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Agronomy Journal 94:975-980 (2002)
© 2002 American Society of Agronomy

PRODUCTION PAPER

Yield Responses to Narrow Rows Depend on Increased Radiation Interception

Fernando H. Andrade*,a, Pablo Calviñob, Alfredo Ciriloc and Pablo Barbieria

a Unidad Integrada INTA Balcarce-Facultad de Ciencias Agrarias UNMP, CC 276, 7620 Balcarce, Buenos Aires, Argentina
b Unidad Integrada INTA Balcarce-Facultad de Ciencias Agrarias UNMP and AACREA, CC 276, 7620 Balcarce, Buenos Aires, Argentina
c INTA Pergamino, CC 276, 7620 Balcarce, Buenos Aires, Argentina

* Corresponding author (fandrade{at}balcarce.inta.gov.ar)

Received for publication November 27, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The response of grain yield to narrow rows can be analyzed in terms of the effect on the amount of radiation intercepted by the crops. The objective of this work was to study the effect of row spacing on grain yield and radiation interception (RI) during the critical period for grain set in three crop species. Ten experiments were conducted with maize (Zea mays L.), sunflower (Helianthus annuus L.), or soybean [Glycine max (L.) Merr.] under irrigation or under dryland conditions without severe drought during flowering and grain filling. The treatments consisted of two row distances combined with other factors such as plant density, cultivar, defoliation, etc. Grain yield responses to decrease distance between rows were inversely proportional to RI achieved with the wide-row control treatment during the critical period for grain number determination (r2 = 0.62, 0.54, and 0.86 for maize, soybean, and sunflower, respectively). Moreover, when row spacing was reduced, grain yield increases and RI increases during the critical periods for grain set were significantly and directly correlated in the three crop species (r2 = 0.71, 0.64, and 0.94 for maize, soybean, and sunflower, respectively). For the conditions of these experiments, grain yield increase in response to narrow rows was closely related to the improvement in light interception during the critical period for grain set.

Abbreviations: MG, maturity group • PAR, photosynthetically active radiation • RI, radiation interception


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
DECREASING ROW SPACING at equal plant densities produces a more equidistant plant distribution. This distribution decreases plant-to-plant competition for available water, nutrient, and light and increases radiation interception (RI) and biomass production (Shibles and Weber, 1966; Bullock et al., 1988). It also reduces the leaf area index required to intercept 95% of the incident radiation due to an increase in the light extinction coefficient (Flenet et al., 1996). However, the benefits of more equidistant spacing for crops grown without important water and nutrient deficiencies are variable. Some researchers reported grain yield increases (Hunter et al., 1970; Scarsbrook and Doss, 1973; Olson and Sanders, 1988; Bullock et al., 1988; Porter et al., 1997; Ethredge et al., 1989; Parvez et al., 1989; Board et al., 1992; Egli, 1994), but others have not (Robinson, 1978; Beatty et al., 1982; Zaffaroni and Schneiter, 1991; Blamey and Zollinger, 1997; Ottman and Welch, 1989; Rumawas et al., 1971; Nunez and Kamprath et al., 1969; Westgate et al., 1997).

There are times during the crop cycle that are most critical for yield determination. These times comprise the period bracketing flowering in maize (Kiniry and Ritchie, 1985; Fischer and Palmer, 1984) and sunflower (Chimenti and Hall, 1992, Connor and Sadras, 1992; Cantagallo et al., 1997) and extend to more advanced reproductive stages in soybean (Shaw and Laing, 1966; Board and Tan, 1995; Egli, 1997). Higher crop growth rates during these periods would allow more grains to be set and thus higher grain yields (Andrade et al., 1999). Crop growth rate is directly related to the amount of radiation intercepted by the crop (Gardner et al., 1985). Therefore, the response of grain yield to narrow rows can be analyzed in terms of the effect on the amount of RI at the critical periods for kernel set. In some cases, full RI during these periods may not be achieved with wide rows. Examples of this situation could be late soybean plantings, early maize plantings, defoliation or stress at early stages, use of short-season cultivars, use of erect-leaf maize hybrids, etc.

Our working hypothesis is that the yield increase in maize, soybean, and sunflower in response to decreased distance between rows is a result of an increase in light interception at the critical periods for grain set. Responses are expected to be inversely proportional to RI achieved with the wider row spacing at those critical periods for grain yield determination.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The experiments were conducted at Balcarce (37°45' S lat), Tandil (37°15' S lat), and Pergamino (33°56' S lat), Argentina, during different growing seasons. The soils were typical Argiudols with an organic matter content of 6 to 8% in Balcarce and Tandil and 2% in Pergamino in the first 25 cm of depth. Nitrogen and P were applied following fertilizer recommendations derived from soil analysis. Soybean seeds were inoculated with Bradirhizobium japonicum. In Exp. 3, 4, 5, and 6, soil water in the 1-m depth was kept above 50% of maximum available water by sprinkler irrigation. In Exp. 1, a complementary irrigation of 45 mm was applied at flowering. The rest of the experiments (2, 7, 8, 9, and 10) were conducted under dryland conditions; however, plants were not exposed to severe drought during flowering and grain filling. In all cases, weeds, insects, and diseases were controlled. The size of the experimental plots ranged from 28 to 39 m2, and the number of replications varied from three to four.

Crop, cultivars, plant density at harvest, sowing date, row spacing, and other treatments are shown in Table 1. In Exp. 1, treatments were a factorial arrangement of two maize hybrids and two row distances (Table 1). In Exp. 2, the treatments were a factorial arrangement of two maize hybrids, three defoliation treatments (an untreated control, defoliation at V3 stage, and defoliation at V5 stage; Ritchie and Hanway, 1982), and two row distances (Table 1). Defoliation consisted of removing all exposed leaf blades. Experiment 1 and 2 were analyzed as randomized complete block designs with three replications. In Exp. 3, 4, and 5, the treatments were a factorial arrangement of two to six maize hybrids, three plant densities, and two row distances, with three replications (Table 1). The experiments were split-split plot designs in which densities corresponded to the main plots in randomized complete blocks, row spacing to the subplots, and hybrids to the sub-subplots. Data from Exp. 6 were taken from Bodrero (1988). The experiment was conducted under irrigation, and the treatments were a factorial arrangement of six soybean cultivars [maturity group (MG) III and MG IV] and two row distances (Table 1). In Exp. 7, the treatments were a factorial arrangement of two soybean cultivars (MG II and MG III), three plant densities, and two row distances (Table 1). The analyzed values correspond to the average of two sowing dates (4 and 19 Dec. 1999). In Exp. 8, the treatments were a factorial arrangement of two soybean cultivars (MG III and MG IV) and two row distances. This experiment was conducted in deep (>1 m) and shallow (0.6 m) soils. Experiments 6, 7, and 8 were randomized complete block designs with three replications. In Exp. 9 and 10, the treatments were a factorial arrangement of two sunflower cultivars and two row distances in a randomized complete block design with three replications (Table 1).


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  		Table 1. Species, cultivars, plant density at harvest, sowing date, row spacing, and other treatments used in the different experiments.

 
Radiation interception was calculated as (1 - It/I0) x 100, where It is incident photosynthetically active radiation (PAR) just below the lowest layer of photosynthetically active leaves and I0 is incident PAR at the top of the canopy. The values of It and I0 were obtained with an AccuPAR radiometer (Decagon Devices, Pullman, WA) in Exp. 3 to 5 and with a LI-COR 188 B meter (LI-COR, Lincoln, NE) connected to a line quantum sensor 191 SB in the rest of the experiments. Determinations were taken near flowering in maize and sunflower and at R3 in soybean (Fehr and Caviness, 1977). The number of observations were at least five per experimental unit. The measurements were confined to the midday period (1100–1300 h) and were taken on sunny days only following the technique described by Gallo and Daughtry (1986). At harvest, 7.15 m of each of the two center rows in maize and sunflower and 5 m of the central rows in soybean were harvested to determined total grain yield. Grain moisture was determined with a Tesma A-79 moisture meter (Tesma SAIC, Buenos Aires, Argentina), and yield was expressed at a moisture content of 140, 110, and 120 g kg-1 in maize, sunflower, and soybean, respectively. Linear regressions between percentage yield increase in response to a reduction in row spacing and RI achieved with wide rows were established for the three crops. Moreover, linear regressions between percentage yield increase in response to a reduction in row spacing and RI increase in response to the same treatment were also calculated for the three species. Radiation interception increase was expressed as [(RI in narrow rows - RI in wide rows)/RI in wide rows] x 100. Data were also processed by analysis of variance.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
In maize, the largest increases in RI at flowering and in grain yield in response to narrow rows were observed in Exp. 2 (Table 2). Within this experiment, the largest increase in grain yield (p < 0.05) in response to narrow rows was found in crops that were defoliated at V5 and reached <65% RI in wide rows. Responses to decreases in row spacing were lower when crops were not defoliated because they achieved much higher RI in wide rows at flowering. In the experiments conducted under irrigation (1, 3, 4, and 5), close to maximum RI was achieved with wide row spacing, and small or no grain yield responses (p > 0.05) to narrow rows were observed. Combining the data from all of the experiments, percentage grain yield increase in response to decrease in row spacing was significantly and inversely associated (r2 = 0.62, p < 0.001) with RI achieved with wide rows (Table 3). Moreover, when row spacing was reduced, percentage yield increase was significantly related to percentage increase in RI (r2 = 0.71, p < 0.001) (Table 4).


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  		Table 2. Radiation interception (RI) and grain yield as a function of row spacing in maize, soybean, and sunflower in Exp. 1 to 10. Data in Exp. 7 correspond to the average of two December planting dates.

 

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  		Table 3. Ordinate (a), slope (b), and coefficient of determination (r2) of the linear regressions between percentage yield increase in response to a reduction in row spacing and radiation interception observed in wide rows for maize, soybean, and sunflower. Standard error of a and b are also shown. N is the number of observations.

 

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  		Table 4. Ordinate (a), slope (b), and coefficient of determination (r2) of the linear regressions between percentage yield increase and radiation interception (RI) increase when row width was reduced for maize, soybean, and sunflower. Radiation interception increase was expressed as [(RI in narrow rows - RI in with wide rows)/RI in wide rows] x 100. Standard error of a and b are also shown. N is the number of observations.

 
In soybean, the greatest increases in RI at R3 and in grain yield in response to narrow rows were observed in crops planted in December (Table 2, Exp. 6 and 7) (p < 0.05). The smallest responses were observed when RI at R3 in wide rows was close to or >90%. Combining the data from the three soybean experiments, percentage grain yield increase from decreased row spacing was inversely associated (r2 = 0.54, p < 0.01) with RI in wide rows at R3 (Table 3). Moreover, when row spacing was reduced, percentage yield increase was significantly and positively related to percentage increase in RI (r2 = 0.64, p < 0.01) (Table 4).

In sunflower, increases in grain yield in response to narrow rows were only observed in the short-season hybrid Zenit (Table 2) (p < 0.05). This cultivar showed the lowest RI at flowering in wide rows. Maximum RI at flowering in wide rows was achieved with the long-season hybrid Rancull, and no positive grain yield response to narrow rows was observed. Again, percentage grain yield increase in response to decrease in row spacing was inversely associated (r2 = 0.86, p < 0.1) with RI achieved with wide rows at flowering (Table 3). Moreover, when row spacing was reduced, percentage yield increase was positively and directly related to percentage increase in RI (r2 = 0.94, p < 0.05) (Table 4).

A common relationship for the three species combined was determined (Fig. 1 and 2) . Average grain yield response to narrow rows was close to zero when RI in wide rows was >90%. The average grain yield responses increased to 4.5 and 8.8% for RI values in wide rows of 80 to 90% and 70 to 80%, respectively (Fig. 1). On the other hand, when row distance was reduced, the relative increase in grain yield was approximately 50% of the relative increase in RI (Fig. 2).



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  		Fig. 1. Relationship between percentage grain yield increase in response to reduction in row spacing and radiation interception (RI) observed in wide rows for maize (circles), soybean (triangles), and sunflower (squares). Radiation interception was expressed as percentage of incident radiation and measured at flowering in maize and sunflower and at R3 in soybean. Average standard errors of all experiments were 1.4 and 3.4% for x and y variables, respectively. Y = 42.85 - 0.45x; r2 = 0.60; p < 0.001.

 


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  		Fig. 2. Relationship between percentage grain yield increase in response to narrow rows and radiation interception (RI) increase in response to the same treatment for maize (circles), soybean (triangles), and sunflower (squares). Radiation interception increase was expressed as [(RI in narrow rows - RI in wide rows)/RI in wide rows] x 100. Average standard errors of all experiments were 1.9 and 3.4% for x and y variables, respectively. Radiation interception was measured at flowering in maize and sunflower and at R3 in soybean. Y = 0.17 + 0.52x; r2 = 0.62; p < 0.001.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Most of the yield response of the maize crop to reductions in row distance was related to improvements in RI at the critical flowering period. When available data from the literature were combined (Westgate et al., 1997; Scarsbrook and Doss, 1973; Ottman and Welch, 1989; Bullock et al., 1988), a similar relationship was found between percentage yield increase and percentage RI increase at flowering in response to narrow rows (slope = 0.54, r2 = 0.49, p < 0.01, n = 11). In soybean, there was an association between yield response to reductions in row spacing and RI increase at R3. A similar conclusion was obtained when data from the literature, in which RI at R3 or close to R3 was reported (Board et al., 1992; Egli, 1994; Board and Harville, 1996), were combined and analyzed (slope = 0.45, r2 = 0.44, p < 0.01, n = 16). Thus, soybean yield responses to reductions in row spacing can be partly attributed to improvements in RI at the critical pod-setting period. Finally, in sunflower, yield response to narrow rows was observed for the short-season hybrid only, and again, it was related to improvements in RI at flowering. In most cases, the literature indicates little or no effect of row spacing on sunflower grain yield (Zaffaroni and Schneiter, 1991; Robinson, 1978; Vijayalakshmi et al., 1975). This lack of response is probably because sunflower has a high capacity to achieve full light interception at flowering, provided that adapted cultivars are grown without serious water deficits or other adversities during the vegetative period.

In our work, maximal increases in RI with narrow rows did not differ much among the three species. In the literature, however, reported increases are often >25% in late-planted soybean (Board et al., 1992; Egli, 1994; Board and Harville, 1996) and generally <15% in maize (Scarsbrook and Doss, 1973; Bullock et al., 1988; Ottman and Welch, 1989; Westgate et al., 1997). Greater responses to decreases in row spacing are expected in those crop species whose plants are closer together within the row. Similarly, the response of maize to narrow rows is low or null at low plant densities (Fulton, 1970) because the decrease in transmitted PAR between the rows is compensated by an increase in transmitted PAR between the plants in the row.

In the three crops, full light interception can probably not be achieved when (i) short-season and/or erect leaf cultivars are grown (Anderson et al., 1998) or (ii) plants are defoliated (frost, hail, insects, etc.) or subjected to water or nutrient stress at vegetative stages (Alessi et al., 1977; Barbieri et al., 2000). Because drought or nutrient deficiencies at vegetative periods limit leaf area expansion (Trapani and Hall, 1996; Salah and Tardieu, 1997), they would increase the probability of response to narrow rows. Early plantings in maize and late plantings in soybean would also increase the response to narrow rows because these management practices produce small plants with fewer leaves (Andrade et al., 1996; Duncan et al., 1973; Weaver et al., 1991).

There are suggestions in the literature that reduced row distance can increase soybean yield without affecting light interception at the critical period for grain number determination (Duncan, 1986; Egli, 1994; Wells, 1991). Such responses would be explained by an increase in potential grain number due to an increase in node number and vegetative biomass, which would increase dry matter partitioning to reproductive structures and the number of grains set per unit growth during the critical periods for grain set. Increases in potential grain number in response to plant biomass are greatest in soybean, intermediate in sunflower, and smallest in maize (Vega et al., 2001). However, Vega (2001) showed that dry matter partitioning during the critical period for grain set was highly stable in soybean and sunflower. Our data suggest that grain yield increases in response to narrow rows closely relates to the improvement in light interception during the critical period for grain set.

Examples of other advantages of reduced row spacing are a decrease in water evaporation from the soil surface (Yao and Shaw, 1964; Nunez and Kamprath, 1969; Karlen and Camp, 1985), an inhibition of weed growth (Forcella et al., 1992; Teasdale, 1994), and an improved uptake of limiting nutrients from the soil (Stickler, 1964; Rosolem et al., 1993, Barbieri et al., 2000). In some of these cases, responses to narrow rows could be greater than predicted by RI. Contrarily, narrow rows would decrease yield when crops are subjected to progressive drought because enhanced early cover would increase water use (Zaffaroni and Schneiter, 1989), resulting in a more severe water stress at the critical moments for grain set (Fulton, 1970).

We developed a very simple model to describe the responses of crops to narrow rows. Grain yield response to decreased distance between rows was inversely proportional to RI achieved with the wide-row control treatment during the critical period for grain number determination. Similar association patterns were observed in the three species examined (Fig. 1). This approach, although simple, does not consider the real improvement in RI with narrow rows. A more meaningful and elaborated approach to analyze the effect of reducing row spacing was to relate grain yield increases to RI increases during the critical periods for grain set. These two variables were significantly and directly correlated in the three crop species (Fig. 2). These results highlight the importance of the critical periods for grain number determination. For the conditions of these experiments, management practices should ensure that the crop reaches full RI at these moments to maximize the number of reproductive sinks set.


   ACKNOWLEDGMENTS
 
Thanks go to Dr. Dennis Egli for helpful comments on the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This work was supported by Instituto Nacional de Tecnología Agropecuaria (INTA), Facultad de Ciencias Agrarias UNMP, CREA Tandil, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), and Monsanto Argentina.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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