Impact of Variability in Plant Spacing on Corn Yield R.W. Heiniger and T.A. Smith

1. Impact of Variability in Plant Spacing on Corn Yield R.W. Heiniger and T.A. Smith • INTRODUCTION • Newer hybrids are more stress tolerant (Tollenaar, 1991) and less likely to lodge under high plant populations (Tollenaar, 1989). To take advantage of this characteristic corn growers have increased seeding rates with the goal of increasing yield (Widdicombe and Thelen, 2002). As plant population increases the distance between plants in the row decreases. This increases the likelihood of interplant competition which could be amplified by errors resulting from improper seed placement. Achieving a perfectly uniform stand with equidistant within-row spacing is impossible to achieve under actual field conditions. Low planter precision in seed metering, variations in seed germination , uneven seedbed conditions, and careless planting operation can all cause variation in plant-to-plant spacing. Emerging plants within the row can be too far apart, resulting in gaps, or too close together, resulting in multiple plants within close proximity to one another. This variability in plant spacing could negate the yield increase from the use of higher seeding rates. • There are conflicting opinions regarding how changes in plant spacing variability (PSV) affect yield. Several researchers have reported that moderate increases in PSV showed no effect on corn yield, suggesting that the increases in water and nutrient stress experienced by plants situated too close together were offset by plants spaced too far apart (Liu et al, 2004a; Lauer and Rankin, 2004; Muldoon and Daynard, 1981; Tollenaar et al., 2006). Others have suggested that yield losses increase as PSV increases (Doerge et al, 2002; Nielsen, 1991; Nielsen, 2004). These researchers report that the compensation by plants spaced further apart did not balance the reduced grain production from plants that were crowed together. Nielsen (2004) suggests that the differences in these findings were primarily due to the way plant spacing variability was achieved in the field studies. He states that differences in PSV must be created at the time of planting to capture early effects on plant growth and that repeatable PSV treatments must closely mimic the range of field conditions that occur in the “real world”. Therefore, studies in which corn was planted and then thinned or where close seed placement was not achieved are not representative of field conditions. • The majority of the work examining PSV has been conducted in Corn Belt states of Kansas, Illinois, Indiana (Nielsen, 2004), Wisconsin (Lauer and Rankin, 2004), or in Canada (Liu et al. 2004a, Muldoon and Daynard, 1981, Tollenaar et al., 2006). Growing conditions in the Southeastern US are quite different. Subsoils are acidic, which generally limits root growth to the upper 15 cm  of soil. Additionally, soils have lower water holding capacities, which require the crop to depend heavily on timely precipitation. Crowding stresses in these types of environments may be more severe locally, and neighboring plants under less stress may not have the resources to compensate. Therefore, work examining PSV in the Southeastern US is needed to determine how crop performance is affected in these environments. • OBJECTIVES • To determine the yield loss caused by PSV at higher seeding rates in North Carolina. • To determine the level of PSV commonly found in grower fields in North Carolina. • MATERIALS AND METHODS • Objective No. 1: Six sites were selected in eastern North Carolina (Table 1) to represent corn growing areas of the state. Two sites were planted in 2005, three in 2006, and one in 2007. At each site corn was planted in 76 cm rows into soybean residue using a Kinze no-till planter at a target seeding rate of 13 360 seeds ha-1 (33 000 seeds acre-1) The experimental design was a split plot with site-year as the main block and subplots consisting of five treatments replicated four times. These five treatments were: • A. Careful planting at 4.8 km h-1 using a calibrated Kinze finger mechanism (check) • B. Planting at 12.9 km h-1 mph using the Kinze finger mechanism • C. Planting at 4.8 km h-1 using an Almaco cone seeder (Fig. 1) • D. Planting at 4.8 km h-1 using the cone seeder with random slots covered (Fig. 1) • E. Planting at 4.8 km h-1 using the cone seeder with either one or two slots covered Fig. 1) • For treatments C, D, and E enough seed for a 12.16 m length of row was counted and dropped into the cone unit such that each uncovered slot contained one or more seeds. The cone was calibrated so that it made one revolution in 12.16 m. Plots were four rows wide by 12.16 m long. A 17-17-0 starter fertilizer was applied in a 2 x 2 band at planting. At sidedress, 300 kg ha-1 of N was applied using 30% UAN. Plots were kept weed free throughout the season by using preplant and layby herbicides combined with hand weeding. The distances between individual plants in two 6.1 m sections of the center two rows of each plot were measured at V6. Grain yield was measured from these same center two rows using a plot combine with a HarvestMaster grain gauge. Table 1. Sites selected for the study along with information on soil type and the management practices used. PLANT POPULATION AND PLANT SPACING VARIABILITY The only population difference among treatments was between Treatments A, B, C, and D and Treatment E (Fig. 2). The large variability in the stand in Treatment E often resulted in multiple plants at a single site. This created competition for light which often led to the loss of one of the plants. Even so differences in plant population among the treatments were small with little or no impact on final yield. In comparison, there were large and consistent differences among treatments in PSV (Fig. 3). The use of cone units resulted in predictable levels of PSV across locations. Furthermore, the differences ranged from an almost perfect spacing (Treatment A) to large gaps and multiple plants at given locations (Treatment E) that were present from seeding throughout the season (see inset photos). Because of the way seed dropped from the cone units the patterns in plant spacing were similar to what was observed in farm fields. YIELD RESPONSE TO PLANT SPACING There were significant differences in corn yield among each the treatments with the check having the highest yield and Treatment E, with severe skips and gaps, having the lowest yield (Fig. 4). The pattern of declining yield among the treatments was similar to the linear pattern in increasing PSV. Plots of yield as a function of PSV revealed linear relationships across all six sites with R2 ranging from 0.46 at Currituck in 2007 to 0.81 at Pasquotank in 2005 (Fig. 5). Comparisons of the slopes of the linear equations which represent the amount of yield lost as the variability in spacing between plants increases using full and reduced models found no significant differences in slopes at the Hyde, Pasquotank, Pamlico, Beaufort, and Perquimans sites. However, the slope at the Currituck site differed from all of the other sites except for Perquimans. This indicates that for most of these sites a single relationship can be used to measure grain loss from increasing levels of variability in plant spacing (Fig. 6). Figure 1. Slot arrangements for the Almaco cone for treatments D and E. All 31 slots were open for treatment C. Figure 6. Relationships between yield loss and increases in PSV for the combined sites of Hyde05, Pasquotank05, Pamlico06, and Beaufort06. The slope of the linear response differed at Currituck in 2007 possibly due to the use of a different hybrid. CONCLUSIONS Where it was possible to combine sites, a common relationship between PSV and yield (Fig. 6) indicated that, on average, corn yield was reduced by 0.126 Mg ha-1 for each cm increase in PSV (5.1 bu acre-1 for each inch). This loss is twice as high as the losses reported by Nielsen (1991; 2004) of 2.2 and 2.5 bu acre-1, and considerably higher than the losses reported by Liu et al. (2004b) of 1.5 bu acre-1 or Doerge et al. (2002) at 3.4 bu acre-1. Yield losses at the Currituck location (0.061 Mg ha-1 for each cm or 2.44 bu acre-1 per inch) were similar to previous reports. Since Currituck was the only location where a different hybrid was used differences in yield losses to PSV may be related to the hybrid grown. Clearly, variability in plant spacing can cause significant yield losses in the southeastern US. A survey of sixteen grower fields in North Carolina found PSV values ranging from 7.04 cm (2.77 in) to 16.61 cm (6.54 in) with an average of 12.29 cm (4.84 in). This means that growers are losing 1.55 Mg ha-1 or 24.6 bu acre-1 due to non-uniform plant spacing. These losses could be reduced by careful planter calibration and, as shown by the difference between planting at 4.8 and 12.9 km h-1, reductions in planting speed particularly in rough no-till conditions. An increase in planter speed alone increased PSV by 2 cm and decreased yield by 0.4 Mg ha-1 (6.4 bu acre-1). As corn growers continue to increase plant populations it is important that they take steps to slow down planting speed and to calibrate the planter if they are to realize increases in yield potential. E. D. ANALYSIS The Proc Mixed procedure in SAS (SAS Institute, 2006) was used to determine if there were differences in PSV (measured as the standard deviation of the distances between plants in the row), plant population, and grain yield among the five treatments. When differences were detected Fisher’s Protected LSD was used to separate means. Regression analysis was used to examine the relationships between PSV and grain yield at each site. Full and reduced models were used to determine if the slopes of these relationships differed among site-years. RESULTS The site-year by spacing treatment interaction was not significant for any of the variables of interest indicating similar responses of grain yield, plant population, and PSV to changes in plant spacing across the environments tested in North Carolina (Table 2). The main effect of spacing treatment was significant for all three variables (Figs. 2, 3, and 4). Table 2. Significance of the influence of site-year and spacing treatment on PSV, plant population, and grain yield. REFERENCES Doerge, Tom, Tom Hall, and Doug Gardner. 2002. New research confirms benefits of improved plant spacing in corn. Crop Insights Vol. 12, No. 2. Pioneer Hi-Bred Int’l, Johston, IA. Available online at http://www.pioneer.com/usa/agronomy/corn/1202.html (verified 10/10/2009) Lauer, Joseph and Mike Rankin. 2004. Corn response to within row plant spacing variation. Agron. J. 96(5):1464-1468. Liu, Wiedong, Matthijs Tollenaar, Greg Stewart, and William Deen. 2004a. Within-row plant spacing variability does not affect corn yield. Agron. J. 96:275-280. Liu, Wiedong, Matthijs Tollenaar, Greg Stewart, and William Deen. 2004b. Impact of planter type, planting speed, and tillage on stand uniformity and yield of corn. Agron J. 96:238-240. Muldoon, J.F., and T.B. Daynard. 1981. Effects of within-row plant uniformity on grain yield of maize. Can. J. Plant Sci. 61:887-894. Nielsen, R.L. 1991. Stand establihment variability in corn. Purdue Univ. Agronomy Dept. Publication AGRY-91-01. Available online at http://www.agry.purdue.edu/est/pubs/AGRY-91-01_v5.pdf. (verified 10/10/2009). Nielsen, R.L. 2004. Effect of plant spacing variability on corn grain yield. Purdue Univ. Agronomy Dept. Corny News Network. Available online at http://www.kingcorn.org/research/psv/Update2004.html. (verified 10/10/2009). Tollenaar, M. 1989. Genetic improvement in grain yield of commercial maize hybrids grown in ontario from 1959 to 1988. Crop Sci. 29:1365-1371. Tollenaar, M. 1991. Physiological basis of genetic improvement of maize hybrids in Ontario from 1959 to 1988. Crop Sci. 31:119-124. Tollenaar, M., W. Deen, L. Echarte, and W. Liu. 2006. Effect of crowding stress on dry matter accumulation and harvest index in maize. Agron. J. 98:930-937. Widdicombe, W.D., and K.D. Thelen. 2002. Row width and plant density effects on corn grain production in the Northern Corn Belt. Agron J. 94:1020-1023.   Figures 2 and 3. Plant population and plant Spacing Variability achieved by the five changes in either speed or seed drop mechanisms. Error bars indicate that only treatment E had differences in plant population and that consistent and large differences were found in PSV across locations. Figures 4 and 5. Yield response to the plant spacing treatments across locations and to changes In PSV within each location. Severe Skip Uniform Spacing Single Skip Skips with Doubles Higher Speed