compared with 45% with glyphosate alone 2 WAT. CGA 277476, cloransulam-methyl, imazaquin, and pyrithiobac were antagonistic to hemp sesbania fresh weight reduction when compared with the expected response, but fresh weights were still reduced more than with the same rate of glyphosate alone.

The increased use of conservation tillage in cotton production requires that information be developed on the role of cover crops in weed control. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue, conservation-tillage, nontransgenic cotton production system. Black oat, rye, and wheat were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were used: no herbicide, preemergence (PRE) herbicides alone, and PRE plus postemergence (POST) herbicides. The PRE system consisted of pendimethalin at 1.12 kg ai/ha plus fluometuron at 1.7 kg ai/ha. The PRE plus POST system contained an additional application of fluometuron at 1.12 kg/ha plus DSMA at 1.7 kg ai/ha early POST directed (PDS) and lactofen at 0.2 kg ai/ha plus cyanazine at 0.84 kg ai/ha late PDS. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was used with PRE herbicides, weed control was similar to the PRE plus POST system. Rye and black oat provided more effective weed control than wheat in conservation-tillage cotton. The winter fallow, PRE plus POST input system yielded significantly less cotton in 2 of 3 yr compared to systems that included a winter cover crop. Use of black oat or rye cover crops has the potential to increase cotton productivity and reduce herbicide inputs for nontransgenic cotton grown in the Southeast.

The significance of glyphosate and the appearance of glyphosate-resistant weeds have raised concerns about glyphosate sustainability. Resistance-prevention strategies, however, should first consider the mechanisms for resistance. For example, target-site resistance can provide virtual immunity, ensuring that every herbicide application successfully selects for resistance. However, metabolism and exclusion mechanisms provide lower magnitudes of resistance and are dependent on dosage. This discussion proposes that the relative risk of weed resistance is most highly correlated to mode of action (MOA), due to the respective principal mechanism for resistance. The development of data correlating agronomic practices with weed resistance vs. herbicide/MOA choices will be critical to the design of effective prevention strategies. Because resistance to glyphosate in weeds is typically of a low magnitude, using a high-dose strategy should minimize the potential for the selection of resistance and thus help to make the use of glyphosate sustainable. Maximizing weed control is the key to successful agronomic practice with limited weed resistance.

Seeds of a putative 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide–resistant tall waterhemp biotype from Henry County, IA, were collected from a seed corn field in fall 2009 after plants were not controlled following a POST application of mesotrione plus atrazine. The response of this biotype to various herbicide modes of action was evaluated in greenhouse and field tests. Under greenhouse conditions, the suspect biotype showed an eightfold decrease in sensitivity to mesotrione with a 50% control rate of 21 g ha−1 compared with 2.7 g ha−1 for the susceptible biotype. The biotype also had a 10-fold decrease in sensitivity to atrazine and a 28-fold decrease in sensitivity to thifensulfuron. Under field conditions, tall waterhemp was not controlled POST at the label rate of 1,100 g ha−1 of atrazine. Tall waterhemp control was less than 60% at the label rates of three commonly used POST HPPD-inhibiting herbicides in seed corn: 105 g ha−1 of mesotrione, 92 g ha−1 of tembotrione, or 18 g ha−1 of topramezone. Thus, this new tall waterhemp biotype is resistant to three herbicide modes of action: HPPD inhibitors, photosystem-II inhibitors, and acetolactate synthase (ALS) inhibitors.

Field studies were conducted in 2000, 2001, and 2002 at Urbana, IL, to examine the interference potential of common waterhemp that emerged at soybean growth stages VE, V2-V3, V4-V5, R1-R2, and R3-R4 in 19- and 76-cm row soybean. Soybean row width and common waterhemp emergence timing significantly influenced common waterhemp density, biomass, seed production, mortality, and soybean yield loss. Common waterhemp density declined as emergence timings were at later soybean growth stages. This decline happened at earlier growth stages in narrow-row soybean. Significant reductions in common waterhemp biomass and seed production occurred at the V2-V3 and V4-V5 emergence timings for the narrow- and wide-row soybean, respectively. Common waterhemp seed production was more than 23,000 seeds per plant at the VE emergence timing for both soybean row widths. Survival of common waterhemp that emerged after the V4-V5 soybean growth stage was less than 20% in both row widths. Common waterhemp interference reduced soybean seed yield at the VE, V2-V3, and the V4-V5 emergence timings. Row width affected the magnitude of yield reductions at these interference timings, with reductions being less in narrow-row soybean. This research suggests that control measures need to be implemented to common waterhemp plants that emerge before V4-V5 soybean to protect soybean yield and reduce common waterhemp seed production.

Corn and soybean growers across Indiana were surveyed during winter 2003/2004 to assess their perceptions about the importance of glyphosate-resistant weeds and management tactics to prevent development of resistant populations. The survey showed two intriguing observations. First, 65% of survey respondents expressed moderate or low levels of concern about weeds developing resistance to glyphosate, whereas 36% expressed a high level of concern. Second, when asked an open-ended question regarding the factors that contribute to development of glyphosate-resistant weeds, 58% of the responses included repeated use of the same mode of action. Other factors such as poor application techniques or timing (33%), unique weed characteristics (8%) and changes in tillage practices (1%) were also mentioned. The survey showed that even though a relatively low percentage of respondents were highly concerned about resistance, they still expressed a willingness to use field scouting, tank-mix partners with glyphosate for burn-down and postemergence weed control, and soil-applied residual herbicides as resistance management strategies. This survey also showed that growers who farm 800 ha or more were more concerned about glyphosate resistance and more likely to adopt resistance management strategies than smaller growers.

Predicting crop yield reductions from weeds continues to challenge weed science. Useful methods based upon weed densities and upon area weeds influence have been developed. Results vary greatly from year to year and location to location for several reasons which are reviewed: 1) the shift in crop yield from a weed density-dependent to a weed density-independent relationship; 2) the effect of density on essential growth factor competed for; 3) differences among weed species in relative competitiveness for essential growth factors; 4) the differential effect of environmental conditions on the competitiveness of weed species; and 5) the effect of time of emergence on competition. To reduce the variability in prediction methods will necessitate developing a method based upon the factor(s) responsible for yield reduction.

Imazethapyr at 0.07 and 0.10 kg ai ha-1 applied preplant incorporated (PPI), preemergence (PRE), and postemergence (POST) was evaluated at two locations in 1988 and 1989 for safety to dry edible beans. Bean stunting, leaf crinkling, and interveinal chlorosis were evident from imazethapyr and varied from 1 to 57%. Imazethapyr significantly reduced bean height and delayed maturity. PPI and POST applications of imazethapyr at 0.07 and 0.10 kg ha-1 did not reduce bean seed yields compared to yields of the untreated control. Bean cultivar by herbicide interactions were significant for bean injury but varied with year and location.

The value of glyphosate has been compromised in some fields where weed populations have developed resistance or tolerant species increased. Three case studies related to reduced control from glyphosate are: (1) a weed population that has become resistant to glyphosate, with horseweed in Tennessee as an example; (2) a weed population increases due to lack of control in “glyphosate only” systems, with tropical spiderwort in Georgia cotton used as an example; and (3) the hypothetical resistance of common waterhemp to glyphosate in Illinois. For each of these case studies, an economic analysis was performed using a partial budget approach. This economic analysis provides the cost of control to the farmer when glyphosate fails to control these weeds and gives a critical time in years to compare different glyphosate resistance management philosophies (applicable only before resistance has evolved). The cost of glyphosate-resistant horseweed in cotton-soybean-corn rotation in Western Tennessee was calculated to be $30.46/ha per year. The cost of tropical spiderwort in cotton in southern Georgia was calculated to be $35.07/ha per year. The projected cost if common waterhemp were to develop glyphosate resistance in a corn-soybean rotation in southern Illinois was projected to be $44.25/ha per year, and the critical time was determined to be greater than 20 yr, indicating that a resistance management strategy would extend the value of glyphosate-resistant crops.

Eight field studies were conducted in soybeans at seven locations in Kansas over a 3-yr period to examine the efficacy of using reduced rates of the herbicides acifluorfen, bentazon, chlorimuron, and tank mixes of acifluorfen and bentazon. POST applications of these herbicides at 1/2X rates at 2 wk after planting (WAP) resulted in broadleaf weed control similar to that obtained from standard treatments of 1X rates applied at 4 WAP at six of seven studies with acifluorfen, bentazon, and acifluorfen plus bentazon and at five of seven studies with chlorimuron. One-quarter rates applied 2 WAP were equivalent to standard treatments for broadleaf weed control in four of seven studies with acifluorfen and chlorimuron, five of seven studies with bentazon, and six of seven studies with acifluorfen plus bentazon. One cultivation at 4 WAP, increased the broadleaf weed control with all herbicide treatments.

Barnyardgrass at 10 or more plants per 1 m of row caused over 80% root yield loss in spring-sown sugarbeets in California. The yield loss decreased to between 5 and 20% when barnyardgrass densities were one plant ever 2 to 3 m of crop row. The relationship between crop yield and barnyardgrass density was described by a rectangular hyperbolic function. The economic threshold density was in the range of one barnyardgrass plant every 5 to 20 m or row. Barnyardgrass plants growing at densities of less than one plant per 1 m in sugarbeets experienced only interspecific competition. Such individual barnyardgrass plants were estimated to produce from 30 000 to over 200 000 seeds, with an average of 100 000 seeds per plant. This seed production equated to a seed rain of about 4000 to nearly 20 000 seeds per m from a density of one weed every 10 m of crop row. These data suggest that using single-year economic thresholds for barnyardgrass is not a sound management practice because even low densities can produce large quantities of seed.

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss.

This research examined dicamba measurements following an application to soil inside a humidome. The dicamba formulations examined were the diglycolamine (DGA) and diglycolamine plus VaporGrip® (DGA+VG), both applied with glyphosate. Post-application dicamba measurements were related to ambient temperature, with more dicamba detected as the temperature increased. There also appeared to be a minimum temperature of ~15 C at which dicamba decreased to low levels. The addition of glyphosate to dicamba formulations decreased the spray mixture pH and increased the observed dicamba air concentrations. Adding glyphosate to DGA+VG increased detectable dicamba air concentrations by 2.9 to 9.3 times across the temperature ranges examined. Particle drift would not be expected to be a factor in the research, as applications were made remotely before treated soil was transported into the greenhouse. The most probable reason for the increased detection of dicamba at higher temperatures and with mixtures of glyphosate is via volatility.

Glyphosate typically controls Palmer amaranth very well. However, glyphosate-resistant (GR) biotypes of this weed are present in several southern states, requiring the development of effective alternatives to glyphosate-only management strategies. Field experiments were conducted in seven North Carolina environments to evaluate control of glyphosate-susceptible (GS) and GR Palmer amaranth in narrow-row soybean by glyphosate and conventional herbicide systems. Conventional systems included either pendimethalin or S-metolachlor applied PRE alone or mixed with flumioxazin, fomesafen, or metribuzin plus chlorimuron followed by fomesafen or no herbicide POST. S-metolachlor was more effective at controlling GR and GS Palmer amaranth than pendimethalin; flumioxazin and fomesafen were generally more effective than metribuzin plus chlorimuron. Fomesafen applied POST following PRE herbicides increased Palmer amaranth control and soybean yield compared with PRE-only herbicide systems. Glyphosate alone applied once POST controlled GS Palmer amaranth 97% late in the season. Glyphosate was more effective than fomesafen plus clethodim applied POST. Control of GS Palmer amaranth when treated with pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST was equivalent to control achieved by glyphosate applied once POST. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST. Systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE without fomesafen POST controlled GR Palmer amaranth less than 30% late in the season. Systems of pendimethalin or S-metolachlor PRE followed by fomesafen POST controlled GR Palmer amaranth less than 60% late in the season.

Management of corn (Zea mays) row spacings and populations has been used for many years to increase corn productivity. In 1998 and 1999, nonirrigated corn was grown in 38-, 56-, and 76-cm row spacings at populations averaging 59,300, 72,900, and 83,900 plants/ha in Michigan. Glufosinate at 0.29 kg/ha was applied to common lambsquarters (Chenopodium album L.) averaging 5 cm in height in each plot. Corn population and row spacing did not influence weed emergence following application of glufosinate. Common lambsquarters biomass and seed production were reduced when grown under canopies of corn planted in populations exceeding 72,900 plants/ha. Common lambsquarters biomass was reduced as corn row spacings were reduced from 76 to 38 cm. Early-season interception of photosynthetic active radiation (PAR) by corn canopies increased as row spacings decreased, but differences were not apparent later in the season. Interception of PAR was similar throughout the season when corn populations exceeded 72,900 plants/ha. Corn yields were not affected by row spacing, but they were increased with corn populations of 72,900 plants/ha or higher.

The growth regulator herbicides 2,4-D and dicamba are used to control glyphosate-resistant horseweed before crops are planted. With the impending release of 2,4-D–resistant and dicamba-resistant crops, use of these growth regulator herbicides postemergence will likely increase. The objective of this study was to determine the effectiveness of various growth regulators on Indiana horseweed populations. A greenhouse dose–response study was conducted to evaluate the effectiveness of 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of four populations of horseweed in the greenhouse. Population 66 expressed twofold levels of tolerance to 2,4-D ester and diglycolamine salt of dicamba. Population 43 expressed an enhanced level of tolerance to diglycolamine salt of dicamba but not to the other herbicides. Diglycolamine salt of dicamba provided the best overall control of populations 3 and 34. Additionally, a field study was conducted to evaluate standard use rates of 2,4-D amine, 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of various sized glyphosate-resistant horseweed plants. Control of plants 30 cm or less in height was 90% or greater for all four herbicides. On plants greater than 30 cm tall, diglycolamine salt of dicamba provided 97% control while 2,4-D amine provided 81% control. Diglycolamine salt of dicamba provided the highest level of control of glyphosate-resistant horseweed, followed by dimethylamine salt of dicamba, 2,4-D ester and 2,4-D amine, respectively. This research demonstrates that horseweed populations respond differently to the various salts of 2,4-D and dicamba, and it will be important to determine the appropriate use rates of each salt to control glyphosate-resistant horseweed.

About 15 years ago, I heard several speakers saying that our crop yields were “leveling off.” This stimulated me to assemble data on the subject. The result is shown in Table 1, which gives the U.S. average yields for 10-yr periods during this century for nine crops. The increases are spectacular, varying from two- to sevenfold. Factors contributing to these increases differ from crop to crop. To obtain information on this subject, several experts on each crop were consulted, and their conclusions are summarized in the following text. Only those factors contributing to increased yields are covered. However, great improvements in efficiency of production, product quality, and reduction in soil erosion also have occurred as cultivars and production practices have changed. For most of the crops, increases in yields started in the 1940s and have increased dramatically during the rest of the century. We have not attempted to predict the future at this time, but the rate at which yields are increasing does not appear to be slowing.

Vegetation development bordering the Middle Rio Grande, as with most major southwestern U.S. tributaries, has historically undergone rapid and dynamic change. The introduction of saltcedar (or Tamarisk, genus Tamarix) and other exotic species into this environment within the 20th century has contributed to this process. These plants are now an integral component of the riparian vegetation mix. Manpower, logistics, and financial resources constrain the degree to which a desired riparian habitat can be restored from saltcedar thickets on the Bosque del Apache National Wildlife Refuge near Socorro, NM. Saltcedar clearing is accomplished using a combination of herbicide, burning, and mechanical control techniques costing from $750 to $1,300/ha. Soil salinity and depth to water are the principal physical features limiting revegetation efforts. Cottonwood and black willow plantings and natural regeneration after timed irrigations have produced diverse habitats that support a wide array of faunal species in areas previously occupied by homogeneous saltcedar.