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Radiation-use efficiency in relation to row spacing for late-planted soybean
Field Crops Research, 36 (1994) 13-19
ELSEVIER
Field Crops Research
Radiation-use efficiency in relation to row spacing for late-planted soybean J.E. Board*, B.G. Harville, M. Kamal Department of Agronomy, Louisiana State University Agricultural Center, Baton Rouge, LA 70803-2110, USA (Accepted 13 October 1993)
Abstract Greater soybean [Glycine max (L.) Merr.] yield can be achieved at late planting dates by using row spacings narrower than conventional wide rows (76 to 100 cm). Objectives were to determine the relative yield performances of 100-, 50-, and 25-cm row spacings in late-planted soybean and to relate yield differences to changes in yield components, several growth dynamic parameters, and radiation interception. Field studies were conducted at Baton Rouge, LA (30°N Lat.) from 1989 through 1991 at a late planting date with 'Centennial' soybean (Maturity Group VI) planted on 100-, 50-, and 25-cm row spacings. The test was conducted on a silty clay soil. Highest yield occurred with 50-cm row width. Yield in the 25- vs. 50-cm rows was less, even though light interception (LI) patterns were similar. Growth dynamic analysis revealed that radiation-use efficiency (RUE) during the first half of the seed filling period was significantly lower in 25- vs. 50- or 100-cm rows. Reduced RUE in 25-cm row spacing was related to reduced pod number associated with lower crop growth rate (CGR) from pod initiation to 10 days after seed initiation. Results indicate that factors other than optimal LI (95%) affect yield responses to row width. Key words: Glycine; Management practice; Radiation-use efficiency; Soybean
I. Introduction Previous research has indicated that narrow-row cutture is a viable means to increase soybean yield at late planting dates in the southeastern USA (Boerma and Ashley, 1982; Boquet et al., 1982; Board et al., 1990a). Wide-row culture is typically defined as an interrow spacing of 100 cm, whereas narrow-row culture is considered to be 75 cm or less (Tanner and Hume, 1978). Studies aimed at determining the optimum row spacing have given conflicting results. Some (Boerma and Ashley, 1982; Boquet et al., 1982; Board et al., 1990a) *Corresponding author. Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript number 93-09-7143. 0378-4290/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0378-4290(93) E 0 0 6 1 - 2
noted increase whereas others (Hartwig, 1957; Caviness, 1966; Taylor, 1980; Heatherly, 1988) have not. Several researchers have reported optimum yield at equidistant interrow and intrarow plant spacings (Wiggans, 1939; Johnson, 1987; Parvez et al., 1989). Cooper (1977) also obtained his largest yield at a very narrow-row spacing (20 cm). These studies indicate that higher yields can be obtained as row spacing is reduced where interrow and intrarow plant spacings are similar. Yield increases are attributed to minimization of competition for resources such as water, minerals, and light. This theory is not supported by other research, which reported little yield improvement when row spacing was less than 50 cm (Beaver and Johnson, 1981; Helsel et al., 1981) or reduced yield (Board et al., 1992). Mineral and water
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J.E. Board et al. / Field Crops Research 36 (1994) 13-19
uptake studies have not indicated any advantage of narrow vs. wide rows (Bennie et al., 1982; Mason et al., 1982). Furthermore, greater LI in narrow-row culture has been shown to be related more to increased leaf area index (LAI), associated with greater total dry matter (TDM), than to increased LI per unit LAI resulting from more equidistant plant arrangement (Board and Harville, 1992). Thus, research has not supported the theory that equidistant spacing increases yield by reducing competition for resources. Greater yield in narrow- vs. wide-row culture has also been explained as resulting from increased LI during critical developmental stages (Shibles and Weber, 1966; Board et al., 1992). Some reports (Shibles and Weber, 1966; Taylor et al., 1982) have identified the seed formation perod as a critical stage. However, reducing row width from 100- to 75- and then to 50cm (Board et al., 1992) resulted in greater LI, higher CGR, and increased yield component development between emergence and seed initiation. Similar findings have been reported from other studies (Egli et al., 1987; Board et al., 1990a). Narrow-row yield increases were greater at late vs. optimal planting dates in these studies. In a previous report (Board et al., 1992), it was shown yield increased in association with LI, CGR, and TDM as row spacing was reduced from 100- to 50-cm and then declined with a further reduction to 25 cm. Therefore, the objective of this paper was to seek an explanation for that decline through examination of yield components, dry matter production, and light relations.
2. Materials and methods
2. I. Culture Field studies were conducted during 1989 through 1991 at the Ben Hur Farm near Baton Rouge, LA (30°N Lat.) on a Mhoon silty clay soil (fine-silty, mixed, nonacid, thermic, Typic, Fluvaquents; USDA Soil Taxonomy) provided with subsurface drainage. Fertilizer was applied prior to planting at a rate 0-41-69 kg h a - 1 ( N - P - K ) following recommendations based on soil tests. Herbicides and hand hoeing were used for weed control. Insects and diseases were controlled by use of
recommended pesticides. Supplemental irrigation was used in 1990 to avoid drought stress. Tests were hand-sown on 26 July 1989, 10 July 1990 and 16 July 1991. All sowing dates were after the optimal sowing period for Louisiana (early May through mid-June). 'Centennial' soybean (Maturity Group VI) was used because it had demonstrated yield responses to row spacing in previous late-planted studies. Experimental unit size differed between years, depending on the sample size needed. In 1989 and 1990, units were 6.1 by 11.5 m to accomodate yield, yield components, and growth samples taken during the growing season. In 1991, growth samples were not collected and the experimental unit was decreased to 6.1 by 4.0 m. Within any given year, similar plant densities between row spacings were achieved by seeding at a high rate and then hand thinning shortly after emergence to a population of 325 000 plants ha ~. Stand counts were made at maturity to verify plant densities within each treatment.
2.2. Data obtained, data analysis, and experimental design Developmental stages Throughout the study, developmental stages were recorded twice weekly, following Fehr and Caviness (1977), on five randomly selected plants per plot.
Seed yield and yield components Seed yield (adjusted to 130 g k g - ~ moisture) was measured on a 4.3-m 2 interior plot area in 1989 and 1990. The experimental design was a randomized complete block with two years and four replications as block factors. Treatments were row spacings of 100, 50, and 25 cm. A fourth row spacing (75 cm) was also included in the study. Relationships among the 100-, 75-, and 50-cm row spacings were discussed in a companion publication (Board et al., 1992). The purpose here is to compare narrow (50 cm) with very narrow (25 cm) row spacings and seed yields from only the 100-, 50-, and 25-cm row widths are presented. Seed yield was subjected to analysis of variance and LSDs were employed for mean separation. In 1989 and 1990, yield components were measured at maturity on random selection of 20 plants per plot. Plants were harvested at ground level and separated into main stems and branches, and determinations made
J.E. Board et al./ Field CropsResearch 36 (1994) 13-19 for total branch length, branch number, and the following yield components on both main stems and branches: node number, fertile node number, percentage fertile nodes, pods per fertile node, pods m - 2, seeds per pod, seed m - 2, and seed size. The dry matter of these samples were summed and used for determination of TDM and reproductive dry matter (RDM) at maturity. Harvest Index, defined as RDM/TDM, was also obtained. All TDM parameters, Harvest Index, and yield components were subjected to analysis of variance and mean separation done according to LSD. Leaf area index, light interception, total dry matter, crop growth rate, and radiation-use efficiency In 1989 and 1990, 1-m row lengths were sampled for TDM and LAI at 32, 39, 47, 54, 60, 68, and 75 days after emergence (DAE). No sampling was done in 1991. Total dry matter (g m -z) was determined after drying to constant weight at 60°C in a forced-air dryer. Crop growth rate was determined by a stepwise regression analysis of TDM against time (Hunt and Parsons, 1981 ), described in the companion paper (Board et al., 1992). Leaf area index was measured from the same samples used to determine CGR. Twenty-five per cent (by fresh weight) of the leaf blades were placed through a Li-Cor 3000 (Li-Cor Inc., Lincoln, NE) portable leaf-area meter and LAI determined. On the same sampling dates used for TDM and LAI, LI was determined with a Li-Cor Line Quantum Sensor (1 m long) connected to a LI-1000 data logger. Photosynthetically active radiation (PAR) was measured in ~mol m - 2 s - ~ at ground level, starting adjacent to the plant row and then at 12.5-cm intervals across the interrow space to the next plant row, and at the top of the canopy. Light interception percentage was based on the average of the ground-level recordings and the ambient flux. All recordings were made parallel to the rows and taken between 1200 and 1300 h central standard time under full-sun conditions. In 1991, at seed initiation, PAR was recorded at mid-canopy height at 12.5-cm intervals between rows. These data were used to calculate LI in the upper and lower halves of the canopy. Radiation-use efficiency (RUE) was determined by dividing TDM increase between seed initiation and the mid-point of seed filling by total solar radiation (Louisiana Agriclimatic Information Service; Robbins, 1991 ) intercepted by the canopy during this period. This
15
reproductive period was chosen to avoid the rapid leaf abscission of the second half of the seed filling period. Radiation-use efficiency before seed initiation was not measured, because the relationship between TDM at seed initiation with LI between emergence and seed initiation appeared consistent across row spacings. The experimental design for LI and LAI was a randomized complete block in a split plot arrangement. Years and replications were considered as random block factors. Main plots and split plots were row spacing and DAE, respectively. Experimental design for RUE was a randomized complete block with years and replications as block factors. Light interception, LAI, and RUE data were analyzed by analysis of variance using appropriate error terms. Mean separation was done according to LSD. Photomorphogenic light ratios within the canopy In 1991, red (R) and far-red (FR) light irradiances (645 nm (red) and 735 nm (far red); Kasperbauer, 1987) were determined on 25-, 50-, and 100-cm row spacings at the 7-leaf stage and again at seed initiation using a Li-Cor 1800 portable spectroradiometer (band width 1 nm) equipped with a fiber optic probe. R/FR is the ratio of the two irradiances. Measurements were taken adjacent to the main stem terminal apex and were the average of five recordings based on irradiance (W m -2) coming from above, north, south, east, and west directions (Kaul and Kasperbauer, 1988). Ambient incoming R / F R was also recorded. Measurement at the main stem apex was suggested (M.J. Kasperbauer, pers. commun.) because meristematic tissue is rich in phytochrome. Recordings were made between 1600 to 1800 h in the afternoon (clear-sky conditions) because end-of-day R/FR has been shown (Kasperbauer, 1987) to affect some of the yield components studied in our test. Experimental design was a randomized complete block in a split plot arrangement with four replications. Main plots were the three row spacings and split plots were the two developmental stages. Data were analyzed by analysis of variance and mean separation was done with LSDs. 3. Results and discussion
3.1. Seed yield and yield components Seed yield (Table 1) was significantly affected by row spacing, but not by years or the year X row spacing
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J.E. Board et al. / Field Crops Research 36 (1994) 13-19
Table 1 Means for seed yield in 25-, 50-, and 100-cm row spacing averaged over two years, 1989-90, Baton Rouge, Louisiana Row spacing (cm)
Seed yield (kg ha -~)
Std. error
25 50 100
2940 3670*** 2730
123 5I 85
3.2. Light interception, leaf area index, and radiation-use efficiency Light interception patterns during the 1989 and 1990 growing seasons (Figs. 1 and 2) were essentially identical for both 25- and 50-cm row spacings. Both row spacings had significantly higher LI than 100-cm rows. Increased LI was closely associated with greater LAI in 50-cm rows in both years, and for the 25-cm row width in 1989. However, in 1990, 25- and 100-cm rows had similar LAI throughout the growing season. Greater LI in this case resulted from greater interception per unit LAI (Board and Harville, 1992). Optimal LI (95%) was achieved by 25- and 50-cm rows by pod initiation. This corresponded to 44 DAE in 1989 and 49 DAE in 1990. Optimal LI was maintained until the last 10 days of the seed filling period. Similar LI patterns during the first half of seed filling were not matched by similar TDM accumulation. As measured by RUE (Table 3), solar radiation was transferred into TDM about 75% more efficiently in 50- vs. 25-cm rows during this period. Averaged across years, RUE in 50-cm rows was 0.87 g M J - 1. This figure is comparable to that reported by Loomis and Connor (1992) for soybean production in Iowa. In contrast, RUE was only 0.50 g M J - 1for the 25-cm row spacing. Because most of the dry matter accumulated during this period was reproductive material, lower RUE in 25-cm rows was caused by fewer pods (seed weight and seed per pod were unaffected), a yield component determined shortly after seed initiation (Andrews, 1966; Fraser et al., 1982).
***Mean in 50-cm row spacing is significantly larger compared with 25- or 100-cm row spacing at the 0.001 probability level.
interaction. When averaged across three years, significantly greater yield occurred in 50-compared with 100- or 25-cm row spacings. Seed yields in 25- and 100-cm row spacings were similar. Data in the current and companion paper (Board et al., 1992) demonstrate that 50-cm rows yielded better than 25-, 75-, or 100cm rows. Yield components are presented in Table 2. Greater yield in 50-cm rows was not related to TDM at seed initiation or Harvest Index (data not shown), but rather to larger final TDM and RDM (data not shown) related to greater pod production (Table 2). Seeds per pod and seed size were unaffected (data not shown). Greater pod number was related to more pods per fertile node on the main stem, more branches, and more fertile nodes on branches. Thus, the yield decline from 50- to 25-cm row spacings resulted from mechanisms similar to those shown to explain yield decline between 50and 100-cm rows (Board et al., 1990b, 1992).
Table 2 Main stem ( MS ) and branch (BR) yield components related to soybean yield differences between 100-, 50-, and 25-cm row spacings, averaged a over 1989 and 1990, Baton Rouge, Louisiana Row spacing (cm)
100 50 25
BR numbers (no. m -2)
161 167" 141
Node number (no. m 2)
Fertile node number (no. m 2)
Pods per fertile node (no. m 2)
Pod number (no. m -2)
MS
BR
Total
MS
BR
Total
MS
BR
Total
MS
BR
Total
446 532 520
335 353* 280
780 885* 800
236 267 256
194 317"* 233
529 584** 489
3.60 3.93** 3.50
2.11 2.14 2.13
2.78 2.97 2.89
852 1041"** 894
616 673** 505
1467 1714"** 1399
*, **, ***Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. ~Significant year X row spacing interactions occurred for MS node number and MS pods per fertile node.
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
100
Table 3 Means a and mean squaresb for radiation-use efficiency (RUE) during the first half of seed filling (R5-R6.5) in 100-, 50-, and 25-crn row spacings, averaged over 1989 and 1990, at Baton Rouge, Louisiana
. .~.~t.r~_~t ...~h . . 4 p . . ~ t
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.
I 411
O
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I .........
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-'"
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I
/ ~ / ~" / '-/
100 - cm rows
1 ..... ~! ..... R.3....,.R.S. ...............
30
40
50
60
RUE (g M J - 1)
100 50 25
0.87** 0.87** 0.50
Anova source
Summary df
RUE
Year Rep (year) Row space Year × row space Error CV
1 6 2 2 12 27.3
NS NS 0.359** NS 0.041
"|
2
o
Row spacing (cm)
.~7.,-~,
70
DAYS AFTER EMERGENCE (DAE)
Fig. I. Leaf area index ( h A l ) and light interception (hi) in lO0-, 50-, and 25-cm row spacings for soybean grown in 1989. Vertical bars represent LSD,~.os~ for LAI and LI.
100 80 _%
I~
6O
~'~~'~
4O 2O
0 5
..~,~
i .........
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4 st
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.
2 1
oi
m 30
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...a.~........ .~5........ ~7..-..~. 50
60
70
DAYS AFTER EMERGENCE (DAE)
Fig. 2. Leaf area index (LAI) and light interception (LI) in 100-, 50-, and 25-cm row spacings for soybean grown in 1990. Vertical bars represent LSD,~os~ for LAI and L1.
3.3. Possible explanations for row spacing effects on radiation-use efficiency Reduced RUE in 25- vs. 50-cm row spacing may have resulted from reduced light penetration into the
**Significant at the 0.01 probability level. ~Mean in 100- or 50-cm row spacing is significantly grreater than 25-cm row spacing at the indicated probability level. ~Mean squares are significant at the probability level indicated.
canopy during the period when pod number was determined. Previous studies have indicated that light availability during pod formation affects pod number and yield (Schou et al., 1978; Wiebold et al., 1981). Because yield component differences between 25- and 50-cm row spacings occurred during pod formation, less light penetration during this period into the canopy of the 25-cm rows may have been the cause. This hypothesis was not supported by canopy PAR data obtained in 1991. Despite greater yield in 50- vs. 25cm rows (data not shown), flux levels within the canopy and LI for the top and bottom halves of the canopy were nearly identical for both row spacings at seed initiation. Approximately 95% of the intercepted irradiance was absorbed in the top half of the canopy. A second hypothesis to explain differences in pod production between these two row spacings involves possible R/FR differences within the canopy. Through alterations of row orientation and row spacing, Kasperbauer (1987) showed that lower R/FR was associated with fewer branches and pods per plant. Reduced yield in 25- vs. 50-cm rows in the current study was related to reduced branch and pod number, but results in 1991 indicated that R/FR at the main stem apex actually was significantly larger in 25-cm rows (0.40) compared with 50- (0.28) or 100-cm rows (0.24).
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
18
Because no developmental stage × row spacing interaction occurred, data were the averages of recordings at the 7-leaf stage and seed initiation. Thus, the trend was for R/FR to increase, rather than decrease, as row spacing was decreased from 100- to 50- and then to 25cm row widths. This result might be expected, however, because plants in the 25-cm rows were the least crowded within the row and, therefore, more red light would penetrate to the apex. A third hypothesis to explain lower RUE in 25-cm rows is lower canopy assimilatory capacity during pod formation and the 10- to 12-day period after seed initiation during which final pod number is set. Previous studies have indicated that CGR during pod formation and pod set has a strong positive relation with pod number (Board and Harville, 1993). Although both row spacings had greater LI during this period relative to 100-cm rows (Figs. 1 and 2), CGR did not differ between 25- and 100-cm row spacings (Table 4). In contrast, CGR was always significantly greater in 50vs. 100-cm rows. In many instances, CGR was also significantly greater in 50- compared with 25-cm rows. Thus, the most likely hypothesis to explain greater pod number and hence greater RUE in 50- vs. 25-cm rows Table 4 Crop growth rates (CGR) in 100-, 50-, and 25-cm row spacings near seed initiation (R5) for soybean grown in 1989 and 1990 at Baton Rouge, Louisiana Year
Days after emergence
Row spacing (cm)
CGR ( g m : day t)
1989
47
R5 = 51 days
54
100 50 25 100 50 25 100 50 25 100 50 25 100 50 25 100 50 25
13.22 16.52" 15.63 12.62 18.00"* 14.21 10.70 17.19**" 11.62 13.75 19.24***b 14.72 14.28 19.09**h 14.11 12.71 15.65 *b 10.88
60
1990
54
R5 = 6 0 d a y s
60
68
*, **, ***CGR in 50- compared with 100-cm row spacing is significantly greater at the 0.05, 0.01, and 0.001 probability levels, respectively. "'bCGR in 50- compared with 25-cm row spacing is significantly greater at the 0.05 and 0.01 probability levels, respectively.
was greater assimilatory capacity (as measured by CGR; Jones, 1992) during pod formation and pod set. Future work will focus on determining why, despite favorable LI during this period, CGR differences between 25- and 50-cm rows occurred. Although previous research demonstrated that row spacing does not affect plant nutrition (Bennie et al., 1982), root distribution and mineral uptake may affect CGR in certain situations.
3.4. Implications for row spacing studies Differences in RUE between narrow-row spacings having similar LI pattems help explain why some researchers have reported narrow-row yield responses whereas others have not. In cases where no yield increase was reported, the researcher may have been using a narrow-row width having less RUE than some other narrow-row width. Thus, choice of optimum row spacing may involve selection for optimum RUE, as well as greater LI between emergence and seed initiation (Board et al., 1992). Yield, yield component, and growth dynamic data indicate neither equidistant spacing nor light interception theories completely explain yield responses to narrow-row culture. The most equidistant spacing ( 25 cm) resulted in significantly smaller CGR and yield compared with a less equidistant spacing (50 cm). Because LI patterns in 25- and 50-cm row spacings were similar, CGR and yield should have been similar according to the light interception theory. Previous studies have demonstrated the role of LI in yield increases as row spacing is narrowed from 100 cm (Shibles and Weber, 1966; Egli et al., 1987; Board et al., 1992). Results of the current study indicate that narrow-row spacings having similar LI can differ in CGR and RUE, thus influencing yield.
References A n d r e w s , C.H., 1966. S o m e aspects of p o d and seed d e v e l o p m e n t in Lee soybeans. Ph. D. Dissertation, Mississippi State Univ., State College (Diss. Abstr. 7 1 : 0 7 3 1 1 ). Beaver, J,S. a n d Johnson, R.R., 1981. Response of determinate and indeterminate soybeans to varying cultural practices in the northern USA. Agron. J., 73: 8 3 3 - 8 3 8 . Bennie, A.T.P., Mason, W.K. a n d Taylor, H.M., 1982. Responses of s o y b e a n s to two r o w spacings and two soil water levels. III.
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
Concentration, accumulation, and translocation of 12 elements. Field Crops Res., 5: 31-43. Board, J.E. and Harville, B.G., 1992. Explanations for greater light interception in narrow- vs. wide-row soybean. Crop Sci., 32: 198-202. Board, J.E. and Harville, B.G., 1993. Soybean yield component responses to a light interception gradient during the reproductive period. Crop Sci., 33: 772-777. Board, J.E., Harville, B.G. and Saxton, A.M., 1990a. Narrow-row seed-yield enhancement in determinate soybean. Agron. J., 82: 64-68. Board, J.E., Harville, B.G. and Saxton, A.M., 1990b. Branch dry weight in relation to yield increases in narrow-row soybean. Agron. J., 82: 540-544. Board, J.E., Kamal, M. and Harville, B.G., 1992. Temporal importance of greater light interception to increased yield in narrowrow soybean. Agron. J., 84: 575-579. Boerma, J.R. and Ashley, D.A., 1982. Irrigation, row spacing, and genotype effects on late and ultra-late planted soybean. Agron. J., 74: 995-999. Boquet, D.J., Koonce, K.L. and Walker, D.M., 1982. Selected determinate soybean cultivar yield response to row spacings and planting dates. Agron. J., 74: 136-138. Caviness, C.E., 1966. Spacing studies with Lee soybeans. Arkansas Agric. Exp. Stn. Bull. 713. Cooper, R.L., 1977. Response of soybean cultivars to narrow rows and planting rates under weed-free conditions. Agron. J., 69: 8992. Egli, D.B., Guffy, R.D. and Heitholt, J.J., 1987. Factors associated with reduced yields of delayed plantings of soybean. J. Agron. Crop Sci., 159: 176-185. Fehr, W.R. and Caviness, C.E., 1977. Stages of soybean development. Iowa Agric. Exp. Stn. Spec. Rep. 80. Fraser, J., Egli, D.B. and Leggett, J.E., 1982. Pod and seed development in soybean cultivars with differences in seed size. Agron. J., 74: 81-85. Hartwig, E.E., 1957. Row width and rates of planting in the Southern states. Soybean Digest, 17: 13-16. Heatherly, L.G., 1988. Planting date, row spacing, and irrigation effects on soybean growth on clay soil. Agron. J., 80: 227-231. Helsel, Z.R., Johnston, T.J. and Hart, L.P., 1981. Soybean production in Michigan. Mich. State Univ. Bull. E-1549. Hunt, R. and Parsons, E.T., 1981. Plant Growth Analysis: Users Instructions for the Stepwise and Spline Programs. Unit of Comparative Ecology, University of Sheffield, Sheffield, UK. Johnson, R.R., 1987. Crop management. In: J.R. Wilcox (Editor),
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Soybeans: Improvement, Production and Uses (2nd ed. ). American Society of Agronomy, Madison, WI, pp. 355-383. Jones, H.G., 1992. Plants and Microclimate. A Quantitative Approach to Environmental Plant Physiology (2nd ed.). Cambridge Univ. Press, Cambridge, England. Kasperbauer, M.J., 1987. Far-red light reflection from green leaves and effects on phytochrome-mediated assimilate partitioning under field conditions. Plant Physiol., 85: 350--354. Kaul, K. and Kasperbauer, M.J., 1988. Row orientation effects on FR/R light ratio, growth and development of field-grown bush bean. Physiol. Plant., 74: 415-417. Loomis, R.S. and Connor, D.J., 1992. Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge Univ. Press, Cambridge, UK. Mason, W.K., Rowse, H.R., Bennie, A.T.P,, Kaspar, T.C. and Taylor, H.M., 1982. Response of soybeans to two row spacings and two soil water levels. II. Water use, root growth and plant water status. Field Crops Res., 5: 15-29. Parvez, A.Q., Gardner, F.P. and Boote, K.J., 1989. Determinate- and indeterminate-type soybean cultivar responses to pattern, density, and planting date. Crop Sci., 29: 150-157. Robbins, K.D., 1991. A user's manual for the LAIS climatic database management system. Louisiana Agric. Exp. Stn. Bull. Computer Applications Series No. 101. Schou, J.B., Jeffers, D.L. and Streeter, J.G., 1978. Effects of reflectors, black boards, or shades applied at different stages of plant development on yield of soybean. Crop Sci., 18: 29-34. Shibles, R.M. and Weber, C.R., 1966. Interception of solar radiation and dry matter production by various planting patterns. Crop Sci., 6: 55-59. Tanner, J.W. and Hume, D.J., 1978. Management and production. In: A.G. Norman (Editor), Soybean Physiology, Agronomy, and Utilization. Academic Press, New York, pp. 157-212. Taylor, H.M., 1980. Soybean growth and yield as affected by seasonal water supply. Agron. J., 72: 543-547. Taylor, H.M., Mason, W.K., Bennie, A.T.P. and Rowse, H.R., 1982. Responses of soybeans to two row spacings and two soil water levels. I. An analysis of biomass accumulation, canopy development, solar radiation interception and components of seed yield. Field Crops Res., 5: 1-14. Wiebold, W.J., Ashley, D.A. and Boerma, J.R., 1981. Reproductive abscission levels and patterns for 11 determinate soybean cultivars. Agron. J., 73: 43-46. Wiggans, R.G., 1939. The influence of space and arrangement on the production of soybean plants. J. Am. Soc. Agron., 31 : 314321.
ELSEVIER
Field Crops Research
Radiation-use efficiency in relation to row spacing for late-planted soybean J.E. Board*, B.G. Harville, M. Kamal Department of Agronomy, Louisiana State University Agricultural Center, Baton Rouge, LA 70803-2110, USA (Accepted 13 October 1993)
Abstract Greater soybean [Glycine max (L.) Merr.] yield can be achieved at late planting dates by using row spacings narrower than conventional wide rows (76 to 100 cm). Objectives were to determine the relative yield performances of 100-, 50-, and 25-cm row spacings in late-planted soybean and to relate yield differences to changes in yield components, several growth dynamic parameters, and radiation interception. Field studies were conducted at Baton Rouge, LA (30°N Lat.) from 1989 through 1991 at a late planting date with 'Centennial' soybean (Maturity Group VI) planted on 100-, 50-, and 25-cm row spacings. The test was conducted on a silty clay soil. Highest yield occurred with 50-cm row width. Yield in the 25- vs. 50-cm rows was less, even though light interception (LI) patterns were similar. Growth dynamic analysis revealed that radiation-use efficiency (RUE) during the first half of the seed filling period was significantly lower in 25- vs. 50- or 100-cm rows. Reduced RUE in 25-cm row spacing was related to reduced pod number associated with lower crop growth rate (CGR) from pod initiation to 10 days after seed initiation. Results indicate that factors other than optimal LI (95%) affect yield responses to row width. Key words: Glycine; Management practice; Radiation-use efficiency; Soybean
I. Introduction Previous research has indicated that narrow-row cutture is a viable means to increase soybean yield at late planting dates in the southeastern USA (Boerma and Ashley, 1982; Boquet et al., 1982; Board et al., 1990a). Wide-row culture is typically defined as an interrow spacing of 100 cm, whereas narrow-row culture is considered to be 75 cm or less (Tanner and Hume, 1978). Studies aimed at determining the optimum row spacing have given conflicting results. Some (Boerma and Ashley, 1982; Boquet et al., 1982; Board et al., 1990a) *Corresponding author. Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript number 93-09-7143. 0378-4290/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0378-4290(93) E 0 0 6 1 - 2
noted increase whereas others (Hartwig, 1957; Caviness, 1966; Taylor, 1980; Heatherly, 1988) have not. Several researchers have reported optimum yield at equidistant interrow and intrarow plant spacings (Wiggans, 1939; Johnson, 1987; Parvez et al., 1989). Cooper (1977) also obtained his largest yield at a very narrow-row spacing (20 cm). These studies indicate that higher yields can be obtained as row spacing is reduced where interrow and intrarow plant spacings are similar. Yield increases are attributed to minimization of competition for resources such as water, minerals, and light. This theory is not supported by other research, which reported little yield improvement when row spacing was less than 50 cm (Beaver and Johnson, 1981; Helsel et al., 1981) or reduced yield (Board et al., 1992). Mineral and water
14
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
uptake studies have not indicated any advantage of narrow vs. wide rows (Bennie et al., 1982; Mason et al., 1982). Furthermore, greater LI in narrow-row culture has been shown to be related more to increased leaf area index (LAI), associated with greater total dry matter (TDM), than to increased LI per unit LAI resulting from more equidistant plant arrangement (Board and Harville, 1992). Thus, research has not supported the theory that equidistant spacing increases yield by reducing competition for resources. Greater yield in narrow- vs. wide-row culture has also been explained as resulting from increased LI during critical developmental stages (Shibles and Weber, 1966; Board et al., 1992). Some reports (Shibles and Weber, 1966; Taylor et al., 1982) have identified the seed formation perod as a critical stage. However, reducing row width from 100- to 75- and then to 50cm (Board et al., 1992) resulted in greater LI, higher CGR, and increased yield component development between emergence and seed initiation. Similar findings have been reported from other studies (Egli et al., 1987; Board et al., 1990a). Narrow-row yield increases were greater at late vs. optimal planting dates in these studies. In a previous report (Board et al., 1992), it was shown yield increased in association with LI, CGR, and TDM as row spacing was reduced from 100- to 50-cm and then declined with a further reduction to 25 cm. Therefore, the objective of this paper was to seek an explanation for that decline through examination of yield components, dry matter production, and light relations.
2. Materials and methods
2. I. Culture Field studies were conducted during 1989 through 1991 at the Ben Hur Farm near Baton Rouge, LA (30°N Lat.) on a Mhoon silty clay soil (fine-silty, mixed, nonacid, thermic, Typic, Fluvaquents; USDA Soil Taxonomy) provided with subsurface drainage. Fertilizer was applied prior to planting at a rate 0-41-69 kg h a - 1 ( N - P - K ) following recommendations based on soil tests. Herbicides and hand hoeing were used for weed control. Insects and diseases were controlled by use of
recommended pesticides. Supplemental irrigation was used in 1990 to avoid drought stress. Tests were hand-sown on 26 July 1989, 10 July 1990 and 16 July 1991. All sowing dates were after the optimal sowing period for Louisiana (early May through mid-June). 'Centennial' soybean (Maturity Group VI) was used because it had demonstrated yield responses to row spacing in previous late-planted studies. Experimental unit size differed between years, depending on the sample size needed. In 1989 and 1990, units were 6.1 by 11.5 m to accomodate yield, yield components, and growth samples taken during the growing season. In 1991, growth samples were not collected and the experimental unit was decreased to 6.1 by 4.0 m. Within any given year, similar plant densities between row spacings were achieved by seeding at a high rate and then hand thinning shortly after emergence to a population of 325 000 plants ha ~. Stand counts were made at maturity to verify plant densities within each treatment.
2.2. Data obtained, data analysis, and experimental design Developmental stages Throughout the study, developmental stages were recorded twice weekly, following Fehr and Caviness (1977), on five randomly selected plants per plot.
Seed yield and yield components Seed yield (adjusted to 130 g k g - ~ moisture) was measured on a 4.3-m 2 interior plot area in 1989 and 1990. The experimental design was a randomized complete block with two years and four replications as block factors. Treatments were row spacings of 100, 50, and 25 cm. A fourth row spacing (75 cm) was also included in the study. Relationships among the 100-, 75-, and 50-cm row spacings were discussed in a companion publication (Board et al., 1992). The purpose here is to compare narrow (50 cm) with very narrow (25 cm) row spacings and seed yields from only the 100-, 50-, and 25-cm row widths are presented. Seed yield was subjected to analysis of variance and LSDs were employed for mean separation. In 1989 and 1990, yield components were measured at maturity on random selection of 20 plants per plot. Plants were harvested at ground level and separated into main stems and branches, and determinations made
J.E. Board et al./ Field CropsResearch 36 (1994) 13-19 for total branch length, branch number, and the following yield components on both main stems and branches: node number, fertile node number, percentage fertile nodes, pods per fertile node, pods m - 2, seeds per pod, seed m - 2, and seed size. The dry matter of these samples were summed and used for determination of TDM and reproductive dry matter (RDM) at maturity. Harvest Index, defined as RDM/TDM, was also obtained. All TDM parameters, Harvest Index, and yield components were subjected to analysis of variance and mean separation done according to LSD. Leaf area index, light interception, total dry matter, crop growth rate, and radiation-use efficiency In 1989 and 1990, 1-m row lengths were sampled for TDM and LAI at 32, 39, 47, 54, 60, 68, and 75 days after emergence (DAE). No sampling was done in 1991. Total dry matter (g m -z) was determined after drying to constant weight at 60°C in a forced-air dryer. Crop growth rate was determined by a stepwise regression analysis of TDM against time (Hunt and Parsons, 1981 ), described in the companion paper (Board et al., 1992). Leaf area index was measured from the same samples used to determine CGR. Twenty-five per cent (by fresh weight) of the leaf blades were placed through a Li-Cor 3000 (Li-Cor Inc., Lincoln, NE) portable leaf-area meter and LAI determined. On the same sampling dates used for TDM and LAI, LI was determined with a Li-Cor Line Quantum Sensor (1 m long) connected to a LI-1000 data logger. Photosynthetically active radiation (PAR) was measured in ~mol m - 2 s - ~ at ground level, starting adjacent to the plant row and then at 12.5-cm intervals across the interrow space to the next plant row, and at the top of the canopy. Light interception percentage was based on the average of the ground-level recordings and the ambient flux. All recordings were made parallel to the rows and taken between 1200 and 1300 h central standard time under full-sun conditions. In 1991, at seed initiation, PAR was recorded at mid-canopy height at 12.5-cm intervals between rows. These data were used to calculate LI in the upper and lower halves of the canopy. Radiation-use efficiency (RUE) was determined by dividing TDM increase between seed initiation and the mid-point of seed filling by total solar radiation (Louisiana Agriclimatic Information Service; Robbins, 1991 ) intercepted by the canopy during this period. This
15
reproductive period was chosen to avoid the rapid leaf abscission of the second half of the seed filling period. Radiation-use efficiency before seed initiation was not measured, because the relationship between TDM at seed initiation with LI between emergence and seed initiation appeared consistent across row spacings. The experimental design for LI and LAI was a randomized complete block in a split plot arrangement. Years and replications were considered as random block factors. Main plots and split plots were row spacing and DAE, respectively. Experimental design for RUE was a randomized complete block with years and replications as block factors. Light interception, LAI, and RUE data were analyzed by analysis of variance using appropriate error terms. Mean separation was done according to LSD. Photomorphogenic light ratios within the canopy In 1991, red (R) and far-red (FR) light irradiances (645 nm (red) and 735 nm (far red); Kasperbauer, 1987) were determined on 25-, 50-, and 100-cm row spacings at the 7-leaf stage and again at seed initiation using a Li-Cor 1800 portable spectroradiometer (band width 1 nm) equipped with a fiber optic probe. R/FR is the ratio of the two irradiances. Measurements were taken adjacent to the main stem terminal apex and were the average of five recordings based on irradiance (W m -2) coming from above, north, south, east, and west directions (Kaul and Kasperbauer, 1988). Ambient incoming R / F R was also recorded. Measurement at the main stem apex was suggested (M.J. Kasperbauer, pers. commun.) because meristematic tissue is rich in phytochrome. Recordings were made between 1600 to 1800 h in the afternoon (clear-sky conditions) because end-of-day R/FR has been shown (Kasperbauer, 1987) to affect some of the yield components studied in our test. Experimental design was a randomized complete block in a split plot arrangement with four replications. Main plots were the three row spacings and split plots were the two developmental stages. Data were analyzed by analysis of variance and mean separation was done with LSDs. 3. Results and discussion
3.1. Seed yield and yield components Seed yield (Table 1) was significantly affected by row spacing, but not by years or the year X row spacing
16
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
Table 1 Means for seed yield in 25-, 50-, and 100-cm row spacing averaged over two years, 1989-90, Baton Rouge, Louisiana Row spacing (cm)
Seed yield (kg ha -~)
Std. error
25 50 100
2940 3670*** 2730
123 5I 85
3.2. Light interception, leaf area index, and radiation-use efficiency Light interception patterns during the 1989 and 1990 growing seasons (Figs. 1 and 2) were essentially identical for both 25- and 50-cm row spacings. Both row spacings had significantly higher LI than 100-cm rows. Increased LI was closely associated with greater LAI in 50-cm rows in both years, and for the 25-cm row width in 1989. However, in 1990, 25- and 100-cm rows had similar LAI throughout the growing season. Greater LI in this case resulted from greater interception per unit LAI (Board and Harville, 1992). Optimal LI (95%) was achieved by 25- and 50-cm rows by pod initiation. This corresponded to 44 DAE in 1989 and 49 DAE in 1990. Optimal LI was maintained until the last 10 days of the seed filling period. Similar LI patterns during the first half of seed filling were not matched by similar TDM accumulation. As measured by RUE (Table 3), solar radiation was transferred into TDM about 75% more efficiently in 50- vs. 25-cm rows during this period. Averaged across years, RUE in 50-cm rows was 0.87 g M J - 1. This figure is comparable to that reported by Loomis and Connor (1992) for soybean production in Iowa. In contrast, RUE was only 0.50 g M J - 1for the 25-cm row spacing. Because most of the dry matter accumulated during this period was reproductive material, lower RUE in 25-cm rows was caused by fewer pods (seed weight and seed per pod were unaffected), a yield component determined shortly after seed initiation (Andrews, 1966; Fraser et al., 1982).
***Mean in 50-cm row spacing is significantly larger compared with 25- or 100-cm row spacing at the 0.001 probability level.
interaction. When averaged across three years, significantly greater yield occurred in 50-compared with 100- or 25-cm row spacings. Seed yields in 25- and 100-cm row spacings were similar. Data in the current and companion paper (Board et al., 1992) demonstrate that 50-cm rows yielded better than 25-, 75-, or 100cm rows. Yield components are presented in Table 2. Greater yield in 50-cm rows was not related to TDM at seed initiation or Harvest Index (data not shown), but rather to larger final TDM and RDM (data not shown) related to greater pod production (Table 2). Seeds per pod and seed size were unaffected (data not shown). Greater pod number was related to more pods per fertile node on the main stem, more branches, and more fertile nodes on branches. Thus, the yield decline from 50- to 25-cm row spacings resulted from mechanisms similar to those shown to explain yield decline between 50and 100-cm rows (Board et al., 1990b, 1992).
Table 2 Main stem ( MS ) and branch (BR) yield components related to soybean yield differences between 100-, 50-, and 25-cm row spacings, averaged a over 1989 and 1990, Baton Rouge, Louisiana Row spacing (cm)
100 50 25
BR numbers (no. m -2)
161 167" 141
Node number (no. m 2)
Fertile node number (no. m 2)
Pods per fertile node (no. m 2)
Pod number (no. m -2)
MS
BR
Total
MS
BR
Total
MS
BR
Total
MS
BR
Total
446 532 520
335 353* 280
780 885* 800
236 267 256
194 317"* 233
529 584** 489
3.60 3.93** 3.50
2.11 2.14 2.13
2.78 2.97 2.89
852 1041"** 894
616 673** 505
1467 1714"** 1399
*, **, ***Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. ~Significant year X row spacing interactions occurred for MS node number and MS pods per fertile node.
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
100
Table 3 Means a and mean squaresb for radiation-use efficiency (RUE) during the first half of seed filling (R5-R6.5) in 100-, 50-, and 25-crn row spacings, averaged over 1989 and 1990, at Baton Rouge, Louisiana
. .~.~t.r~_~t ...~h . . 4 p . . ~ t
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.
I 411
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I .........
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17
-'"
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I
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100 - cm rows
1 ..... ~! ..... R.3....,.R.S. ...............
30
40
50
60
RUE (g M J - 1)
100 50 25
0.87** 0.87** 0.50
Anova source
Summary df
RUE
Year Rep (year) Row space Year × row space Error CV
1 6 2 2 12 27.3
NS NS 0.359** NS 0.041
"|
2
o
Row spacing (cm)
.~7.,-~,
70
DAYS AFTER EMERGENCE (DAE)
Fig. I. Leaf area index ( h A l ) and light interception (hi) in lO0-, 50-, and 25-cm row spacings for soybean grown in 1989. Vertical bars represent LSD,~.os~ for LAI and LI.
100 80 _%
I~
6O
~'~~'~
4O 2O
0 5
..~,~
i .........
i .........
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4 st
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.
2 1
oi
m 30
40
...a.~........ .~5........ ~7..-..~. 50
60
70
DAYS AFTER EMERGENCE (DAE)
Fig. 2. Leaf area index (LAI) and light interception (LI) in 100-, 50-, and 25-cm row spacings for soybean grown in 1990. Vertical bars represent LSD,~os~ for LAI and L1.
3.3. Possible explanations for row spacing effects on radiation-use efficiency Reduced RUE in 25- vs. 50-cm row spacing may have resulted from reduced light penetration into the
**Significant at the 0.01 probability level. ~Mean in 100- or 50-cm row spacing is significantly grreater than 25-cm row spacing at the indicated probability level. ~Mean squares are significant at the probability level indicated.
canopy during the period when pod number was determined. Previous studies have indicated that light availability during pod formation affects pod number and yield (Schou et al., 1978; Wiebold et al., 1981). Because yield component differences between 25- and 50-cm row spacings occurred during pod formation, less light penetration during this period into the canopy of the 25-cm rows may have been the cause. This hypothesis was not supported by canopy PAR data obtained in 1991. Despite greater yield in 50- vs. 25cm rows (data not shown), flux levels within the canopy and LI for the top and bottom halves of the canopy were nearly identical for both row spacings at seed initiation. Approximately 95% of the intercepted irradiance was absorbed in the top half of the canopy. A second hypothesis to explain differences in pod production between these two row spacings involves possible R/FR differences within the canopy. Through alterations of row orientation and row spacing, Kasperbauer (1987) showed that lower R/FR was associated with fewer branches and pods per plant. Reduced yield in 25- vs. 50-cm rows in the current study was related to reduced branch and pod number, but results in 1991 indicated that R/FR at the main stem apex actually was significantly larger in 25-cm rows (0.40) compared with 50- (0.28) or 100-cm rows (0.24).
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
18
Because no developmental stage × row spacing interaction occurred, data were the averages of recordings at the 7-leaf stage and seed initiation. Thus, the trend was for R/FR to increase, rather than decrease, as row spacing was decreased from 100- to 50- and then to 25cm row widths. This result might be expected, however, because plants in the 25-cm rows were the least crowded within the row and, therefore, more red light would penetrate to the apex. A third hypothesis to explain lower RUE in 25-cm rows is lower canopy assimilatory capacity during pod formation and the 10- to 12-day period after seed initiation during which final pod number is set. Previous studies have indicated that CGR during pod formation and pod set has a strong positive relation with pod number (Board and Harville, 1993). Although both row spacings had greater LI during this period relative to 100-cm rows (Figs. 1 and 2), CGR did not differ between 25- and 100-cm row spacings (Table 4). In contrast, CGR was always significantly greater in 50vs. 100-cm rows. In many instances, CGR was also significantly greater in 50- compared with 25-cm rows. Thus, the most likely hypothesis to explain greater pod number and hence greater RUE in 50- vs. 25-cm rows Table 4 Crop growth rates (CGR) in 100-, 50-, and 25-cm row spacings near seed initiation (R5) for soybean grown in 1989 and 1990 at Baton Rouge, Louisiana Year
Days after emergence
Row spacing (cm)
CGR ( g m : day t)
1989
47
R5 = 51 days
54
100 50 25 100 50 25 100 50 25 100 50 25 100 50 25 100 50 25
13.22 16.52" 15.63 12.62 18.00"* 14.21 10.70 17.19**" 11.62 13.75 19.24***b 14.72 14.28 19.09**h 14.11 12.71 15.65 *b 10.88
60
1990
54
R5 = 6 0 d a y s
60
68
*, **, ***CGR in 50- compared with 100-cm row spacing is significantly greater at the 0.05, 0.01, and 0.001 probability levels, respectively. "'bCGR in 50- compared with 25-cm row spacing is significantly greater at the 0.05 and 0.01 probability levels, respectively.
was greater assimilatory capacity (as measured by CGR; Jones, 1992) during pod formation and pod set. Future work will focus on determining why, despite favorable LI during this period, CGR differences between 25- and 50-cm rows occurred. Although previous research demonstrated that row spacing does not affect plant nutrition (Bennie et al., 1982), root distribution and mineral uptake may affect CGR in certain situations.
3.4. Implications for row spacing studies Differences in RUE between narrow-row spacings having similar LI pattems help explain why some researchers have reported narrow-row yield responses whereas others have not. In cases where no yield increase was reported, the researcher may have been using a narrow-row width having less RUE than some other narrow-row width. Thus, choice of optimum row spacing may involve selection for optimum RUE, as well as greater LI between emergence and seed initiation (Board et al., 1992). Yield, yield component, and growth dynamic data indicate neither equidistant spacing nor light interception theories completely explain yield responses to narrow-row culture. The most equidistant spacing ( 25 cm) resulted in significantly smaller CGR and yield compared with a less equidistant spacing (50 cm). Because LI patterns in 25- and 50-cm row spacings were similar, CGR and yield should have been similar according to the light interception theory. Previous studies have demonstrated the role of LI in yield increases as row spacing is narrowed from 100 cm (Shibles and Weber, 1966; Egli et al., 1987; Board et al., 1992). Results of the current study indicate that narrow-row spacings having similar LI can differ in CGR and RUE, thus influencing yield.
References A n d r e w s , C.H., 1966. S o m e aspects of p o d and seed d e v e l o p m e n t in Lee soybeans. Ph. D. Dissertation, Mississippi State Univ., State College (Diss. Abstr. 7 1 : 0 7 3 1 1 ). Beaver, J,S. a n d Johnson, R.R., 1981. Response of determinate and indeterminate soybeans to varying cultural practices in the northern USA. Agron. J., 73: 8 3 3 - 8 3 8 . Bennie, A.T.P., Mason, W.K. a n d Taylor, H.M., 1982. Responses of s o y b e a n s to two r o w spacings and two soil water levels. III.
J.E. Board et al. / Field Crops Research 36 (1994) 13-19
Concentration, accumulation, and translocation of 12 elements. Field Crops Res., 5: 31-43. Board, J.E. and Harville, B.G., 1992. Explanations for greater light interception in narrow- vs. wide-row soybean. Crop Sci., 32: 198-202. Board, J.E. and Harville, B.G., 1993. Soybean yield component responses to a light interception gradient during the reproductive period. Crop Sci., 33: 772-777. Board, J.E., Harville, B.G. and Saxton, A.M., 1990a. Narrow-row seed-yield enhancement in determinate soybean. Agron. J., 82: 64-68. Board, J.E., Harville, B.G. and Saxton, A.M., 1990b. Branch dry weight in relation to yield increases in narrow-row soybean. Agron. J., 82: 540-544. Board, J.E., Kamal, M. and Harville, B.G., 1992. Temporal importance of greater light interception to increased yield in narrowrow soybean. Agron. J., 84: 575-579. Boerma, J.R. and Ashley, D.A., 1982. Irrigation, row spacing, and genotype effects on late and ultra-late planted soybean. Agron. J., 74: 995-999. Boquet, D.J., Koonce, K.L. and Walker, D.M., 1982. Selected determinate soybean cultivar yield response to row spacings and planting dates. Agron. J., 74: 136-138. Caviness, C.E., 1966. Spacing studies with Lee soybeans. Arkansas Agric. Exp. Stn. Bull. 713. Cooper, R.L., 1977. Response of soybean cultivars to narrow rows and planting rates under weed-free conditions. Agron. J., 69: 8992. Egli, D.B., Guffy, R.D. and Heitholt, J.J., 1987. Factors associated with reduced yields of delayed plantings of soybean. J. Agron. Crop Sci., 159: 176-185. Fehr, W.R. and Caviness, C.E., 1977. Stages of soybean development. Iowa Agric. Exp. Stn. Spec. Rep. 80. Fraser, J., Egli, D.B. and Leggett, J.E., 1982. Pod and seed development in soybean cultivars with differences in seed size. Agron. J., 74: 81-85. Hartwig, E.E., 1957. Row width and rates of planting in the Southern states. Soybean Digest, 17: 13-16. Heatherly, L.G., 1988. Planting date, row spacing, and irrigation effects on soybean growth on clay soil. Agron. J., 80: 227-231. Helsel, Z.R., Johnston, T.J. and Hart, L.P., 1981. Soybean production in Michigan. Mich. State Univ. Bull. E-1549. Hunt, R. and Parsons, E.T., 1981. Plant Growth Analysis: Users Instructions for the Stepwise and Spline Programs. Unit of Comparative Ecology, University of Sheffield, Sheffield, UK. Johnson, R.R., 1987. Crop management. In: J.R. Wilcox (Editor),
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Soybeans: Improvement, Production and Uses (2nd ed. ). American Society of Agronomy, Madison, WI, pp. 355-383. Jones, H.G., 1992. Plants and Microclimate. A Quantitative Approach to Environmental Plant Physiology (2nd ed.). Cambridge Univ. Press, Cambridge, England. Kasperbauer, M.J., 1987. Far-red light reflection from green leaves and effects on phytochrome-mediated assimilate partitioning under field conditions. Plant Physiol., 85: 350--354. Kaul, K. and Kasperbauer, M.J., 1988. Row orientation effects on FR/R light ratio, growth and development of field-grown bush bean. Physiol. Plant., 74: 415-417. Loomis, R.S. and Connor, D.J., 1992. Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge Univ. Press, Cambridge, UK. Mason, W.K., Rowse, H.R., Bennie, A.T.P,, Kaspar, T.C. and Taylor, H.M., 1982. Response of soybeans to two row spacings and two soil water levels. II. Water use, root growth and plant water status. Field Crops Res., 5: 15-29. Parvez, A.Q., Gardner, F.P. and Boote, K.J., 1989. Determinate- and indeterminate-type soybean cultivar responses to pattern, density, and planting date. Crop Sci., 29: 150-157. Robbins, K.D., 1991. A user's manual for the LAIS climatic database management system. Louisiana Agric. Exp. Stn. Bull. Computer Applications Series No. 101. Schou, J.B., Jeffers, D.L. and Streeter, J.G., 1978. Effects of reflectors, black boards, or shades applied at different stages of plant development on yield of soybean. Crop Sci., 18: 29-34. Shibles, R.M. and Weber, C.R., 1966. Interception of solar radiation and dry matter production by various planting patterns. Crop Sci., 6: 55-59. Tanner, J.W. and Hume, D.J., 1978. Management and production. In: A.G. Norman (Editor), Soybean Physiology, Agronomy, and Utilization. Academic Press, New York, pp. 157-212. Taylor, H.M., 1980. Soybean growth and yield as affected by seasonal water supply. Agron. J., 72: 543-547. Taylor, H.M., Mason, W.K., Bennie, A.T.P. and Rowse, H.R., 1982. Responses of soybeans to two row spacings and two soil water levels. I. An analysis of biomass accumulation, canopy development, solar radiation interception and components of seed yield. Field Crops Res., 5: 1-14. Wiebold, W.J., Ashley, D.A. and Boerma, J.R., 1981. Reproductive abscission levels and patterns for 11 determinate soybean cultivars. Agron. J., 73: 43-46. Wiggans, R.G., 1939. The influence of space and arrangement on the production of soybean plants. J. Am. Soc. Agron., 31 : 314321.