Although glyphosate controls many plant species, certain broadleaf weeds in Nebraska's cropping systems exhibit various levels of tolerance to the labeled rates of this herbicide, including ivyleaf morningglory, Venice mallow, yellow sweetclover, common lambsquarters, velvetleaf, kochia, Russian thistle, and field bindweed. Therefore, two field studies were conducted in 2004 and 2005 at Concord and North Platte, NE, to evaluate performance of (1) seven preemergence (PRE) herbicides and (2) glyphosate tank mixes applied postemergence (POST) at three application times for control of eight weed species that are perceived as problem weeds in glyphosate-resistant soybean in Nebraska. The PRE herbicides, including sulfentrazone plus chlorimuron, pendimethalin plus imazethapyr, imazaquin, and pendimethalin plus imazethapyr plus imazaquin provided more than 85% control of most weed species tested in this study 28 d after treatment (DAT). However, sulfentrazone plus chlorimuron and pendimethalin plus imazethapyr plus imazaquin were the only PRE treatments that provided more than 80% control of most weed species 60 DAT. In the POST glyphosate tank-mix study, the level of weed control was significantly affected by the timing of herbicide application; control generally decreased as weed height increased. In general, glyphosate tank mixes applied at the first two application times (early or mid-POST) with half label rates of lactofen, imazamox, imazethapyr, fomesafen, imazaquin, or acifluorfen, provided more than 80% control of all species that were 20 to 30 cm tall except ivyleaf morningglory, Venice mallow, yellow sweetclover, and field bindweed. Glyphosate tank mixes applied late POST with lactofen, imazethapyr, or imazaquin provided more than 70% control of common lambsquarters, velvetleaf, kochia, and Russian thistle that were 30 to 50 cm tall. Overall, glyphosate tank mixes with half label rates of chlorimuron or acifluorfen were the best treatments; they provided more than 80% control of all the studied weed species when applied at early growth stages. Results of this study suggested that mixing glyphosate with other POST broadleaf herbicides, or utilizing soil-applied herbicides after crop planting helped effectively control most problematic weeds in glyphosate-resistant soybean in Nebraska.

Venice mallow is an emerging weed problem in many regions of the world in various crops. Studies on its hard seed coat were conducted by scanning electron, light, and fluorescence microscopy. The effects of environmental factors on germination and seedling emergence were examined in laboratory experiments. Seeds possessed physical dormancy (90%) that required immersion for 0.5 h in concentrated sulfuric acid to break without damaging the seed. After scarification, 29% of seeds exhibited primary embryo dormancy. Exposure of seeds to even short periods of 15 d of prechilling induced secondary dormancy (10% germination). The surface of hard seeds had a velvety appearance from numerous papillate structures and deposits (hydrophilic material). The location of the water barrier was very superficial in the outer seed coat. Although, in hard seeds, the hilar area appeared to have vertical ruptures and the hilum fissure appeared to be open, there was no water entry. It was observed that microruptures could be the region of structural weakness of hard seeds in relation to water permeability during prolonged burial (8 mo).

The highest (60%) germination was recorded at a day/night temperature of 30/20 C with a 12-h photoperiod. No germination was measured at either 10 or 45 C constant temperature. Germination was recorded with a broad range of pH (3-11) and seeds were rather tolerant to low water potential (20% germination at –1.2 MPa). Seedling emergence was higher for seeds buried at 2 cm than for those placed on the surface (54 vs. 38%, respectively). These results showed that Venice mallow is a rather unique species that possesses a complex mechanism of dormancy (physical, primary, and secondary). Information gained in this study will be used in developing management strategies for this species.

Weed seed persistence in the soil seedbank is central to weed population dynamics; however, limited knowledge of mechanisms regulating seed survival in soil remains an obstacle to developing seed-bank management practices. Weed seeds are rich in carbon and nitrogen, and therefore may represent an important nutritional resource to soil microbes. The objective of this study was to test the hypothesis that weed seed mortality due to microbial predation is limited by soil inorganic N availability and soil C:N ratio. A factorial of N fertilizer rate (0, 14, and 28 mg N kg soil−1) and corn stover addition rate (0 and 3,000 mg stover kg soil−1) was applied to bioassay units containing Illinois field soil (silt loam, 3.8% organic carbon) and seeds of one of eight annual weed species common to Illinois field crops: giant foxtail, green foxtail, yellow foxtail, wooly cupgrass, giant ragweed, redroot pigweed, velvetleaf, and Venice mallow. Seeds were incubated for 2 mo, after which they were recovered from the soil and tested for viability. Only three of the eight species, velvetleaf, giant ragweed, and wooly cupgrass, responded to the experimental treatments. Velvetleaf seed mortality was 40% lower in the corn stover–amended treatment than in the unamended treatment. Both giant ragweed and wooly cupgrass showed a more complex interaction between N fertilizer and corn stover treatments. Path analysis supported the hypothesis that the influence of soil N on seed mortality in velvetleaf was because of the direct effect of soil N on microbial predation of velvetleaf seeds, whereas for giant ragweed and wooly cupgrass, the effect on seed mortality appeared to be mediated through soil N effects on germination. Mechanisms underlying soil N fertility effects on weed seed mortality appear to be species-specific. Future investigations of this phenomenon should include quantitative measures of seed coat composition and quality.

Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various Venice mallow densities and duration of interference on sugarbeet. Sucrose concentration was not affected by Venice mallow interference. The effect of Venice mallow density on sugarbeet root and sucrose yield loss was described by the rectangular hyperbola model. Root and sucrose yield loss increased as Venice mallow density increased. The estimated asymptote, A (percent yield loss as density approaches infinity) was 61% for both root and sucrose yield loss, and the estimated parameter, I (percent yield loss per unit weed density as density approaches zero) was 6% for both root and sucrose yield loss. Sugarbeet root yield decreased as the duration of Venice mallow interference increased. The critical timing of weed removal to avoid 5 and 10% root yield loss was 30 and 43 d after sugarbeet emergence, respectively. Results show that Venice mallow is competitive with sugarbeet implying that it should be managed appropriately to reduce negative effects on yield and prevent seed bank replenishment and re-infestation in subsequent years.