Maryse L. Leblanc

The use of thermal time to model common lambsquarters (Chenopodium album) seedling emergence in corn

Abstract A mathematical model was developed to predict common lambsquarters seedling emergence in southwestern Quebec. The model was based on the thermal-time concept, using air temperatures in the double-sine calculation method. The model was built using data from five experiment-years for corn naturally infested with weed populations. Once developed, the model was calibrated using different crop seedbed preparation times. The base temperature was then adjusted for each time of seedbed preparation. A power regression function was used to relate adjusted base temperatures and the accumulated thermal units at seedbed preparation time. A modified Weibull function was then fitted to the field emergence data, expressed as the cumulative proportion of the total seedling emergence over the growing season as a function of cumulative thermal units. The simplicity and accuracy of this model would make it an excellent tool to predict common lambsquarters seedling emergence in field situations, facilitating the determination of the timing of scouting in integrated weed management systems. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; corn, Zea mays L. ‘Pioneer 3921’.

Weed response to flame weeding at different developmental stages.

Abstract Flame weeding is often used for weed control in organic production and other situations where use of herbicides is prohibited or undesirable. Response to cross-flaming was evaluated on five common weed species: common lambsquarters, redroot pigweed, shepherds-purse, barnyardgrass, and yellow foxtail. Dose-response curves were generated according to species and growth stage. Dicot species were more effectively controlled than monocot species. Common lambsquarters was susceptible to flame treatment with doses required for 95% control (LD95) ranging from 0.9 to 3.3 kg/km with increasing maturity stage. Comparable levels of control in redroot pigweed required higher doses than common lambsquarters, but adequate control was still achieved. Flaming effectively controlled shepherds-purse at the cotyledon stage (LD95  =  1.2 kg/km). However, the LD95 for weeds with two to five leaves increased to 2.5 kg/km, likely due to the rosette stage of growth, which allowed treated weeds to avoid thermal injury. Control of barnyardgrass and yellow foxtail was poor, with weed survival > 50% for all maturity stages and flaming doses tested. Flame weeding can be an effective and labor-saving weed control method, the extent of which is partially dependent on the weed flora present. Knowledge of the local weed flora and their susceptibility to flame weeding is vital for the effective use of this method. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; common lambsquarters, Chenopodium album L. CHEAL; redroot pigweed, Amaranthus retroflexus L. AMARE; shepherds-purse, Capsella bursa-pastoris (L.) Medik. CAPBP; yellow foxtail, Setaria pumila (Poir.) Roemer and J.A. Schultes SETLU.

Susceptibility of Dry Edible Bean (Phaseolus vulgaris, Cranberry Bean) to the Rotary Hoe1

Abstract: A 3-yr study was conducted to assess cranberry bean susceptibility to mechanical weeding using a rotary hoe at preemergence, hook, cotyledon, unifoliate, and first to fourth trifoliate stages of bean development and at different combinations of stages. The experiment was conducted in a weed-free environment. Cultivation with the rotary hoe reduced bean yield only for the treatment that received four cultivations at four different bean growth stages. Three cultivations improved yield compared with no cultivation. Single cultivation done at any of the eight crop growth stages did not affect yield. Crop density at harvest was decreased 6% in the treatments receiving two cultivations and 9% in the treatments receiving four cultivations compared to no cultivation. The effects of the cultivations on grain moisture were not consistent and differed from year to year. Seed weight did not differ among treatments in either year. Because this study was conducted under weed-free conditions, the beneficial effects of cultivating with the rotary hoe are probably mostly related to breaking the soil crust, improving soil aeration, preserving soil moisture, or promoting mineralization of the nutrients required by the crop. Nomenclature: Cranberry bean, Phaseolus vulgaris L. ‘SVM Taylor’. Additional index words: Bentazon, cultivation, fenoxaprop-p-ethyl, mechanical weeding, physical weed control, trifluralin.

Calibration and validation of a common lambsquarters (Chenopodium album) seedling emergence model

Abstract Studies were conducted to calibrate and validate a mathematical model previously developed to predict common lambsquarters seedling emergence at different corn seedbed preparation times. The model was calibrated for different types of soil by adjusting the base temperature of common lambsquarters seedling emergence to the soil texture. A relationship was established with the sand mineral fraction of the soil and was integrated into the model. The calibrated model provided a good fit of the field data and was accurate in predicting cumulative weed emergence in different soil types. The validation was done using data collected independently at a site located 80 km from the original experimental area. There were no differences between observed and predicted values. The accuracy of the model is very satisfactory because the emergence of common lambsquarters populations was accurately predicted at the 95% probability level. This model is one of the first to take into consideration seedbed preparation time and soil texture. This common lambsquarters emergence model could be adapted to model other weed species whose emergence is limited by low spring temperature. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL.; corn, Zea mays L. ‘Pioneer 3921’.

Effect of the Presence or Absence of Corn on Common Lambsquarters (Chenopodium album L.) and Barnyardgrass [Echinochloa crus-galli (L.) Beauv.] Emergence1

A 3-yr study was conducted to establish if the presence of corn had an effect on the emergence patterns and total weed seedling density under growing conditions in southwestern Québec. Weed seedling emergence was monitored in permanent quadrats throughout the growing season in the presence and absence of growing corn. Common lambsquarters and barnyardgrass were prevalent in most site-years. The presence of corn did not affect the patterns of common lambsquarters and barnyardgrass emergence nor their total weed seedling density except in 1994. Corn canopy was probably not sufficiently developed to affect light levels or soil temperature needed for weed germination and, consequently, seedling emergence. In 1994, in the absence of corn, some soil crusting was observed on a fine-textured soil, and the total number of seedlings was reduced. The results of these weed emergence studies in corn can be extended to other crops growing with wide row spacing and relatively slow canopy closure similar to those of grain corn. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; corn, Zea mays L. ‘Pioneer 3921’. Additional index words: Amaranthus retroflexus L., AMARE, crop competition, redroot pigweed, weed density, weed seedling emergence. Abbreviations: CHU, corn heat units; PAR, photosynthetically active radiation.

Susceptibility of Row-Planted Soybean (Glycine max) to the Rotary Hoe

ABSTRACT A three-year study was conducted to determine soybean susceptibility to physical damage from cultivations done with the rotary hoe in a weed-free situation. Plot size was large enough to enable the rotary hoe to be used at a speed of 15 km h−1. The soybeans were systematically cultivated at eight growth stages, from pre-emergence to fourth trifoliate leaf. Two, three and four cultivations were done on a combination of growth stages. Soybean population decreased with the number of cultivations but yields were either not affected or significantly increased compared with the uncultivated control. Cultivations with the rotary hoe could be done up to the 4th trifoliate leaf growth stage without risk of decreasing yield.

Mechanical Weed Control in Agriculture

Weeds are plants that are considered undesirable in a crop at a given time.Weeds are harmful for a number of reasons. They reduce crop yields, interfere with the harvest, support pathogens and insect pests and contaminate seeds.

Seed Dormancy and Seed Morphology Related to Weed Susceptibility to Biofumigation

Biofumigation is practiced to control soilborne pests and weeds in agronomic fields. The objectives of this research were to assess the dose response of weed seeds to Indian mustard biofumigation and associate responses to seed dormancy state, initial dormancy, and seed parameters. A petri dish biofumigation methodology was developed to expose seeds of common lambsquarters, bird vetch, wild carrot, common ragweed, green foxtail, velvetleaf, hairy galinsoga, and red clover to allelochemicals produced after rehydrating 0 (control), 1.94, 2.90, 5.81, 11.61, and 17.41 mg cm-2 of dried mustard powder. Weed species expressed specific dose responses, estimated ED50, LD50, and maximal mortality. Hairy galinsoga and wild carrot were consistently the most affected by biofumigation, with maximal mortality reaching 97% and 95%, ED50 values for germination were 1.91 and 2.68 mg cm-2, and LD50 values were 3.31 and 3.69 mg cm-2 of dried mustard tissue, respectively. Initial dormancy was assessed by germination and tetrazolium tests. Seed parameters such as testa thickness, relative weight of the testa, and seed size were measured directly by manual dissection, weighing seed structures, and stereomicroscopic imaging software measurements. Linear regression analyses revealed initial dormancy to be positively related to ED50 and LD50 values with a significant interaction with seed surface and seed width, respectively. Exposure to 5.81 mg cm-2 of dried mustard powder increased common ragweed seed mortality for after-ripened seeds by 293% and by 58% for primary dormant seeds compared with untreated seeds. Mortality of common lambsquarters secondary and primary dormant seeds increased by 730% and 106%, respectively, and for wild carrot by 1,193 and 156%, respectively. Results underline the potential to incorporate biofumigation into weed management programs for repression of susceptible weed species. Nomenclature: Indian mustard, Brassica juncea (L.) Czern. ‘cv. Caliente 199’; velvetleaf, Abutilon theophrasti Medik. ABUTH; bird vetch, Vicia cracca L. VICCR; common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; green foxtail, Setaria viridis (L.) Beauv. SETVI; hairy galinsoga, Galinsoga quadriradiata Cav. GASCI; red clover, Trifolium pratense L. TRFPR; wild carrot, Daucus carota L. DAUCA. ED50: estimated half maximal effective dose of dry mustard biomass that decrease germination; LD50: lethal dose of dry mustard biomass that kills 50% of viable seeds.

Mechanical Weed Control in Corn (Zea mays L.)

Mechanical weed control in corn was practised as early as the 19th century. Over the past 30 years, however, effective selective herbicides have more or less replaced mechanical cultivation (Lampkin 1990). Although cultivation, or tillage, is still done because of the benefits to the soil, weed control is performed through an early-season application of herbicide. Tillage not only controls weeds but loosens the soil and breaks the surface crust, a common problem in corn growing. Crusts tend to form in silty clay soils after a period of rain followed by hot, windy weather. The crust slows oxygen diffusion and reduces heat transfer, makes emergence difficult for corn seedlings, and has a negative impact on crop uniformity. Removing the surface crust by cultivation also promotes mineralization of the nutrients required by corn (Souty and Rode 1994). In addition, cultivation helps to preserve soil moisture needed for plant growth, since the layer of loosened soil limits the capillary rise of moisture. This function of cultivation is most effective in regions with a dry climate and when the corn root system is not very well developed. When the roots are well distributed throughout the soil or the foliage provides shade, little moisture is lost even if the field has not been cultivated.