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RICE

Rice Seeding and Nitrogen Rate Effects on Yield and Yield Components of Two Rice Cultivars

^a Delta Res. and Ext. Center, Mississippi State Univ., P.O. Box 197, Stoneville, MS 38776
^b RiceTec, Inc., 822 Woodruff, Sikeston, MO 63801
^c Rice Research Station, Louisiana State Univ. AgCenter, 1373 Caffey Rd., Rayne, LA 70578. Joint contribution of the Louisiana and Mississippi Agric. Exp. Stn. Louisiana State Univ. AgCenter Publ. 07-61-0198. Mississippi Agric. Exp. Stn. Publ. J-11162. This research was funded in part by the Louisiana Rice Res. Board, the Mississippi Rice Promotion Board, and USDA-CSREES

^* Corresponding author (JBond{at}drec.msstate.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field research was conducted for 2 yr to investigate the relationship between rice (Oryza sativa L.) seeding rate and preflood nitrogen (N) rate utilizing long-grain rice cultivars planted into clay and silt loam soils. Rice cultivars included ‘Cheniere’ and ‘Wells’ seeded at 162, 323, and 646 seeds m^–2. Nitrogen was applied before flooding at 67, 134, and 202 kg ha^–1. No response to soil texture and no interaction between seeding rate and N rate were detected for the parameters examined. The lowest applied N rate had lower yield than the other two N rates. Rough rice yields were 7564 for 67 kg N ha^–1, 8520 for 134 kg N ha^–1, and 9000 for 202 kg N ha^–1 averaged over all cultivars, seeding rates, and soil textures. Similarly, when head rice yield was pooled across soil texture and seeding rate, head rice yield of Cheniere was independent of N rate, but head rice yield of Wells increased when the N rate was increased from 67 to 202 kg ha^–1. Panicle density responded to N rate similar to rough rice yield. Panicle density increased with seeding rate up to 418 panicles m^–2 at a seeding rate of 646 seeds m^–2. Filled grain panicle^–1 was highest at a seeding rate of 162 seeds m^–2. Cheniere produced more filled grain panicle^–1 while Wells had a higher 1000-grain weight. Grain yield and yield components of Cheniere and Wells respond to seeding rates and N rates independently when planted into clay or silt loam soils.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
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.

Received for publication March 26, 2007.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
RICE GROWN UNDER FIELD CONDITIONS does not function as a homogeneous population of plants (Wu et al., 1998). Rather, it is comprised of a population of tillers formed at different times and having specific growth characteristics. The rice plant's ability to tiller is an important characteristic because tillering impacts panicle production (Miller et al., 1991), which is highly correlated with grain yield (Counce and Wells, 1990; Miller et al., 1991; Gravois and Helms, 1992). Miller et al. (1991) reported that rice grain yield in a continuously flooded, water-seeded cultural system was dependent on final tiller density, with rice grain yield increasing as final tiller density increased up to 700 tillers m^–2.

Plant population density is a principal factor affecting tiller production (Counce et al., 1992; Schnier et al., 1990). The plant population density required to produce optimum rice grain yield is affected by cultivar and seeding rate (Gravois and Helms, 1992). Counce (1987) reported optimum plant populations for non semidwarf rice cultivars grown in a drill-seeded cultural system in Arkansas to be 130 to 172 plants m^–2. No significant rice grain yield losses occurred in that research with higher plant populations. Excessive plant population densities can lead to greater plant height and weaker culms, increasing the potential for losses due to lodging and disease (Dofing and Knight, 1994).

Nitrogen is one of the most yield-limiting nutrients in lowland rice production, and proper N management is essential for optimizing rice grain yields (Fageria et al., 1997). However, N fertilizer is one of the most expensive inputs for rice production and N deficiencies are widely reported in lowland rice soils (Fageria and Baligar, 1996; Kundu et al., 1996). Nitrogen recovery efficiency for lowland rice grown in the tropics is typically 30 to 50% of applied N (De Datta, 1986; Fageria and Baligar, 2001). Research in the southern United States examining the influence of application timings and N management strategies on N use efficiency reported N recovery of 17 to 61% of the applied N at rice maturity depending on N management strategy (Norman et al., 1989; Westcott et al., 1986).

Rice cultivars commonly grown in the USA require and respond to large amounts of N (Bollich et al., 1994; Norman et al., 2003; Wells and Johnston, 1970). Nitrogen use efficiency in a delayed-flood, drill-seeded cultural system is optimized when N is applied in a single preflood application under optimum conditions and intensive management (Bollich et al., 1994; Norman et al., 1999). This single preflood application is made to a dry soil when rice is at the four- to five-leaf stage and before flood establishment (Bollich et al., 1994; Bollich et al., 1999; Wilson et al., 2001). Furthermore, the N fertilizer rate that produced maximum grain yield also produced the highest head rice (whole milled rice) yield (Bond and Bollich, 2007; Jongkaewwattana et al., 1993). Nitrogen rates for optimum grain yield vary based on cultivar and soil texture (Bond et al., 2006; Norman et al., 2005; Walker, 2006). Rice grown on clay soils typically requires more fertilizer N, even though native soil N concentrations are greater on clay soils. Diffusion of the ammonium ion is up to 12 times less per day in clay compared with silt and/or sandy loam soils (Trostle et al., 1998).

Extensive work has been conducted in the southern USA rice-growing region examining the response of rice cultivars to seeding rates or N rates. However, few of these studies have examined the interaction between these two factors. One experiment examined a single semidwarf cultivar planted into a silt loam soil in Arkansas in 1972 and 1973 (Wells and Faw, 1978). Although a second report compared semidwarf and conventional-height cultivars, the research was conducted only on a clay soil in Arkansas from 1983 to 1985 (Counce et al., 1992). Cultivars examined in previous research have all been replaced with more vigorous, high-yielding cultivars. Furthermore, rice can respond differently to applied N fertilizer when grown on a clay soil compared with a silt loam soil (Chen et al., 1989; Westcott et al., 1986). Therefore, the objective of the current research was to characterize the relationship between rice seeding rate and N rate utilizing two long-grain rice cultivars currently in cultivation in the southern USA when planted into soil of different textures (clay and silt loam).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
An experiment was conducted on clay- and silt loam-textured soils in 2005 and 2006 in Louisiana, Mississippi, and Missouri to evaluate the response of two rice cultivars to different seeding rates and N fertilizer rates. Specific details of each location appear in Table 1 . Plots were drill-seeded from late March to early May, with dates corresponding to the optimum planting period for rice from southern Louisiana to southern Missouri. Rice was grown in an upland condition until the five-leaf growth stage at which time preflood N rates were broadcasted onto dry soil within 2 d before flood establishment. Standard agronomic and pest management practices were used during the growing season according to state recommendations (Linscombe et al., 1999; Miller and Street, 1999). At maturity, plots were drained approximately 2 wk before harvest. Rice was harvested with a small-plot combine at a moisture content of approximately 200 g kg^–1 using a 45- to 50-cm cutting height.

Cheniere rice and Wells rice were provided for each location from foundation seed grown in Louisiana and Mississippi, respectively. In each year for each location, the average 1000-grain weight was determined from five random samples of each cultivar to determine grains kg^–1 for each cultivar. Grain of each cultivar for individual plots was then packaged based on weight. Grain of each cultivar was treated with giberellic acid at 150 ppm giberellic acid to improve seedling emergence and mancozeb (ethylene bisdithiocarbamate) at 130 ppm to protect seedlings against pathogens. All experiments were drill seeded using grain drills equipped with double-disk openers and press wheels with 18 cm between each row. Individual plots consisted of 12 rows measuring 7.6 m in length at Louisiana sites, seven rows measuring 5.2 m in length at Mississippi sites, and nine rows measuring 3.7 m in length at Missouri sites.

When rice grain reached approximately 200 g kg^–1 moisture content for each cultivar and treatment, a randomly selected area of 1 m² from each plot was hand-harvested to determine yield components (panicle density, filled grain panicle^–1, and 1000-grain weight) and head rice yield. The remaining area in each plot was harvested with a small-plot combine to determine rough rice yield after all yield component and head rice yield subsamples had been collected. Rough rice yield was adjusted to 120 g kg^–1 moisture content. The total number of panicles in each hand-harvested sample was counted to determine panicle number m^–2 (panicle density). Ten rice panicles were then randomly selected from each hand-harvested sample, threshed, and the number of filled grains was counted to determine the average number of filled grain panicle^–1 for each treatment. The remaining portion of the hand-harvested samples was then threshed with a plot thresher, combined with grain from the 10-panicle subsamples, and dried to approximately 120 g kg^–1 moisture content. Five 1000-grain subsamples were then weighed to determine 1000-grain weight. Head rice yield was estimated from 125-g samples of cleaned rough rice using the procedure outlined by Adair et al. (1972). Rough rice was mechanically hulled, milled in a McGill no. 2 miller for 30 s, and size separated with a no. 12 (4.76-mm) screen. Head rice yield was calculated as a mass fraction of the original 125-g sample of rough rice.

All data were subjected to analysis of variance (SAS Institute, 2003) with site year being used as a random-effect parameter testing all possible interactions of soil texture, cultivar, seeding rate, and N rate. Site years, replications (nested within site years), and all possible interactions containing these effects were considered random effects; all other factors (soil texture, cultivar, seeding rate, and N rate) were considered fixed effects. Considering site year an environmental or random effect permits inferences about treatments to be made over a range of environments (Carmer et al., 1989). Least square means were calculated and mean separation (P 0.05) was produced using PDMIX800 in SAS, which is a macro for converting mean separation output to letter groupings (Saxton, 1998).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Rough Rice Yield
The main effects of rice seeding rate and N rate were significant for rough rice yield (Table 2 ). The main effects of cultivar, soil texture, and all interactions containing either of these main effects were not significant at P = 0.05. Rough rice yield increased with seeding rate up to 8478 kg ha^–1 at 323 seeds m^–2, but no further yield increase was realized from increasing seeding rate to 646 seeds m^–2 (Table 3 ).

View this table: [in this window] [in a new window]   Table 3. Effect of seeding rate on rough rice yield, panicle density, and filled grain panicle–1 in Louisiana, Mississippi, and Missouri in 2005 and 2006.

View this table: [in this window] [in a new window]   Table 4. Effect of N rate on rough rice yield and panicle density in Louisiana, Mississippi, and Missouri in 2005 and 2006.

Results of the current research apparently contrast those of earlier research, which demonstrated that more N was required for rice grown on clay soils compared with silt loam soils. Westcott et al. (1986) reported higher grain yield at St. Joseph, LA (Sharkey clay soil) compared with the Crowley, LA location (Crowley silt loam soil). However, Chen et al. (1989) reported that rice grown in 1982 at Beaumont, TX (Beaumont clay) responded more to N than rice grown at Eagle Lake, TX (Nada sandy loam), but responses were similar for the two locations in other years. The main effect of soil texture and all interactions containing soil texture were not significant in the current research. The research of Chen et al. (1989) demonstrates that the rice response to N fertilizer on different soil textures can vary depending on year. Furthermore, cultivars in the current research may be more adapted to a broad range of environments and soil textures than previous cultivars, which may explain, at least in part, the conflicting results of our research from what had previously been reported.

Head Rice Yield
The main effects of soil texture and seeding rate and all interactions containing either of these factors were not significant for head rice yield (Table 2). The cultivar by N rate interaction was significant. Pooled across soil texture and seeding rate, head rice yield of Cheniere was independent of N rate, but head rice yield of Wells increased when the N rate was increased from 67 to 202 kg ha^–1 (Table 5 ). Head rice yield of Cheniere was 22 and 15% greater than Wells with 67 and 134 kg N ha^–1, respectively. Jongkaewwattana et al. (1993) reported that head rice yield was highest following the N rate producing maximum grain yield. Rough rice yield in the current study did not increase with N rates greater than 134 kg ha^–1 (Table 4) regardless of cultivar. Head rice yield of Cheniere and Wells did not increase with N rates higher than 67 and 134 kg ha^–1, respectively (Table 5).

View this table: [in this window] [in a new window]   Table 5. Effect of cultivar and N rate on head rice yield in Louisiana, Mississippi, and Missouri in 2005 and 2006.

No response to seeding rate was observed for head rice yield in the current study (Table 2). Gravois and Helms (1996) reported that ‘Millie’ and ‘Adair’ head rice yields increased with increasing seeding rate, but head rice yield decreased with seeding rate for ‘Katy’ and ‘Kaybonnet’. They concluded that head rice response to seeding rate should be assessed on a cultivar basis. Data from the current research indicate that Cheniere and Wells head rice yields are independent of seeding rate.

Yield Components
Panicle density and filled grain panicle^–1 were influenced by the main effect of rice seeding rate (Table 2). Panicle density increased with seeding rate up to 418 panicles m^–2 at a seeding rate of 646 seeds m^–2 (Table 3). Filled grain panicle^–1 was highest at a seeding rate of 162 seeds m^–2. Yield compensation between panicle density and filled grain panicle^–1 may explain the effect of seeding rate on filled grain panicle^–1. Other researchers have reported similar trends in panicle density and filled grain panicle^–1 (Gravois and Helms, 1992; Jones and Snyder, 1987; Wells and Faw, 1978). Panicle density had the highest effect on rice grain yield, even at low seeding rates where filled grain panicle^–1 increased to compensate for decreased panicle density (Gravois and Helms, 1992). The relationship between panicle density and filled grain panicle^–1 may explain the rough rice yield response to seeding rate (Table 3). Although panicle density increased up to the highest seeding rate, rough rice yield and filled grain panicle^–1 did not increase at seeding rates higher than 323 and 162 seeds m^–2, respectively.

Pooled across soil texture, cultivar, and seeding rate, panicle density responded to N rate similar to rough rice yield (Table 4). The main effect of cultivar was significant for filled grain panicle^–1 and grain weight (Table 2). The semidwarf Cheniere produced more filled grain panicle^–1 while the conventional-height Wells exhibited a higher grain weight (Table 6 ). The trend observed for panicle density indicates that it is the yield component most closely associated with rough rice yield. Of the three yield components important for determining rough rice yield, panicle density, as a function of plant density or tiller development, is determined first physiologically.

View this table: [in this window] [in a new window]   Table 6. Effect of cultivar on filled grain panicle–1 and grain weight in Louisiana, Mississippi, and Missouri in 2005 and 2006.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

The differences in agronomic characteristics and yield potential between cultivars in the current research and those used in earlier research are the most likely explanation for the different seeding rate and N rate responses between this study and those conducted previously. However, similar to the findings of Wells and Faw (1978), yield was related to panicle density, which was maximized in the current research at the 134 kg ha^–1 rate of N. Relative to seeding rate, panicle density in the current research increased with seeding rate up to 646 seeds m^–2; however, grain yield did not increase when seeding rate was increased from 323 to 646 seeds m^–2. The grain yield plateau could be attributed to the fact that filled grain panicle^–1 was greatest at 323 seeds m^–2. Although the magnitude of panicle density in the current research was not as great as that reported by Wells and Faw (1978) (771 to 1214 plants m^–2 at a seeding rate of 303 seeds m^–2), filled grain panicle^–1 at a seeding rate of 323 seeds m^–2 was at least 2.5 times greater than that reported by Wells and Faw (1978) (27 to 43 florets panicle^–2 at a seeding rate of 303 seeds m^–2) at a similar a seeding rate. This indicates that panicle size has increased since the research of Wells and Faw (1978) due possibly to greater N use efficiency as a function of improved genetics.

Cultivars tested in the current research are semidwarf or stiff-strawed, conventional-height cultivars with high yield potential. These newer cultivars respond differently to inputs than older cultivars. Grain yield and yield components of Cheniere and Wells responded to seeding rates and N rates independently when planted into clay or silt loam soils, and producers should employ seeding and N rates that maximize yield potential.

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	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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