Research
Cover Crop Termination Timing on Rice Crop Production in a No-Till System A. S. Nascente,* C. A. C. Crusciol, T. Cobucci, and E. D. Velini
ABSTRACT Measuring shikimic acid accumulation in response to glyphosate applications can be a rapid and accurate way to quantify and predict glyphosate-induced damage to sensitive plants. The objective of this paper was to evaluate the effect of cover crop termination timing by glyphosate application on rice (Oryza sativa L.) yield in a no-till system. A factorial experiment, arranged in a split-plot design, was conducted for 2 yr. Treatments consisted of cover crops (main plots) and timed herbicide applications (subplots) to these cover crops (30, 20, 10, and 0 d before rice planting). There was a decrease in rice yield from 2866 kg ha-1 to 2322 kg ha-1 when the herbicide was applied closer to the rice planting day. Glyphosate application on cover crops increased shikimate concentrations in rice seedlings cultivated under palisade grass (Brachiaria brizantha), signal grass (B. ruziziensis), guinea grass (Panicum maximum), and weedy fallow (spontaneous vegetation) but not under millet (Pennisetum glaucum), which behaved similarly to the control (clean fallow, no glyphosate application). Glyphosate applications in the timing intervals used were associated with stress in the rice plants, and this association increased if cover crops took longer to completely dry and if higher amounts of biomass were produced. Millet, as a cover crop, allowed the highest seedling dry matter for upland rice and the highest rice yield. Our results suggest that using millet as a cover crop, with glyphosate application far from upland rice planting day (10 d or more), was the best option for upland rice under a no-tillage system.
A.S. Nascente and T. Cobucci, Brazilian Agricultural Research Corporation (EMBRAPA), Rice and Beans Research Center, P.O. Box 179, 75.375-000, Santo Antônio de Goiás, Goiás, Brazil; C.A.C. Crusciol and E.D. Velini, São Paulo State Univ. (UNESP), College of Agricultural Science, Dep. of Crop Science, P.O. Box 237, 18.610-307, Botucatu, São Paulo, Brazil. Received 22 Jan. 2013. *Corresponding author (
[email protected]). Abbreviations: a.i., active ingredient; NTS, no-till system; SOM, soil organic matter.
U
pland rice (Oryza sativa L.), which is cultivated in Asia, Africa, and the Americas, has been increasing in global importance because of the decreasing availability of water for flood-irrigated varieties (Tao et al., 2006; Crusciol et al., 2013; Nascente et al., 2013). Because available water resources have been reduced owing to the competing demands of industry and population, alternatives are being sought that allow a greater efficiency of water usage (Saito et al., 2005; Tao et al., 2006). Some alternatives include growing rice under aerobic conditions such as no-tillage systems (NTS) that allow better conservation of soil moisture with the use of cover crops (Curran et al., 1996; Dabney et al., 2001; Nascente et al., 2011). Because grain crops when used as cash crops do not normally produce enough straw to allow yearlong soil coverage, perennial forage species such as Brachiaria and Panicum, which produce large amounts of straw and remain longer on the soil surface because of their high C/N ratio (Crusciol et al., 2012; Nascente et al., 2012), have attracted interest and have been used either in rotation with cash crops or as a cover crop. These grasses also grow quickly and create aggressive root systems that favor nutrient cycling by improving the physical properties and increasing the biological activity and the organic matter of the soil (Dabney et al., 2001; Published in Crop Sci. 53:2659–2669 (2013). doi: 10.2135/cropsci2013.01.0047 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
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Nascente and Crusciol, 2012). Millet is another option; it has rapid straw degradation and quickly releases nutrients to the soil, allowing for increased upland rice yields (Nascente et al., 2013). The management of cover crops before planting grain is normally accomplished with glyphosate, a total systemic herbicide (Powles et al., 1998; Ng et al., 2004; Binkholder et al., 2011). This herbicide has a broad spectrum of action for controlling annual and perennial weeds and cover crops (Pratley et al., 1999). Glyphosate acts by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase, thereby causing the rapid accumulation of high levels of shikimic acid in affected plant tissues (Duke et al., 2003; Matallo et al., 2009). The measurement of shikimic acid accumulation in response to glyphosate application is a rapid and accurate way to quantify and predict glyphosateinduced damage in sensitive plants (Harring et al., 1998; Pline et al., 2002; Henry et al., 2007; Matallo et al., 2009). Glyphosate injury in plants is slow, taking several days to cause plant death (Velini et al., 2010). Thus, applying glyphosate close to a cash crop planting day may allow for the cover crops to be alive and standing. In this regard, the cover crops can impair planting operations or provide initial shading for rice seedlings, causing damage to their development and a reduction in crop yield (Constantin et al., 2008). Furthermore, there is the possibility of root exudation of the glyphosate from the cover crops to the rice, especially if the roots of the cover crops treated with herbicide are abundant and close to the crop roots (Coupland and Lutman, 1982). Nunes et al. (2009) determined that the most appropriate time for the chemical management of tropical forage with glyphosate is between 7 and 14 d before sowing of soybeans. Monquero et al. (2010), working with palisade grass, signal grass, and millet, determined that the desiccation of all cover crops was better for soybean development when it occurred between 7 and 14 d before crop sowing. Ricce et al. (2011) reported that the burn-down of black oats (Avena strigosa Schreb.) and rye grass (Lolium multiflorum Lam.) close to the soybean-planting day did not cause any damage to soybean grain yields. Nascente and Crusciol (2012) showed that glyphosate applications on guinea grass and palisade grass near the soybean planting date (10 or less days) caused significant damage to the cash crop yield. Nascente et al. (2012) reported that signal grass must be desiccated 20 d or more before upland rice sowing to avoid crop loss (this study was performed under greenhouse conditions). Constantin et al. (2008) asserted that planting corn soon after cover crop desiccation might lead to uneven germination and inappropriate early development (shading) in corn seedlings. They recommended an interval of at least 2 to 3 wk between the chemical management and the planting of corn. Although a great deal is known about this subject concerning soybean and corn crops, little is known about the effects on rice yield of glyphosate 2660
application timing on cover crops before rice planting. Therefore, the objective of this paper was to evaluate the effect of cover crop termination timing by glyphosate application on rice yield in a no-till system (NTS).
MATERIAL AND METHODS Site Description A field experiment was conducted in Santo Antônio de Goiás County, State of Goiás, Cerrado Region, Brazil (16°27´ latitude, 49°17´ longitude, and 823 m a.s.l.). The regional climate is tropical savanna, classified as Aw according to Köppen. There are two well-defined seasons: dry from May to September and rainy from October to April. The annual mean rainfall is 1500 mm. The local annual mean temperature is 22.7°C, varying annually between 14.2°C and 34.8°C. The soil is a kaolinitic, thermic Typic Haplorthox in gently undulating topography with a ratio of clay 540 g kg-1, silt 110 g kg-1, and sand 350 g kg-1. A soil chemical characterization was performed before initiating the experiment (Table 1). The study was conducted in an area that had been in NTS for 6 yr (2001–2002to 2006–2007) in rotations with corn (2001, 2003, and 2005) and soybeans (2002, 2004, and 2006) in the rainy season and fallow in the dry season.
Experimental Design and Treatments Experiments were performed for two growing seasons (2008– 2009 and 2009–2010). The experiment was arranged in a splitplot design with three replications, where type of cover crop was assigned to the main plot and termination timing of glyphosate application was the subplot unit. The experiment also included a no-cover crop as a control treatment. In these plots, no cover crop was used, but conventional tillage (one plowing and two disking, called clean fallow) was performed at 30 d before rice sowing. The main plots (6 × 40 m) were planted with the five cover crops. The cover crops used were as follows: (i) Weedy fallow [spontaneous vegetation, predominantly Bidens pilosa L., Commelina benghalensis L., Conyza bonariensis (L.) Conquist and Cenchrus echinatus L.], (ii) guinea grass (Panicum maximum Jacq.), (iii) signal grass (Brachiaria ruziziensis R. Germ. and C.M. Evrard), (iv) palisade grass [Brachiaria brizantha (Hoschst. ex A. Rich.) Stapf.] ‘Marandu’, and (v) millet [Pennisetum glaucum (L.) R. Br.] ‘BN-2’. The subplots (6 × 10 m) were used for the four application timings (30, 20, 10, and 0 d before rice planting) of the herbicide. There was a 1-meter-wide alley between each subplot. There were 17 rows in each subplot; the three central rows (rows 8, 9, and 10) were used for yield measurements, and row 6 was used for seedling evaluation. Cover crop desiccation was achieved by applying glyphosate (Roundup Original, 1800 g acid equivalent ha-1, Monsanto Brazil). A boom sprayer was used with a spray volume of 200 L ha-1.
Crop Management Cover Crops In November of 2007 and March of 2009, tropical perennial forages were sown; millet was sown in March 2008 and March 2009. The cover crops were planted in 0.20-m row spacings with a mechanical planter set to distribute 10 kg ha-1 of pure live seeds as recommended by Crusciol et al. (2010).
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Table 1. Soil chemical properties at the depths 0 to 0.05, 0.05 to 0.10, and 0.10 to 0.20 m in a composite sample obtained from sampling 20 subsamples with a metal auger in each layer at the experimental area. Depth Properties†
0–0.05
0.05–0.10
0.10–0.20
————————————— m ————————————— 2008 6.6 6.2 5.9
pH (water) SOM‡ (g kg-1) P (mg kg ) K (mg kg-1) Ca2+ (cmolc kg-1) Mg2+ (cmolc kg-1) Al3+ (cmolc kg-1) Cu (mg kg-1) Zn (mg kg-1) Fe (mg kg-1) Mn (mg kg-1) -1
pH (water) SOM (g kg ) P (mg kg-1) K (mg kg-1) Ca2+ (mmolc kg-1) Mg2+ (mmolc kg-1) Al3+ (mmolc kg-1) Cu (mg kg-1) Zn (mg kg-1) Fe (mg kg-1) Mn (mg kg-1) -1
25.3 14.5 233.3 3.0 1.2 0.0 1.9 4.7 35.3 25.0
23.2 13.4 83.4 1.9 0.6 0.1 2.0 4.2 35.4 21.2
5.7
24.1 17.2 129.2 2.3 0.8 0.0 1.9 4.3 36.2 22.6 2009 5.7
27.6 17.5 230.3 3.4 1.7 0.0 1.8 5.9 36.7 30.2
21.0 18.5 173.9 2.5 1.0 0.0 2.2 5.3 25.1 25.1
16.2 19.1 129.2 2.2 0.6 0.1 2.3 4.8 38.8 22.1
5.5
The pH was determined in water (1:2.5). Phosphorus and K were extracted by Mehlich 1 extracting solution (0.05 M HCl in 0.0125 M H2SO4); the P content was determined colorimetrically and the K through flame photometry. Calcium, Mg, and Al were extracted with 1 M KCl. Aluminum was determined by titration with NaOH and Ca and Mg by titration with EDTA from the extracted solution. Micronutrients were determined in a portion of the extract for P through atomic absorption spectrophotometry. Soil analysis methods used in this study are described in a soil analysis manual published by EMBRAPA (1997).
†
‡
SOM, soil organic matter.
Upland Rice Rice (cultivar BRS Sertaneja) was sown in November 2008 and 2009 at 0.35 m between rows and 80 seeds per meter. Fertilizers, on the basis of soil analyses (Table 1), were applied at sowing for 20 kg N ha-1 as urea, 52.4 kg P ha-1 as triple super phosphate, and 49.8 kg K ha-1 as potassium chloride. Additional urea was applied at 44 kg N ha-1 1 d after upland rice was seeded and 44 kg N ha-1 at 45 d after rice emergence. Crop management was performed in accordance with standard practices for an upland rice crop. We treated the seeds with tricyclazole (225 g a.i. 100 kg of seeds -1) to protect against rice blast fungus [Magnaporthe grisea (T.T. Hebert) M.E. Barr] and fipronil (62.5 g a.i. 100 kg of seeds -1) to protect against Oryzophagus oryzae Costa Lima, Syntermes molestus Burmeister, and Procornitermes triacifer Silvestri. We had no subsequent problems with pests in our trial and did not need to apply any insecticide. However, we did apply the fungicide tricyclazole (300 g a.i. ha-1) twice (at the full tillering stage and 15 d afterward) to guard against rice blast fungus.
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Sampling and Analyses Cover Crop Dry Matter
Cover crops were sampled at upland rice planting day in each subplot. This procedure was performed using a metal square 1.0 m × 1.0 m, selected randomly and sampled one time in each subplot. The collected plant material was placed in paper bags and dried in a forced ventilation oven at 65°C; the material was weighed, and the dry matter was converted to Mg ha-1.
Shikimic Acid Concentration To analyze the shikimic acid concentration, samples were taken from each subplot where glyphosate was applied and from the control treatment (without glyphosate application). All the seedlings of upland rice present in the central meter of row 6 were collected 7 d after emergence. The seedling plants were placed in an oven of forced ventilation at 60°C until dry; they were then weighed and ground in a Wiley mill (0.5 mm mesh). Data from the weight and number of these seedlings were also used to analyze the initial stand and dry matter accumulation by the rice plants. The shikimic acid concentration was assessed by a method proposed by Matallo et al. (2009). A rate of 400 mg of each sample and 10 mL of water at pH 7.0 were added to a glass beaker and placed in a microwave for 20 s at 100 W. After cooling, the samples were filtered through number 1 filter paper. The concentration of shikimic acid in the extracted solution was determined using a Shimadzu LC 2010 chromatograph equipped with a Class VP 6.0 software, an autoinjector, and a photodiode array detector using a detection wavelength of 212 nm. A Phenomenex Gemini C18 110 A° column (250 mm × 4.0 mm; 5-µm particle size) was used with an injection volume of 20 μL. The isocratic system used 95:5 deionized water at pH 3.0: methanol and a flow rate of 1.0 mL min-1. The total running time was 10 min, with shikimate retention time at 5.1 min. A seven-point calibration curve with shikimate concentrations ranging from 2.04 to 407.2 µg mL -1 was used to quantify the shikimate levels. The relationship between the amount of shikimate and the peak area was linear, with a correlation coefficient of 0.999. With this method, the detection limit for shikimate was 0.395 µg mL-1.
Upland Rice Grain Yields and Yield Components A manual harvest was conducted when approximately 90% of panicles had grains of typical mature coloration. Panicles were dried in the sun for 1 to 2 d and later submitted to mechanical threshing in a research plot thresher. The following evaluations were performed: the number of panicles m-2 (obtained by counting the number of panicles in 2.0 m of plants in two central rows from the usable area of each subplot), the total number of spikelets panicle-1 (obtained by counting the number of spikelets in 20 panicles at the useable area), the spikelet fertility [obtained using the following function: (number of grain-bearing spikelets/total number of spikelets per panicle) × 100], the 1000-grain weight (evaluated by the random collecting and weighing of four samples of 1000 grains from each subplot [130 g kg-1 wet basis]), and the grain yield (obtained from unhulled grain weight collected from three central rows of five meters in each subplot, eliminating 2.5 m on each side, correcting their moisture content to 130 g kg-1 wet basis and converting to kg ha-1).
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Table 2. ANOVA (F probability) significance for amount of straw on the soil surface at rice (Oryza sativa L.) sowing day, seedling stand, shikimic acid concentration in the seedlings, seedling dry matter, panicle m-2, spikelet panicle-1, spikelet fertility, 1000grain weight, and grain yield of upland rice as a function of the blocks, cover crops species, cover crop termination timing by glyphosate application and year.
Factors/Variables Blocks Year (Y) Cover crop species (C) Herbicide timing (H) Y´C Y´H C´H Y´C´H
Cover crops dry matter
Seedling stand
Shikimic acid content
0.2179 0.0434 < 0.001 < 0.001
0.571 < 0.001 0.089 0.096
0.395 0.297 < 0.001 0.323
0.213 0.098 < 0.001 < 0.001
0.749 < 0.001 < 0.001 < 0.001
0.271 0.033 0.027 0.342
0.667 0.375 < 0.001 0.767
0.773 0.041 0.012 0.098
0.363 0.044 0.017 0.023
0.1159
0.883
0.457
0.223
0.871
0.753
0.761
0.754
0.257
0.3278
0.321
0.325
0.195
0.434
0.218
0.478
0.432
0.325
< 0.001 0.5429
0.047 0.764
0.651 0.765
0.897 0.512
0.219 0.342
0.884 0.754
0.159 0.653
0.769 0.312
0.115 0.532
Seedling Spikelets dry matter Panicle m-2 panicle-1
Spikelets fertility
1000-grain weight Grain yield
Figure 1. Cover crop dry weight (Mg ha-1) as affected by cover crop type and termination timing. Data was the average across the 2 yr. **, significant at P < 0.01.
Statistical Analyses Two-way ANOVAs were run for all data (cover crop dry matter, rice seedling number and weight, shikimate concentration, upland rice yield and yield components) results, using Statistical Software Package SAS (SAS Institute, 1999). Cover crops and herbicide timing were considered fixed effects. Two error terms were considered in the analysis of the data; one was associated with the cover crops and the other associated with the herbicide timing and the interaction (cover crop ´ herbicide timing). A combined analysis for years was performed for assessing the overall treatment comparisons. Mean separations were conducted using an LSD test. Effects were considered statistically significant at p £ 0.05. A polynomial regression analysis was also performed according to the cover crop dry matter and upland rice yield with the desiccation time of the straw. A Pearson correlation coefficient was used at p £ 0.05 between (i) the amount of cover crops and the upland rice yield and its components, and (ii) the glyphosate application day and the upland rice yield and its components.
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RESULTS AND DISCUSSION Cover Crops
The desiccation time significantly affected the cover crop dry matter on the soil surface at the rice-sowing day (Table 2). All cover crops had significant linear growth (Fig. 1). Herbicide applications closer to the day of sowing rice allowed for higher amounts of cover crop dry matter on the soil surface. Plants desiccated by glyphosate stopped growing and immediately began the process of straw degradation (Nascente et al., 2012). Desiccation farther in advance of rice sowing may have caused a reduction in dry matter accumulation, which could be observed on the day of sowing rice. On the other hand, the plants dried closer to rice-sowing day had more time to develop and accumulate dry matter. Millet provided the lowest amount of dry matter on the soil surface on the day of sowing rice in all periods, with values similar to weedy fallow and differing from the other cover crops (Fig. 1). Millet produced the lowest level of dry matter because it is an annual; the plant completed its life cycle in July and dropped its seeds. With the subsequent onset of the rainy season (September–October), these
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Table 3. Cover crop dry weight (CCDW), seedling stand, panicle m-2, spikelets panicle-1, 1000-grain weight and grain yield of upland rice (Oryza sativa L.) plants as affected by year. Year
CCDW
Seedling stand
Panicle m-2
Spikelets panicle-1
1000-grain weight
Grain yield
2008–2009 2009–2010
Mg ha-1 8.8 a† 6.2 b
number 50 b 56 a
number 74 b 92 a
number 165 b 183 a
grams 25.4 a 23.0 b
Kg ha-1 2343 b 2846 a
†
Means followed by the same letter vertically do not differ by LSD test at p < 0.05.
seeds germinated and reestablished the plants. Therefore, millet plants had less time to accumulate biomass relative to the weedy fallow treatment and the perennial forage species. These results were similar to those found by Torres et al. (2005) and Pacheco et al. (2011), who obtained 3.6 and 4.1 Mg ha-1 of dry matter, respectively, for millet in the Cerrado region. In addition, millet was extremely sensitive to glyphosate, such that 70% of the plant was totally dried 7 d after treatment, similar to reports by Monquero et al. (2010). In the weedy fallow treatment, which was composed of weeds, the amount of dry matter ranged from 3.3 to 7.4 Mg ha-1 (Fig. 1). Weeds generally exhibit rapid growth and dry matter accumulation, along with greater efficiency in extracting water and nutrients from the soil. These plants typically have greater adaptability than crop plants under harsh conditions as a survival strategy (Nascente et al., 2004). However, despite weeds accumulating dry matter that could be used as straw in the NTS, they also have nonuniform growth because of the varying composition of species. Desiccated weeds do not provide uniform distribution of straw in the soil. Furthermore, if not properly controlled, weeds can compete for water, light and nutrients, causing a reduction in the grain yields of cash crops. Weeds can also serve as hosts for pests and diseases, hinder harvesting operations and, if allowed to finish their life cycle, spread their seeds into crop areas (Nascente et al., 2004). Therefore, it is not a recommended practice to use weedy fallow for the formation of straw in the NTS. According to Balbinot et al. (2009), soils under weedy fallow increased weed infestation, and the use of cover crops significantly reduced weed incidence. Favero et al. (2000) reported that treatments under weedy fallow conditions produced lower amounts of dry biomass and accumulated fewer nutrients than treatments with legumes. As for the perennial forage treatments, guinea grass, signal grass, and palisade grass showed no significant differences in the amount of straw in the four periods of desiccation (Fig. 1). These perennial forages already had established root systems in the beginning of the growing season, and after the onset of summer rains in September, they quickly began to grow again. As a consequence, we found significant amounts of dry matter on the soil surface on the day of sowing rice in all periods of desiccation, reaching values higher than 11 Mg ha-1. In corroborating this information, it was noted in several studies that these species are known crop science, vol. 53, november– december 2013
to produce large amounts of biomass and have tolerance to drought and ample capacity for regrowth after the resumption of rains (Timossi et al., 2007; Nascente and Crusciol, 2012; Nascente et al., 2012; Crusciol et al., 2012). According to Lopes et al. (1987), 6 to 7 Mg ha-1 of dry matter is necessary to cover 100% of the soil. It was found that these perennial forages took longer to be desiccated by glyphosate. Supporting this information, Monquero et al. (2010) reported that 14 d after glyphosate application, on average, only 65% of the plants were completely dried. In our study, millet required approximately 10 d to dry completely, fallow 15 d, and the perennial forages 28 d. In general, the plants produced more dry matter in the 2008–2009 growing season than in the 2009–2010 growing season (Table 3). This difference most likely occurred because in the first growing season, with the exception of millet, the cover crops were planted early in the rainy season (November 2007), while in the second growing season, the seeding of cover crops occurred in March 2009, after the rice harvest. Thus, the cover crops had a longer growing season in the first year than the second year. However, it must be noted that in both growing seasons, even across the entire dry period after the onset of the rainy period, subsequent cover crops accumulated sufficient amounts of dry matter to provide a uniform and well-distributed straw on the soil surface to be used in the NTS. The exception occurred in the weedy fallow treatment, which produced uneven and poorly distributed dry matter.
Shikimic Acid There was no effect of herbicide timing on shikimic acid concentrations (Table 2). With the exception of millet (0.7 µg g-1), all cover crops resulted in higher quantities of shikimic acid (2.7, 3.7, 4.5 and 4.0 µg g-1 for weedy fallow, guinea grass, signal grass, and palisade grass, respectively) in rice seedlings (Table 4). These results indicate that there was translocation of glyphosate from the cover crops to the rice seedlings. Additionally, shikimic acid concentrations under the control treatment (clean fallow, no glyphosate application) did not differ from the millet treatment. The low concentration of shikimic acid in rice seedlings growing on millet could have occurred because this cover crop is very sensitive to glyphosate (Crusciol et al., 2010; Monquero et al., 2010; Pacheco et al., 2011; Nascente and Crusciol, 2012). The millet was killed very quickly (10 d) after applying the herbicide and may not have had
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Table 4. Effect of cover crops species on shikimic acid concentration in the seedlings, seedling dry matter, panicle m-2, spikelet panicle-1, spikelet fertility, 1000-grain weight (1000-grain W), and grain yield of upland rice (Oryza sativa L.) in a 2-yr experiment. Cover crops
Shikimic acid content
Seedling dry matter
Panicle m-2
Spikelets panicle-1
Spikelets fertility
1000-grain W
Grain yield
Weedy fallow Guinea grass Signal grass Palisadegrass Millet Clean fallow‡
µg g 2.7 a† 3.7 a 4.5 a 4.0 a 0.7 b 0.3 b
gm 4.6 b 3.1 c 3.2 c 3.4 c 5.9 a 6.9 a
number 96 ab 86 b 68 c 67 c 98 a 100 a
number 172 b 162 c 163 bc 173 b 201 a 190 a
% 72.6 b 75.0 a 72.3 b 73.9 b 77.6 a 78.2 a
g 25.7 a 23.7 c 23.4 c 23.7 c 24.8 b 24.7 b
kg ha-1 3069 b 2429 c 1855 d 1982 d 3648 a 3521 a
-1
-2
†
Means followed by the same letter vertically do not differ by the LSD test at P < 0.05.
‡
Control treatment, fallow with conventional tillage (one plowing and two disking).
Figure 2. Effect of cover crops termination timing by glyphosate application on seedling stand of upland rice (Oryza sativa L.) in a 2-yr field experiment. *, significant at P < 0.05; **, significant at P < 0.01; ns, not significant.
enough time to translocate the herbicide to the rice seedlings. On the other hand, the other cover crop species are less sensitive to glyphosate and take longer to completely dry (approximately 28 d). In this sense, we can infer that the herbicide remains longer in these cover crops. Thus, a more likely explanation for the higher levels of shikimic acid in the rice seedlings under the cover crops is that the herbicide is being picked up by the emerging seedlings from the foliage of the cover crop. Glyphosate is poorly transmitted through the roots of plants. However, the presence of shikimate in the rice seedlings does indicate that the plants are being exposed to glyphosate in some way. According to Koger et al. (2005) and Matallo et al. (2009), when glyphosate is applied to cover crops close to a cash crop sowing day, herbicide can be translocated to the nontarget plants. This translocation can be verified by the shikimic acid measurement.
Seedling Stand The seedling stands of upland rice were affected by the year and the interaction between the cover crop and the herbicide timing (Table 2). For guinea grass, signal grass, and palisade grass, there were linear increases of seedling stands 2664
when herbicide was applied further from the rice planting day (Fig. 2). On the other hand, there was no effect in the cover crops millet and weedy fallow. As for the years, a greater stand was observed in the 2009–2010 growing season (56 seedling m-2) than in 2008–2009 (50 seedling m-2) (Table 3). Since 2008–2009 had a greater amount of straw than 2009–2010, we could infer that the greater amount of perennial forages had a significant reduction on seedling emergence. According to Constantin et al. (2007), the effects of glyphosate in the nontarget plants could be increased owing to the greater amount of dry matter from the cover crops. Millet and weedy fallow had lower amounts of straw and did not cause a significant reduction in the plant stand.
Seedling Dry Matter The seedling dry matter of the upland rice was affected by the main effect of cover crops and the herbicide timing (Table 2). Millet allowed for the largest values, similar to the control (clean fallow) and differing from the weedy fallow, palisade grass, signal grass, and guinea grass (Table 4). As for the herbicide timing, there was a significant linear effect, with the highest values at 30 d and lowest at rice-sowing day (Fig. 3). Thus, soils with greater amounts
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crop science, vol. 53, november– december 2013
Figure 3. Effect of cover crops termination timing by glyphosate application on seedling dry matter of upland rice (Oryza sativa L.) in a 2-yr field experiment. **, significant at P < 0.01.
of biomass, in addition to presenting planting difficulties, such as physical impediments to the emergence of crops and shading in the initial period of crop development, can provide translocation of the herbicide to the nontarget plants and possibly reduce subsequent vegetative growth, which may have an adverse effect on crop yields (Constantin et al., 2007; Monquero et al., 2010; Nascente and Crusciol, 2012; Nascente et al., 2012). This effect could be seen in the seedling dry matter of the upland rice. Applying herbicide further from the seeding day (and allowing more time for glyphosate to kill the cover crop) was associated with higher amounts of seedling dry matter in upland rice (9.6 g m-2 in 30 d and 6.5 g m-2 0 d before upland rice planting) (Fig. 3). Thus, farmers should apply herbicide several days before planting. This method would result in cover crop degradation earlier and facilitate the seeding operation, reducing the amount of cover crop biomass and the nutrient sequestration by the cover crop and the possible allelopathic effects. There are reports about allelopathic effects of Brachiaria decumbens L. when its green plants are incorporated into the soil (Souza et al., 2006). Additionally, Rodrigues et al. (2012) stated that palisade grass had an allelopathic effect on the seed germination of Stylosanthes guianensis L. However, to the best of our knowledge there are no reports of any allelopathic effects of perennial grasses on upland rice under field conditions. According to Barbosa et al. (2008), Constantin et al. (2009), Monquero et al. (2010), and Nascente and Crusciol (2012), the benefits of cover crop chemical management before cash crop planting (10 d or more) are reduced competition for water in the early crop development, promotion of the decomposition of cover crop residues or weeds (which can provide nutrients for the crops and reduce the possibility of allelopathic effects), improved uniformity of planting (tractor and planting machines), crop science, vol. 53, november– december 2013
reduced possibility of herbicide translocation to nontarget plants, and increased productivity.
Upland Rice Yield Components Cover crop millet resulted in a higher number of panicles m-2, which differed from that of guinea grass, signal grass, and palisade grass but not from the weedy fallow and the control (clean fallow) (Table 4). Regarding the number of spikelets panicle -1, millet and the control differed from all other treatments. In the characteristic spikelet fertility, millet, the control, and guinea grass achieved better results than signal grass, palisade grass, and weedy fallow. In the yield component 1000-grain weight, fallow had the highest values of all other treatments, followed by millet, the control, guinea grass, signal grass, and palisade grass. The timing of herbicide application on the cover crop affected the number of panicles m-2 of upland rice, with reduction from 92 (herbicide application 30 d before upland rice planting day) to 75 (herbicide application on upland rice planting day) panicles m-2 (Fig. 4). The year affected both the number of panicles m-2 (2008: 74, and 2009: 92) and number of spikelets panicle -1 (2008: 165, and 2009: 183) (Table 3). This difference could be caused by the greater amount of cover crop biomass in the 2008–2009 growing season than in 2009–2010, which could have provided higher herbicide translocation and/or less nutrient or water availability to the rice plants. Thus, it is likely that these situations damaged rice plant development and these yield components (Table 4). In the 1000-grain weight yield component, the opposite was observed, with higher values in 2008–2009 (25.4 g) than in 2009–2010 (23.0 g). With an increased number of panicles (from 74 to 92) there is a tendency for reduced panicle size, resulting in lower 1000-grain weight (from 25.4 g to 23.0 g) (Fageria et al., 2011).
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Figure 4. Effect of cover crops termination timing by glyphosate application on number of panicle m-2 of upland rice (Oryza sativa L.) in a 2-yr field experiment. **, significant at P < 0.01. Table 5. Pearson correlation coefficients and F probability between the day of glyphosate application or cover crop dry matter on the soil surface at upland rice (Oryza sativa L.) sowing day and upland rice grain yield and its yields components. Variable
Grain yield
1000-grain weight
Spikelet fertility
Spikelet panicle-1
Panicles m-2
Day of glyphosate application
0.23 0.009
0.05 0.585
0.01 0.901
0.11 0.226
0.27 0.003
-0.26 0.004
-0.02 0.811
-0.56
