Characterization of Emergence Flows of Volunteer Corn as Function of the Type of Harvest Grain Loss

Volunteer corn resistant to glyphosate is constant as weed in soybeans planted in succession. This work aimed to identify the emergence flows of volunteer corn plants in the period of time from the harvest of corn planted following soybean (autumn corn) to the plenty establishment of the canopy of the following soybean crop (summer soybean), as a function of different types of propagules generated by preceding corn harvest losses. Four field experiments were carried out in 2013 and 2014, at a Cerrado location (Sinop, MT) and a subtropical location (Londrina, PR), Brazil. Treatments included the distribution of corn crop residues (factor “A”) either on soil surface or incorporated into superficial soil layers. Four types of propagules (factor “B”) were characterized as ears with whole husk; ears with half husk; broken ears, no husk; and loose grains. The density of emerged plants was recorded fortnightly between August and December. When partially incorporated into soil, propagules generate an increased density of emerged plants as compared to the surface deposition treatments. The main sources of volunteer plants, in descending order of importance, were: (1) loose grains, (2) broken ears, (3) ears with damaged husk, and (4) ears with intact husk. Ears emerged later compared to loose grains or broken ears. Climatic conditions influence the emergence pattern. For the climate of (Savannah-like) Cerrado, the beginning of the rainy season is preponderant for the start of corn emergence. Under subtropical climatic conditions, mild low temperatures, associated or not to rainfall, determine the emergence peaks.


Introduction
The most important crop rotation in grain-producing areas of Brazil is the cultivation of soybeans in spring/summer, immediately followed by corn planted in summer/fall.Soybean resistant to glyphosate (GR) was officially approved in Brazil to be planted in 2006, and the GR corn in 2010.Since their commercialization in Brazil, GR corn and soybean were rapidly adopted by growers as they make weed management easier.In 2017, more than 70% of corn and 90% of soyeban planted were GR.After the approval of the GR corn it in fact became itself one of the main weeds to be controlled into GR soybean fields.Glyphosate, which was previously the main tool to provide control of volunteer corn in GR soybeans, has become a useless tool in its control, due to the insertion of the resistance gene to this herbicide in corn.Volunteer plants come from grains lost during corn harvest in fall/winter, and start to emerge with the first rain events at or right after soybean sowing, in the next planting season.Volunteer corn plants are very skilled competitors in depleting natural resources such as water, light and nutrients.Where there is no control of volunteer corn plants, soybean yield losses of approximately 25% is expected with volunteer corn densities of 1 plant m -2 (Alms et al., 2016).The degree of interference of volunteer corn on soybean depends both on its density and its emergence timing.Marquardt et al (2012) reported that 0.5-16 corn plants m -2 caused 10-40% of reduction in soybean grain yield.However, the soybean yield was not affected when corn, at these densities, emerged later compared to soybean.In addition to competing for environmental resources, volunteer corn can serve as a host, or green bridge, for pests and diseases, causing even more losses to soybeans.Pests which affect several crops as armyworm (Spodoptera frugiperda), may migrate from volunteer corn to soybean and impact its plant stand and productivity (Silva et al., 2017).The presence of volunteer corn during fallow may also contribute do increases in the population of soybean cyst nematode (Heteroda glycines) (Fonte).
Control of volunteer corn is, therefore, fundamental for full expression of crop yields in successional crops such as soybeans.Reducing crop losses through better adjustment of combines and harvesting at the appropriate time help to decrease the emergence of volunteer corn plants (Shauck et al., 2010).Harvesting losses create a variety of propagules that may lead to the emergence of volunteer corn in successional soybeans, such as grains detached from ears, damaged ears with and without husk, as well as whole and broken ears.Such remaining propagules can be either deposited on the soil surface or partially incorporated into soil at shallow depths, as a result of the machinery traffic during harvest, sowing or pesticide applications.
Due to the prevailing no-tillage cropping systems adopted by Brazilian farmers, weed management relies primarily on the use of herbicides.Although the soil tillage may be considered as tool to manage volunteer corn, it is undesirable in Brazilian environmental conditions where the dry matter degradation rate is naturally high; soil tillage would result in chopping of the mulching and its partial incorporation into soil, increasing the degradation rate (Busari et al., 2015).
Control of volunteer plants emerged from ear segments, unlike those emerged from scattered grains, tends to be more troublesome because seeds adhered to the ear will emerge unevenly in time, what can result in several flows along the soybean cycle (Petter et al., 2015).Volunteer corn emerged from grains still attached to ears predominate in areas where lodging and/or breakage of plants occurs and; in this situation, volunteer corn occurs in clumps with high density of plants at specific spots into the field (Deen et al., 2006).Thus, it is to be expected that harvest losses as ear segments tend to to cause more damage to soybean performance than the interference caused by individual plants originated from grains detached from the ear.López-Ovejero et al. (2016) reported yield losses between 18 and 70% in soybean yield when the corn residues varied between one and four ear segments per m 2 .
The control of glyphosate-tolerant volunteer corn in GR soybeans is primarily performed with the use of post-emergence herbicides of the acetyl-coenzyme-A carboxylase inhibitors (ACCase), since the most pre-emergence soybean herbicides provide inefficient control of volunteer corn (Chahal et al., 2014;Marquart & Johnson, 2013).Controlling volunteer corn at the right timing also exerts a great influence on soybean yield.Volunteer corn emerging later in the season cause lower soybean yield loss compared to the plants emerged during early soybean growth stages (Chahal & Jhala, 2016).In order to establish an efficient management strategy, it is therefore relevant to understand the dynamics of the emergence flow and the control strategy that best applies to each reality.This work aimed to identify the emergence flows of volunteer corn plants in the period of time from the harvest of corn planted following soybean (autumn corn) to the plenty establishment of the canopy of the following soybean crop (summer soybean), as a function of different types of propagules generated by preceding corn harvest losses.

Materials and Methods
Field experiments were conducted at two locations during 2013 and 2014.The first experiment was conducted at Embrapa Agrossilvipastoril, Sinop, Mato Grosso (MT) (Lat.: 11°51′37″ S; 55°36′19″ W), and the second at Embrapa Soja, Londrina, Paraná (PR), (Lat.: 23°11′30″ S; 51°10′58″ W), Brazil.Both experimental locations have Clay texture soil (Sinop-MT: pH 5.2, 34% sand, 22% silt, 44% clay and 1.9% organic matter-OM; Londrina-PR: pH 5.6, 77% clay, 7% silt, 16% sand and 2.7% OM).Mean climatic inputs recorded throughout the experiments are presented in the respective figures, along with emergence of volunteer corn seedlings.The treatments were composed by four different corn propagule types (factor "B"), either distributed on soil surface or partially incorporated into soil (factor "A") (Table 1).Corn propagules used at the present study were collected at the time of corn harvest, on neighboring fields, and immediately deposited/planted (depending on the treatment) in the area of the experiment.The "partially incorporated" treatments (T5-T7) consisted on the shallow incorporation of one half of each ear into soil (treatments with ears), or grain coverage with a layer of 2 cm of soil (treatments with spare grains) (T8), simulating the compaction caused by the combine wheels.The plots consisted of a 1 m 2 area, where the different types of propagules were evenly distributed, with four replications.Each experimental plot comprised six ears of corn or 0.5 kg of grains (corresponding to the average amount of grains from six corn ears), according to the treatment (Table 1).Considering that the present study aimed to assess the behavior of emergence flow of corn, a big amount of grains was supplied per square meter of the plots aiming to give higher reliability to the dataset.
The experiments were carried out from August to December in both years, during the period between corn harvest and the establishment of the successional soybean canopy.Both experiments were installed in traditionally cropped areas, with no corn or soybean planting in the previous two years.Areas were kept free from weeds during the course of the experiments by a burndown application with glyphosate prior to implantation and regular applications of paraquat dose of 600 g ha -1 , for controlling the volunteer corn flows after each evaluation.All herbicide applications were always carried out with a CO 2 pressurized sprayer, coupled to an application bar with nozzles 110.02, spaced in 0.50 m, regulated for a spraying volume of 120 L ha -1 .Fortnightly counts of the density of corn seedlings emerged per plot were carried out.
Data were analyzed by descriptive statistics.In the first step of the analysis, the emergence of corn seedlings was correlated to available local climatic parameters for each location: mean air temperature (ºC) for Paraná; mean maximum soil temperature (ºC) for Mato Grosso; and relative air humidity (%) and rainfall (mm) for both locations.
Corn emergence was described by bar graphs for the fortnight periods between the first half of August and the second half of December.On the bar graph, the climatic parameters were illustrated by a smoothed line, from climatic data obtained from local automatic climatic stations, as average values for ten-days periods (three means per month).The climatic parameter curve was smoothed by the Loess method (Cleveland & Devlin, 1988), by using the quadratic model, in order to better characterize the local microclimate, presenting 95% confidence intervals.The correlation of corn emergence to the climatic parameters was run separately for each year of the experiment.
The second step of the analysis consisted on analyzing emergence patterns of corn seedling for each of the two years (2013 and 2014).The third step consisted on the characterization of the emergence of volunteer corn as a function of a period of the year (1 st half of August-2 nd half of December), considering the different treatments (Table 1).For the second and third steps, the emergence data was smoothed by the Loess quadratic model (Cleveland & Devlin, 1988), and the curves were compared on the basis of their respective 95% confidence intervals (Cumming et al., 2007).The curves were considered distinct in sections where their respective confidence intervals did not overlap.
The fourth step of the analysis consisted on the characterization of each of the four crop loss types (factor "B") as a function of soil deposition (factor "A"), and the results were organized in circular percent graphs.All the analyzes were performed in the statistical software "R" (R Core Team, 2016), using functions available in the base and ggplot2 packages.

Results and Discussion
The emergence flows vary depending on the type of propagule and on climatic conditions specific to each growing environment.The results are discussed separately according to the locality of the trial, due to the edaphoclimatic differences between these regions. jas.ccsenet.

Sinop-
When pro emergence in the seco soybean so concentrat

Figure disposit
In       eased unteer corn infestation can be ranked in the following descending order of importance: (1) loose grains, (2) broken ears, (3) ears with damaged husk and (4) ears with intact husk.Ears present later emergence flows.Climatic conditions influence the emergence pattern of volunteer corn.For the Cerrado climate, the onset of the rainy season is preponderant for the beginning of the emergence start.Under subtropical climate conditions, mild cold temperatures, whether or not this condition is associated with intense rainfall, determines the main peaks of emergence.

Table 1 .
Treatments screened for voluntee2014ergence of corn plants in the period preceding planting, and during initial development of soybean, at Sinop-MT and atLondrina-PR, Brazil, in 2013 and2014