Plant canopy radiation-use efficiency (RUE), defined as unit of dry biomass produced per unit of absorbed photosynthetically active radiation (APAR), has been widely studied both in experimental and theoretical contexts because the use of this relationship greatly simplifies estimating biomass production in plant growth models. Previous studies have indicated that RUE may be sensitive to changes in the fractions of diffuse and direct radiation; RUE has been shown experimentally to increase under conditions of increased diffuse light caused either by atmospheric conditions or by shading from other plants in intercrops. Therefore, we hypothesized that weed species RUE would be greater for weeds grown in mixed weed–crop communities than for weeds grown in more uniform and less dense monotypic communities. To address this question, field experiments were conducted during 2001 and 2002 to determine the vegetative-stage RUE of giant ragweed, velvetleaf, woolly cupgrass, and wild-proso millet grown in monotypic communities or in corn. ANOVA indicated that the effect of community type on RUE was significant (P < 0.0001) and that the interaction between species and year effects was significant (P = 0.0152). Paired comparisons showed that giant ragweed RUE differed from velvetleaf RUE in 2001 (P = 0.0381) and that giant ragweed RUE differed between years (P = 0.0455). Pooled across species types and years, RUE was approximately 50% greater for weeds grown in weed–corn communities than for weeds grown in monotypic communities. These results indicate that more complex canopy architecture in mixed-species communities (i.e., greater total leaf area index [LAI] and heterogeneity of height among individuals) was associated with greater weed RUE. Including weed RUE response to the competitive environment may be one approach to improving the predictive accuracy of process-based growth models for weed biomass accumulation in mixed-species communities.

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2 s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3 levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Field experiments were conducted to quantify cumulative and annual rates of woolly cupgrass seedling emergence and seed mortality and to characterize woolly cupgrass seedling emergence patterns. Woolly cupgrass seed bank decline was rapid, declining by an average of 73, 96, and 99.5% after 1, 2, and 3 yr, respectively. Woolly cupgrass seed mortality accounted for a much greater portion of seed loss from the seed bank (80%) than germination and emergence (19.5%) during the 3-yr period. Annual rates of emergence ranged from 3 to 17% of the fall seed bank and were similar between seed banks established in different years when compared within the same year. Annual rates of mortality ranged from 50 to 92% and varied between seed banks established in different years when compared within the same year; older seed banks had higher rates of mortality than younger seed banks. For first-year seed banks, 97 to 99% of the total season emergence occurred within the first 3 wk of emergence. However, for second- and third-year seed banks, a greater percentage of the total season emergence occurred later in the season compared with emergence that occurred during the first year. The data suggest that in addition to various environmental and seed-source factors, seed bank age may also play a role in seed mortality rate and seedling emergence pattern.

Annual emergence and seed persistence of common waterhemp, velvetleaf, woolly cupgrass, and giant foxtail were characterized in central Iowa for 4 yr following burial of seeds collected and buried in autumn 1994. First-year emergence as a percentage of the original seed bank ranged from 5 to 40%, and the relative order was common waterhemp < velvetleaf < giant foxtail < woolly cupgrass. During the second and third years, there were no differences in percent emergence among species, with emergence percentages ranging from 1 to 9% of the original seed bank. During the fourth year, seedlings continued to emerge from only the velvetleaf and common waterhemp seed banks. A greater percentage of common waterhemp seed persisted each year and 12% of the original seed was recovered after 4 yr of burial. Five percent of the velvetleaf was recovered at the end of the fourth year. No woolly cupgrass and giant foxtail seed was recovered after the third and fourth years. The proportion of the seed that was accounted for from year to year through emergence and seed recovery varied by species and year. Total recovery of velvetleaf ranged from 61 to 87%, common waterhemp from 50 to 81%, woolly cupgrass from 29 to 79%, and giant foxtail from 23 to 79%. Based on the results of this research, velvetleaf and common waterhemp form more persistent seed banks than woolly cupgrass and giant foxtail. Therefore, woolly cupgrass and giant foxtail should be more amenable to management through seed bank depletion than velvetleaf and common waterhemp.

Base temperature (Tb) for leaf appearance and the thermal time interval between appearance of successive leaves (phyllochron) were determined for six common weed species from a series of growth chamber experiments. Mean leaf appearance Tb values for giant ragweed, velvetleaf, redroot pigweed, large crabgrass, woolly cupgrass, and wild-proso millet were 1.3, 8.0, 8.5, 4.5, 2.2, and 5.1 C, respectively, and mean phyllochron values were 37.2, 34.4, 17.3, 42.2, 65.2, and 34.2 growing degree-days per leaf, respectively. Phyllochron values increased slightly with mean temperature for each weed species. To our knowledge, these are the first reported leaf appearance Tb and phyllochron values for giant ragweed, woolly cupgrass, and wild-proso millet. The results confirm leaf appearance Tb and phyllochron values reported previously for velvetleaf and redroot pigweed. However, large crabgrass leaf appearance Tb and phyllochron values in our study varied somewhat from those reported previously. The results provide information needed for parameterization of plant growth models that predict leaf development based on thermal time.

Weed biotypes putatively resistant to acetolactate synthase (ALS)–inhibiting herbicides were reported by Iowa farmers from 1997 to 2001. Greenhouse studies confirmed cross-resistance to triazolopyrimidine sulfonanilide and sulfonylurea (SU) herbicides in giant ragweed from Scott County, IA (Werner Farm), which corresponded to resistance to susceptibility (R:S) GR50 (50% growth reduction) ratios of 21 and 28 to cloransulam and primisulfuron + prosulfuron, respectively. At the enzyme level, this represented a 49- and 20-fold I 50 (50% enzyme inhibition) increase. Cross-resistance to imidazolinone (IMI) and SU herbicides was also observed in common sunflower from Cherokee, IA. Compared with a susceptible biotype, the resistant common sunflower biotype demonstrated GR50 R:S ratios of 36 and 43 to imazethapyr and chlorimuron, respectively. Shattercane from Malvern, IA, was susceptible to nicosulfuron but was resistant to imazethapyr (GR50 R:S ratio = 29). The woolly cupgrass biotypes from Union County, IA (Pettit Farm and Travis Farm), were reportedly resistant but were identified susceptible to both IMI and SU herbicides. Using an in vivo ALS assay, extractable endogenous 2,3-diketone concentrations ranged from 25 to 71 nmol g−1 fresh weight for all species. Compared with susceptible biotypes, 2,3-diketone levels accumulated to at least twofold higher levels in treated resistant plants 120 h after herbicide application. Field history data suggested that resistance evolved independently in three environments where ALS-inhibiting herbicides represented an important component of the selection pressure.