Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m-2. Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.

Greenhouse and field studies were conducted to compare root development of jointed goatgrass to winter wheat. Time of seminal root development in jointed goatgrass was similar to that of winter wheat and to root development predicted by a model. The only exception was the epiblast node roots (−1A and −1B), which developed approximately 1 phyllochron later in jointed goatgrass than in winter wheat. At crown nodes, A and B roots developed later in jointed goatgrass than in winter wheat, but development of X and Y roots at these nodes was similar for both species. First-order branching at a given root axis in jointed goatgrass roots began approximately 1 phyllochron after development of that axis, compared to 2.5 phyllochrons for winter wheat and the model. Second- and third-order branching of jointed goatgrass roots began 1 and 0.5 phyllochrons earlier than the respective branching of winter wheat roots. Leaf and tiller development followed the same pattern with time for both species. Maximum distance between crown roots was 1.5 to 3 times greater in winter wheat than in jointed goatgrass. Root length density at the 0- to 10-cm depth was less for jointed goatgrass than for winter wheat, but there were no differences at greater depths. The frequency of branching of first-order laterals was greater for jointed goatgrass than for winter wheat. In the field, the relationship of jointed goatgrass shoot development to accumulated growing degree-days was linear (R2 = 0.97), with a slope and y-intercept similar to winter wheat (R2 = 0.95). These data, showing slight differences in root growth and development between jointed goatgrass and winter wheat, may be used in predictive modeling to better understand the biology and ecology of each species, and may be used in conjunction with other models to establish weed threshold levels and improve selective placement of fertilizers and herbicides to benefit crop development.

Greenhouse and laboratory studies were conducted to determine the effect of atrazine on efficacy, absorption, translocation, and metabolism of primisulfuron applied to velvetleaf and giant foxtail. Efficacy of primisulfuron was reduced by 18 and 22% when applied at 20 and 40 g ai ha−1, respectively, in combination with 1.7 kg ai ha−1 atrazine to velvetleaf. Efficacy of primisulfuron was reduced by 15 and 16% when applied at 30 or 60 g ai ha−1, respectively, in combination with 1.7 kg ai ha−1 atrazine to giant foxtail. Foliar absorption of 14C-primisulfuron by either weed species was not affected by addition of atrazine to the treatment solution. Atrazine had no effect on metabolism of 14C-primisulfuron by either weed species. In the absence of atrazine, translocation of absorbed 14C from primisulfuron out of treated leaves of velvetleaf and giant foxtail averaged 19 and 29%, respectively, across sampling times. These values were reduced to an average of 9 and 16% in velvetleaf and giant foxtail, respectively, when 14C-primisulfuron was applied in combination with atrazine. The majority of translocated 14C from primisulfuron was transported acropetally in velvetleaf and basipetally in giant foxtail. Atrazine significantly reduced 14C translocation from primisulfuron to these meristematic sinks in both weed species. Reduced translocation was positively correlated with reduced control of these weeds when primisulfuron was tank mixed with atrazine.

Field studies were conducted at three locations over 2 yr in southern Ontario to determine the critical period of weed control in soybean. This period generally consisted of two discrete periods, a critical weed-free period and a critical time of weed removal. The critical weed-free period was relatively short in duration and consistent across locations and years. A period of weed control lasting up to the fourth node growth stage (V4), approximately 30 days after emergence (DAE), was adequate to prevent a yield loss of more than 2.5%. The critical time of weed removal was variable across locations and years and ranged, for example, from the second node growth stage (V2) to the beginning pod growth stage (R3), approximately 9 to 38 DAE, at a 2.5% yield loss level. A phenologically based period of most rapid yield loss due to weed interference occurred from beginning bloom stage (R1) to beginning seed stage (R5). The short and consistent critical weed-free period indicates the duration of residual herbicide control necessary in soybean and supports use of nonresidual, postemergence herbicides and mechanical weed control.

Field experiments were conducted in 1986 and 1987 to determine the effects of shoot and root interference of common cocklebur and soybean. Plants were grown in porous membrane envelopes. Common cocklebur plants had a longer vegetative growth period than soybean and were twice as tall as soybean at maturity. Soybean root and shoot dry weight and seed yield were reduced by shoot and whole plant (root and shoot) interference of common cocklebur, with whole plant interference giving the greatest reduction. Common cocklebur growth was affected little by soybean interference. Common cocklebur shoot interference alone reduced total soybean seed weight by 48%, the amount caused by common cocklebur whole plant interference.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter rye biomass and seed yield. Downy brome reduced yields most when it emerged within 3 wk of rye, but densities of more than 100 downy brome m-2 were required to reduce yields by 20 to 30%. The greatest reductions in rye biomass (28%) and seed (33%) yields over the 3 yr occurred when 400 downy brome m-2 emerged with the crop. Downy brome, at densities up to 400 plants m-2, emerging 6 wk after rye or in early spring, reduced rye biomass and seed yield less than 10% in all years. Winter rye effectively shaded downy brome (40 to 90%) for much of the growing season.

To understand persistence strategies of meadow and western salsify, ecophysiological characteristics of their seed germination were studied. Anaerobiosis (immersion in deoxygenated water) induced secondary dormancy in seeds of both species. Dormancy could be induced in 86% of meadow salsify seeds and in 65% of western salsify seeds by a 1-d anaerobiosis treatment. The induced dormancy was gradually released during storage of air-dried secondary dormant meadow salsify seeds, and the rate of this release was influenced by storage temperature; 30 C was more effective than 10 or 20 C in releasing secondary dormancy. These results suggest that the two species may rely on induced dormancy as an option in their persistence strategy. The optimum temperature for germination of nondormant seeds of both species was 15 C. Maximum germination percentages for both species were established within 4 to 6 d of incubation at 15 C and within 14 to 28 d of incubation at 25 C. Nondormant seeds did not germinate below 10 or above 30 C. Stratification (at 5 C for 2 to 10 wk) stimulated germination of secondary dormant seeds of meadow salsify. This stratification requirement can be important in preventing germination of dormant salsify seeds in the fall, thereby avoiding high seedling mortality in the winter. Light (red and far-red) had no effect on germination of seeds in secondary dormancy. In a separate study, seeds of both species were planted in pots at depths of 2 to 14 cm and seedling emergence was observed. Maximum emergence occurred when seeds were buried 2 cm deep. Seeds planted 8 cm or deeper germinated but did not emerge.

Dose-response studies estimating GR40 values indicated different levels of propanil resistance in junglerice populations from fields previously treated with propanil, compared to a check population collected where this herbicide had never been used. The GR40 for susceptible populations ranged from 0.36 to 0.50 kg ai ha−1 and for resistant populations ranged from 1.10 to 3.10 kg ai ha−1. Considerable variability in growth and morphology existed among populations. Variability in cumulative leaf area, aboveground biomass, mean relative growth rate, mean net assimilation rate, and mean leaf area ratio could not be related to propanil resistance. Competitiveness was not related to propanil resistance either. of several vegetative and reproductive parameters measured at maturity, only grain weight per plant and number of grains per plant were correlated with GR40 (r = −0.73, P = 0.06). This trend towards lower reproductive fitness in propanil-resistant junglerice plants may reduce its ecological success when growing with propanil-susceptible plants in the absence of this herbicide.

In a replacement series study, barley was more competitive than green foxtail and wild oat at 28/22 and 22/16 C. Wild oat was more competitive with green foxtail at 22/16 C than at 28/22 C. Maximum green foxtail dry weight and leaf area was produced at 28/22 C. As temperature increased, maximum dry weight and leaf area occurred earlier during plant growth. Similarly, maximum wild oat leaf area occurred earlier with increasing temperature, but the greatest leaf area was observed at 16/10 C. Leaf area ratio (LAR) and relative growth rate (RGR) of wild oat did not differ markedly between temperature regimes. Green foxtail LAR was higher while RGR was lower at 16/10 C than at 22/16 or 28/22 C. Under the 16/10 C regime green foxtail produced little mature seed. Greatest seed numbers were produced at 28/22 C. Wild oat produced mature seed under all temperature regimes with the greatest seed numbers produced at 22/16 C.

Progression of crenate broomrape population density (PD, number of emerged plants m-2) in faba bean was studied over 8 yr in Spain. Spatial dispersion and effect of climatic conditions on parasite population growth rate (PGR) also were studied. With repeated cropping of faba bean, infestations of crenate broomrape increased from an initial PD of 0.15 to an average of 26, with a maximum of about 40 to 45. The average population growth rate (PGR, ratio between the PD of any 2 consecutive years) was approximately 3. However, this figure varied widely among localities and years, from 0.8 to 7.7. A highly significant relationship (P = 0.01) was found between PGR and rainfall and soil temperatures during December to February, months of crop vegetative growth. Dispersion of crenate broomrape infestations mainly followed direction of crop rows, most likely due to the effect of tillage and harvesting operations, which were the same direction as sowing.

Goatsrue, a perennial legume, is an exotic noxious weed currently found in the United States only in Cache County, Utah. It infests irrigated pastures, alfalfa fields, and noncropland areas. In order to compare their responses to temperature, goatsrue and alfalfa were grown in artificially illuminated controlled-environment chambers in 16 day/night temperature regimes ranging from 15/4 to 36/25 C. Growth analysis was used to evaluate effects of temperature on dry matter accumulation, leaf area production, and biomass allocation. Both species grew best at day/night temperatures of 22/25, 29/ 18, and 29/25 C. Leaf appearance rates were linearly related to mean daily temperature. Goatsrue produced fewer but larger leaves and a greater total leaf area than alfalfa. Biomass partitioning to leaves was greater in goatsrue, whereas partitioning to stems was greater in alfalfa. Response of vegetative dry matter production to temperature closely paralleled response of leaf area duration in both species. Alfalfa generally had a higher net assimilation rate, but the greater leaf area duration of goatsrue resulted in greater dry matter accumulation in this species after 50 d of growth. Overall responses to temperature were similar in the two species. Thus it seems likely that goatsrue could become a much more widely distributed weed in alfalfa.

Hairy vetch was grown as a winter annual cover crop and evaluated for weed suppression when desiccated by paraquat or left alive until natural senescence in a 3-yr field experiment. Total weed density and biomass were variable in the desiccated hairy vetch treatment relative to a bare soil treatment but were consistently lower in the live hairy vetch treatment relative to the desiccated or bare soil treatments. An average of 87% of sites under live hairy vetch compared to 8% of sites under desiccated hairy vetch transmitted less than 1% of unobstructed sunlight. The red (660 nm) to far-red (730 nm) ratio of transmitted light was reduced by 70% under live hairy vetch compared to 17% under desiccated hairy vetch. Daily maximum soil temperature and diurnal soil temperature amplitude were reduced by live hairy vetch > desiccated hairy vetch > bare soil. Soil moisture content was greater under both live and desiccated hairy vetch compared to bare soil during droughty periods. Changes in light extinction, red to far-red ratio, and diurnal soil temperature amplitude were sufficient to explain greater weed suppression by live than desiccated hairy vetch.

A fundamental component of modeling crop interference is the effect of understory photosynthetic photon flux density (PPFD) on weed architecture and growth. The effect of decreased PPFD on spaced redroot pigweed dry matter accumulation, distribution, and plant architecture was quantified by increasing corn density and delaying weed planting date. As canopy-transmitted PPFD declined, total dry matter accumulation decreased and relative dry matter distribution was greater to main-stem components than to branch components. Increased rectangularity in understory weed architecture was associated with a concomitant decrease in branch number. The proportion of leaf area and dry matter in the upper segment of the redroot pigweed increased as PPFD declined with increased corn density. Results suggest that changes in plant architecture, as influenced by canopy-transmitted PPFD, may be as important as those of total dry matter and leaf area when describing and predicting the effects of crop-weed interference.

Itchgrass biotypes from 34 countries or territories were evaluated under controlled and natural day length at Houma, LA. They could be placed in five broad groups based primarily on the effect of day length on flowering but also on general morphology and pattern of growth. The Group 1 biotypes (Argentina, Bolivia, Costa Rica, Cuba-1, Ecuador-1, Florida, Guadeloupe, Honduras-1, Louisiana-1, North Carolina, Papua New Guinea-1, Panama, Peru, Philippines-1, and Venezuela) were not affected by day lengths of 12 and 14 h (day length neutral) and flowered 35 to 57 d after germination. The Group 2 biotypes (Australia, Kenya-1, Reunion, South Africa, and Zimbabwe) also were not greatly affected by photoperiod and flowered 54 to 67 d after germination, and were taller with fewer and larger diameter culms than the Group 1 biotypes. The Group 3 biotypes (Brazil, Colombia, Jamaica, and Philippines-2) were moderately sensitive to photoperiod. They took longer to flower at 14 h than at 12 h and flowered in 63 to 95 d. The Group 4 biotypes (Cuba-2, Dominican Republic, Ecuador-2, Honduras-2, India, Louisiana-2, Malaysia, Papua New Guinea-2, Puerto Rico, Taiwan, and Thailand) were short-day plants that did not flower in the screenhouse until August or September (130 to 163 d), after the natural photoperiod had decreased to 13 h. The Group 5 biotypes (Indonesia, Kenya-2, Nigeria, Sudan, Tanzania, and Trinidad) were strict short-day plants that flowered only in September to November (167 to 226 d), after the day length had decreased to about 12 h. Most biotypes were diploids with 2n = 20 chromosomes, but some were polyploids (2n = 40 or 2n = 60). The shape of the glume tip on the pedicellate spikelet was found to be acuminate, acute, or obtuse for the biotypes with 2n = 20, 40, and 60 chromosomes, respectively, and was a reliable marker to distinguish diploids from polyploids.

Homozygous, sethoxydim-tolerant corn was field tested at two locations in 1989 and 1990. Sethoxydim at 0.22, 0.44, and 0.88 kg ha−1 was applied to sethoxydimtolerant corn in the 3- and 7-leaf stages. None of the sethoxydim treatments caused visible injury to the sethoxydim-tolerant corn, but all treatments were lethal to a parental corn line used as a control. Sethoxydim applied at either stage of corn development had no effect on number of days to 50% silk emergence, plant height, or grain yield, compared to nontreated plants. Sethoxydim-tolerant corn was also tolerant to mixtures of sethoxydim plus other postemergence herbicides that control dicotyledonous weeds. Sethoxydim mixed with atrazine or sethoxydim applied in sequential applications with dicamba or 2,4-D gave annual grass control similar to sethoxydim applied alone. However, the sethoxydim plus bentazon treatment resulted in reduced grass control in comparison to sethoxydim alone. When the broadleaf herbicides were mixed with sethoxydim or applied as sequential treatments, broadleaf weed control was the same as when the broadleaf herbicides were applied alone. The high level of corn tolerance to sethoxydim and the broad spectrum of weed control resulting from combinations of sethoxydim plus other postemergence herbicides indicates that sethoxydim-tolerant corn hybrids could increase the options available for weed control in corn.

The influence of light, soil moisture, and temperature on establishment of selected species through hairy vetch residue on the soil surface was investigated under controlled conditions in the greenhouse. Hairy vetch residue at rates ranging from 0 to 616 g m−2 had no effect on corn, slightly reduced velvetleaf and green foxtail establishment, and severely inhibited common lambsquarters establishment under full sunlight conditions. The same rates of hairy vetch residue reduced velvetleaf, green foxtail, and common lambsquarters establishment more under a shade cloth with 9% light transmittance than under full sunlight. Day/night temperatures of 24/16 or 32/26 C had no effect and soil moistures of 50 or 133% field capacity had little effect on response of all species to residue rates. Weed establishment was similar under shade cloth without residue as under residue with an equivalent light transmittance, suggesting that light was more important than allelopathy or physical impedance for weed suppression by hairy vetch residue.

Sicklepod was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. Maximum dry weight, leaf area, plant height, node number, and leaf number after 46 d occurred at 29/26 and 34/26 C. Temperatures of 29/21 C or lower reduced dry weight by more than 50%. Leaf number, leaf weight, and leaf area were more sensitive to changes in day temperature, whereas plant height and root, stem, and total dry weight were more sensitive to night temperature. Dry matter production was more closely correlated with leaf area duration than with its other component, net assimilation rate. Leaf appearance rate and dry matter production were linearly related to average daily temperature. The low-temperature threshold for leaf production was 13 ± 1 C. Observations of plant development in photoperiods ranging from 10 to 16 h confirmed that sicklepod is a short-day plant with a critical day length of 13 to 14 h. No reproductive development occurred in photoperiods of 15 or 16 h. Seedlings that emerged in 10-h photoperiods required more than 1-wk exposure to short days to initiate and continue reproductive development. Plants from a North Carolina population flowered earlier than plants from a Florida population in photoperiods of 12, 13, or 14 h, but in an 11-h photoperiod the two populations flowered at the same time.

Greenhouse and field experiments were conducted to examine foliar absorption and activity of nicosulfuron and primisulfuron by quackgrass with various adjuvants. Foliar absorption of 14C-nicosulfuron and 14C-primisulfuron plus petroleum oil adjuvant (POA) was completed by 4 h after application. Absorption of nicosulfuron plus POA and primisulfuron plus POA increased from 11 and 2% of applied, respectively, to 51 and 12% with the addition of urea-ammonium nitrate liquid fertilizer (UAN). At least 83% of the absorbed 14C from either herbicide penetrated the leaf epicuticular waxes. Absorption of 14C-labeled herbicides was greatest with the following adjuvants: POA + UAN > nonionic surfactant (NIS) + UAN = methylated seed oil. The addition of UAN to either NIS or POA significantly increased 14C-herbicide uptake. In greenhouse studies, nicosulfuron and primisulfuron applied with POA plus UAN provided greater quackgrass control than with POA alone. Despite the differences in foliar uptake in the greenhouse, few differences were observed between these adjuvants in 1989 or 1990 field efficacy trials. Quackgrass control was reduced by the addition of atrazine to nicosulfuron plus POA in 1989 and to primisulfuron plus POA in 1990. Combinations with atrazine did not reduce 14C-nicosulfuron or 14C-primisulfuron uptake by quackgrass.

Jointed goatgrass is a serious weed in winter wheat, and presently no herbicides are available for its selective control. This study examined the effect of time of emergence and removal on jointed goatgrass interference in winter wheat, as well as its rate of development and soil water extraction. The goal of this study was to suggest cultural practices that minimize jointed goatgrass interference in winter wheat. Jointed goatgrass development was identical to ‘Vona’ winter wheat in two crop seasons, even though precipitation differed drastically between seasons. Depth of soil water extraction of both species was also similar. Jointed goatgrass at 18 plants m−2 reduced grain yield 27 and 17% when emerging 0 and 42 d after Vona, respectively. The relationship between time of jointed goatgrass emergence after winter wheat and grain yield loss was Y = 30.6–0.29X (X = days, r = 0.72), indicating that plants emerging in late fall still caused yield loss. Removing jointed goatgrass by early March prevented winter wheat grain yield loss. The interference data suggests that producers assess infestation levels and plan control measures in early March.

Growth and morphological characteristics of four weed and two crop species were analyzed to determine how small-seeded weeds can compete with large-seeded crops despite the initial size disadvantage. Small-seeded weeds had higher relative growth rates because of increased percentages of biomass devoted to leaves. This morphological scheme was achieved primarily through a reduction in the percentage of biomass devoted to roots. Yet, small-seeded weeds were able to develop larger mean plant root lengths as a result of having both roots of smaller diameter and higher rates of root length increase.