Field studies were conducted in 1996 and 1997 to determine the effects of RP-201772 and RP-201772 tank mixtures applied to emerged Zea mays. RP-201772 at 105 g ha−1 and tank-mixed metolachlor/benoxacor or atrazine were applied preemergence and to spike, 2-leaf, and 4-leaf Z. mays. Herbicide treatments over the four application timings provided greater than 90% Chenopodium album, Ambrosia artemisiifolia, Amaranthus retroflexus, and Abutilon theophrasti control. However, Setaria faberi control varied among years and application timings. Severe Z. mays injury, 70% and 40%, was observed when RP-201772 tank-mixed with metolachlor/benoxacor was applied to 2-leaf and 4-leaf Z. mays, respectively. Greenhouse studies confirmed Z. mays sensitivity from delayed applications of RP-201772 tank-mixed with metolachlor/benoxacor. Similarly, increased Z. mays injury was observed from postemergence applications of RP-201772 tank-mixed with acetochlor/MON-13900. Herbicide absorption, translocation, metabolism, and retention studies were conducted to determine the physiological basis for the observed Z. mays injury from delayed applications of the RP-201772 tank mixture with metolachlor/benoxacor. Metolachlor/benoxacor increased radiolabeled RP-201772 absorption when applied to spike, 2-leaf, and 4-leaf Z. mays. RP-201772 translocation and metabolism did not explain enhanced Z. mays injury. However, RP-201772 retention increased fivefold when metolachlor/benoxacor was present in the spray solution and applied to 2-leaf and 4-leaf Z. mays. Increased RP-201772 absorption and retention appeared to be the basis for Z. mays injury when tank-mixed with metolachlor/benoxacor and applied to emerged Z. mays.

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.

Greenhouse and laboratory studies were conducted to determine the physiological basis for CGA-248757 and flumiclorac selectivity in five plant species. CGA-248757 and flumiclorac selectively control weeds postemergence (POST) by inhibiting protoporphyrinogen oxidase (Protox). Injury symptoms from CGA-248757 and flumiclorac include rapid desiccation and necrosis similar to injury from diphenyl ether and bipyridinium herbicides. Species sensitivity to CGA-248757 and flumiclorac was evaluated by comparing the dry weight reduction from POST applications. Abutilon theophrasti was sensitive to both herbicides, Amaranthus retroflexus was more sensitive to flumiclorac than CGA-248757, Brassica kaber was sensitive to CGA-248757 but tolerant of flumiclorac, and Zea mays and Glycine max were tolerant of both herbicides. Studies evaluated CGA-248757 and flumiclorac retention, absorption, translocation, and metabolism. Enhanced herbicide metabolism contributed to the tolerance of A. retroflexus to CGA-248757 and B. kaber to flumiclorac. Decreased herbicide retention, absorption, and translocation coupled with increased metabolism contributed to Z. mays tolerance of CGA-248757 and flumiclorac. Decreased herbicide retention and increased herbicide metabolism provided G. max tolerance of both herbicides.

The effect of ammonium sulfate (AMS) on efficacy, absorption, and translocation of glufosinate in various weed species was investigated in greenhouse and field experiments. Adding ammonium sulfate at 20 g L−1 increased the efficacy of glufosinate on Echinochloa crus-galli, Setaria faberi, and Abutilon theophrasti, but not on Amaranthus rudis or Chenopodium album. AMS increased foliar absorption of 14C from 14C-glufosinate to the greatest extent in A. theophrasti and S. faberi and to the least extent in C. album. AMS increased the translocation of 14C out of the treated leaf 24 h after treatment in A. theophrasti and S. faberi, but not in C. album. These results suggest that AMS can increase the efficacy of glufosinate on A. theophrasti and S. faberi by increasing foliar absorption and subsequent translocation of glufosinate.

Mulches on the soil surface are known to suppress weed emergence, but the quantitative relationships between emergence and mulch properties have not been clearly defined. A theoretical framework for describing the relationships among mulch mass, area index, height, cover, light extinction, and weed emergence is introduced. This theory is applied to data from experiments on emergence of four annual weed species through mulches of selected materials applied at six rates. Mulch materials, in order from lowest to highest surface-area-to-mass ratio, were bark chips, Zea mays stalks, Secale cereale, Trifolium incarnatum, Vicia villosa, Quercus leaves, and landscape fabric strips. The order of weed species' sensitivity to mulches was Amaranthus retroflexus > Chenopodium album > Setaria faberi > Abutilon theophrasti, regardless of mulch material. The success of emergence through mulches was related to the capacity of seedlings to grow around obstructing mulch elements under limiting light conditions. Mulch area index was a pivotal property for quantitatively defining mulch properties and understanding weed emergence through mulches. A two-parameter model of emergence as a function of mulch area index and fraction of mulch volume that was solid reasonably predicted emergence across the range of mulches investigated.

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.