Weed seeds were germinated in incubators at constant temperatures ranging from 4 to 32 C increments. Percent germination per day was regressed against temperature with the x-intercept representing the low threshold temperature for development. Low threshold temperatures for development (low TTD's) calculated for wild proso millet (Panicum miliaceumL. # PANMI), barnyardgrass [Echinochloa crus-galli(L.) Beauv. # ECHCG], common lambsquarters (Chenopodium albumL. # CHEAL), and redroot pigweed (Amaranthus retroflexusL. # AMARE) were 6.9, 9.7, 6.0, and 10.0 C, respectively.
In a 3-y field study, the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine] at 1.7 kg ai/ha and sulphosate [trimethyl sulfonium salt of N-(phosphonomethyl)glycine] at three rates, a split application of 0.6 + 0.6 or single applications of 1.1 or 1.7 kg ai/ha satisfactorily controlled swamp dodder (Cuscuta gronoviiWilld. ex R. & S. # CVCGR) in cranberry (Vaccinium macrocarponAit.). While none of these treatments decreased cranberry yield, sulphosate at 1.1 or 1.7 kg/ha did increase yield. Glyphosate at 0.4 kg/ha applied late in the life cycle of carrot (Daucus carotaL.) when swamp dodder was in full flower satisfactorily controlled swamp dodder while increasing yield of carrot roots. Glyphosate applied early in the life cycle of carrot and dodder controlled swamp dodder but lowered yield of carrot roots.
Research was conducted to characterize the phenology of common lambsquarters growth parameters as influenced by climatic variation among years. Treatments included soybean or corn grown alone, common lambsquarters with soybean or corn, and common lambsquarters grown alone. Common lambsquarters leaf area and plant height phenology differed among years and was variable within treatments. Conversely, crop leaf area and plant height phenology did not differ among years and was less variable within a treatment than common lambsquarters. Weed relative leaf area and relative volume differed among years because of differences in crop and common lambsquarters leaf area and plant height phenology. Differences in common lambsquarters relative leaf area and relative volume among years may explain differences in previously reported crop yield responses to weed infestations between sites and years. Although common lambsquarters relative leaf area and relative volume differed among years, variability as indicated by regression coefficients of determination was also high within year and treatment. Crop leaf area and plant height phenology were well described by regression equations, with r 2 values greater than 0.68. Therefore, low coefficients of determination for relative leaf area and relative volume models were attributed to variability in common lambsquarters within a treatment.
The relative competitive ability of common lambsquarters and giant foxtail in mixed weed–corn communities was characterized in 1998 and 1999 using empirical models that described late-season weed biomass on the basis of weed density, early-season relative leaf area, or early-season relative shoot volume. Competition coefficients estimated from weed density were inconsistent between years because they indicated that giant foxtail was more competitive than common lambsquarters in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the competition coefficients based on relative leaf area and relative volume were consistent between years. Competition coefficients estimated from relative leaf area indicated that giant foxtail was more competitive than common lambsquarters in each year. Competition coefficients estimated from weed relative volume indicated that the relative competitive ability of each weed species was similar in each year. Weed relative competitive abilities were characterized further by describing the mechanisms of competition related to shoot height and width growth. Giant foxtail was taller than common lambsquarters shortly after emergence each year, but plasticity of common lambsquarters growth resulted in reduced height differential between the weed species over time. Even so, giant foxtail was taller than common lambsquarters at physiological maturity each year. Coefficients that described the ability of each weed species to crowd neighbors indicated that giant foxtail shoot width was affected more by increased common lambsquarters density and proportion than was common lambsquarters shoot width by giant foxtail. The greater ability of common lambsquarters to crowd neighbors relative to giant foxtail was attributed to the greater leaf area density (LAD) of common lambsquarters compared with that of giant foxtail. Although characterization of shoot height, width, LAD, and biomass elucidated in part the mechanisms of competition between these species, models that accounted for differences in early-season relative plant size were consistent between years, indicating that giant foxtail was equally or more competitive than common lambsquarters in corn.
Widespread use of crop yield loss models based on weed density has been limited on account of spatial and temporal variability. Furthermore, research characterizing crop yield loss associated with two or more weed species is lacking for many cropping systems. Therefore, research was conducted to characterize giant foxtail and common lambsquarters leaf area, height, and shoot volume in soybean, to quantify the relative competitive ability of giant foxtail and common lambsquarters in a mixed–weed species environment, and to assess weed density, weed relative leaf area, and weed relative volume as predictors of soybean yield loss. Based on weed density, coefficient estimates of percent soybean yield loss as giant foxtail or common lambsquarters densities approached zero differed between years. In contrast, coefficient estimates of maximum soybean yield loss were similar between years. Based on weed relative leaf area, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters leaf area differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail leaf area did not change over time within a growing season or between years. Based on weed relative volume, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters volume differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail volume did not change over time within a growing season or between years. Based on weed density, weed relative leaf area, or weed relative volume, giant foxtail was more competitive than common lambsquarters in terms of soybean yield loss. Temporal variability of weed density, weed relative leaf area, and weed relative volume indicates that additional parameters may be required to accurately predict weed–crop interactions in a multiple–weed species community.
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