Palavras-chave: espaçamento entre linhas, modelos matemáticos, Oryza sativa, Echinochloa spp.ABSTRACT -High yield losses in irrigated rice occur due to weed interference, as weeds are also adapted to the flooded environment where the crop is cultivated. The objective of this study was to evaluate the degree of interference and determine the economic weed threshold of populations of barnyardgrass in irrigated rice, according to crop arrangement. Thus, an experiment was carried out under field conditions with rice crop, under the conventional system. The treatments consisted of three arrangements of rice plants, one cultivar BRS Pelota (17 and 32 cm, hand sown) and nine populations of the competitor barnyardgrass present in each sowing method. The rectangular hyperbolic nonlinear regression model appropriately estimates rice productivity losses in the presence of barnyardgrass plants. Manually sown rice crop shows greater competitive ability against barnyardgrass in relation to the 17 and 32 cm spacings. The variable leaf area fits better to the hyperbolic model,compared to the variables plant population, shoot dry weight and soil covering. Sowing the crop by hand increases the economic weed threshold, justifying the adoption of measures to control barnyardgrass at higher population levels. Increased grain yield, rice price, herbicide efficiency and control cost reduction decrease the economic weed threshold values, justifying the adoption of control measures in low populations of barnyardgrass.

O monitoramento da ocorrência de plantas de capim-arroz resistentes ao herbicida quinclorac foi realizado no Rio Grande do Sul e em Santa Catarina, visando determinar a origem da resistência e sua disseminação, bem como detectar práticas de manejo ou condições edafoclimáticas provavelmente envolvidas na seleção e distribuição geográfica do biótipo resistente. As sementes foram coletadas, purificadas e homogeneizadas, sendo os estudos realizados em laboratório e casa de vegetação. Em laboratório, foi conduzido teste de germinação padrão, embebendo as sementes dos biótipos nas doses de 0x, 1x, 2x, 6x, 16x e 32x a concentração recomendada de quinclorac (375 g ha-1), sendo avaliadas a curva de germinação e a porcentagem de controle aos 14 dias após semeadura (DAS); em casa de vegetação, foram utilizadas as mesmas doses, aspergidas sobre as plantas aos 20 dias após a emergência (DAE), com as plantas no estádio de quatro folhas a um perfilho. Foram avaliadas a porcentagem de controle e a massa seca aos 35 DAE; biótipos com coeficiente de resistência (RI) superior a quatro foram considerados resistentes. Neste estudo foram encontradas sementes de capim-arroz resistentes ao herbicida quinclorac. Elaborou-se mapa de distribuição dos biótipos resistentes nas áreas amostradas dos Estados do Rio Grande do Sul e Santa Catarina. O herbicida profoxydim é alternativa de controle dos biótipos de capim-arroz resistentes a quinclorac.

The lack of registered herbicides for weed control in sweet sorghum has been a limiting production factor. This study aimed at evaluating the selectivity of herbicides in sweet sorghum and weed control efficiency. A randomized blocks design, with four replications, was used. Treatments consisted of sweet sorghum cultivars (BRS 506, BRS 509 and BRS 511) and herbicides applied alone at pre-emergence (atrazine - 1,500 g ha-1; atrazine + s-metolachlor - 1,665 g ha-1 + 1,035 g ha-1; flumioxazin - 50 g ha-1; s-metolachlor - 1,440 g ha-1) and post-emergence (tembotrione - 100.8 g ha-1) and sequentially at pre- and post-emergence (atrazine + s-metolachlor + tembotrione - 1,665 g ha-1 + 1,035 g ha-1 + 100.8 g ha-1; atrazine + tembotrione - 1,500 g ha-1 + 100.8 g ha-1; flumioxazin + tembotrione - 50 g ha-1 + 100.8 g ha-1), plus two controls, one weeded and one unweeded. The phytotoxicity rate to sorghum was 98.0 %, 98.0 % and 100 % for tembotrione; 100 %, 98.7% and 100 % for flumioxazin + tembotrione; 100 %, 100 % and 100 % for s-metolachlor + atrazine + tembotrione; and 98.7 %, 98.7 % and 99.7 % for atrazine + tembotrione, respectively to the BRS 509, BRS 506 and BRS 511 cultivars. Tembotrione, flumioxazin + tembotrione, atrazine + s-metolachlor + tembotrione and atrazine + tembotrione showed a good control rate of Ipomoea indivisa, Digitaria ciliaris and Urochloa plantaginea, but did not present selectivity to the sorghum cultivars. Atrazine, in general, showed higher selectivity to the yield components of the BRS 509 and BRS 506 cultivars.

Weedy rice (Oryza sativa L.) exhibits a great variability of morphological traits. To detect if this variability can affect its growth behavior, two experiments were performed on 10 Italian weedy rice populations grown as pure stand and in competition with rice. Five awnless and five awned populations were grown in field conditions in 15‐L pots. In the pure stand experiment, each pot hosted a single plant of weedy rice, while in the competition experiment the weedy rice plant was surrounded by 10 plants of cultivar Sirio CL. Plant height, tiller, and leaf numbers were recorded six times during the growing season. In competition, leaf area, culm weight, and leaf weight were also assessed. In pure stand, no significant differences between awned and awnless groups were found for all the considered parameters. Differences were found in plant height (from 70.7 to 91.9 cm) and leaf weight (from 5.64 to 9.85 g plant−1) among awned populations only. In competition, weedy rice showed lower and more variable growth indices. The least and most affected growth variables were plant height (16% of average reduction in comparison with pure stand) and leaf weight (70.3% of average reduction), respectively. Awned populations showed higher and more variable values of growth parameters, suggesting a stronger competitiveness and a wider phenotypic plasticity. Knowledge of growth behavior related to weedy rice variability could improve modeling of infestation dynamics and highlights the need of an integrated weed management approach.

No abstract

O objetivo deste trabalho foi avaliar a influência do início da irrigação, associado a doses dos herbicidas penoxsulam e clomazone, no controle de plantas daninhas e rendimento de grãos da cultura do arroz. O experimento foi conduzido em blocos casualizados e parcelas subsubdivididas, com entradas de água (19, 24 e 29 dias após emergência) e os herbicidas penoxsulam (18, 36, 54 e 72 g ha-1) e clomazone (300, 400, 500 e 600 g ha-1) aplicados em préemergência da cultura e das plantas daninhas. Os herbicidas penoxsulam, entre 18 e 72 g ha¹, e clomazone, entre 400 e 600 g ha-1, aplicados em pré-emergência na cultura do arroz irrigado, permitiram o início da irrigação até os 29 dias após emergência sem prejuízos no controle de plantas daninhas ou no rendimento de grãos da cultura.

Summary The introduction of imidazolinone‐tolerant rice varieties has made selective Oryza sativa (weedy rice) control possible. We hypothesised that Italian weedy rice populations have variable degrees of susceptibility to imazamox prior to imidazolinone‐tolerant variety introduction. To this end, 149 Italian weedy rice populations collected from fields never before cultivated with imidazolinone‐tolerant varieties were tested in a glasshouse‐based, whole‐plant response screening study. Imazamox was applied to all populations post‐emergence at a rate of 70 g a.i. ha−1, resulting in 70–90% shoot biomass reduction in the majority of cases. The results prompted a second study of the seedling dose response of four weedy rice populations from the initial study group. Three imidazolinone‐tolerant and one conventional rice variety were also included. The seedling roots were cut six days after germination and exposed to different concentrations of imazamox. The root regrowth associated with each concentration‐exposure was then measured. Imazamox concentrations to inhibit weedy rice root growth by 50% varied by about two orders of magnitude, or between 0.0018 and 0.12 mm. Even with this result, imidazolinone‐tolerant varieties were at least 31.8 times less susceptible than weedy rice populations, suggesting that Italian weedy rice populations were not tolerant to imazamox before introduction of these varieties.

RESUMO -Objetivou-se com este trabalho avaliar a eficiência do método mecânico de controle de plantas daninhas na cultura do sorgo e determinar o período crítico de competição da cultura com a comunidade infestante em terras baixas de clima temperado, considerando o estádio de desenvolvimento das plantas de sorgo. O experimento foi instalado em condições de campo, em Planossolo hidromórfico cultivado nos últimos cinco anos com arroz, com população elevada de plantas daninhas, principalmente capim-arroz (Echinochloa sp.) e papuã (Brachiaria plantaginea). Os tratamentos consistiram de diferentes períodos de convivência das plantas daninhas com a cultura ou de manutenção da cultura no limpo. Esses períodos foram da emergência até o sorgo atingir três, cinco, sete e nove folhas. Foram mantidas, ainda, uma testemunha constantemente limpa e uma permanentemente infestada. Concluiuse que o controle das plantas daninhas na cultura do sorgo, cultivado em terras baixas de clima temperado, deve ser realizado no período entre a emissão da terceira e a da sétima folha, podendo-se utilizar com segurança o controle mecânico para esse fim, sem prejuízos no rendimento de grãos da cultura.Palavras-chave: arroz vermelho, capim-arroz, competição.ABSTRACT -The objective of this work was to evaluate the efficiency of the mechanical method of weed control in sorghum crop, and to determine the critical period of competition between weeds and the culture under lowland fields in temperate climate areas, considering the development stage of the sorghum plants. The trial was installed under field conditions, in lowland where flooded rice had grown in the previous five years, with high infestation levels of barnyardgrass (Echinochloa sp.) and alexandergrass (Brachiaria plantaginea). The treatments were intercropping periods or absence of weeds in the experimental unitis. These periods were from emergence until the crop stage of 3, 5, 7 or 9 completely expanded leaves. Besides the treatments that involved the presence or absence of weeds, there were one control always free of infestation and one with no weed control. It was concluded that weed control in the sorghum crop, grown in lowland fields of temperate climate areas, should be conducted between the emission of the third and the seventh leaves of the crop, with the adoption of mechanical control being viable for this objective, with no damage for the sorghum grain yield.