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Conyza species are important weeds in global agriculture, especially due to their capacity to evolve resistance to multiple herbicide mechanisms of action.We aimed to evaluate the frequency and distribution of resistance to glyphosate and chlorimuron-ethyl in Conyza spp. populations from Brazil. Seed samples were collected from grain production areas across nine Brazilian states over five consecutive years ( 2014to 2018). Prior to resistance monitoring trials, dose-response assays were conducted to determine a single dose of glyphosate or chlorimuron-ethyl to discriminate resistant and susceptible populations. Resistance monitoring based on plant responses to the application of discriminatory doses of glyphosate (960 g ha -1 ) or chlorimuron-ethyl (20 g ha -1 ). Populations were classified as resistant, moderately resistant, or susceptible to either herbicide. While glyphosate resistance was highly frequent (71.2%) in all the five years, chlorimuron-ethyl resistant populations occurred at 39.8% of the total. The frequency of multiple resistance to both herbicides (35.3%) was proportional to the occurrence of chlorimuron-ethyl resistance (39.6%). Resistance to glyphosate and to chlorimuron-ethyl were found across all states evaluated.

ResumoO nitrogênio pode mitigar os danos ocasionados pela redução de área foliar por influenciar a divisão celular. Este trabalho teve como objetivo avaliar a eficiência da aplicação de doses de nitrogênio em cobertura como estratégia de manejo da desfolha do colmo em diferentes estádios fenológicos do milho. O experimento foi instalado em Lages, SC, nos anos agrícolas de 2008/2009 e 2009/2010. O delineamento experimental utilizado foi o de blocos ao acaso dispostos em parcelas subdivididas. Na parcela principal testaram-se as épocas de desfolha: sem desfolha; desfolha em V8 (oito folhas expandidas); desfolha em V15(quinze folhas expandidas) e desfolha em VT (pendoamento). Nas subparcelas avaliaram-se quatro doses de nitrogênio: 0, 50, 100 e 200 kg ha -1 , aplicadas no dia da desfolha em cada estádio fenológico e em V8 na testemunha não desfolhada. A desfolha realizada em V8 não reduziu o rendimento de grãos do milho, em relação à testemunha não desfolhada, independentemente da dose de N aplicada. A aplicação de nitrogênio em V15 aumentou o rendimento de grãos, reduzindo os prejuízos ocasionados pela desfolha. As desfolhas realizadas em VT causaram grandes prejuízos à produtividade do milho, onde não houve recuperação pela aplicação subseqüente de nitrogênio em cobertura. O sucesso da aplicação de N como estratégia para atenuar os prejuízos ocasionados pela desfolha depende do estádio em que a perda de área foliar ocorre e da dose de N aplicada. Palavras-chave: Zea mays, nitrogênio, área foliar, rendimento de grãos AbstractNitrogen can mitigate damages caused by leaf area reduction due to its influence on cell division. This work was carried out aiming to evaluate the efficiency of side-dressing different rates of nitrogen as a management strategy to maize stem defoliation at different growth stages. The experiment was set in Lages, during the 2008/2009 and 2009/2010 growing seasons. The experimental design was a randomized block with split plots. Three defoliation times were tested in the main plot: eight expanded leaves (V8), fifteen expanded leaves (V15) and tasseling (VT), plus a control without defoliation. Four nitrogen rates were assessed in the split-plots: 0, 50, 100 and 200 kg ha -1 of N. Nitrogen was side-dressed at the defoliation day of each growth stage and at V8 in the control. Defoliations performed at V8 did not reduce grain yield, in comparison to the control, regardless of N rate. Side-dressing N rates at V15 increased grain yield, mitigating damages caused by defoliation. Defoliation carried out at VT promoted

-In Brazil, ryegrass (Lolium multiflorum) has been identified as resistant to glyphosate, becoming a major problem, especially in crops cultivated in the winter season. This herbicide can indirectly affect photosynthesis by inhibiting biosynthesis of many compounds. Thus, the aim of this study was to investigate the influence of glyphosate on the physiological profile of susceptible and resistant ryegrass populations to the herbicide. The experimental design was completely randomized with two treatments (720 and 1,080 g e.a. ha ) and four replications plus control with no treatment. Two ryegrass populations were sown, one susceptible and another one resistant to glyphosate. After the treatments, evaluations were carried out at 1, 3, 7 and 28 days after application (DAA). Variables analyzed were: CO 2 net assimilation rate, stomatal conductance, CO 2 internal concentration, transpiration, water use efficiency and instantaneous carboxylation efficiency. The glyphosate herbicide caused irreversible damage in a susceptible population which at 28 DAA in all variables analyzed this population was already dead and it was impossible to analyze it, but it was shown that the effects of this herbicide were intensified from the third day after application. In the case of the resistant population, at 3 DAA in all variables, it suffered significant effects comparing to the control, showing that even with a high level of resistance the herbicide can affect its metabolism. Keywords

Nitrogen fertilizer broadcast application over maize (Zea mays L.) is becoming more common as narrow row spacing is adopted and larger fields are cultivated. The contact of N fertilizers with plant leaves can partially damage leaf tissues and consequently affect crop yield. This study was conducted to evaluate the effect of source, method, and time of application of N fertilizer on maize leaf area and grain yield. A field trial was conducted in Brazil during two growing seasons. Urea (UR) and ammonium nitrate (AN) were applied at V5 and V10 maize growth stages broadcasted over the canopy or banded on the soil at a rate of 200 kg N ha−1. Solid fertilizers were broadcasted over dry and wet leaves whereas for applications to the soil solid fertilizers or solutions were used. In the first season, broadcasting AN reduced the leaf area at silking by 15.7% compared to soil application regardless of leaf moisture. In the following year, broadcasting AN reduced by 7.5% the leaf area at silking compared with UR. In the first growing season, the grain yield with UR was 10.1% greater than that with AN when both fertilizers were broadcasted. In the second season for the same application method, the grain yield from crops fertilized with UR was 32% higher than those where AN was used. In addition, the yield was 18% greater when fertilizers were applied to the soil than when they were broadcasted. Broadcasting N fertilizers causes leaf phytotoxicity and may decrease grain yield, especially when AN is used.

Resumo -O objetivo deste trabalho foi verificar se os perfilhos podem mitigar os prejuízos ocasionados pela desfolha do colmo principal em milho. Os experimentos foram implantados em Lages, SC, em 20/10/2006 e 31/10/2008. O delineamento experimental utilizado foi o de blocos ao acaso, com parcelas subdivididas. Na parcela principal, foram testados dois destinos dos perfilhos: manutenção até a colheita e remoção quando o colmo principal apresentava nove folhas expandidas (V9). Nas subparcelas, avaliaram-se quatro épocas de desfolha do colmo principal: V9, V15 (quinze folhas expandidas), VT (pendoamento) e folhas preservadas até a colheita. Avaliou-se o híbrido P30F53, à densidade de 55 mil plantas por hectare. Determinaram-se: a produtividade total, a do colmo principal e a dos perfilhos. As desfolhas realizadas em V15 e VT diminuíram a produtividade, em comparação ao tratamento sem desfolha, independentemente do destino dos perfilhos. Quando o milho foi desfolhado em V15 e VT, a produtividade total das parcelas em que os perfilhos foram mantidos foi maior do que daquelas em que eles foram removidos. Isto indica que os perfilhos foram benéficos ao colmo principal do milho, quando houve restrições à produção de carboidrato oriundas da perda de área foliar próximo do florescimento.Termos para indexação: Zea mays, perfilho, massa de grãos, produtividade. Tillering as a trait to mitigate damages caused to maize by the main stem defoliationAbstract -The objective of this work was to verify if tillers can mitigate damages caused by the main stem defoliation in maize. The experiments were set up in Lages, SC, Brazil, on 10/20/2006 and 10/31/2008. The experimental design was a randomized block with split plots. Two tiller destinations were tested in the main plots: tillers preserved until harvesting, and tillers removed when the main stem had nine expanded leaves (V9). Four main stem defoliation times were assessed in the split plots: V9, V15 (15 expanded leaves), VT (tasseling), and leaf maintenance until harvesting. Hybrid P30F53 was tested at 55 thousand plants per hectare. Total grain yield, tiller grain yield and main stem grain yield were determined. Defoliations performed at V15 and VT decreased total productivity, in comparison to the control, regardless of tiller destination. When the main stem was defoliated at V15 and VT, plots where tillers were kept promoted higher total grain yield than those where tillers were removed. This indicates that tillers were instrumental to maize main stem when there were restrictions to the production of carbohydrates due to leaf area loss close to flowering.Index terms: Zea mays, tiller, grain mass, productivity. IntroduçãoOs perfilhos são ramificações laterais que se desenvolvem a partir de gemas axilares dos nós abaixo da superfície do solo. São estruturas utilizadas pelas plantas da família Poaceae como estratégia para compensação de espaços vazios dentro da comunidade (Li et al., 2003;Almeida et al., 2004). Nas espécies em que o perfilhamento é comum, como o trigo, o arroz ...

The combination of herbicides with different modes of action has been adopted not only to improve weed control but also to increase the environmental sustainability of plant-protection products. In this study, we showed a synergistic effect of the auxin herbicide 2,4-D amine with the PSII-inhibiting herbicide metribuzin to control the global grass weed wild oat (Avena sterilis) population and investigated the underlying mechanisms. Pretreatment with 2,4-D amine did not change the foliar absorption of metribuzin but did increase metribuzin translocation to the roots and new leaves, although enhancement of the metribuzin metabolism was also observed. Considering that the expression level of the target site psbA gene is significantly higher in leaves than in roots, increased metribuzin translocation to new leaves is likely the major cause of the observed synergism, even though enhanced metribuzin metabolism may offset the metribuzin efficacy. This is the first report on the synergistic mechanism between 2,4-D amine and metribuzin in weed control.