Summary Thermal requirements for the germination of Amaranthus quitensis, a common annual weed in Argentina, were studied. In addition, temporal changes in dormancy from seeds produced at different times during the growing season were examined. For this second objective, thermal and light requirements for germination were tested in seeds buried at different depths, with or without crop residues. Base and optimum temperatures for germination rates were 12.8°C and 37°C respectively. At dispersal time, maximum percentage germination was 60–70% and this was generally recorded at 35°C/25°C in a 14‐h photoperiod. Seed germination tended to increase in later seed collection dates. Seeds of A. quitensis showed seasonal changes in germinability in the soil. In winter, germination of retrieved seeds increased to over 90% until summer, after which there was a decrease until the following winter when germination was close to 40%. There were no differences in germinability between burial depths and crop residue levels. Germination requirements for alternating temperatures and light tended to disappear after burial. Initial viability was 99% and declined slightly during burial. Soil temperature seems to play a crucial role not only by regulating seasonal changes in dormancy, but also by defining the percentage and the germination rate in non‐dormant seeds.

The effect of spurred anoda competition in narrow- (35 cm) and wide-row (70 cm) soybean was studied in field experiments for 2 yr. Vigorous early soybean growth in narrow- compared with wide-row soybean resulted in lower radiation transmitted through the canopy, which can partially account for greater competitiveness of narrow-row than wide-row soybean. Soybean plant height was not significantly influenced by the row spacing. Relative yield total (RYT), which is the relationship between yield in mixtures and in monocultures of the crop or the weed and indicates resource complementarity, was equal to 1 with 12 spurred anoda/m2 in the year with less precipitation. Regardless of the row spacing, spurred anoda gave resource use complementarity with the crop (RYT > 1) in all other treatments; therefore, partial avoidance of competition in mixed species was evident. Soybean aggressivity, which takes into account the effect of competition on both the crop and the weed and indicates competitive ability, decreased with weed density in both row spacings. Soybean yield loss at harvest was linearly related to relative dry weight 40 d after planting. Weed-free narrow- and wide-row soybean produced similar yields. In the presence of the spurred anoda, soybean yield was greater in narrow-row compared with wide-row soybean only in the most humid year. A management system that uses quick canopy closure with narrow-row soybean can provide excellent soybean yield and suppression of low spurred anoda densities.

-Field studies were established in Zavalla and Oliveros, Argentina, during four years in order to optimize Johnsongrass (Sorghum halepense (L.) Pers.) chemical control by means of the thermal calendar model in comparison with other criteria (weed height or days after sowing). The effect of three application dates of postemergence herbicides was determined by visual control, density of tillers originated from rhizome bud regrowth, and from crown and shoot bud regrowth, and soybean yield. Following the thermal calendar model criterion, applications during the second date afforded the best control. Weed height for the first date showed little variability between experiments but was highly variable in the second and third application dates, achieving in some cases values greater than 120 cm. For all years, no significant differences were detected for crop yield between the first and second application dates, and yields were always lower for the third date. The greatest rhizome bud regrowth was observed for the earliest application date and the highest crown and shoot bud regrowth was determined for the last application date. Parameters associated with control efficiency showed the best behaviour for the second date. However, plant height at this moment may interfere with herbicide application and the variability exhibited by this parameter highlights the risk of determining control timing using only one decision criterion.Index terms: weeds, herbicides, application timing. CRITÉRIOS DE DECISÃO PARA OTIMIZAR O CONTROLE DE CAPIM-MASSAMBARÁ EMCONDIÇÕES DE PÓS-EMERGÊNCIA NA CULTURA DA SOJA, NA ARGENTINA RESUMO -Foram conduzidos experimentos de campo durante quatro anos agrícolas em Zavalla e Oliveros, Argentina, com o objetivo de otimizar o controle químico de capim-massambará (Sorghum halepense (L.) Pers.), usando o modelo do calendário térmico em comparação com outros critérios de decisão (altura da planta daninha ou dias após a semeadura). O efeito do herbicida foi determinado por avaliação visual, densidade de perfilhos originados de rebrota de rizomas e de touceira e caule, e rendimento de grão de soja. As aplicações segundo o critério do calendário térmico mostraram o melhor controle. A altura da planta daninha na primeira época evidenciou pouca variabilidade entre tratamentos, mas foi muito variável pela segunda e terceira época, atingindo, em alguns casos, valores maiores que 120 cm. Os rendimentos da cultura na primeira e segunda época não diferiram entre si e superaram o rendimento da terceira época. A maior densidade de rebrota de rizomas foi observada na primeira época de aplicação, e o maior rebroto de touceira e caule foi determinado na terceira época. Os parâmetros associados com a eficiência mostraram o melhor comportamento na segunda época. Contudo, a altura da planta nesse momento pode interferir com a aplicação do herbicida e a variabilidade mostrada por esse parâmetro salienta o risco de determinar a época de controle usando só um critério de decisão.Termos para indexação: planta daninha, he...

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