Field Crops Analysis of growth and yield of maize, sunflower and … · 2015-12-08 · ELSEVIER Field Crops Research 41 (1995) 1-12 Field Crops Research Analysis of growth and yield

E L S E V I E R Field Crops Research 41 (1995) 1-12

Fie ld Crops R e s e a r c h

Analysis of growth and yield of maize, sunflower and soybean grown at Balcarce, Argentina

Fernando H. Andrade Unidad Integrada EEA INTA Balcarce, Facultad de Ciencias Agrarias UNMP, CC 276 (7620) Balcarce, Buenos Aires province, Argentina

Received 31 May 1994; accepted 11 November 1994

Abstract

The objectives of this study are: ( 1 ) to analyze the capacity of maize, sunflower and soybean to produce dry matter and seed yield, including the responses to shading during flowering, and shading and thinning during seed filling; and (2) to evaluate effects of plant density and sowing date on growth and yield. This report integrates data obtained during seven years of research at Balcarce Experimental Station, Argentina. In these experiments, nutrients and water were not limiting to growth. Maize produced the most biomass because of sustained ground cover and high light conversion efficiency. It also had the largest harvest index on a dry weight basis. When dry matter was expressed in glucose equivalents, differences among harvest indices for the three crops were smaller. Flowering in maize, seed filling in soybean and flowering and seed filling in sunflower were critical periods in determination of grain yield. Sunflower had more capacity than maize to compensate for fewer grains through greater grain weight. Maize and sunflower had low stability in grain number at less than optimal plant densities. Finally, delays in sowing date significantly reduced grain yields of the three crops. These reductions were due to decreases in number of grains per m 2 and in grain weight.

Keywords: Crop comparison; Maize; Management practices; Soybean; Sunflower; Yield

1. Introduct ion

Grain yield is related to light interception, to radiation-use efficiency and to photoassimilate partitioning ( Charles-Edwards, 1982; Gifford et al., 1984). Aspects of this simple physiological approach have been used to analyze growth of maize (Zea mays L.), sunflower ( Helianthus annuus L. ), and soybean ( Glycine max L. Merr.) crops (Shibles and Weber, 1966; Williams et al., 1968; Kiniry et al., 1989; Trapani et al., 1992). However, comparative analyses of physiological determinants of growth and yield among the three crops, accounting for differences in photosynthate requirements per unit of biomass production, have not been made.

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Critical stages for grain yield determination have been defined for maize (Tollenaar, 1977; Kiniry and Ritchie, 1985), sunflower (Chimenti and Hall, 1992) and indeterminate soybeans (Constable and Hearn, 1978; Snyder et al., 1982). To our knowledge, no comparative responses of the three crops to growth reductions at flowering and at grain filling have been reported. Moreover, these three species are important crops of local cropping systems and more information is needed about effects of stresses that occur at various times in the season. Similarly, while maize, sunflower and soybean have been studied individually for their responses to variation in plant population density (e.g. Robinson, 1978; Parks et al., 1982; Tetio Kagho and Gardner, 1988) and to sowing date (e.g. Hunger, 1980;

2 F.H. Andrade / Field Crops Research 41 (1995) 1-12

Knapp and Shaw Reid, 1981; Anderson and Vasilas, 1985) their responses have not been compared and interpreted in terms of radiation interception, radiation- use efficiency and biomass partitioning.

The objectives of the present paper are: ( 1 ) to analyze the capacity of maize, sunflower and soybean to produce dry matter and seed yield, including their responses to shading during flowering and shading and thinning during seed filling; and (2) to evaluate the effects of plant density and sowing date on growth and yield of these three crops. The work builds on similar- ities and contrasts among these crops in ecophysiological traits related to crop productivity. Grain yield is analyzed in terms of (i) radiation interception ( e i ) ,

(ii) efficiency of conversion of intercepted radiation into biomass (radiation-use efficiency, ec) and (iii) dry matter partitioning between vegetative and reproductive structures, taking into account the amount of glucose required for synthesis.

This report integrates data obtained during seven years of research at Balcarce Experimental Station, Buenos Aires Province, Argentina. Nutrients and water were not limiting to growth. Even though the experiments were independent and structured around different hypotheses, they had a common aim at understanding processes involved in growth and yield formation. Considerable intraspecific genetic variabil- ity is found with each of the traits considered here. The cultivars used in these experiments do, however, rep- resent the general features of maize, sunflower and indeterminate soybean adapted to the southeast of the Buenos Aires province. Relevant literature is used to explain, complete or reaffirm the data obtained locally.

2. Methods and material

The data presented in this work were obtained in experiments conducted at the INTA Balcarce Experi- mental Station (37°45'S, 58°18'W, altitude 130 m) from the 1986/87 through the 1993/94 growing seasons, from which treatments without water or nutrient limitations were chosen. Table 1 presents climatic data for this period. This area is characterized by low average temperatures during the growing season and a frost- free period of about 150 days. The soil was a Typic Argiudol (USDA Taxonomy) with an organic matter content of 5.6%.

In one set of experiments, maize hybrid SPS 240 was sown in mid-October. Plant densities at harvest in the 86/87, 87/88, and 88/89 growing seasons were 7.8, 8.3 and 9.1 plants m -2, respectively. The number of replications were eight, three, and four for the first, second and third years. Sunflower hybrids Topflor ( 88 / 89 growing season) and Dekalb G 100 (91 /92 growing season) were sown toward the end of October with final plant densities of 6.7 plants m-2 and with four replications. Soybean cultivar Asgrow 3127 was sown on 10 November 1986 and 11 November 1987 with a final plant density of 33 and 27 plants m -a, respectively. The number of replications were five and three, respectively. The size of the experimental units exceeded 40 m 2 in all cases. Weeds and insects were adequately controlled. Above-ground dry-matter accumulation was followed during the growing cycle by taking samples of 10 (maize and sunflower) or approximately 30 (soybean) plants every 15 to 20 days. These samples were oven-dried (with air circulating at 60°C) to constant weight.

Global radiation was monitored with a pyranometer (SIAP Bimetallic, Model PL1; La Plata, Argentina) and transformed to photosynthetically active radiation by multiplying by 0.48. Percent PAR interception was estimated from photosynthetic photon flux measurements (PPFD) as 100× [1 - ( I t / Io ) ] , where It is the incident PPFD at ground level and I o is the incident PPFD at the top of the canopy. The values for It and Io were obtained with a LI-COR 188 B radiometer (LI- COR, Lincoln, NE) connected to a line quantum sensor, LI-COR 191 SB. Determination were taking every 15 days following the technique described by Gallo and Doughtry ( 1986) for sensor placement and number of observations. The measurements were confined to the midday period (1130-1300 h) and were taken on sunny days only. Incident PAR was multiplied by the fraction of intercepted radiation to obtain the PAR intercepted by the crop. Radiation-use efficiency was calculated as the slope (forced through the origin) of the relationship between cumulative dry matter and cumulative intercepted PAR. Grain yield was determined at physiological maturity by harvesting 10 m 2 of maize and sunflower or 5.6 m 2 of soybean crop. Additional details on irrigation, fertilization, weed control, sampling, and other matters, were reported in Andriani et al. ( 1991 ), Uhart and Andrade ( 1991 ) and Andrade et al. (1992).

F.H. Andrade / Field Crops Research 41 (1995) 1-12

Table 1 Daily solar radiation and daily maximum and minimum temperature. Monthly means for 1986/1993

Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May

Solar radiation (MJ m 2 day- ~) 13.5 16.9 21.0 23.5 23.4 20.9 16.2 12.5 8.2 Max. temperature (°C) 16.0 19.0 22.4 25.4 27.4 26.7 24.3 20.3 15.6 Min. temperature (°C) 4.9 7.2 9.8 12.1 13.8 13.7 12.2 8.6 5.5

In another set of experiments, conducted with optimal management during the 1992/93 and 1993/94 growing seasons, effects of shading and thinning on grain yield of the three crops were evaluated. Maize hybrid Dekalb 636 was sown in early to mid-October with a final density of approximately 8.5 plants m-2; sunflower hybrid Dekalb G100 was sown in mid- to late October with a final density of 5.7 plants m - 2; and soybean cultivar Asgrow 3127 was sown in early November with a final plant density of 48 (first year) or 34 ( second year) plants m-2. Three treatments were applied to each crop: (1) 45% shading during seed filling; (2) thinning during seed filling; and (3) an untreated control. Plots were shaded with black synthetic mesh cloth stretched above the crop on cane and wire structures. Shading was applied from 17 days after beginning of flowering to physiological maturity in maize and sunflower, and from R5 to physiological maturity in soybean (Fehr and Caviness, 1977). In the thinning treatment, plant density was reduced to one fourth of the original plant density. A fourth treatment consisting of 45% shading during a one-month period encompassing flowering in maize and sunflower and right after the beginning of flowering in soybean was included in the second year. The experiment was conducted with four replications.

Effects of sowing date and planting density were evaluated in a third set of experiments conducted during the 1990/91 and 1992/93 seasons. Experimental details were similar to those described above. Sun- flower hybrid Dekalb G 100 and maize hybrids Dekalb 636 and Dekalb 4F91 were sown in mid-September, mid-October, mid-November and mid-December at optimal plant densities. These same hybrids also were sown at optimal dates with final plant densities of 5.7 and 8.6 m -2 for maize and 3.7 and 6.7 m -2 for sunflower. In addition, the performance of soybean cultivar Asgrow 3127 sown on 29 December 1986 was compared with that of early sowings. A detailed description of this experiment was given in Bodrero (1988).

Nitrogen in plant material was determined following the method reported by Nelson and Sommers (1973). Oil content was determined by the Standart AOCS method (AOCS, 1978) and by nuclear magnetic res- onance analysis (Robertson and Morrison, 1979) in sunflower grains. Ash content was determined by weight after the complete combustion of the plant sam- ple. Carbohydrate content was calculated as the resid- ual fraction. Lignin concentration in residues was assumed to be 9% in all crops.

Data were processed by analysis of variance proce- dures and by linear regression analysis. Appropriate standard errors of the means were calculated. Curves in the figures were fit by eye.

3. Results and discussion

3.1. Crop growth

Dry matter production

Fig. 1 illustrates above-ground dry matter accumulation of the three crops when grown without limitations of water or nutrients. Final production was 2250, 1350 and 1200 g m -2 for maize, sunflower and soybean, respectively. Similar values have been reported in the literature for irrigated maize (Williams et al., 1968; Muchow et al., 1990; Liang et al., 1991), sunflower (Cox and Joliff, 1986; Trapani et al., 1992), and soybean (Cox and Joliff, 1986).

At the beginning of their respective growing cycles, sunflower and maize had similar rates of dry matter accumulation and soybean showed the lowest rates. Crop growth rates, measured from 40 to 80 days after emergence, were 30 + 1.2, 31 + 1.4 and 18 _+ 0.8 g m - 2 for maize, sunflower, and soybean, respectively. Maize had the largest crop growth rates during grain filling, which explains why sunflower had accumulated at flowering a greater proportion of the total dry matter produced during its growing season than maize (80

F.H. Andrade / Field Crops Research 41 (1995) 1-12

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Fig. 1. Above-ground dry matter accumulation as a function of days after emergence. (a) Maize hybrid SPS 240; (b) sunflower hybrids DK G100 and Topflor; (c) soybean cultivar Asgrow 3127; and (d) comparison of maize (M), sunflower (G) and soybean ( S ). Exper- iments conducted at INTA Balcarce Experimental Station without water or nutrient limitations. Vertical bars indicate SE of the means. F = flowering.

and 40% respectively). By contrast, the indeterminate soybean had accumulated only 9% of its total dry matter production by flowering. Soybean and maize accumulated approximately 30 g m -2 of nitrogen in above- ground parts by physiological maturity compared to only 20 g m-2 for sunflower (Andrade et al., 1994). Biological fixation provided one third of the total nitrogen accumulated by soybean (Gonzalez, 1994). Maize accumulated the most phosphorus during the season (5.4 g m-2) , compared to soybean (3.1 g m -2) and sunflower (2.0 g m -2) (Andrade et al., 1994).

Radiation interception Fig. 2 illustrates interception of photosynthetically

active radiation (PAR) by the crops, over the season. Early in its growing cycles, sunflower surpassed maize in PAR interception. This is explained by differences in the crops in canopy leaf arrangement and leaf expansion. Maize, with its more erectophile leaves (see Wil- liams et al., 1968; Pepper et al., 1977; Rawson et al., 1984; Zaffaroni and Schneiter, 1989), has a smaller extinction coefficient than sunflower. Thus, it needed more leaf area to intercept a given proportion of the incoming radiation. In addition, the maize crop had less favorable thermal conditions for leaf expansion than sunflower at the beginning of its growth (Table 1).

Base temperatures during the vegetative period are 8°C for maize (Cirilo and Andrade, 1992), rather lower for sunflower ( <5°C; Goyne et al., 1989) and from 6 to 10°C for soybean (Hesketh et al., 1973; Wilkerson et al., 1985).

With optimal sowing dates and appropriate densities, all of the crops eventually reached 95% interception of the incoming radiation, allowing attainment of their respective maximal crop growth rates. The maize crop maintained this for a longer period of time, however, from just before flowering until a few days before physiological maturity. By contrast, percentage radiation interception by sunflower dropped markedly during its reproductive growth. Sunflower leaves senesce rapidly after flowering losing their photosynthetic capacity (Rawson and Constable, 1980). Soybean, a species that presents strong monocarpic senescence (Sinclair and de Wit, 1975), behaved in an intermediate way.

Cumulative photosynthetically active radiation intercepted by the crops over the course of the growing season was 820, 700 and 720 MJ m - : for maize, sunflower and soybean, respectively.

Conversion efficiency of intercepted radiation to above-ground biomass

Conversion efficiency was highest for maize (2.77 ___ 0.08 g M J- J ), lowest for soybean ( 1.74 -t- 0.06 g MJ- ' ) and intermediate for sunflower (Fig. 3). Sim-

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Fig. 2. Radiation interception (PAR) by the crop, expressed as a percentage of incident radiation, as a function of days after emergence for (a) maize, (b) sunflower, (c) soybean, and (d) comparison of maize (M), sunflower (G) and soybean (S). Vertical bars indicate SE. For more detail, see caption of Fig. 1.

20 40 60 80 100 120 140


F.H. Andrade / Field Crops Research 41 (1995) 1-12 5

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Fig. 3. Dry matter accumulation as a function of intercepted cumulative PAR for (a) maize, (b) sunflower, (c) soybean, and (d) comparison of maize (M), sunflower (G) and soybean (S). Average conversion efficiencies are 2.77 + 0.08, 1.74 + 0.06, 2.73 + 0.22 and 1.32+0.31 g MJ 1 for maize, soybean, sunflower at early stages and sunflower at late stages, respectively. For more details see caption of Fig. 1.

ilar values have been reported in the literature (Gosse et al., 1986; Kiniry et al., 1989; Trapani et al., 1992). The scatter in the maize data was mainly attributable to variation among years (Andrade et al., 1992).

The efficiencies were small during early stages of growth which may be explained by low temperatures (mainly for maize, Andrade et al., 1993a), dry matter partitioning to roots during this period and saturation of photosynthesis (mainly in sunflower and soybean, Trapani et al., 1992).

During reproductive growth, efficiencies were highest for maize and lowest for sunflower. Maize kernels had a high carbohydrate content (84.5%); whereas

Table 2

sunflower cipsellas had high lipid content (46.5 % ) and soybean seeds were high in both protein (36%) and lipid ( 19.1% ) (Table 2). To produce a gram of maize, sunflower or soybean grain, 1.39, 2.22 and 1.93 glucose were required, respectively (Table 2). Sinclair and de Wit (1975) calculated similar values. Thus, sunflower grain is more expensive, in terms of glucose required for synthesis, than maize grain. This explains, at least partly, the substantial drop in sunflower conversion efficiency (the slope of the relationship between dry matter accumulation and cumulative intercepted radiation) following the onset of lipid synthesis after flowering (Fig. 3). By contrast, maize varied little in the energetic value of its dry matter over the growing cycle (Table 2; Varlet Grancher et al., 1982). Soybean conversion efficiency was constant over its growing cycle despite the higher energetic value of its reproductive than of its vegetative biomass. This is probably explained by its indeterminate growth habit that results in its reproductive and vegetative growth being coin- cident over a long period.

Glucose equivalents required for total biomass production were 3046, 2155 and 1872 g m -2 for maize, sunflower and soybean, respectively (Table 2). Con- version efficiencies expressed as g glucose M J - l of intercepted PAR were 3.71, 3.08, and 2.6 for maize, sunflower and soybean. These values are independent of biomass composition and differ due to differences in photosynthetic efficiencies and canopy structures.

The erectophile leaves of maize allow a more uni- form distribution of the incoming radiation within the crop canopy. This is an advantage for canopy photosynthesis and conversion efficiency at high values of leaf area index. Reductions of 30% in crop growth or productivity have been calculated in response to

Grain and residue yield, grain composition, glucose required for synthesis of grain and residues and harvest index for maize, sunflower and soybean. Glucose equivalents were calculated according to Penning de Vries (1974). Grain and residue composition were obtained from different experiments conducted under irrigation and fertilization with maize hybrid SPS240, sunflower hybrid Dekalb G100 and soybean cultivar Ax3127

Grain (g m- 2) Residues (g m- 2) Harvest index

dry matter lipid protein carbohydrates ash glucose equiv, dry matter glucose equiv, dry matter glucose equiv.

Maize 1013 40.5 101 856 14.8 1409 1237 1637 0.45 0.463 Sunflower 392 182.3 63 133 13.1 870 958 1285 0.29 0.404 Soybean 444 84.8 160 168 31.4 856 756 1016 0.37 0.457


2500 (0.45)

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Maize Sunf lower Soybean

Fig. 4. Grain yield (0% moisture), total above-ground dry matter (including fallen leaves) and harvest index (in brackets) for maize, sunflower and soybean. Hybrids SPS 240 (maize) DK G100 and Topflor (sunflower) and soybean cultivar Asgrow 3127 were grown without limitations of water or nutrients. Maximum SE of the means for grain yield were 25.4, 24.9, and 17.0 g m 2 for maize, sunflower and soybean, respectively.

increases in extinction coefficient with a leaf area index of 5 (Pearce et al., 1967; Pepper et al., 1977). In addition, maize does not have detectable photorespiration. Thus, its leaf photosynthetic rate is 30 to 40% greater than those of C3 species (Laing et al., 1974). Even though sunflower has planophile leaves and C3 photosynthetic characteristics, it had high conversion efficiencies in glucose equivalents. It has high photosynthetic efficiency, especially during the vegetative period (Rawson and Constable, 1980) because of (i) elevated specific rubisco activity, (ii) high sto- matal conductance and (iii) efficient chloroplast elec- tron transport (CETIOM, 1983; Connor and Sadras, 1992).

Grain y ie ld

Fig. 4 illustrates grain yields (0% moisture) and total biomass production for the three crops. Similar maximal grain yields have been reported (Duncan et al., 1973; Robinson, 1978; CETIOM, 1983; Parks et al., 1989; Muchow et al., 1990; Cooper et al., 1991). Har- vest index was greatest for maize (0.45), intermediate for soybean (0.37) and least for sunflower (0.29), similar to values presented in the literature (Ashley and Ethridge, 1978; CETIOM, 1983; Pandey et al., 1984; Cox and Joliff, 1986; Sinclair et al., 1990; Liang et al., 1991 ). The differences in harvest index were mainly the result of differences in energetic values per unit of reproductive biomass. When harvest index was expressed in glucose required for synthesis, much more

similar values were obtained for the three crops (Table 2). Maize yields were reduced dramatically (DK 636 by more than 50%; P < 0.05) when incident radiation was reduced by shading (45%) during a 30-day period around flowering, but other hybrids are more tolerant (Uhart and Andrade, 1991 ). Sunflower yields dropped approximately 40% in response to similar shading treatments (P < 0.05 ). In contrast, soybean yields were hardly affected (9% reduction, differences not significant at P < 0.05) by shading during 30 days beginning at flowering. These effects were mainly explained by differences in number of grains set.

Shading during seed filling reduced seed yield per plant in all three crops (P < 0.05 ). The reductions were greatest for sunflower, intermediate for soybean and least for maize (Table 3). In sunflower and soybean grain number was affected more than grain weight (data not shown). In sunflower, however, lipid concentration dropped from 46.5 to 39.1%. Yield per plant was greatly increased in soybean and sunflower and hardly affected in maize when source per plant during seed filling was increased by thinning (Table 3 ). Grain number was the yield component mostly affected in soybean and grain weight in sunflower. Vasilas et al. (1989) found similar results for soybean. Thus, mod- ification of source capacity during grain filling had more impact on plant yield in sunflower and soybean than in maize.

The most critical periods for yield determination were flowering in maize, seed filling in soybean and

Table 3 Relative seed yield per plant as a function of source intensity during grain filling. In the thinning treatment, plant density was reduced to 1/4 of the original value. Shading intensity was 45%. Means followed by different letters differed at P < 0.05 according to Duncans multiple range test

Crop Relative seed yield per plant (%)

shading control thinning

Maize year 1 76 b 100 a 100 a year2 82c 100b l17a mean 79 100 109

Sunflower year 1 64 c 100 b 153 a year2 66c 100b 135a mean 65 100 144

Soybean year 1 75 c 100 b 190 a year2 75c 100b 186a mean 75 100 188


¢c

200

100

u m ' - ~ .

20 40 60 80 1 O0

Cipsellas/haad or kernels/ear (%)

Fig. 5. Seed weight response (in percent) to relative reductions in number of cipsellas per head in sunflower ( • ) or kernels per ear in maize (O) achieved by preventing pollination. Data from Leon et al. (1987) and Arguissain and Andrade (unpublished). Vertical bars indicate SE.

both in sunflower. The physiological condition of the crop at flowering greatly affected final grain number per m 2 in maize and the number of filled cipsellas in sunflower. Data presented by Tollenaar (1977), Edmeades and Daynard (1979), CETIOM (1983), Fischer and Palmer (1984), Uhart and Andrade (1991), Chimenti and Hall (1992), and Connor and Sadras (1992) agree with these results. Because these crops have determinate growth habits, grain yield dropped when grain number was reduced at flowering. However, the seed-filling period was more critical in sunflower.

The number of seeds per m e in soybean (indeterminate cultivar) was a function of the physiological condition of the crop at advanced stages. Stress during early reproductive stages had little impact on seed yield of soybean because the crop continued to produce reproductive structures after released from stress and the eventual decrease in number of pods was partially compensated by an increase in seeds per pods and in seed weight (data not shown). In contrast, grain filling was more critical in this crop. This agrees with data presented by Andriani et al. ( 1991 ).

Sunflower had more capacity than maize to compensate through heavier grains for a low number of grains per unit area. In other experiments, an 80% reduction in number of kernels per ear or in number of cipsellas per head (achieved by preventing pollination, a treatment that did not affect potential grain weight) increased grain weight only 30% in maize (Arguissain and Andrade, unpublished data) and up to 100% in sunflower (Leon et al., 1987) (Fig. 5). Kiniry et al. (1990) found a similar behavior for maize.

3.2. Plant density at harvest and crop yield

Decreasing plant density from 8.6 to 5.7 plant m-2 in irrigated maize resulted in less radiation interception at the critical flowering stages (Fig. 6). This reduced ground cover was associated with smaller crop growth rates. Moreover, there is evidence that conversion efficiency in maize is lower with low plant densities (Andrade et al., 1993b). On the other hand, reducing plant density from 6.7 to 3.7 plants m -2 in irrigated sunflower did not affect radiation interception during the critical stages of grain yield determination, but only at early growth stages (Fig. 6). In the literature, similar responses have been reported for soybean over a range from 40 to 6 plants m -2 (Shibles and Weber, 1966). Maize presented the lowest stability in percentage interception when plant density was reduced because it does not tiller much and does not show plasticity in leaf expansion. Sunflower's high stability in ground cover with variations in plant density results from variations in leaf size while that of soybean derives from variations in branch number.

Reducing sunflower (DK G 100) density from 6.7 to 3.7 plants m -2 reduced the number of full achenes by 1300 m -e (8800 to 7500; P < 0.05) and significantly

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Fig. 6. Fractional radiation (PAR) interception as a function of days after emergence for sunflower DK G100 at 3.7 (C]) and 6.7 ( I ) plants m -2 and for maize DK 636 at 5.7 (©) and 8.6 ( 0 ) plants m-2. October sowing dates. Verticals bars indicate SE of the means. Experiments conducted at Balcarce with irrigation and fertilization.

8 F.H. Andrade/Field Crops Research 41 (1995) 1-12

1 5 0 0

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,~ 5o0 --I ....

....... ~ "'L'LL'Z'.L'~

0 i J i i i i

20 40 60 80 1 O0 120

S o w i n g da te (days af ter 1 sep tember )

Fig. 7. Grain yield (14% moisture) of maize (O), sunflower ( • ) and soybean ( • ) as a function of sowing date expressed as days after 1 September. Hybrids DK 636 (maize) and DK G100 (sunflower) and soybean cultivar Asgrow 3127 were grown at Balcarce with no limitations by water and nutrients. Final plant densities were 8.5, 6.7, and 30.0 plant m -2 for maize, sunflower, and soybean, respectively. SE are 24.6 and 22.6 g m -a to compare sowing dates within maize and sunflower crops, respectively, and 19.0 and 10.0 g m -2 for early and late soybean, respectively.

increased achene weight by 26%. Grain yield did not differ significantly between the two densities. On the other hand, changing plant density from 8.6 to 5.7 m - 2 in maize (DK 4F91) reduced grain yield because it caused a decrease of 760 grains m -2 (3690 to 2930; P < 0 . 0 5 ) and only a 3% increase in individual grain weight. Hybrid DK 636 behaved similarly.

Thus, both maize and sunflower had low stability in grains or cipsellas per m 2 in response to decreases in plant density below optimal values. The literature presents similar results (Williams et al., 1968; Robinson, 1978; CETIOM, 1983; Tollenaar et al., 1992). Low ground cover in maize at low densities did not fully explain its low reproductive plasticity as the number of grains set per unit PAR intercepted and per unit crop

100

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0

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I I I I I I 20 40 60 80 1 O0 120

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Fig. 8. Days from emergence to flowering for maize (O) and sunflower ( • ) and from emergence to R5 in soybean ( • ) as a function of sowing date expressed as days after 1 September. Hybrids DK 636 (maize) and DK G100 (sunflower) and soybean cultivar Asgrow 3127 were grown at Balcarce with irrigation, fertilization and appropriate plant densities.

growth rate over a 30-day period around silking was also less at suboptimal densities (Andrade et al., 1993b). Sunflower reproductive plasticity, expressed as cipsellas per unit area, did not follow its leaf plasticity. This contrasted with the behavior of soybean, which has a high capacity to produce additional reproductive structures per plant in response to such treatments (Parks et al., 1982).

Although no differences in grain yield between densities of 3.7 and 6.7 plants m -z were found for DK G100 sunflower grown under irrigation and fertilization in Balcarce, low densities frequently limit sunflower yields in high-yielding environments (Robinson, 1978; CETIOM, 1983) because the larger number of fruits per head and larger fruit weight do not fully compensate for fewer heads per ha. At plant densities greater than optimum, maize had low yield stability because ear development was suppressed strongly. Detasseling and the use of male sterile isolines partially reversed these effects (more details are given in Frugone et al., 1992). Mostut and Marais (1982) reported similar results. Early sowings were more tolerant of high densities than late sowings (Cirilo and Andrade, unpublished data) and hybrids showed vari- ability in this trait. The low yield stability of maize with variations in density results in the need for careful adjustment of plant density to environmental conditions and input levels.

3.3. Sowing date and crop yield

Delayed sowing resulted in substantial reductions in grain yields of all three crops (Fig. 7), as was also reported by Duncan et al. (1973), Hunger (1980), Knapp and Shaw Reid ( 1981) and Anderson and Vas- ilas (1985). With delayed sowing, development was hastened (Fig. 8) because the crops encountered higher temperatures during the vegetative growth (Major et al., 1975; Goyne et al., 1989; Cirilo and Andrade, 1992) and, for soybean, also because of photoperiodic effects mainly during reproductive stages (Major et al., 1975). Shortening of the growing cycle decreased the amount of radiation intercepted during the growing season and, thus, total dry matter at harvest (data not shown).

Delays in sowing date (up to mid-November) resulted in larger maize plants but smaller sunflower and soybean plants at flowering. Thus, up to flowering,


greater crop growth more than compensated for accel- erated crop development in maize while the opposite was true for sunflower and soybean. In maize, conversion efficiency during the vegetative period was higher for the late-November sowings than for earlier ones (Andrade et al., 1993a). Contrarily, sunflower conversion efficiencies during the same stages were not affected by sowing date.

Starting from the mid-October sowing, grains m-2 dropped 20 and 54% for 1- and 2-month delays in maize sowing date, respectively. For similar delays in sunflower sowing date, the declines were 21 and 35%. In soybean, a 1.5-month delay (from optimal date) resulted in a 24% decrease in seed density.

Reductions in crop growth rate during the critical stages for determination of grain number were responsible for the small number of grains set. The smaller crop growth rates during those stages were mainly the result of less incident radiation at that time and, for soybean, also of less soil cover (Bodrero, 1988). The high plasticity of soybean in interception was not expressed in late sowings because development accel- erated, preventing the plant from expressing its branch- ing potential. Moreover, short photoperiods induced shortening of the critical stages in late-planted soybean.

The large allocation of dry matter to vegetative structures in late-sown maize could be an additional factor

responsible for the small number of grains set (Fischer and Palmer, 1984; Cirilo and Andrade, 1994b).

Late sowings reduced grain weight in spite of the fact that grain number was also reduced. Starting from the mid-October sowing, delays of 1 and 2 months reduced grain weight by 15 and 28% in maize and 0 and 14% in sunflower, respectively. Late soybean ( 1.5- month delay) had a 23% reduction in this yield component.

In late sowings, part of the grain-filling period occurred at times with low incident radiation and temperatures low enough to affect the conversion efficiency of maize (Cirilo and Andrade, 1994a), sunflower (Warren Wilson, 1967) and soybean (Hof- stra and Hesketh, 1975). Moreover, the seed-filling period of soybean is shortened by such conditions (Major et al., 1975).

4. Conclusions

Of the three crops, maize produced the most biomass because of sustained ground cover and high conversion efficiency. Its harvest index on a dry weight basis was also the largest. When harvest index variable was expressed in glucose equivalents, the values for the three crops were more similar.

Table 4 Comparative ecophysiological traits of maize, sunflower and soybean grown at Balcarce, Argentina

Maize Sunflower Soybean

Radiation interception High during grain filling Conversion efficiency High C4 Crop growth rate during vegetative period High Crop growth rate during grain filling High

period Total biomass (g m 2) 2250 Harvest index High Grain yield (g m 2) 1013 Critical period for yield determination Short around flowering

Capacity to compensate a low number of Low grains with heavier grains

Yield stability to variation in plant Low density

Yield response to delay in sowing date Decrease Vegetative biomass response to delay in Increase

sowing date

Strong decrease postflowering Medium; it falls postflowering High Low

1350 Low 392 Flowering and grain filling

High

Medium

Decrease Decrease

Low at the beginning; high plasticity Low; typical C3 Low Low

1200 Medium 444 Grain filling; extensive period for determination of seed number Medium

High

Decrease Decrease


Flowering in maize, seed filling in soybean, and flowering and seed filling in sunflower were critical periods for grain yield determination. Sunflower had more capacity than maize to compensate a low number of grains with greater weight per grain. Maize and sunflower had low stability in grain number per unit area in response to variations in plant density. Finally, sowing date delays resulted in significant reductions in grain yield of the three crops. These reductions were due to smaller numbers of grain per m 2 and smaller grain weights.

Table 4 presents a comparative summary of these aspects. Optimal crop management should aim to max- imize radiation interception, conversion efficiency of intercepted radiation to biomass and partitioning to reproductive structures at critical stages of yield defi- nition. This work provides elements to guide crop management in this direction.

Acknowledgements

This work was supported by Instituto Nacional de Tecnologfa Agropecuaria, Consejo Nacional de Inves- tigaciones Cientfficas y T&nicas, Dekalb Argentina SA, Fundaci6n Antorchas and the Facultad de Ciencias Agrarias de la Universidad de Mar del Plata.

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