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CROP ECOLOGY, PRODUCTION & MANAGEMENT

Soybean Light Interception and Yield Response to Row Spacing and Biomass Removal

Dep. of Plant Science, Cook College, Rutgers Univ., 59 Dudley Road, New Brunswick, NJ 08901-8520

Corresponding author (singer{at}aesop.rutgers.edu)

	ABSTRACT

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

 
Biomass removal effects on soybean [Glycine max (L.) Merr.] light interception and yield under different row spacings have not been documented. Objectives for this study were to evaluate biomass removal, light interception, seed yield, and yield components of soybean under seven clipping treatments. Treatments in this study were clipping at V5, R1, R4, V5 + R1, V5 + R4, R1 + R4, and V5 + R1 + R4, and a control in 18- or 20-cm and 76-cm rows. In 1998, a dry year, biomass removal exhibited a negative linear relationship with seed yield in narrow rows (r² = 0.62). A row spacing–clipping interaction was observed. In wide rows, clipping at V5 or R1 produced a compensatory response by increasing yield 12 and 5%, respectively. Generally, soybean in 76-cm rows maintained seed yield by adjusting the number of pods or seed weight, except when clipping occurred at V5 + R4 or at three growth stages. In 1999, under severe moisture stress, row spacing influenced seed yield. Averaged across clipping treatments, narrow rows yielded 16% more (301 vs. 254 g m^-2) than wide rows. Generally, soybean yield declined as clipping timing was delayed and as clipping frequency increased in narrow rows, except when clipping occurred at V5 + R1. Clipping at V5 and V5 + R1 did not affect yield in wide rows. All other clipping treatments suffered yield reductions ranging from 20 to 58%. A quadratic relationship (R² = 0.83) was observed between biomass removal and seed yield.

Abbreviations: DAP, days after planting • HI, harvest index • LAI, leaf area index • PAR, photosynthetically active radiation

	INTRODUCTION

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

 
SOYBEAN PRODUCERS suffer annual yield losses because of deer (Odocoileus virginianus) browsing. A report by Rutgers University Center for Wildlife Damage Control (unpublished, 1998) summarizing a survey of New Jersey farmers concluded that deer were responsible for 70% of wildlife damage to crops and that 34% of cultivated acreage in the state was affected. Deer browsing is reported to produce the greatest crop loss nationwide from wildlife and has steadily increased during the last 30 yr (Conover and Decker, 1991). Although deer damage many crops, apparently, they demonstrate a preference for soybean (Waer et al., 1992). Determining the extent of damage has relied upon visual estimation. Flyger and Thoerig (1962) found that soybean producers either exaggerated their losses or underestimated the degree of damage.

A North Carolina study (DeCalesta and Schendeman, 1978) concluded that the most severe deer damage to soybean occurred during the first week after germination and that damage was greater in field borders than field interiors. Damage inflicted by simulating deer depredation during the first week after germination reduced yield 80% per plant (DeCalesta and Schendeman, 1978). Garrison and Lewis (1987) simulated deer damage by varying the extent of defoliation of fully developed leaf nodes and found that a single 100% defoliation before V5 (Fehr and Caviness, 1977) significantly lowered yield, but no differences were observed when defoliation also included terminal bud removal. DeCalesta and Schendeman (1978) studied deer damage in natural settings and concluded that only leaves were consumed; flowers and pods were untouched. More recently, soybean producers have observed damage over entire fields and at all growth stages (Leidner, 1994; Wallace, 1995). Francoeur (1995) evaluated severe simulated deer damage in 1-m rows during vegetative and reproductive growth and concluded that damage imposed at V10 lowered yield in 1 of 2 yr but damage at R4 decreased yield in both years.

Previous research has shown that reductions in light interception of defoliated treatments related directly or indirectly to yield reductions (Higley, 1992; Hunt et al., 1994; Haile et al., 1998). Hunt et al. (1994) concluded that removal of the leaf area delayed the time to achieve a critical leaf area index (LAI) of 3.5, thus limiting light interception and dry matter accumulation. Haile et al. (1998) imposed defoliation treatments at R2 and concluded that yields were directly related to the light interception capacity of soybean canopies after defoliation. Many of the recent defoliation studies have focused on insect defoliation using either single-day or sequential defoliation approaches. Moreover, most defoliation studies have evaluated soybean response to defoliation in row spacings greater than 68 cm. The objective of our study was to study the effect of severe biomass removal during vegetative and reproductive growth in narrow and wide rows under dryland conditions on soybean light interception, yield, and yield components.

	MATERIALS AND METHODS

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

 
A 2-yr field study was initiated in 1998 on a Quakertown silt loam (fine-loamy, mixed, mesic Typic Hapludult) at the Snyder Research and Extension Farm near Pittstown, NJ (40°30' N, 75°00' W). Indeterminate soybean ‘Golden Harvest H-1357RR’ was planted on 4 June 1998 and 27 May 1999, at 494 000 seeds ha^-1. A John Deere 8300¹ grain drill was used in 1998 and an ALMACO (Nevada, IA) small-plot grain drill was used in 1999 for narrow rows (18 and 20 cm in 1998 and 1999, respectively), while a John Deere 7200 was used for wide rows (76 cm) in both years. Fertilizer was applied according to soil test recommendations at a rate of 0-58-58 kg ha^-1 (N-P-K). Soybean was evaluated each year following a rye cover crop (Secale cereale L.). Preemergence herbicides were used for weed control in both years. Plot size was 6.1 m long by 2.0 m wide in narrow rows and 6.1 m long by 3.0 m wide in wide rows.

The experimental design was a randomized complete block in a split-plot arrangement with three replications in 1998 and four in 1999. The main plot was narrow versus wide row spacing. Sub-plot treatments were a control, clipping at V5, R1, R4, and all combinations for a total of seven clipping treatments. Clipping was accomplished using hedge clippers to remove topgrowth by measuring the height of each treatment and removing approximately 30% of the average height. Growth stages were determined using the control plot as a reference. All removed biomass was collected and dried in a forced-air oven at 60°C to a constant moisture and weighed. Biomass removal was converted to an area basis.

Light interception was measured at 47, 54, 63, 69, 76, and 85 d after planting (DAP) in 1998 and 49, 57, 60, 63, 67, 71, 77, 81, 83, 90, and 96 DAP in 1999 by taking the average of six parallel (Board and Harville, 1992) measurements per plot, three along the base of the row and three in the middle of the row. All measurements were made between 1200 and 1400 h in full-sun conditions. A 1-m long Delta-T Devices Ltd Sunscan Probe (Cambridge, England) was used to measure light interception of photosynthetically active radiation (PAR) below the canopy and a beam fraction sensor on a tripod was used to simultaneously measure incident light above the canopy. Light interception was calculated as the difference between incident and transmitted light divided by incident light.

Ten plants per plot were randomly sampled at harvest to determine pod number, seed per pod, seed weight, sample seed yield, total dry matter, branches per plant, and harvest index (HI) (sample seed weight/total dry matter). Pod number was converted to an area basis based on harvest stand counts. Yield, adjusted to 130 g kg^-1, was measured by combining the harvest of two interior 76-cm rows and eight interior 18-cm and 20-cm rows in 1998 and 1999, respectively, that had been end-trimmed to 4.6 m. Combined seed yield was converted to an area basis. Precipitation was recorded hourly at the experimental site. Because limited long-term weather data are available at the research farm, the closest station (11 km) with long-term data was used for normal precipitation.

All data were analyzed by analysis of variance (ANOVA) procedures with the SAS Statistical Software Package (1991). The Bartlett test on the full model indicated that error terms for most data sets were not homogeneous, so a separate analysis is presented for each year. Regression was used to determine the relationship between seed yield and biomass removal. Because the same seeding rate was used for both row spacings, ANCOVA was conducted using stand density as a covariant for all data sets. Only pods m^-2 in 1999 was significant so ANOVA output is presented for all statistical calculations minus pods m^-2 in 1999 where adjusted means are presented. Mean separation for main effects and interactions were obtained by Fischer's least significant difference (LSD), as described by Little and Hills (1978). Effects were considered significant in all statistical calculations if P-values were 0.05.

	RESULTS AND DISCUSSION

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

 
1998
The 1998 growing season can be characterized as dry. Below-normal precipitation was recorded from the period June through September (Table 1). Only 8 mm was received during the period 13 to 25 DAP, delaying emergence, while only 18 mm was recorded from 27 to 57 DAP. Timely rains occurred during reproductive growth.

View larger version (36K): [in this window] [in a new window]   Fig. 1. Light interception of soybean in narrow and wide rows for a control and clipping at V5, R1, R4, and all combinations in 1998 near Pittstown, NJ. Vertical bars indicate LSD (0.05)

Although clipping at R4, V5 + R1, and R1 + R4 did not affect branch number in narrow rows (Table 2), clipping at all three growth stages decreased branch number (4.63, 4.63, and 4.57 vs. 3.67 branches plant^-1). Branch number was affected more in wide rows. Clipping at V5 or any combination of clipping at two growth stages significantly lowered branch number plant^-1, compared with clipping solely at R4. Harvest index was greatly influenced by row spacing and clipping treatment. In narrow rows, clipping at R4, or at any combination including R4, had significantly lower HI's compared with treatments not clipped at R4 or the control, which is not consistent with Board and Harville (1998), who found no difference in HI under partial defoliation from R1 to R5 in 50-cm rows. Defoliation intensity and single versus sequential defoliation may explain this inconsistency. In wide rows, HI was maintained except when clipping occurred at all three growth stages. A significant row spacing x clipping interaction was observed: In wide rows, clipping at V5 + R4 and R1 + R4 had HI's of 0.59 and 0.58, respectively (compared with the control-maintained HI of 0.59); in narrow rows, clipping at V5 + R4 and R1 + R4 had HI's of 0.53 and 0.47, respectively (compared with the control-maintained HI of 0.58).

Soybean in wide rows was able to maintain or increase seed yield when clipped, excluding clipping at V5 + R4 and at all three growth stages, which is similar to Francoeur (1995). Clipping at V5 and R1 increased yield 12 and 5%, while clipping at V5 + R1 only lowered yield 2% compared with the control. Seed yield was similar in the control compared with clipping at R4 or R1 + R4. It is unclear why clipping at V5 + R4 responded differently.

Pod numbers were significantly lower when clipping occurred at either R1 + R4 or V5 + R1 + R4, compared with the control in narrow rows. Pod number increased sharply when soybean was clipped at V5 + R1. The increase in pod number offset a decline in seed weight. Pod number differences were not apparent in wide rows, except comparing clipping at V5 and R1 + R4. A row spacing–clipping interaction was observed because of the inability of narrow rows to maintain pod number when clipping occurred at R1 + R4 or V5 + R1 + R4. No differences were detected in the number of seed pod^-1 in wide rows.

Seed number differences among clipping treatments occurred for soybean grown in narrow rows. Clipping reduced seed number compared with the control, except when clipping occurred at either R1 or V5 + R1. Seed numbers were lowest when clipping occurred at V5 + R4, R1 + R4, and V5 + R1 + R4. Likewise, seed weight was reduced in these treatments. Apparently, seed weight and number are affected before pod number when clipping frequency increases during reproductive growth in narrow rows. A negative linear relationship was detected between biomass removal and seed yield in 1998 in both row spacings (Fig. 2) . Nevertheless, narrow rows were more variable, particularly when biomass removal ranged between 50 and 110 g m^-2. Soybean in wide rows was able to compensate for biomass removal by adjusting yield components to adapt to the timing and frequency of defoliation.

View larger version (23K): [in this window] [in a new window]   Fig. 2. Relationship between biomass removal and seed yield in narrow and wide rows in 1998 and 1999 for soybean planted near Pittstown, NJ

Light interception behaved differently in 1999 compared with 1998 (Fig. 3) . No differences were detected in either row spacing at 48 DAP. At 57 DAP in narrow rows, treatments clipped at R1 intercepted less light than all other treatments. Reduced light interception was observed in these treatments until 67 DAP, when no difference was detected among all treatments. Light interception declined in most treatments from 57 to 67 DAP because of severe water stress that caused leaf wilting. Clipping at R4 reduced light interception on average about 20% compared with treatments not clipped at this growth stage. Treatments clipped at R4 did not recover leaf area and averaged about 15% less PAR interception than treatments not clipped at R4.

View larger version (39K): [in this window] [in a new window]   Fig. 3. Light interception of soybean in narrow and wide rows for a control and clipping at V5, R1, R4, and all combinations in 1999 near Pittstown, NJ. Vertical bars indicate LSD (0.05)

At the last light interception measurement at 97 DAP, treatments in narrow rows not clipped at R4 intercepted 95% of PAR compared with 78% in wide rows, while treatments in narrow rows clipped at R4 intercepted 82% of PAR compared with 55% in wide rows. Furthermore, light interception was declining rapidly among treatments in wide rows, while only two of four treatments clipped at R4 in narrow rows had decreased light interception.

Light interception was not related to branch number plant^-1 in narrow rows (Table 3). Row spacing affected branch number (3.66 vs. 4.51 in narrow and wide rows, respectively). Differences were detected in wide rows when clipping occurred at V5 compared with either R4 or R1 + R4 (5.38 vs. 3.98 and 3.60 branches plant^-1) and probably contributed to decreased light interception in those treatments.

Unlike in 1998, soybean in wide rows in 1999 was unable to maintain HI when clipping occurred at the R4 growth stage. Clipping at V5, R1, and V5 + R1, although not statistically significant, increased HI compared with the control. Seed yield was 29 and 26% lower for the control in narrow and wide rows in 1999 compared with 1998.

Similar to 1998, clipping at V5 was not different than the control in narrow rows. Unlike 1998, no difference was detected clipping at V5 + R1 in 1999 compared with the control. Clipping combinations that included R1 other than V5 + R1 reduced yield. Combinations of clipping including R4 reduced yield, compared with single clipping not including R4. Clipping at V5 + R4 resulted in the lowest yield.

Differences in seed yield in wide rows were more pronounced in 1999 because of soybean's inability to maintain yield as clipping timing was delayed and as clipping frequency increased. A row spacing–clipping interaction was not observed. Clipping at V5 and R1 did not reduce seed yield compared with the control, and only produced a 4% compensatory increase when clipping occurred at V5. Clipping at V5 + R1 reduced yield 23%, while clipping at R4, V5 + R4, R1 + R4, and V5 + R1 + R4 reduced yield 54, 57, 50, and 54%, respectively. Fehr et al. (1977) observed a 38% yield reduction, averaged across cultivars and years, at R4 on indeterminate soybean using a 100% half-plant cut-off technique, which is similar to the 34% reduction we observed, when averaged across years. Soybean could not overcome severe water stress during vegetative and reproductive growth (until R5), which decreased light interception.

Yield loss was attributable to decreased pod numbers in both row spacings, although no difference was observed between row spacings, which is not consistent with the results of Bullock et al. (1998), who observed linear decreases in pod number as row width increased. Although pod number was greater in narrow rows at V5 compared with the control, this did not translate into greater seed yield. Clipping at R4 had the greatest impact on pod number because of the removal of pods. In 1999, plants were shorter (data not shown) than 1998, which accounted for greater pod removal when removing the top 30% of the plant. Clipping at R1 + R4 did not affect pod number, presumably because of increased pod set at lower nodes. Clipping at R4, V5 + R4, and at all three growth stages produced the lowest pod numbers because of greater pod set at upper nodes.

Pod number in wide rows behaved somewhat differently. A positive response to clipping was observed at R1 and V5 + R1. Only V5 + R4 had significantly lower pod numbers compared with the control. Clipping at R1 + R4 and at all three growth stages did not negatively impact pod number. But unlike 1998, in 1999 soybean was not able to maintain seed weight when clipping occurred at R4 or as clipping frequency increased, regardless of row spacing, except for clipping solely at R4 in wide rows. An increase in seed weight at V5 in wide rows offset a decrease in seed number, but no differences were observed for seed number pod^-1.

The relationship between biomass removal and seed yield was more tenuous in narrow rows in 1999 (Fig. 2). Greater variability between 100 and 140 g m^-2 biomass removal weakened the relationship compared with 1998; nevertheless, biomass removal did a better job of predicting seed yield when >140 g m^-2 biomass was removed. Wide rows demonstrated a quadratic relationship between biomass removal and seed yield. Removing >70 g m^-2 of soybean biomass had little influence on seed yield. Apparently, biomass removal was not limiting soybean yield beyond this threshold.

	CONCLUSIONS

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

 
Soybean exhibited a varied response to biomass removal. Under dry conditions in narrow rows, a negative linear relationship was observed. Soybean yield progressively declined as biomass removal was delayed to latter growth stages and as clipping frequency increased. In wide rows, a compensatory response was observed when biomass removal occurred during vegetative or early reproductive growth. Soybean in wide rows was generally able to maintain yield, except when clipping occurred at V5 + R4 or at all three growth stages, by adjusting pod number or seed weight. Our results suggest that factors other than light interception at full pod affect yield responses to biomass removal.

Under severe drought during vegetative and reproductive growth (until R5), soybean compensated for biomass removal at V5 and V5 + R1, but not R1 in narrow rows. Apparently, biomass removal in soybean can produce compensatory responses under extreme environmental conditions. Furthermore, soybean clipped at all three growth stages yielded 7, 26, and 11% greater than soybean clipped at R4, V5 + R4, and R1 + R4. Biomass removal in wide rows under extreme drought affected soybean differently than under the dry conditions of 1998. Again, no difference was detected when clipping at V5 or R1, but the compensatory response observed in 1998 at R1 was eliminated. Pod number or seed weight did not buffer seed yield. Consequently, any treatment including biomass removal at R4 significantly reduced soybean yield. Reductions in light interception were most likely responsible for the inability of soybean to maintain seed yield in wide rows as the timing of clipping was delayed and as clipping frequency increased.

	NOTES

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

 
Research supported by Rutgers Univ. Center for Wildlife Damage Control and the New Jersey Agric. Exp. Stn.

¹ Mention of a trademark, proprietary product, or vendor does not constitute a guarantee of warranty for the product, and does not imply its approval to the exclusion of other products or vendors that may be suitable.

Received for publication March 28, 2000.

	REFERENCES

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

 

• Board, J.E., and B.G. Harville. 1992. Explanations for greater light interception in narrow- vs. wide-row soybean. Crop Sci. 32:198–202.[Abstract/Free Full Text]
• Board, J.E., and B.G. Harville. 1998. Late-planted soybean yield response to reproductive source/sink stress. Crop Sci. 38:763–771.[Abstract/Free Full Text]
• Bullock, D., S. Khan, and A. Rayburn. 1998. Soybean yield response to narrow rows is largely due to enhanced early growth. Crop Sci. 38:1011–1016.[Abstract/Free Full Text]
• Conover, M.R., and D.J. Decker. 1991. Wildlife damage to crops: Perceptions of agricultural and wildlife professionals in 1957 and 1987. Wildlife Soc. Bull. 19:46–52.
• DeCalesta, D.S., and D.B. Schendeman. 1978. Characterization of deer damage to soybean plants. Wildlife Soc. Bull. 19:46–52.
• Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean development. Iowa Agric. Exp. Stn. Spec. Rep. 80. Ames, IA.
• Fehr, W.R., C.E. Caviness, and J.J. Vorst. 1977. Response of indeterminate and determinate soybean cultivars to defoliation and half-plant cut-off. Crop Sci. 17:913–917.[Abstract/Free Full Text]
• Flyger, V., and T. Thoerig. 1962. Crop damage caused by Maryland deer. Proc. Ann. Conf. Southeast. Assoc. Game Fish Comm. 16:45–52.
• Francoeur, L.C. 1995. Evaluating the effects of simulated deer damage on soybean growth and yield. M.S. thesis, Clemson University, Clemson, SC.
• Garrison, R.L., and J.C. Lewis. 1987. Effects of browsing by white-tailed deer on yields of soybean. Wildlife Soc. Bull. 15:555–559.
• Haile, F.J., L.G. Higley, and J.E. Specht. 1998. Soybean cultivars and insect defoliation: Yield loss and economic injury levels. Agron. J. 90:344–352.[Abstract/Free Full Text]
• Higley, L.G. 1992. New understanding of soybean defoliation and their implication for pest management. p. 56–65. In L.G. Copping et al. (ed.) Pest management in soybean. Elsevier Appl. Sci. Publ., London.
• Hunt, T.E., L.G. Higley, and J.F. Witkowski. 1994. Soybean growth and yield after simulated bean leaf beetle injury to seedlings. Agron. J. 86:140–146.[Abstract/Free Full Text]
• Leidner, J. 1994. Better yields, healthier deer. Prog. Farmer. 109 (9):100–102.
• Little, T.M., and F.J. Hills. 1978. Agricultural experimentation: Design and analysis. John Wiley and Sons, New York.
• SAS Institute. 1991. SAS user's guide. Statistics. SAS Inst., Cary, NC.
• Waer, N.A., H.L. Stribling, and M.K. Causey. 1992. White-tailed deer preferences for planted forage crops. Ala. Agric. Exp. Sta. 39:4.
• Wallace, S.U. 1995. Managing difficult pests: Deer. p. 37–41. In Proc. 1995 Southern Soybean Conf., Memphis, TN. United Soybean Board, Trent-Jones, Inc., Princeton, NJ.

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