Susan E. Weaver

The biology of Canadian weeds. 115. Conyza canadensis

Conyza canadensis (L.) Cronq. (Canada fleabane, horseweed, mare’s-tail) is a winter or summer annual, native to North America, and found in all provinces of Canada except Newfoundland. It is a weed of orchards, vineyards, roadsides, and arable fields where tillage has been reduced or eliminated. Most seedlings emerge from late August through October and form rosettes which overwinter. Large numbers of small, wind-dispersed seeds, ranging to over 200 000 seeds per plant, are produced in late summer. Populations of C. canadensis in more than ten countries have evolved resistance to herbicides such as paraquat, atrazine, chlorsulfuron or glyphosate. Several paraquat resistant populations were found in orchards in Essex Country, Ontario. It serves as a wild host of the tarni shed plant bug, and of aster yellows, a mycoplasma disease transmitted by the aster leaf hopper. Key words: Canada fleabane, ERICA, Conyza canadensis, Erigeron canadensis, horseweed

Impact of lamb's-quarters, common ragweed and green foxtail on yield of corn and soybean in Ontario

Field studies were conducted in Harrow, Ontario, from 1990 to 1993 to quantify the relationship between yield of field corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] and density of lamb’s-quarters (Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), and green foxtail [Setaria viridis (L.) Beauv.]. Experiments were conducted separately for each weed and crop combination. Weeds emerged at the same time as or within a week of the crop. Coefficients of the rectangular hyperbolic damage function were estimated for each year and pooled over years. In both crops, lamb’s-quarters was the most competitive of the three weed species, and green foxtail was the least competitive at low densities. Maximum yield loss at high weed density varied with weed species in field corn, but not in soybean. The estimated competition coefficients of the damage function have been incorporated in a decision support system for integrated weed management in Ontario. Key words: Weed interference, yield loss

Prickly lettuce (Lactuca serriola) interference and seed production in soybeans and winter wheat

Abstract Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m−2 or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m−2 in 2002 to 1.2 plants m−2 in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m−2 caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height of the main stem. Seed production per plant ranged from 2,200 to 67,000 in soybeans, and up to 87,000 in a noncrop area at the edge of the field. Winter wheat harvest interrupted prickly lettuce flowering, and only about 25 to 30% of the plants present in the wheat crop survived harvest and flowered in untreated stubble. These plants produced less than 4,000 seeds per plant. Postharvest control with glyphosate, mowing, or cultivation prevented prickly lettuce seed production in wheat stubble. This study suggests that prickly lettuce populations could build up quickly in continuous no-till soybeans, but rotation with winter wheat and control of plants at the edge of the field would help to limit population growth. Nomenclature: Prickly lettuce, Lactuca serriola L. LACSE; soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.

Response of paraquat-resistant and -susceptible horseweed (Conyza canadensis) to diquat, linuron, and oxyfluorfen

Abstract Horseweed is a winter annual weed that has evolved resistance to multiple herbicide modes of action in 11 countries worldwide. A paraquat-resistant horseweed population in an Ontario orchard that was being managed by a rotation of herbicides began to show increased tolerance to the herbicide linuron. Experiments were conducted in the greenhouse to compare the response of this population and several paraquat-susceptible populations to linuron, diquat, and oxyfluorfen. Plants were sprayed with a range of doses of each herbicide when they were from 5 to 10 wk old, and the ED50, or dose at which shoot dry weight was reduced by 50%, was estimated. There was a sevenfold difference in the ED50 values of the paraquat-resistant and -susceptible populations in response to diquat and a threefold difference in response to linuron. The response to oxyfluorfen was age dependent. The ratio of resistance to susceptible ED50 values was estimated as 57 for 5-wk-old plants and 11 for 8-wk-old plants in response to oxyfluorfen. Ten-week-old plants from both populations showed no response to oxyfluorfen at rates up to 4.8 kg ai ha−1. Nomenclature: Diquat; linuron; oxyfluorfen; paraquat; horseweed, Conyza canadensis L. ERICA.

Correlations among relative crop and weed growth stages

Abstract A study was undertaken to determine whether the relative leaf stages of common annual weeds and crops could serve as a reliable indicator of the time of weed emergence. Ten annual broadleaved and grass weeds were sown at successive intervals in field corn and soybean at Harrow, ON, Canada, in 1997, 1998, and 1999. All weeds emerging at a particular crop leaf stage were assigned to a cohort. Leaf numbers of the crop and different weed cohorts were recorded at 2- to 3-d intervals up to the eight-leaf stage of corn and the fourth trifoliate of soybean. For each weed species, categorical data analysis revealed a high degree of association between the leaf stage of a crop and the leaf number expected for an individual weed of a given cohort. For example, by the third trifoliate of soybean, most of the weeds emerging with the crop (VE cohort) had 8 to 10 leaves, whereas weeds in the V1, V2, and V3 cohorts averaged about seven, four, and two leaves, respectively. Year to year variation in the correspondence between crop and weed leaf numbers generally was small once variation due to time of weed emergence was removed, with one exception. Dry surface soil conditions during emergence of the VE cohort in corn in 1999 resulted in delayed leaf appearance of many weeds with respect to the crop. The relationship between weed and crop leaf stages can provide information for management decisions in two ways: (1) it indicates the relative time of weed emergence in assessing the need for control, and (2) it indicates the crop stage at which scouting for particular weed leaf stages should occur. Nomenclature: Corn, Zea mays L. ‘Pioneer 3573’; soybean, Glycine max (L.) Merr. ‘NK 2492’.

Responses of spreading orach (Atriplex patula) and common lambsquarters (Chenopodium album) to soil compaction, drought, and waterlogging

Abstract Root traits and growth of spreading orach and common lambsquarters were compared in response to soil compaction, drought, and waterlogging under controlled environment conditions. On the basis of the typical habitats occupied, the hypothesis was that spreading orach would be more tolerant of compaction and waterlogging and common lambsquarters more tolerant of drought. When grown in buckets with two soil bulk densities (1.2 and 1.6 g cm−3) for 8 wk, the two species responded similarly to compaction, with the fraction of fine roots reduced by 10%, total root length by 70%, root and shoot dry weight and leaf area by 50 to 60%, and plant height by 30% at the high compared with the low bulk density. When grown for 6 wk in soil columns 1 m long, which were watered daily or allowed to dry, common lambsquarters was deeper rooted than spreading orach at both moisture levels and better able to sustain growth in the drying columns. The watering regime did not alter the rooting depth of either species. Total root length in successive 10-cm increments declined exponentially from the top to the bottom of the watered columns, but root proliferation was reduced in the upper 20 cm of the drying columns. The average root diameter of both species decreased with drought and increased with soil compaction. When grown in waterlogged soil at 10 or 20 C for 4 wk, seedlings of spreading orach survived with little reduction in growth, whereas survival and growth of common lambsquarters were drastically reduced, particularly under cool soil conditions. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; spreading orach, Atriplex patula L. ATXPA.

Germination Characteristics of the Dimorphic Seeds of Spreading Orach (Atriplex patula)

Abstract Spreading orach is an annual weed that colonizes roadsides, field edges, and increasingly, no-till agricultural fields. It produces dimorphic seeds with different levels of physiological dormancy, but little is known about the germination ecology of the two seed types. Field and controlled-environment studies were conducted to determine seed responses to light and stratification, the pattern of seedling emergence in the field, and the effect of soil water content on the length of cold stratification required to break dormancy for each seed type. The large, brown seeds have three times the mass of the smaller, black seeds, primarily because of a larger embryo, but have a thinner seed coat. Germination of brown and black seeds in petri dishes was 98 and 90%, respectively, after stratification for 3 mo at 5 C, whereas germination of unstratified seeds was 19 and 12%, respectively. Light stimulated germination of both stratified and unstratified black seeds but did not increase germination in stratified brown seeds. Up to 40% of brown seeds germinated in situ during stratification, compared with only 2% for black seeds. Germination in petri dishes and emergence in the field were more rapid for brown seeds than for black seeds. Maximum germination of black seeds occurred after stratification for 2 or 3 mo at 5 C on soil that was waterlogged (pore-water matric potential, ψ  =  0 kPa), wet (ψ  =  −0.38 kPa), or at field capacity (ψ  =  −10 kPa). For shorter periods of stratification, total germination and germination rate of black seeds declined as soil water content decreased from waterlogged to dry (ψ  =  −500 kPa). Seed dimorphism in spreading orach may provide a mechanism to enhance survival in uncertain or variable habitats such as disturbed agricultural fields. Nomenclature: Spreading orach, Atriplex patula L

Velvetleaf (Abutilon theophrasti) Response to Glyphosate on the Field Edge

Abstract Control of weeds growing around field edges to limit seed production is an important component of preventative weed management. POST herbicide rates that are effective on weeds growing within a dense corn or soybean canopy may not be high enough to control weeds at the edge of a field. A study was conducted from 2004 through 2006 to compare velvetleaf growth and fecundity at the edge of the field as opposed to within the crop in response to a range of glyphosate rates. Treatments included position (plot center or edge), time of emergence (VE or V4 crop growth stage) and glyphosate rate (0 to 900 g ae ha−1). Without herbicide application, velvetleaf plants grown on the edge flowered earlier, had thicker stems, and produced more seed capsules than plants grown in the center of the plots. At glyphosate application rates of 200 to 900 g ha−1, the percentage of plants surviving and reproducing was higher on the edge than within the crop. Edge plants treated with 900 g ha−1 of glyphosate produced more seeds than center plants that received no herbicide. Dose–response curves were used to estimate the glyphosate rate that would reduce seed production of surviving plants to 80% of the untreated plants. Plants emerging at the VE stage were estimated to require 300 g ha−1 within the corn or soybean canopy and 668 g ha−1 on the crop edge, whereas plants emerging at the V4 stage would require 0 g ha−1 within the canopy and 280 g ha−1 on the crop edge.