The commercialization of imazethapyr-resistant (Clearfield™, CL) rice in the southern United States has raised serious concerns about gene flow to red rice, producing imazethapyr-resistant red rice populations. Our objectives were to determine the impact of planting date, CL cultivars, and red rice biotypes on outcrossing rate; and to investigate the relative contribution of flowering time of CL rice and red rice biotypes, together with air temperature and relative humidity (RH), on outcrossing rate. Field experiments were conducted at Stuttgart, Rohwer, and Kibler, AR, from 2005 to 2007, at three or four planting times from mid-April to late May. ‘CL161’ (inbred cultivar) and ‘CLXL8’ (hybrid) rice were planted in nine-row plots, with red rice planted in the middle row. Twelve red rice biotypes were used. The flowering of red rice and CL rice, air temperature, and RH were recorded. Red rice seeds were collected at maturity. To estimate outcrossing rate, resistance to imazethapyr was evaluated in subsequent years and confirmed using rice microsatellite markers. CLXL8 rice flowered 2 to 4 d earlier than CL161 rice, and flowering was completed within 1 wk in all plantings. The flowering duration of most red rice biotypes ranged from 4 to 17 d. Flowering synchrony of red rice biotypes and CL rice ranged from 0 to 100% at different plantings. In general, CLXL8 had greater flowering overlap and higher outcrossing rate with red rice than did CL161 rice. The outcrossing rate of red rice biotypes ranged from 0 to 0.21% and 0 to 1.26% with CL161 and CLXL8 rice, respectively. The outcrossing rate differed within each planting date (P < 0.05). Outcrossing was generally lower in mid-May and late May than in mid-April and late April planting times. Flowering synchrony and outcrossing rate were not correlated (r2 < 0.01). Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature of > 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was < 54%, outcrossing was less (0.12%) than when RH was ≥ 54% (0.38%). With CLXL8 rice, a minimum RH of ≥ 54%, from mid-morning to noon, increased outcrossing with red rice. To fully understand the interaction effects of these factors on outcrossing with red rice, controlled experiments are needed.

The selection of herbicide-resistant weed populations began with the introduction of synthetic herbicides in the late 1940s. For the first 20 years after introduction, there were limited reported cases of herbicide-resistant weeds. This changed in 1968 with the discovery of triazine-resistant common groundsel. Over the next 15 yr, the cases of herbicide-resistant weeds increased, primarily to triazine herbicides. Although triazine resistance was widespread, the resistant biotypes were highly unfit and were easily controlled with specific alternative herbicides. Weed scientists presumed that this would be the case for future herbicide-resistant cases and thus there was not much concern, although the companies affected by triazine resistance were somewhat active in trying to detect and manage resistance. It was not until the late 1980s with the discovery of resistance to Acetyl Co-A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors that herbicide resistance attracted much more attention, particularly from industry. The rapid evolution of resistance to these classes of herbicides affected many companies, who responded by first establishing working groups to address resistance to specific classes of herbicides, and then by formation of the Herbicide Resistance Action Committee (HRAC). The goal of these groups, in cooperation with academia and governmental agencies, was to act as a forum for the exchange of information on herbicide-resistance selection and to develop guidelines for managing resistance. Despite these efforts, herbicide resistance continued to increase. The introduction of glyphosate-resistant crops in the 1995 provided a brief respite from herbicide resistance, and farmers rapidly adopted this relatively simple and reliable weed management system based on glyphosate. There were many warnings from academia and some companies that the glyphosate-resistant crop system was not sustainable, but this advice was not heeded. The selection of glyphosate resistant weeds dramatically changed weed management and renewed emphasis on herbicide resistance management. To date, the lesson learned from our experience with herbicide resistance is that no herbicide is invulnerable to selecting for resistant biotypes, and that over-reliance on a weed management system based solely on herbicides is not sustainable. Hopefully we have learned that a diverse weed management program that combines multiple methods is the only system that will work for the long term.

Because of the increasing number of glyphosate-resistant weeds, alternate herbicide-resistant crops and herbicides with different modes of action are required to protect crop yield. Glufosinate is a broad-spectrum POST herbicide for weed control in glufosinate-resistant crops, including soybean. The objective of this study was to compare herbicide programs with glufosinate applied singly at late-POST (LPOST) or sequentially at early POST (EPOST) followed by (fb) LPOST applications and PRE herbicides fb EPOST/LPOST glufosinate alone or tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor in glufosinate-resistant soybean. A field experiment was conducted at the South Central Agriculture Laboratory in Clay Center, NE, in 2012 and 2013. Glufosinate applied in a single LPOST or sequential EPOST fb LPOST application controlled common lambsquarters, common waterhemp, eastern black nightshade, green foxtail, large crabgrass, and velvetleaf ≤ 82% and resulted in a weed density of 6 to 10 plants m−2 by the end of the season. Flumioxazin-, saflufenacil-, or sulfentrazone-based premixes provided 84 to 99% control of broadleaf and grass weeds tested in this study at 15 d after PRE application and a subsequent LPOST application of glufosinate alone controlled broadleaf and grass weeds 69 to 93% at harvest, depending on the herbicide program and weed species being investigated. The PRE application of sulfentrazone plus metribuzin fb EPOST glufosinate tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor controlled the tested broadleaf and grass weeds ≥ 90%, reduced density to ≤ 2 plants m−2, and reduced weed biomass to ≤ 10 g m−2 and produced soybean yields of ≥ 4,450 and 3,040 kg ha−1 in 2012 and 2013, respectively. Soybean injury was 0 to 20% from PRE or POST herbicides, or both and was inconsistent, but transient, during the 2-yr study, and it did not affect soybean yield. Sulfentrazone plus metribuzin applied PRE fb glufosinate EPOST tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor provided the highest level of weed control throughout the growing season and increased soybean yield compared with a single LPOST or a sequential EPOST fb LPOST glufosinate application. Additionally, these herbicide programs provide four distinct mechanisms of action that constitute an effective weed-resistance management strategy in glufosinate-resistant soybean.

Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype.