Influence of Seeding Rate on Weed Density in Soybean Planting System for Southeastern Coastal Plains

Problem statement: Increasing seeding rates may help decrease weed pressure in soybean [Glycine max (L.) Merr.] wide row spacing. Approach: The objective of this study was to evaluate the influence of five glyphosate-resistant soybean Maturity Groups (MG) (IV, V, VI, VII and VIII) and six seeding rates (68,000,136,000, 204,000, 272,000, 340,000 and 408,000 seeds ha) on weed density under dryland conditions on the Southeastern coastal plain in 2007-2009. Results: Weed decrease with increasing seeding rate varied over years. Weed density was generally lower at higher seeding rates for most MG soybeans at 30 and 60 DAP, except MG IV and VIII at 30 DAP in 2007 and MG VI at 30 DAP in 2008. At 60 DAP, soybean Leaf Area Index (LAI) and normalized difference vegetation index (NDVI) were greater with lower weed density. Conclusion: Additionally, negative correlations were observed between weed density and plant LAI/NDVI for all MG in 2008 and MG IV through VI in 2009. These results suggest that increased seeding rates may help decrease weed pressure and improve soybean growth at early growth stages. However the response of weed pressure to seeding rate may vary over years and depend on MG soybean.


INTRODUCTION
The integrated weed management system must encompass, among other areas, the enhancement of crop competitiveness and modeling of crop-weed interference (Swanton and Weise, 1991). Buhler (1999) stated that crop yields are less sensitive to weed control practices when initial weed densities are low. However, weather affects weed populations and crop yield responses to the weed control treatments over years (Buhler, 1999). Measurements of photosynthetically active radiation indicated increased light penetration to the soil surface with increasing row spacing and therefore greater weed pressure (Yelverton and Coble, 1991). Arce et al. (2009) reported less weed biomass at higher seeding rate. Lowering seeding rates below the optimum level reduces soybean competitiveness with weeds, thereby, reducing seed yield (Rich and Renner, 2007). Soybean leaf area and total biomass are reduced by higher weed density, because weeds compete with crops for moisture, nutrients and light (Legere and Schreiber, 1989). The objective of this study was to evaluate the effect of soybean MG and seeding rate on weed pressure and soybean growth.
Plant stand was recorded 2 wks after soybean planting (Table 2). Weed density was visually estimated on a scale from 0-no weed cover to 100% -full weed cover at 30 and 60 DAP prior to glyphosate application. The most prevalent weeds observed in the research plots were palmer amaranth (Amaranthus palmeri) and yellow nutsedge (Cyperus esculentus L.). Leaf area index (LAI) and normalized difference vegetation index (NDVI) were measured on the 3.0 m long two center rows using LAI-2000 (Li-Cor, Lincoln, NE) and GreenSeeker TM (NTech Industries, Inc. Ukiah, CA) at 60 and 90 DAP, respectively. Analysis Of Variance (ANOVA) was separated for each MG using PROC MIXED procedure in SAS (SAS V. 9.2, SAS Institute, Cary, NC). Seeding rate and year were treated as fixed effects and replicate as a random effect. Treatment effects were considered significant at p≤0.05. The PDIFF option in the LSMEANS statement of PROC MIXED procedure was used to compare seeding rate effect for each MG soybeans. PROC NLIN procedure in SAS evaluated relationships between weed density and seeding rates. Correlations between weed density and plant LAI/NDVI at 60 DAP were analyzed using PROC CORR procedure in SAS.

RESULTS
Except for MG IV, the seeding rate by year interactions were significant for weed density at 30 and 60 DAP for MG V-VIII soybeans (Table 3). In general, weed density was greater at lower seeding rates for most MG soybeans at 30 and 60 DAP each year, with the exception of MG IV and VIII at 30 DAP in 2007 and MG VI at 30 DAP in 2008. Figure 1 presents the effect of seeding rate on weed density (% coverage) at 30 and 60 DAP for each MG by year. The variations of weed coverage with seeding rate at 30 DAP and 60 DAP followed a similar trend for each MG: decreasing with seeding rate by linear functions. For the same MG, the intercept and the magnitude of weed density reduction with seeding rate varied with years. However, the decline of weed density with seeding rate was similar at 30 DAP compared to 60 DAP in the same year for almost all MGs except MG IV in 2009 andMG VIII in 2007. (i) (j) Fig. 1: Relationships between weed density (% coverage) and seeding rate for Maturity Group (MG) IV-VIII at 30 and 60 Days After Planting (DAP) in 2007, 2008 and 2009. Each data point represents the mean of four replicates and was regressed against seeding rate, where x is seeding rate (seeds ha −1 ) and y is percent of weed coverage, †, *, **, ***: significant at p ≤ 0.10, 0.05, 0.01 and 0.001, respectively, NS: Not Significant at p≤0.10  Year *** *** SR *** *** Year*SR *** *** VIII Year *** *** SR *** *** Year*SR NS ** ***, **, * indicate significance at P ≤ 0.001, 0.01 and 0.05, respectively, † NS, not significant at P ≤ 0.05   Table 4 presents Pearson correlation coefficients between weed density and LAI/NDVI at 60 DAP. Negative correlations were observed between weed density and LAI/NDVI for all MGs in 2008 andMG IV, V andVI in 2009. There was also a negative correlation between weed density and LAI for MG V and VII in 2007.

DISCUSSION
Decreasing weed pressure with greater seeding rates was similar to observations in organic management of soybeans (Place et al., 2009). Arce et al. (2009) also reported that weed biomass at soybean harvest was inversely related to soybean population.
Soybean canopy usually develops more rapidly at higher seeding rate, which results in lower weed density (Harder et al. (2007).
Negative correlations between weed density and plant LAI/NDVI was expected, because weeds competed with soybeans for resources such water, nutrients and sunlight. Higher weed density would capture more resources and reduce the availability of resources for soybeans, which potentially reduces soybean growth. Though no direct report is available in literature on relationships between weed density and LAI/NDVI, greater soybean LAI and lower weed density at higher seeding rates were previously reported (Rich and Renner, 2007). Legere and Schreiber (1989) also indicated that soybean produced higher LAI accumulation rate over time from weed free stands compared to soybeans in weedy stands.

CONCLUSION
Weed density at 30 and 60 DAP was inversely related to seeding rate for most MG soybeans except MG IV at 30 DAP in 2007. Weed density was generally lower at higher seeding rates for all MG soybeans at 30 and 60 DAP, except MG IV and VIII at 30 DAP in 2007 and MG VI at 30 DAP in 2008. However, weed density reduction with increasing seeding rate varied over years due to mostly different precipitation each growing season. Negative correlations were also observed between weed density and plant LAI/NDVI for all MGs in 2008 and MG IV through VI in 2009. This study indicates that the effect of increasing seeding rate on weed pressure may vary over years and MG soybeans.