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SOYBEANS

Potential of Early-Maturing Soybean Cultivars in Late Plantings

Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091 USA

degli{at}pop.uky.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Late planting reduces soybean [Glycine max (L.) Merr.] yields in soybean–winter wheat (Triticum aestivum L.) double-cropping systems. We evaluated the hypothesis that the use of early-maturing soybean cultivars to shift reproductive growth into a more favorable environment would avoid some or all of this yield penalty. Soybean cultivars Hardin and Kasota [maturity group (MG) I], Burlison and Elgin 87 (MG II), Pioneer 9392 and Probst (MG III), and Stressland and Pennyrile (MG IV) planted in 38-cm rows were used in a 3-yr irrigated experiment with two planting dates (early, mid-May; late, late June) at Lexington, KY (38° N lat). Delayed planting reduced yield (7–36%) of all cultivars as a result of fewer seeds m^-2. Cultivars from MG I and II did not produce higher yields in the late plantings. A combination of narrow rows (19 cm) and high seeding rates (105 seeds m^-2) had no effect on yield of cultivars from MGs I and II in either planting date. However, early maturity did provide an earlier harvest date without significant yield loss. Seed number was significantly related to crop growth rate (CGR) during flowering and pod set (r² = 0.36) and to length of flowering and pod set (r² = 0.56). Radiation use efficiency (g dry matter MJ^-1 intercepted photosynthetically active radiation) was generally reduced in the late plantings for MG III and IV cultivars but not for MG I and II. Early-maturing cultivars in an irrigated environment did not reduce the yield penalty associated with late plantings.

Abbreviations: CGR, crop growth rate • MG, maturity group • PAR, photosynthetically active radiation • RUE, radiation use efficiency

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
THE LONGER GROWING SEASONS in upper and mid-south soybean production regions of the USA result in higher potential crop production (defined by the insolation available when temperatures are suitable for plant growth, de Wit, 1967) than in more northern areas. Although soybean cultivars adapted to these long growing seasons generally have longer growth cycles, yield does not necessarily increase proportionately (Kane and Grabau, 1992; Egli, 1993, 1998), making it difficult to translate the high potential of these regions into higher yield. Double-cropping soybean after winter wheat with two crops produced in the same year utilizes more of the potential productivity. As a result, double-cropping is an important production system in the upper and mid-south, constituting as much as 40% of the soybean hectarage in Kentucky (Williamson and Graham, 1983) and about 20% in the mid-south states of Arkansas, Louisiana, Mississippi, and Tennessee (Wesley, 1999).

Double-cropping soybean after winter wheat delays soybean planting beyond the optimum date, and soybean yields are reduced (Kane et al., 1997a; Wesley, 1999). In most soybean production areas of the USA, delayed planting generally shifts reproductive growth into a less favorable environment where days are shorter and temperatures and insolation lower; there may also be less available soil moisture (Baldwin, 1974; Knapp et al., 1980). The yield loss from delayed planting cannot be eliminated by irrigation (Boerma and Ashley, 1982; Egli et al., 1987). Evaluation of late-planted systems with a crop simulation model (Egli and Bruening, 1992) provided evidence that, in the absence of water stress, lower levels of insolation during reproductive growth were a major contributor to the yield loss, with temperature only becoming important for cultivars that matured in late October or early November.

Combining early-maturing soybean cultivars with early planting to shift reproductive growth into a more favorable environment (i.e., avoid drought) increased yields at several southern USA locations (Kane and Grabau, 1992; Bowers, 1995; Heatherly, 1999). It may be possible to use an analogous strategy with late plantings. Early cultivars in late plantings could also shift reproductive growth into a more favorable environment, although in late plantings the primary objective would be an improvement in insolation (Egli and Bruening, 1992) instead of avoiding drought. Its possible that the earlier occurrence of reproductive growth in early cultivars could reduce the yield penalty associated with late planting dates in double-cropping systems.

Previous studies of early-maturing cultivars in late plantings (Kane et al., 1997a) were not irrigated, so rainfall distribution undoubtedly affected the yield differences between early and late cultivars. Consequently, our primary objective was to evaluate the yield potential of early-maturing soybean cultivars planted in late June, as if double-cropped after wheat, in an irrigated environment. A secondary objective was to investigate further the factors responsible for lower soybean yields in late plantings. Minimizing water stress makes it possible to examine the cultivar maturity–planting date interaction in the absence of the dominating effects of the amount and distribution of rainfall on crop growth and yield.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
The cultivars Hardin and Kasota (MG I), Burlison and Elgin 87 (MG II), Pioneer 9392 and Probst (MG III), and Stressland and Pennyrile (MG IV) were used in these experiments. Cultivars in MG III and IV are full-season cultivars in the Lexington area; cultivars in MG I and II mature early and are not commonly grown by producers in Kentucky. Seeds of these cultivars were planted at Lexington, KY (38° N lat), on two dates in 3 yr with the May planting (hereafter referred to as the early planting) occurring on 21 May 1996, 19 May 1997, and 13 May 1998. The late plantings were made on 25 June 1996, 24 June 1997, and 25 June 1998. The soil was a Lanton silt loam (fine-silty, mixed, thermic Cumulic Epiaquoll) in 1996, a Donerail silt loam (fine, mixed mesic Oxyaquic Argiudoll) in 1997, and a Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalf) in 1998. Individual plots were 6 m long with 6 (Hardin, Burlison, Probst, and Pennyrile) or 12 (Kasota, Elgin 87, Pioneer 9392, and Stressland) 38-cm rows. A slightly higher seeding rate of 20 seeds per m of row (53 seeds m^-2) was used for MG I and II cultivars, compared with 16 seeds per m of row (42 seeds m^-2) for MG III and IV to compensate for their expected smaller plant size. A narrow row (19-cm row spacing with 12 rows per plot)–high population (20 seeds per m of row, 105 seeds m^-2) treatment was included in 1997 and 1998 for all MG I and II cultivars. All plots were irrigated with an overhead sprinkler system as needed to minimize water stress. Vacuum-gauge tensiometers at a depth of 0.25 m were used to schedule irrigation. Air temperatures and insolation data were available from a standard weather station located within 2 km of the experiments.

Four samples (0.46 m from each of four bordered rows) of aboveground plant material were taken between growth stage R1 and R5 (Fehr and Caviness, 1977) from Kasota, Elgin 87, Pioneer 9392, and Stressland in 0.38-cm rows. Linear regression analysis of the dry weight (after drying at 60°C)–time relationship was used to estimate the average CGR between growth stages R1 and R5.

Reproductive growth stages (Fehr and Caviness, 1977) of all cultivar–treatment combinations were taken at weekly intervals (2- to 3-d intervals as growth stage R7 approached) on 10 consecutive plants in a bordered row. The dates of R1 and R5 were determined by linear interpolation and R7 was defined as the date when 50% or more of the plants had reached R7.

Interception of photosynthetically active radiation (PAR) was determined on all cultivar–treatment combinations beginning at R1 and continuing at approximate weekly intervals until the interception of each cultivar–treatment combination averaged over 95%. The PAR below the canopy was measured with a 1.0-m line quantum sensor (LI-195A, Li-Cor, Lincoln, NE) placed on the soil surface perpendicular to the row. Above-canopy measurements were made immediately before and after the three below-canopy readings in each plot within ±1 h of solar noon on relatively clear days. The PAR interception was calculated as the difference between the average above- and below-canopy PAR divided by the above-canopy PAR. Radiation use efficiency (RUE) was estimated by regressing aboveground dry matter from each plot on cumulative intercepted PAR. The PAR interception was greater than 95% when the dry matter sampling was initiated or shortly thereafter on all plots, consequently, no adjustments were made for interception. The PAR was taken as 50% of insolation (Tollenaar and Aguilera, 1992). The saturation vapor pressure deficit during flowering and pod set was estimated from daily maximum and minimum temperatures as described by Stockle and Kiniry (1990).

Individual seed growth rate was determined in 1997 and 1998 on the cultivars used to estimate CGR. Approximately 100 fully developed pods in a bordered row were marked with acrylic paint when the seed first started to swell in the pod. Two 20-pod samples of marked pods were taken 14 d apart during the linear phase of seed growth, and the seed dry weight after drying at 60°C was used to estimate the seed growth rate. Nodes (main stem and branches) were counted on 10 consecutive plants in a bordered row at maturity.

Yield was estimated by harvesting 4.3 m of two bordered rows at maturity (four bordered rows in the narrow row–high population treatment) and threshing the plants in a small plot thresher. Seed size (weight per seed) was estimated by weighing a subsample of 200 seeds from the yield sample, and seed number was calculated from the yield–seed size data.

A split-plot design was used, with planting date assigned to main plots, which were arranged in a randomized complete block design with three replications. The cultivar–population treatments were randomized within each main plot. A combined analysis across years was used only when the treatment structure was consistent across years, limiting the combined analysis to variables determined on the four cultivars used to estimate CGR and RUE.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Analysis of variance indicated that the main effects of planting date, cultivars, and planting date x cultivar interactions were usually significant for yield, yield components (Table 1) , and vegetative and reproductive growth characteristics (not shown). Further evaluation of the data demonstrated that the response of cultivars within each MG to planting date was very similar in all 3 yr; consequently, much of the data was averaged across cultivars for each MG. The combined ANOVA across years for vegetative and reproductive growth characteristics demonstrated that the interactions with years were not significant or, if significant, involved only changes in the magnitude of the response. Consequently, these characteristics were averaged across years (Table 2) .

Late planting significantly (P = 0.05 or 0.10) reduced yield (13–36%) in all comparisons except MG I in 1998 (Table 3) . These reductions are consistent with the results of other late-planted irrigated experiments (Boerma and Ashley, 1982; Egli et al., 1987). In our study, the reduction in yield was associated with fewer seeds per unit area, but there were no consistent effects of late planting on seed size (Table 3). The importance of seed number was also emphasized in previous reports (Egli et al., 1987; Steele and Grabau, 1997), indicating that the processes involved in determining seed number may play an important role in the response to late planting.

A combination of photoperiod and temperature is probably responsible for the early flowering and shorter vegetative growth phase of late-planted soybean (Board and Hall, 1984). The consequences of early flowering [i.e., shorter plants, fewer nodes (Pendleton and Hartwig, 1973), and smaller vegetative mass at the beginning of seed filling (Growth stage R5) (Kane et al., 1997b)] were associated with lower yields in late plantings. Similar trends occurred in these experiments (Table 2). Consequently, we evaluated the early-maturing cultivars in 19-cm rows at double the plant population in 38-cm rows to counteract potential yield limitations from small plants by increasing vegetative mass and nodes per unit area. The narrow row–high population treatment increased nodes per square meter by an average of 57% (data not shown) and probably increased vegetative mass (Board et al., 1996), although it was not measured. However, this treatment did not consistently produce higher yields in either planting date (Table 4) ; in fact, yields were significantly (P = 0.05) higher in only 3 of 16 comparisons. The yield reduction from late planting was not consistently smaller in the narrow row–high population treatment.

Seeds per unit area in soybean is closely associated with canopy photosynthesis (Christy and Porter, 1982) and CGR (Ramseur et al., 1985; Egli and Zhen-wen, 1991; Board et al., 1999) during flowering and pod set. The significant (P = 0.02) linear relationship between seed number and CGR found here (Fig. 1) is consistent with previous reports. Although significant, the regression only accounted for 36% of the variation in seed number. Deleting the two observations that deviated widely from the regression line from the analysis only increased the r² to 0.47. Genetic differences in individual seed growth rate affect seed number (Egli and Zhen-wen, 1991; Egli, 1993), but cultivar differences in seed growth rate in these experiments were generally small (data not shown), and there is little evidence for a cultivar effect in Fig. 1. Apparently other plant or environmental factors directly affected seed number instead of affecting it indirectly through CGR. These data indicate that the determination of seed number may be more complex than the simple model describing seed number as a function of assimilate availability (Egli, 1998).

View larger version (20K): [in this window] [in a new window]   Fig. 1 Relationship between crop growth rate during flowering and pod set and seeds m-2 at maturity, 1996 to 1998, n = 24. The regression was significant at P = 0.002

Radiation use efficiency measures the ability of a crop community to convert solar energy into dry matter and, as such, is an important aspect of crop productivity (Monteith, 1972). Variation in RUE may help explain the variation in productivity across planting dates. The RUE, based on intercepted PAR, varied from 1.2 to 2.0 g MJ^-1 in these experiments (Table 5) . These efficiencies are similar to previous reports from this location (1.3–1.7 g MJ^-1) with a different group of cultivars (Egli, 1993) and to those reported by Board et al. (1994) (maximum of 1.7 g MJ^-1, converted to a PAR basis). However, many of the values in Table 5 are lower than other reports for soybean. Shiraiwa and Hashikawa (1993) reported a range of 2.1 to 2.5 g MJ^-1, while Sinclair and Horie's (1989) theoretical analysis resulted in a maximum value of 2.4 g MJ^-1 (converted to a PAR basis). Great care was taken to minimize water stress in our experiments through frequent irrigation, so water stress should not have limited crop growth and RUE, making it difficult to identify the cause of the generally low RUEs at this location.

The RUE of Pioneer 3932 and Stressland was frequently (4 of 6 comparisons) significantly less in the late plantings, so part of the reduction in yield and seed number of these cultivars in the late planting was a result of this loss of efficiency. This reduction did not occur in the early cultivars and could not be accounted for by variation in environmental conditions during flowering and pod set. The correlations between RUE and mean temperature (r = 0.23), solar radiation (r = -0.12), and saturation vapor deficit (r = -0.13) during flowering and pod set were not significant (P = 0.05, n = 24). Cirilo and Andrade (1994) reported planting date effects on RUE in maize (Zea mays L.), but they were able to associate the variation in RUE with variation in temperature. Whether the decrease in RUE with delayed planting reflects changes in canopy structure (Shiraiwa and Hashikawa, 1993; Yunusa et al., 1993) or in the distribution of N in the canopy (Sinclair and Shiraiwa, 1993) that are associated with the smaller plants in the late planting (Table 2) is not known, but cultivar differences in plant size did not appear to affect RUE in previous experiments (Egli, 1993).

There was also a significant linear relationship between the length of flowering and pod set (R1–R5) and seed number (Fig. 2) . Delayed planting tended to shorten flowering and pod set, which accounted for some of the reductions in seed number in late plantings. This period also tended to be shorter on the early cultivars. Replacing the length of flowering and pod set with the total insolation over the period resulted in only a slight improvement in the fit of the linear regression model (r² increased from 0.56 to 0.61), suggesting that most of the value of the longer period did not come from simply exposing the plant community to more insolation. There was some association between CGR and the length of the flowering and pod set period (r = 0.42, n = 24, P = 0.04). A multiple regression model containing both CGR and the length of flowering and pod set accounted for 71% of the variation in seed number, and both terms in the model were significant (P = 0.05). These results indicate that each variable may have some independent effect on seed number. Apparently both the daily assimilate supply (estimated by CGR) and the length of the period when flowers are available to initiate fruit growth are determinants of seed number. A long flowering and pod set period may partially compensate for a low CGR, while a short flowering and pod set period may limit seed number with a high CGR. However, ultimately there must be some balance between assimilate supply and sink size (seed number and total seed growth rate) (Egli, 1998). Much remains to be learned about the role of the length of the flowering and pod set period in determining this balance.

View larger version (20K): [in this window] [in a new window]   Fig. 2 Relationship between the length of flowering and pod set (growth stage R1–R5) and seeds m-2 at maturity, 1996 to 1998, n = 24. Data were averaged across cultivars within a maturity group. The regression was significant at P = 0.0001

Although seed fill duration is an important determinant of yield (Egli, 1998), delayed planting did not shorten the seed filling period (data not shown). The average seed filling period (R5–R7) was 33 d for both planting dates. These results are consistent with earlier reports (Egli et al., 1987) and also with the minimal contribution of variation in seed size to the reduction in yield from delayed planting in these experiments (Table 2).

The results of 3 yr of experiments did not support our original hypothesis. Early-maturing cultivars in an irrigated environment did not alleviate the yield penalty associated with the late plantings that are characteristic of double-cropping soybean after wheat. Flowering and pod set (R1–R5) was the critical period for determination of yield in late-planted soybean. Although this period occurred earlier in the growing season for MG I than for MG IV (average of 9 d, Table 2), the 20-yr weather average from Lexington indicates that this earliness would result in almost no improvement in temperature (+0.4%) or insolation (+0.5%) during the period. In contrast, the initiation of seed filling (R5) was advanced by 14 d, increasing temperature during this period by an average of 14% and insolation by an average of 19%. Unfortunately, the seed filling period did not seem to be a critical period for the determination of yield in late plantings in these experiments.

The only advantage of the early-maturing cultivars was their earlier maturity without significant yield loss. Use of early-maturing cultivars in a double-cropping system would allow for earlier soybean harvest without sacrificing productivity, an option that may be attractive to some producers.

In these experiments, seed number was associated with both CGR and the length of flowering and pod set. The association with CGR has been widely reported (Ramseur et al., 1985; Egli and Zhen-wen, 1991; Board et al., 1999) and is consistent with the general concept that seed number is determined by the availability of assimilate from photosynthesis (Egli, 1998). Since the developing seed gets most of its assimilate from current photosynthesis, it is not surprising that sink size (seed number) and eventually the total seed growth rate (g m^-2 land area day^-1) would be related to the daily production of assimilate (i.e., rate of canopy photosynthesis). Given this dependence of seed number on the rate of growth, it is hard to understand why there is a relationship between the length of flowering and pod set and seed number. Any explanation of the importance of the period length will probably depend on a better understanding of the flowering and pod set process at individual nodes (Bruening and Egli, 1999). Although there is much that remains to be learned about the determination of seed number in soybean, it seems that both the daily supply of assimilate and the length of the flowering and pod set period are important.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Published with approval of the Director of the Kentucky Agric. Exp. Stn. as Paper 99-06-88.

Received for publication June 28, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 

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