SUMMARYWe aimed to characterize the weed community in sprinkler irrigated rice, previously applied with residual herbicides for two years, as well as to infer about sustainability of the management applied to the fields based on an ecological approach. The experiment was conducted during the cropping seasons 2013/14 and 2014/15, under sprinkler irrigation. Rice was planted in the second-half of October, using the cultivar PUITA INTA CL. Herbicides were applied in pre- and post-emergence (sequential application) of crop and weeds. Ryegrass (Lolium multiflorum) was planted in winter. In May 2015, soil samples were collected for the soil seed bank study. Phytosociological characterization of weeds emerged from the seed bank was conducted from May 2015 to January 2016, until soil seed bank depletion. Echinochloa crusgalli, Fimbristyllis sp., Cyperus esculentus and Killinga brevifolia were the weeds to which most concern should be directed when growing rice under sprinkler irrigation in lowland areas of Southern Brazil. Their competition strategies are based on density. Herbicides used should be efficient in controlling at least these four weed species. A selecting action of herbicides on weed species was botanically characterized, as the weed species reported after two cropping seasons depended on the herbicide treatment applied.

We aimed with this work to evaluate the efficiency of the herbicide [imazapic + imazapyr] in the weed control, especially of the jointvetch, and its reflex in rice plant establishment and grain yield. A field study was conducted, where treatments were: control without herbicide application; Kifix ® 140g ha -1 (24.5 g a.i. ha -1 imazapic + 73.5 g a.i. ha -1 imazapyr) and Kifix ® 280 g ha -1 (49 g a.i. ha -1 imazapic + 147 g a.i. ha -1 imazapyr). We evaluated crop emergence up to 25 days after sowing, crop plant height 12 days after emergence (DAE), jointvetch plant density 31 and 62 DAE, jointvetch dry mass 62 DAE and general weed infestation 62 DAE. At the end of the crop cycle we evaluated grains per panicle, panicle and 1000 grain weight, panicle density and crop grain yield. There is a risk of 15% reduction in the establishment of Clearfield ® rice plants when the maximum dose of the herbicide [imazapic + imazapyr] is applied pre-emergence, but under appropriate conditions, the crop development tends to compensate for this reduction in establishment. The herbicide [imazapic + imazapyr] is effective on jointvetch, but under high infestation conditions, as in the present study, a complementary herbicide application is required to achieve adequate control of jointvetch and avoid rice grain yield reduction due to competition.

Phytosociological surveys have been applied to studies on agroecosystems, especially in relation to weed populations into arable fields. These surveys can indicate trends of variation of the importance of plant populations within a crop, and whether the variations are associated to agricultural practices adopted, which can be further used to support the development of weed management programs. However, to understand the applicability of phytosociological studies for weeds, it is necessary to understand the ecological basis and determine the most appropriate methods to be used when surveying arable fields. Therefore, the aim of the present chapter is to introduce a new approach of phytosociological survey to be used as a tool for the weed science. Throughout the chapter, this new approach is presented in details covering aspects related to methods for sampling and describing weed communities. The following sequence of steps is proposed as the most suitable for a weed phytosociological and association survey: (1) overall infestation; (2) phytosociological tables/graphs; (3) intra-characterization by diversity; (4) inter-characterization and grouping by multivariate analysis; and (5) weeds association through contingency tables.

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Imazethapyr, sulfentrazone, clomazone, diclosulam, trifloxysulfuron-sodium and trifluralin are residual herbicides commonly used for weed control in soybean or sugarcane crops. The sorghum crop implanted succeeding sugarcane, can be affected by the carryover effect of these herbicides. In this context, we aim with this work to evaluate the minimum period between application of herbicides with residual effect (imazethapyr, sulfentrazone, clomazone, diclosulam, trifluralin and trifloxysulfuron-sodium) and the planting of sorghum so that there is no impairment in growth and establishment of this crop due to the herbicide carryover effect. The experiment was installed in randomized blocks design with four replications, under field conditions. The herbicides were applied to the previously tillaged soil, with sorghum being planted 0, 14, 28, 42, 56 and 70 days after herbicide application (DAA). The percentage of germination was evaluated daily from planting, and 7, 14, 21 and 28 days after emergence (DAE) of each planting, the phytotoxicity was evaluated. Thirty five DAE o f each planting season, ten plants were collected per plot for measurement of leaf area, fresh and dry mass of plants, leaves and stems. The minimum time interval for planting sorghum after application of these herbicides varies, but imazethapyr is highlighted by causing high and durable toxicity to sorghum even when planting sorghum after 70 days of its application.