Dead cover, or mulch, consisting of plant residues, plays an important role for the success of diverse agricultural crops, working as an insulating layer protecting the soil from daytime temperature variations and maintaining the soil moist and rich in organic matter. Cowpea is a source of proteins, carbohydrates, vitamins and minerals. Its importance in the North, Northeast and Midwest regions of the country is associated with economic and social aspects, since it is an important food for low-income populations, supplying their nutritional needs. This study was carried out under greenhouse conditions in Manaus, state of Amazonas, with the purpose of assessing the effect of different dead covers on the agronomic characteristics of cowpea cultivars. It consisted of a completely randomized design in a 4 × 4 factorial arrangement. The treatments comprised four cowpea cultivars (BRS Caldeirão, BRS Tumucumaque, BRS Guariba and BRS Tracuateua) and three species of cover plants (Brachiaria decumbens, Brachiaria ruziziensis and Mucuna pruriens) and one control treatment, without soil cover, in a total of 16 treatments, with four replications and two plants per experimental unit. Analysis of variance was applied to the data, and the means were compared by the Scott-Knott’s test at 5% probability level. The following characteristics were examined: number of pods per plant, pod length, number of seeds per pod, weight of shoot dry matter, and grain yield. Mulching provided better results for all characteristics assessed in the four cultivars when compared to the control. BRS Caldeirão is the recommended cultivar for the state of Amazonas and the other regions with similar edaphoclimatic characteristics (high air temperature, rainfall, air humidity, and low-fertility tropical soils) because it exhibited the greatest number of pods per plant, number of seeds per pod, shoot dry matter, and the highest average grain yield (Freire Filho et al., 2011; Souza et al., 2016).

Cassava production in Amazonas state deserves to be highlighted due to its great historical, social, and economic importance. Weed competition severely constrains cassava production in Amazonas. The use of cover crops is safe and very efficient at eliminating weeds while keeping the soil covered. The objective of this study was to evaluate physical properties of soil and glyphosate residues in storage roots as a function of the weed management in cassava. The experiment was carried out in a randomized complete block design with five treatments and five repetitions. The treatments were biological control with two species of cover plants (Brachiaria ruziziensis and Mucuna pruriens), chemical control, mechanical control, and treatment with no weed control. The cover crops characteristics evaluated were dry weight, the percentage of cover, and rate of decomposition of plant residues. In the soil, the bulk density and total porosity were determined. The contamination of the storage roots was evaluated based on the analysis of glyphosate residue. Brachiaria ruziziensis presented more dry weight and higher percentage of cover compared with M. pruriens, and both cover crops showed very similar decomposition rates. The physical properties of soil were unaffected by any treatment evaluated. There was no detection of glyphosate and its metabolite, aminomethylphosphonic acid (AMPA), in any treatment evaluated. Chemical control with glyphosate is not able to contaminate cassava storage roots.

The unsuitable use of herbicides damages many cultures. In cases of high infestations and presence of aggressive weed species in guarana (Paullinia cupana) culture, glyphosate application is advisable, but its impact on guarana physiology is unknown. Therefore, leaf photosynthetic characteristics were measured with the aim of identifying if the photosynthetic performance of guaranazeiro plants is affected in response to glyphosate application. Three glyphosate doses (0 (control); 324 and 432 g a.i. ha-1) were applied to two guaranazeiro cultivars (BRS-Andirá and BRS-Maués) selected on the basis of productive performance. An analysis was made of the effects of these doses on characteristics that represent the photosynthetic process: gas exchange, maximum quantum efficiency of PSII, performance index and chlorophyll content. The application of glyphosate did not affect the short-term responses relative chlorophyll content (SPAD index) and light use (chlorophyll a fluorescence). After 168 h, there were changes only in gas exchange variables. The effects of glyphosate doses on gas exchange was different between guaranazeiro cultivars. The photosynthetic performance of the guaranazeiro seems to be tolerant to the effects of short-term of glyphosate application.

Cassava (Manihot esculenta) yields are severely affected by the interference of weed plants. Using cover crops for weed control appears as a sustainable alternative practice because it maintains the soil covered and reduces the need for herbicides. The aim of this study was to assess cassava crop yields and the soil chemical properties as a function of use of cover crops for weed management. Treatments were three cover crops (Brachiaria ruziziensis, Canavalia ensiformis, and Mucuna pruriens), chemical control, mechanical control, and treatment with no weed control. Cover crops reduced the diversity of species and the quantity of individuals of the weed community in cassava cultivation. The treatments with chemical and physical weed control achieved higher yields. The cover crops B. ruziziensis and C. ensiformis increased cassava yields by 30% and 14%, respectively, when compared with the treatment with no control. The cover crops increased the pH, MO, K, Ca, and Mg values when compared with the treatments with chemical and mechanical weed control. Brachiaria ruziziensis and C. ensiformis are recommended as a cover plants in cassava production systems in the Amazon region. The use of cover crops associated with cassava is a sustainable management option because, in addition to the suppressive effect on weeds, cover crops improve the chemical properties of soil, which may contribute to increasing cassava production in the long term.

Guarana (Paullinia cupana var. Sorbilis (Mart.) Ducke) is a species native from Amazon which has high caffeine concentration. The interactions between guarana and microorganisms in Amazonian soils need to be investigated, especially in relation to the seedling production process, due to its beneficial relationships such as biosynthesis and the supply of stimulating substances, which may result in shortening of the nursery garden or increased productivity. Thus, this research had the objective of evaluating the potential of growth caused by rhizobacteria (Burkholderia ambifaria and Bacillus sp.) in rooting of guarana seedlings. The root length, volume, dry matter of the aerial part, height, number of leaves, leaf area and dry matter of shoot were evaluated at 180 days after planting emergence. Were also determined the carbohydrate and proline content of the aerial part. The rhizobacteria did not influence the growth of the seedlings, however, the carbohydrate contents were higher when the inoculation of the rhizobacteria was carried out with the Burkholderia ambifaria bacteria in the cultivar BRS-Amazonas, with the best results. The proline levels were higher in the control treatment of the two cultivars.