Control of Echinochloa sp. in the Irrigated Rice Crop

The species of Echinochloa (barnyardgrass) stand out among major weeds infesting rice cropping and damages are variable depending on the weed population, rice cultivar, andmanagement practices adopted by rice farmers.The objective of this work was to measure the control of barnyardgrass in rice cropping, cultivar Qualimax 1, due to the early times of flood irrigation, application times, and doses of penoxsulam. The experiment was conducted in the field, where the experimental design used a randomized block design with a split plot design with four replications. The treatments consisted of two application periods (early and late) of penoxsulam three times of irrigation start (1, 15 and 30 days after treatment application—(DAT)) and herbicide doses (0, 24, 36, 48 and 60 g ha). The herbicide penoxsulam revealed that combined with irrigation starting 15 days after herbicide application promoted efficient control of barnyardgrass.


Introduction
The area cultivated with rice in Brazil is estimated at around 2,4 million hectares, with predominance of irrigated rice production with yield of grain average 7133 kg ha −1 [1].The occurrence of weeds in rice fields is a major factor limiting the potential for crop productivity in Brazil, and losses vary according to the plant species, weed population, the rice cultivar, and management practices adopted by rice farmers [2,3].
The weeds present in rice pads stand out as barnyardgrass (Echinochloa spp.), because has similarities morphophysiological with rice plants, wide distribution, and high levels of infestation in crops [4].Furthermore, there is a high adaptability of this species intraspecific variability due to the large, high potential competition for resources [5,6], even at low density, which makes its control essential [7].
Among the weeds present in rice pads, regardless of the system of rice cultivation, chemical control is the main alternative for the management of barnyardgrass, because of its efficiency and practicality [8].However, the use of herbicides should be strategically integrated with other management practices, seeking the agronomic efficiency and cost reduction [9,10].Interactions between irrigation season, time of application, and herbicide rate are important to achieve efficient control of weeds in all cropping systems of rice [11].
Currently, there are commercially available 17 herbicides suitable for control of barnyardgrass in Brazil [6].To obtain an adequate control to prevent and reinfestation, it is necessary that the herbicide has residual effect.The use of herbicides with residual activity in the soil, allows the delay irrigation, because inhibition of germination and emergence of weed that would be exerted by the water are replaced by the action of the herbicide [12].However, the length of this period depends on the product, the dose, the weed species, and the level of infestation of the soil and climatic conditions, ranging from 15 to 30 days after application, depending on the adopted management [8].
The penoxsulam herbicide is characterized as being selective in rice, controlling various weed species [13] and residual effect in the soil, which prevents the emergence of weeds new flow before establishment of flood irrigation [14]. 1 Days after treatment application. 2Means followed by the same capital letters in the column in each application time compare times of early irrigation within dose and followed by the same lowercase line, compare application time within herbicide and times start of irrigation, and do not differ by Tukey's test ( ≤ 0, 05).
In this context, the aim of the research was to evaluate the control of barnyardgrass in the irrigated rice cv Qualimax 1, depending on the time of onset of flooding, application times, and doses of penoxsulam.

Material and Methods
The experiment was conducted in the field, in the Granja Quatro Irmãos, at Rio Grande, RS, crop year 2005/2006.The soil is classified as Lowland Solodic belonging to Unit Mapping Pelotas.The fertilization was performed according to the analysis of soil using 200 kg⋅ha −1 , 5-20-20 formulation at sowing, and 100 kg −1 N⋅ha coverage split applications at the start of tillering and issuance of the primodium leaf (36 and 60 DAE).
The experiment was conducted in a minimum tillage system, and the experiment was a randomized block design with split plots in a factorial design (2 × 3 × 5) with four replications.Each block was composed of an area of 245 m 2 (98.0 m×2.5 m), plots with an area of 75 m 2 , and plots with an area of 15 m 2 (experimental units).
The reagent cultivar was Qualimax 1, where it was shown with populations of 150 kg ha −1 of seed, which led to the establishment of a population of approximately 400 plants⋅m −2 in rows spaced 0.17 m.The choice of cultivar was held to be the most widely grown in the region of conducting the experiment.
For the spraying of herbicides was used a CO 2 pressurized backpack sprayer with bar containing four flat fan nozzles 110.02 working pressure of 20 lb in −2 and spray volume of 150 L ha −1 .At the time of first application (early), rice plants were in the 2-4 leaf stage, the plants of Echinochloa sp.(barnyardgrass) with 1-3 leaves.Already, for the second application (late), rice was meeting with 4 leaves and 1 tiller, plants of Echinochloa sp. with 4 leaves and 2 tillers.The weed population was 524 plants⋅m −2 of Echinochloa sp.

Results and Discussion
The control of Echinochloa sp.there was interaction of the factors studied (Table 1, Figures 1 and 2).The delayed start of irrigation (15 and 30 DAT) for the two lower doses of herbicide, in general, for all evaluation periods reduced the efficiency of control Echinochloa sp.(Table 1).
In the evaluation performed at 10 DAT, it was noted in general that herbicide application development of late season weed control decreased levels of Echinochloa sp., regardless of timing of irrigation and dose used (Table 1).In other periods of evaluation, in general, there was not difference between early and late herbicide application, for all doses tested.This could be because the plants of Echinochloa sp. at the time of application were in advanced stages of development compared to herbicide application when at an early stage of development.
The effect of dose penoxsulam, due to the factors time of flooding and application in control of Echinochloa sp., showed good fit of the data to the model, with  2 of 0.88 and generally low values of QMR data demonstrating the adequacy of the model (Figures 1 and 2).
The anticipation of the onset of flooding, especially in smaller doses of the herbicide reduced the dose penoxsulam needed to obtain effective control of Echinochloa sp., regardless of the time of application and evaluation, as shown by the lower value of the parameter exponential model International Journal of Agronomy (Figures 1 and 2).The comparison between irrigation times based on the values of the exponential model showed that the anticipation of irrigation to 1 DAT, associated with early application, the average of four evaluation, reduced by 7,5 and 9,4 times the value found in the parameter when compared to early irrigation at 15 and 30 DAT, respectively.Performing the same comparison for the late application, it was found that the values were of 1,1 and 1,6 times lower for the early irrigation at 15 and 30 DAT.
In studies with penoxsulam, Pinto et al. [12] observed that at lower doses of herbicide control the best results were seen with the entry of water beforehand, whereas at higher doses the water inlet later allowed for better control results.This could be indicative of the residual effect of the product, which is partially offset from the time of entry of the water due to higher leaching or percolation [15].
According to Pinto et al. [12], it is possible to use lower doses of penoxsulam associated with the water inlet advance (21 DAE) to obtain similar levels of control at higher doses or delay the entry of water into the cultivation and use higher doses of herbicide [12].Thus, Concenc ¸o et al. [17] reported efficient control barnyardgrass with the herbicides clomazone, penoxsulam and when the procedure from the beginning of the irrigation occurred at 19 and 24 DAE in different doses tested.
Settling greater than or equal to 90% as efficient control pattern [6] showed that the last two doses equal or greater than 24 g ha −1 showed penoxsulam control of Echinochloa sp., when irrigation began in the period up to 15 DAT.These results may be due to the high efficiency control Echinochloa sp. by penoxsulam.
The weed control in early development stage comprises the use of lower doses and early applications of herbicide, without changing the control efficiency, since then it immediately start irrigation to prevent reinfestation [6,8].
When testing low doses of penoxsulam mixed with clomazone, Concenc ¸o et al. [16] found an earlier irrigation efficient control of Echinochloa sp.independent of the dose used.Results similar to the present experiment were observed by Bradley and Kendig [18].Evaluating the effects of periods of flooding in controlling Echinochloa sp., Pinto et al. [19] reported that the delayed onset of irrigation in 10 days after the application of penoxsulam reduced by 26% weed control.
The delayed onset of flooding significantly affects grain yield of rice, regardless of dose and application timing penoxsulam, anticipating the start of flooding increases the efficiency of penoxsulam for control of barnyardgrass, and the delay in the entry of water is partially offset by the her-bicide [12].
Whereas the anticipation of irrigation in rice cultivation aids in weed management and serves as a preventive method preventing the emergence of new streams of plants [20], preventing reinfestation in flooded areas, the association of water management with herbicides that have residual power can increase the efficiency of control or allow dose reduction of herbicide.The beneficial effect of water depth on weed control stands out in terms of land cover, eliminating the oxygen available to the roots of the weeds [16,17].
The anticipation of the start of irrigation to 1, 10, or 20 days after herbicide treatment increased the grain yield of rice in competition with Echinochloa sp.[2,3].In a study evaluating the effects of periods of flooding and dose penoxsulam, it was found that the delay of 10 days from the beginning of irrigation after application of this herbicide reduced on average 41% grain yield of rice [19].
Inferring the influence of the management for weed control, Pinto et al. [12] reported that the application of herbicide to retard the entrance of water, or 30 days after emergence (DAE) results in low productivity, and this fact is probably due to reinfestation of the crop due to the lack of additional effect of water on herbicide action and also the lowest residual effect due to the low dose used.This author also points out that regardless of dose or time of herbicide application, with water inlet on the farm at 21 DAE, there is an increase on average of 57% in grain yield of rice compared to 30 DAE.

Table 1 :
Percentage control Echinochloa sp.(barnyardgrass) in irrigated rice, due to periods of flooding, application times, and doses of penoxsulam, Rio Grande, RS, Brazil.