Pedro Jacob Christoffoleti

Períodos de interferência das plantas daninhas na cultura da cana-de-açúcar: III - capim-braquiária (Brachiaria decumbens) e capim-colonião (Panicum maximum)

Um experimento foi conduzido em Sao Joao da Boa Vista-SP, com o objetivo de determinar o periodo anterior a interferencia (PAI) e o periodo total de prevencao a interferencia (PTPI) das plantas daninhas na cultura da cana-de-acucar. A cana foi plantada em abril de 1995, na epoca de plantio caracterizada por maior deficiencia hidrica. A comunidade infestante presente foi variada, sendo Brachiaria decumbens e Panicum maximum as especies mais importantes. Essa comunidade tendeu a apresentar acumulo crescente de materia seca durante todo o periodo de avaliacao e reduziu em ate 40% a produtividade de colmos da cana-de-acucar. A cultura conviveu com a comunidade infestante ate 74 dias apos o plantio, sem sofrer reducao significativa na produtividade (PAI). O periodo minimo de controle para garantir a produtividade foi de 127 DAP (PTPI). Dessa forma, o controle das plantas daninhas foi critico no periodo compreendido entre 74 e 127 dias apos o plantio.

Resistência de plantas daninhas aos herbicidas

Weed herbicide resistance has evolved from weed evolution. The modern agriculture is responsible for this evolution because of the intensive use of herbicides. The knowledge of mechanisms and factors that influence the weed herbicide resistance play an important role in the weed manegement techniques used to avoid or delay herbicide resistance appearence. There are not many report or scientific papers about herbi cide resistance in Brasil. Therefore, this literature review aims to provide information about the main advances and discoveries in the field of weed herbicide resistance.A resistencia de plantas daninhas aos herbicidas ocorre em funcao de um processo evolutivo. O desenvolvimento de biotipos de plantas daninhas resistentes e imposto pela agricultura moderna, atraves da pressao de selecao causada pelo uso intensivo dos herbicidas. O conhecimento dos mecanismos e fatores que favorecem o aparecimento de biotipos de plantas daninhas resistentes e fundamental para que tecnicas de manejo sejam utilizadas no sentido de evitar ou retardar o aparecimento de plantas resistentes em uma area. Sao poucos os relatos ou citacoes de literatura no Brasil. Sendo assim, este trabalho de revisao procura relatar os principais avancos e descobertas na area de plantas daninhas resistentes aos herbicidas.

Períodos de interferência das plantas daninhas na cultura da cana-de-açúcar: I - Tiririca

The objective of this research was to study interference periods between weeds and sugarcane culture in a experimental area located in Pradopolis, Sao Paulo State, Brazil. In these experiment, sugarcane was planted in May of 1995, and harvested 15 months later. The climatic conditions in Sao Paulo State during the months that follow sugarcane planting in the experiment (normal time when growers plant sugarcane), are characterized by negative balance of rain and evapotranspiration and mild temperatures, and the rainy season starting only four months after planting time. According to the results, it was concluded that, in the experiment where purple nutsedge (Cyperus rotundus) was the major infesting weed, sugarcane tolerated only 41 days, after planting, without interference by the weed, due to probably a greater competitive ability and allelopathic effect that have been attributed to this weed during the earlier stages of sugarcane development, interference starts right after initial sugarcane sprouting. On the other hand, purple nutsedge is very sensible to crop canopy shading, and low temperature, so the competition ended at only 22 days after planting (DAP).

Glyphosate sustainability in South American cropping systems

South America represents about 12% of the global land area, and Brazil roughly corresponds to 47% of that. The major sustainable agricultural system in South America is based on a no-tillage cropping system, which is a worldwide adopted agricultural conservation system. Societal benefits of conservation systems in agriculture include greater use of conservation tillage, which reduces soil erosion and associated loading of pesticides, nutrients and sediments into the environment. However, overreliance on glyphosate and simpler cropping systems has resulted in the selection of tolerant weed species through weed shifts (WSs) and evolution of herbicide-resistant weed (HRW) biotypes to glyphosate. It is a challenge in South America to design herbicide- and non-herbicide-based strategies that effectively delay and/or manage evolution of HRWs and WSs to weeds tolerant to glyphosate in cropping systems based on recurrent glyphosate application, such as those used with glyphosate-resistant soybeans. The objectives of this paper are (i) to provide an overview of some factors that influence WSs and HRWs to glyphosate in South America, especially in Brazil, Argentina and Paraguay soybean cropped areas; (ii) to discuss the viability of using crop rotation and/or cover crops that might be integrated with forage crops in an economically and environmentally sustainable system; and (iii) to summarize the results of a survey of the perceptions of Brazilian farmers to problems with WSs and HRWs to glyphosate, and the level of adoption of good agricultural practices in order to prevent or manage it.

Absorção, translocação e metabolismo do glyphosate por plantas tolerantes e suscetíveis a este herbicida

The objective of this research was to characterize foliar absorption, translocation and metabolism of 14C-glyphosate by the weeds Commelina benghalensis, Ipomoea grandifolia, and Amaranthus hybridus. Glyphosate was applied through four drops of 0.5 µL of a solution containing the commercial product, at the recommended dose of 720 g a.e.ha-1 mixed with 14Cglyphosate, on the adaxial surface of the second true leaf of the studied plants. The evaluations were conducted at 2, 4, 8, 12, 24, 48 and 72 hours after herbicide treatment (HAT) for the absorption and translocation experiments and 72 HAT for the metabolism experiment. The absorption and translocation results showed that A. hybridus absorbed more than 90% of the glyphosate applied after 72 HAT. The major part of the herbicide remained in the treated leaf, with a glyphosate translocation rate of 25%. In I. grandifolia, 80% of the herbicide was absorbed after 72 HAT; however, there was a lower translocation, and only 2.2; 3.5 and 4.6% of the 14C-glyphosate were present in the shoot, branch and root, respectively. C. benghalensis presented an absorption rate of 66% after 72 HAT, thus differential glyphosate penetration was the mechanism responsible for tolerance of this weed. In this evaluation, 39% of the glyphosate was present in the treated leaf and 15.2 and 11.6% in the shoot and root, respectively. In the metabolism studies, the metabolite aminomethylphosphonic acid (AMPA) was detected only in C. benghalensis, with differential metabolism being the mechanism of tolerance of this weed. The overall conclusion of this research is that glyphosate tolerance of C. benghalensis is due to the differential absorption and translocation of the herbicide by the weed. In I. grandifolia, the tolerance is due to the lower translocation of the herbicide, with no data evidence that differential metabolism of the herbicide occurs in this weed.

Curvas de dose-resposta de biótipos resistente e suscetível de Bidens pilosa L. aos herbicidas inibidores da ALS

A resistencia de plantas daninhas aos herbicidas e um fenomeno de ocorrencia mundial, sendo caracterizada como uma reducao na resposta de uma populacao a produtos quimicos, em sua dose recomendada, como resultado de sua aplicacao sucessiva. O numero de casos de resistencia registrados no Brasil tem aumentado significativamente nos ultimos anos, porem poucos estudos cientificos tem sido feitos para elucidar este fenomeno. Sendo assim, foram conduzidos experimentos com o objetivo de elaboracao de curvas de dose-resposta comparativa entre dois biotipos da planta daninha picao-preto (Bidens pilosa L.), sendo um resistente (R) e outro suscetivel (S) aos herbicidas inibidores da ALS. Para isso, foram utilizados tres herbicidas do grupo quimico das sulfonilureias e um do grupo quimico das imidazolinonas. No estadio de tres a quatro pares de folhas, as plantas R e S de B. pilosa foram pulverizadas com os herbicidas chlorimuron-ethyl, metsulfuron-methyl, nicosulfuron e imazethapyr, em doses correspondentes a multiplos de 0,0; 0,001; 0,01; 0,1; 1,0; 10; 100 e 1000 vezes a dose recomendada para aplicacao de campo. A partir dos resultados da porcentagem de fitotoxicidade foi feito o ajuste das curvas de dose-resposta. As relacoes entre o C50 do biotipo resistente e o C50 do biotipo suscetivel (R/S) foram de 40,92; 173,84; 57,47 e 57,16 para os herbicidas chlorimuron-ethyl, nicosulfuron, metsulfuron-methyl, e imazethapyr, respectivamente. O biotipo R de B. pilosa apresenta elevado nivel de resistencia cruzada aos herbicidas inibidores da ALS, do grupo quimico das sulfonilureias e imidazolinonas.

Períodos de interferência das plantas daninhas na cultura da cana-de-açúcar: II - capim-braquiária (brachiaria decumbens)

A field trial was carried out in Olimpia, Sao Paulo, Brazil, to evaluate the effects of different periods of weed control on sugarcane yield. In the year before, the field was a Brachiaria decumbens pasture and, for this reason, this grass was the major population in the weed community. Sugarcane was planted on May 1995, at the beginning of the dry season. Despite this constraint, dry matter accumulation in B. decumbens population increased during the whole evaluated period. Thus, its interference in sugarcane plants was very strong, leading to an 82% reduction in the crop yield. Sugarcane plants could grow along with the weed community up to 89 days, without significant productivity reduction. A minimal weed control period of 138 days was required to assure maximum sugarcane productivity. The critical period for weed control was between 89 and 138 days, after planting.

Detection of Sourgrass (Digitaria insularis) Biotypes Resistant to Glyphosate in Brazil

Sourgrass is a perennial weed infesting annual and perennial crops in Brazil. Three biotypes (R1, R2, and R3) of sourgrass suspected to be glyphosate-resistant (R) and another one (S) from a natural area without glyphosate application, in Brazil, were tested for resistance to glyphosate based on screening, dose-response, and shikimic acid assays. Both screening and dose-response assays confirmed glyphosate resistance in the three sourgrass biotypes. Dose-response assay indicated a resistance factor of 2.3 for biotype R1 and 3.9 for biotypes R2 and R3. The hypothesis of a glyphosate resistance was corroborated on the basis of shikimic acid accumulation, where the S biotype accumulated 3.3, 5.0, and 5.7 times more shikimic acid than biotypes R1, R2, and R3, respectively, 168 h after treatment with 157.50 g ae ha−1 of glyphosate. There were no differences in contact angle of spray droplets on leaves and spray retention, indicating that differential capture of herbicide by leaves was not responsible for resistance in these biotypes. The results confirmed resistance of sourgrass to glyphosate in Brazil. Nomenclature: Glyphosate; sourgrass, Digitaria insularis (L.) Mez ex Ekman, TRCIN

Herbicide selectivity by differential metabolism: considerations for reducing crop damages

A seletividade dos herbicidas e uma tecnologia agricola que tem sido vastamente explorada nas estrategias de controle quimico de plantas daninhas. E resultado da acao conjunta de diversos mecanismos que protegem a cultura da fitotoxicidade dos tratamentos herbicidas, mantendo-a com niveis de injurias aceitaveis agronomicamente, ou mesmo na ausencia destas. O principal mecanismo de seletividade dos herbicidas e o metabolismo diferencial desses produtos entre plantas daninhas e cultivadas, em que, nas situacoes de recomendacao agronomica, as plantas daninhas sao menos habeis em realiza-lo. Neste caso, a fitotoxicidade pode ser entendida como a suplantacao da capacidade maxima de protecao oferecida pelos mecanismos de seletividade ou, considerando o metabolismo como o principal mecanismo, como a superacao da capacidade intrinseca da especie em detoxificar determinada molecula. Considerando-se que o metabolismo de herbicidas envolve gasto de energia, os sintomas de fitotoxicidade caracterizam um segundo gasto energetico que nao deve ser aceito como uma resposta fisiologica natural, portanto pode resultar em perdas de rendimento das culturas. Para evitar ou minimizar as perdas ou injurias as culturas, e necessario que as recomendacoes de herbicidas sejam baseadas em trabalhos de seletividade conduzidos com adequado rigor experimental; bem como e importante a conscientizacao dos agricultores quanto a melhor forma de utilizar cada produto.Herbicide selectivity is an agricultural technology largely exploited in chemical strategies of weed control. The joint action of several protection mechanisms avoids phytotoxicity from herbicide treatment, maintaining the level of agronomically accepted damage to a minimum, or even totally avoiding them. The major mechanism of herbicide selectivity derives from the differential metabolism between weed and crop plant species, with weeds presenting a limited ability to perform it under agronomically recommended conditions. In this case, phytotoxicity can be interpreted as an overcoming of the maximum protection capacity offered by the mechanisms of selectivity, or when considering metabolism as the main factor, the overcoming of the inherent plant ability to detoxify a particular molecule. Considering that herbicide metabolism requires energy disposal, symptoms of phytotoxicity characterize an additional waste of energy that should not be accepted as a natural physiologic response; therefore it might result in yield losses. To avoid or minimize crop losses or damages, it is required that herbicide application recommendations are based on results from rigorously conducted selectivity experiments, as well as that there is an increase in the awareness of growers about the best use of each product.

Biologia e manejo de plantas daninhas em áreas de plantio direto

Some important aspects of weed biology and control under no tillage are described to show that the viability of this system depends on weed control efficiently performed. Some of the weeds infesting this cropping system are present in much greater density under the conventional system, this being probably due to the little soil disturbance under no tillage systems, where the occurrence of perennial weeds is more feasible, and changes in the temperature and light incidence on the soil surface influence dormancy of some weed species. Adequate strategy for weed control under the no tillage system requires knowledge on weed seed bank dynamics and must integrate methods of weed control in order to reduce the use of herbicides. The release of alellopathic compounds by some of the cover crops and the suppressive effects of crop residue cover are important measures in order to integrate the chemical control of weeds. However, it is important to be aware of the negative impacts over certain cultivated species of plants. Studies on weed biology and alellopathy of cover crops, associated to herbicide application technology and precision agriculture may contribute to weed control optimization in areas under no tillage system.