RESUMO O arroz é originário de climas tropicais e, quando cultivado em regiões temperadas, pode ter sua produtividade reduzida por exposição ao estresse por frio. Este estudo buscou avaliar as perdas produtivas associadas ao frio em diversas variedades de arroz e a tomada de decisão pela resemeadura da lavoura. A semeadura foi realizada em época precoce, regular ou tardia visando capturar a condição natural de ocorrência de estresse pelo frio em experimento com duração de três anos. As datas de semeadura que obtiveram os maiores rendimentos variaram entre data normal a tardia de acordo com a duração do estresse por frio nos anos com sua ocorrência. Perdas produtivas ocasionadas por condições de estresse por frio variaram entre 0.81 a 2.74 t ha-1, atingindo até 38.6%; Halilbey, Pasali e Mevlutbey foram as variedades mais tolerantes ao frio. O rendimento de grãos correlacionou-se ao número de plantas; uma densidade mínimo 60.8-79.6, e ótimo 132.3 plantas m-2 foi necessária para se atingir o rendimento econômico. Estresse por frio afetou a densidade de plantas de arroz negativamente, e densidades ≤ 60.8-79.6 plantas m-2 indicariam necessidade de eliminação da cultura e sua resemeadura dependendo dos custos de aplicação. O estresse pelo frio ocasionou maiores perdas nos estádios de germinação e plântula em relação a sua ocorrência em estádios mais avançados de desenvolvimento.

ABSTRACT This study aimed to evaluate the cold stress effect in natural field conditions by employing different rice sowing timings over three years, to identify national varieties tolerant to cold stress at the vegetative and generative stages, and to propose a data-derived solution regarding termination and resowing due to cold stress conditions. Early, regular, and late sowing dates were employed to capture natural cold stress conditions in a 3-year-long experiment. Sowing dates resulting in the greatest yields fluctuated from regular to late sowing dates, according to the stress duration in cold stress years. Yield losses resulting from cold stress ranged from 0.810 to 2.740 t ha-1 and reached 38.6%. ‘Halilbey,’ ‘Pasali,’ and ‘Mevlutbey’ were found to be most cold-tolerant varieties. Grain yield was correlated with plant number; the critical minimum level was between 60.8 and 79.6 plants m-2 and the optimum was 132.3 plants m-2 for economical yield. Cold stress negatively affected rice plant density, and plant densities below the critical minimum plant warranted crop termination and resowing, depending on application costs. Cold stress had a far more devastating effect on germination and seedling stages than on later development stages in temperate conditions.

Abstract Background Achieving satisfactory weed control levels in black oat (Avena strigosa) fields is often difficult, owing to the limited number of registered herbicide molecules. Objective To determine novel options for selective, post-emergence chemical weed control in this crop. Methods Eight herbicide treatments were sprayed in the field onto black oat plants at the tillering stage to evaluate crop safety and control of Conyza spp. and Gamochaeta americana, two major weeds of Brazilian black oats. A separate trial was conducted to further assess herbicide safety in a controlled-environment setting, and mesotrione selectivity was then re-evaluated separately in the greenhouse during the following growing season. Crop phytotoxicity was determined using biomass production both in the field and in greenhouse trials. Results Mesotrione, bentazon, 2,4-D, and a 2,4-D+bentazon tank-mix produced light symptoms of crop phytotoxicity, from which plants quickly recovered. Conyza spp. control was achieved via applications of mesotrione (192 g a.i. ha-1), metsulfuron-methyl (3.9 g a.i. ha-1), and a 2,4-D + bentazon tank-mix (502.5 g a.e. ha-1 + 720 g a.i. ha-1, respectively), whereas proper G. americana control was only achieved via applications of either mesotrione or metsulfuron-methyl. Biomass accumulation by black oat plants in the greenhouse was similar across mesotrione-treated and untreated plants. Conclusions Mesotrione could become an option for selective, post-emergence weed control in black oat fields, aiding in the fight against multiple herbicide-resistant Conyza spp. populations which are widespread across major black oat-growing areas in Brazil.