Tolpyralate is a new Group 27 pyrazolone herbicide that inhibits the 4-hydroxyphenyl-pyruvate dioxygenase enzyme. In a study of the biologically effective dose of tolpyralate from 2015 to 2017 in Ontario, Canada, tolpyralate exhibited efficacy on a broader range of species when co-applied with atrazine; however, there is limited published information on the efficacy of tolpyralate and tolpyralate+atrazine relative to mesotrione and topramezone, applied POST with atrazine at label rates, for control of annual grass and broadleaf weeds. In this study, tolpyralate applied alone at 30 g ai ha−1 provided >90% control of common lambsquarters, velvetleaf, common ragweed, Powell amaranth/redroot pigweed, and green foxtail at 8 weeks after application (WAA). Addition of atrazine was required to achieve >90% control of wild mustard, ladysthumb, and barnyardgrass at 8 WAA. Tolpyralate+atrazine (30+1,000 g ai ha−1) and topramezone+atrazine (12.5+500 g ai ha−1) provided similar control at 8 WAA of the eight weed species in this study; however, tolpyralate+atrazine provided >90% control of green foxtail by 1 WAA. Tolpyralate+atrazine provided 18, 68, and 67 percentage points better control of common ragweed, green foxtail, and barnyardgrass, respectively, than mesotrione+atrazine (100+280 g ai ha−1) at 8 WAA. Overall, tolpyralate+atrazine applied POST provided equivalent or improved control of annual grass and broadleaf weeds compared with mesotrione+atrazine and topramezone+atrazine.

Cover crop residues are not widely used for weed control because, as a stand-alone tactic, they do not effectively suppress all weeds and their duration of weed control is too short. Field experiments were conducted in 1995 and 1996, under both irrigated and rainfed conditions, to quantify Amaranthus spp., Setaria spp., and soybean emergence and growth in residues of fall-planted, spring-killed barley, rye, triticale, wheat, and hairy vetch. For both weed species, seedling emergence was reduced 3 wk after soybean planting by rye and wheat residues (≥ 2, 170 kg ha−1) in 1996. In 1996, Amaranthus spp. canopy volume was reduced 38 to 71% by residues 3 wk after planting. Likewise, Setaria spp. canopy biomass was reduced 37 to 97% in residues 5 wk after planting over both years. The response comparison index was used to identify frequency by which weed growth was placed at a disadvantage relative to soybean growth. Amaranthus spp. and Setaria spp. growth suppressions 3 to 5 wk after planting indicate potential times for intervention with other integrated weed management tactics such as reduced postemergence herbicide rates and interrow cultivation.

Imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was evaluated in the field for weed control in flue-cured tobacco (Nicotiana tabacum L.) during 1983 and 1984. The 75 DG formulation of imazaquin was used at 0.28 and 0.42 kg ai/ha. Methods of application were: preplant incorporated, over the top immediately after transplanting, postbed incorporated, and early postemergence. Imazaquin controlled 90%, or better, of common lambsquarters (Chenopodium album L. # CHEAL), common ragweed (Ambrosia artemisiifolia L. # AMBEL), redroot pigweed (Amaranthus retroflexus L. # AMARE), spiny amaranth (Amaranthus spinosus L. # AMASP), ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. # IPOHE], sicklepod (Cassia obtusifolia L. # CASOB), and prickly sida (Sida spinosa L. # SIDSP) with all application methods except early postemergence. It controlled 80 to 89% of large crabgrass (Digitaria sanguinalis L. # DIGSA) by the soil-applied methods. Postemergence applications of imazaquin controlled common ragweed, pigweed spp., ivyleaf morningglory, prickly sida, and sicklepod. Imazaquin was less effective postemergence on lambsquarters and large crabgrass. Tobacco tolerated use of imazaquin by all methods of application except preplant incorporation.

Site properties and weed species abundance are known to vary spatially across fields. The extent to which they covary is not well understood. The objective of this research was to assess how canonical correlation analysis could be used to identify associations among site properties and weed species abundance within an agricultural field. A farmer-managed field rotated between Zea mays and Glycine max in Boone County, IA, was grid-sampled for site properties in 1992 and for weed species abundance between 1994 and 1997. Twelve site properties were considered in relation to five weed species that were identified and counted after all weed control operations were completed. Site properties such as total nitrogen, Bray-1 P, percent organic carbon, and texture were spatially variable. Weed species abundance was also spatially variable such that most weeds were found in patches and much of the field was weed-free. Canonical correlation analysis identified one to four significant correlations between linear combinations of site properties and weed species abundance. The first and second pairs of linear combinations explained the majority of variation in the data and were used to identify associations among site properties and weed species abundance. In years with Z. mays, the first pair of linear combinations described an association between herbicide activity and weed presence, and the second described topography and soil texture associations with weed presence. In years with G. max, the single observed association described a link between soil texture and presence of Setaria species and Polygonum coccineum. Several consistent associations were identified across years, indicating that site properties can influence weed abundance. However, annual variation in the associations may be attributed to differences in agronomic and weed management practices for each crop, as well as temporal weather variation influencing weed abundance from year to year. This multivariate technique is an important tool to identify associations between site properties and weed abundance that could help explain observed patchy patterns of weed abundance. These associations are an important first step in the generation of hypotheses to be tested at the whole field scale.

Identification of associations between site properties and weed species abundance led to the generation of hypotheses as to why weed populations occur where they do, or do not, in agricultural fields. The objective of this research was to use a multivariate statistical technique, canonical correlation analysis, to identify the associations. Two continuous Zea mays production fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site properties and six to seven weed species. Weed species were identified and counted just prior to postemergence weed control in two adjacent quadrats (1 by 0.38 m) at each grid sampling point. These quadrats represented untreated weed populations emerging between crop rows and treated populations that survived preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear combinations of site properties and weed species abundance depending on field site, years, and between- vs. on-crop row weed populations. The first pair of linear combinations consistently described an association that separated weed species across a gradient of topography and soil type. The second pair of linear combinations described associations between weed species and soil fertility. In all cases, it was hypothesized that management practices strongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms for weed patchiness include patchiness in available soil moisture that would influence weed seed germination, emergence, and seedling growth. Additional variation in plant-available preemergence herbicide concentration across the field site would vary weed control efficacy. Another mechanism would be variation in soil fertility that affects the growth, reproduction, and competitive ability of both the crop and the weed.