Echinochloa crus-galli is the main weed in direct dry-seeded rice systems worldwide and is the target of most herbicide applications. Numerous cases of E. crus-galli biotypes with resistance to herbicides have been reported in different regions of the world; however, to date, no cases have been reported in Uruguay. The purpose of this research is to assess the presence of herbicide-resistant E. crus-galli in the rice fields of Uruguay. More than 40 E. crus-galli biotypes were sampled from eastern to northern regions in different years and assessed following the Herbicide Resistance Action Committee (HRAC) protocols of confirmation, using the herbicides propanil, quinclorac, clomazone, bispyribac–sodium, penoxsulam, imazapyr + imazapic, profoxidim and cyhalofop. Herbicides rates included 0, 0.125, 0.25, 0.50, 1, 2, 4 and 8 times the label rate. Most E. crus-galli biotypes (35) resulted as resistant to quinclorac. Furthermore, resistance was confirmed to propanil in at least seven biotypes, 12 to imazapyr + imazapic, and three to penoxsulam. Five biotypes showed multiple resistance to propanil and quinclorac, and one biotype was resistant to quinclorac, penoxsulam and imazapyr + imazapic. No biotype showed confirmed resistance to clomazone, bispyribac-sodium, cyhalofop or profoxidim—herbicides that ensure satisfactory control. The presence of E. crus-galli herbicide resistant-biotypes reduces herbicide options, threatening rice production in Uruguay. In this context, a redesign of the productive systems would represent an opportunity to complement the chemical control, integrating larger-scale cultural and management practices.

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Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv] is the foremost weed in rice (Oryza sativa L.) systems, and its control is crucial to successful rice production. Quinclorac, a synthetic auxin herbicide, has been used effectively to manage E. crus-galli. However, occurrences of quinclorac-resistant genotypes are frequently reported, and its resistance evolution has led to questions about the continued utility of quinclorac for grass control. Identification of the resistance mechanism(s) of resistant genotypes will facilitate development of integrated weed management strategies that sustain quinclorac use for management of E. crus-galli. We evaluated the responses to quinclorac of two contrasting genotypes: E7 (resistant, R) and LM04 (susceptible, S). Quinclorac induced ethylene and cyanide biosynthesis in the S-genotype. Both genotypes responded similarly to an increasing application of exogenous 1-carboxylic acid aminocyclopropane (ACC) and potassium cyanide, and their growth was inhibited at higher doses. The key mechanism for cyanide (HCN) detoxification in plants, β-cyanoalanine synthase (β-CAS) activity, was evaluated in both genotypes, and no significant difference was observed in the basal activity. However, quinclorac significantly induced β-CAS–like activity in the S-genotype, which is consistent with the increased synthesis of ethylene and cyanide. This work suggests that the resistance to quinclorac of the E7 R-genotype is likely related to an alteration in the auxin signal transduction pathway, causing a lower stimulation of ACC synthase and, therefore, limited synthesis of ethylene and HCN after quinclorac treatment.

A semeadura direta de espécies forrageiras de estação fria permite reduzir a marcada estacionalidade da sua produção em campos naturais. Durante quatro anos, conduziu-se um experimento de aplicação de herbicidas sobre campo nativo, em um solo Argissolo Vermelho-Amarelo, de textura arenosa do norte do Uruguai visando introduzir forrageiras de estação fria e estudar o impacto dos herbicidas na composição botânica de espécies estivais. Foram testadas doses de herbicidas (glifosate 1L ha-1, glifosate 4L ha-1, paraquat 3L ha-1 e testemunha), como tratamento principal, aplicadas no ano 1994, em um delineamento blocos ao acaso. A repetição ou não das mesmas doses no ano 1995 constituiu a subparcela e a aplicação ou não das mesmas doses no ano de 1996 constituiu a sub-subparcela. No levantamento de espécies da vegetação existentes no outono de 1998, observou-se que o maior distúrbio sobre a composição botânica do campo nativo foi provocado com a aplicação continuada da dose de 4L ha-1 de glifosate, onde foram identificadas seis espécies. No levantamento de espécies existentes no tratamento testemunha, no outono, foram identificadas onze espécies, sendo que as espécies Andropogon lateralis, Paspalum notatum, Conyza bonariensis, Eryngium horridum, Desmodium incanum, Cyperus sp. e Digitaria sp. constituiram 90% da composição botânica. Com a aplicação de herbicida, ocorreu uma substituição de espécies perenes por anuais.

Accounting for the spatial variability of resources and yields has become both important and feasible in agricultural systems research. Such variability can be detected and addressed at various scales, from that of a small field to a whole region, and completely different problems arise at each scale. Traditionally, agricultural issues have been studied at a small scale (field plots) and then extrapolated in an ad hoc manner to a larger scale (field or region). Results of this process are not always accurate due to the intrinsic differences between the local and the regional level of analysis. In this paper we pursue different approaches as an example of a means of dealing with a particular issue, rice grain quality, at two scales, field and region. At the field level, we test a model that relates head rice (HR) to grain moisture content (GMC) and GMC pattern before harvest. At the region level, we propose a model to predict optimum harvest time for rice varieties in California, based on a degree days (DD) approach. Practical results obtained aid in reducing the risk of loosing HR grain quality at harvest.