Factorial arrangements of treatments are commonly used for agronomic experimental designs when it is suspected that one factor may have a significant influence on the effect of another factor or factors. However, there are several limitations to traditional full factorial designs in agronomic field experiments when a large number of factors are to be evaluated. The most significant limitations of full factorial designs are their large size (and the accompanying issue of increased experimental error), the time and labor requirements for managing such studies and analyzing the data produced, and, above all, the inclusion of treatments and interactions that are frequently neither interesting nor practical. An unfortunate consequence of many studies established with a full factorial arrangement is that the treatment structure is limited to three or fewer factors to keep the number of treatments, experimental area, and fieldwork manageable. To avoid the practical issues associated with complete factorial experiments, we implemented a straightforward treatment structure that included two control treatments (ST and HT) to which we compared five “supplemented” or “withheld” treatments, respectively, in an incomplete factorial design. This treatment structure was explicitly designed to allow three important comparisons, each necessary for answering the objectives of this project: (i) ST versus HT controls, (ii) individual supplemented treatments versus their counterpart ST control, and (ii) individual withheld treatments versus their counterpart HT control.

The objectives of this research were to (i) demonstrate and quantify the corn yield gap in Illinois, (ii) quantify the impact of different management technologies on the reduction of the corn yield gap, (iii) determine the impact of these technologies combined, and (iv) assess the effect of these technologies on yield components (kernel weight and number) as a means to understanding the mechanism behind the yield response.

MATERIALS AND METHODS

Field trials were conducted during the 2009 and 2010 growing seasons at two sites: the Crop Sciences Research and Education Center in Champaign‐Urbana (CU) (40°06′ N, 88°12′ W) in east‐central Illinois and the Dixon Springs Research Center (DS) (37°26′ N, 88°40′ W) in southern Illinois. Different fields were used for each year of the study. The fields in each site were within 3 km of each other and had similar soil types, fertility levels, and management histories. Both sites were nonirrigated and tile‐drained. In CU, soils were level (0–2% slope) and classified as Drummer silty clay loam (fine‐silty, mixed, superactive, mesic Typic Endoaquoll) and Flanagan silt loam (fine, smectitic, mesic Aquic Argiudoll) and, in DS, soils were 2 to 5% slopes and were classified as Grantsburg silt loam (fine‐silty, mixed, active, mesic Oxyaquic Fragiudalf). At CU, the preplanting soil properties at the 0‐ to 15‐cm depth for 2009 and 2010 included, respectively, 44 and 41 g kg−1 organic matter, pH 5.8 and 6.1, 40 and 44 mg kg−1 P, and 153 and 160 mg kg−1 K. At DS, the preplanting soil properties at the 0‐ to 15‐cm depth for 2009 and 2010 included, respectively, 39 and 35 g kg−1 organic matter, pH 6.3 and 6.6, 39 and 45 mg kg−1 P, and 146 and 157 mg kg−1 K. The minerals P and K were extracted using Mehlich III solution. We did not measure nitrate levels at these sites because soil nitrate concentration can change considerably from the time of preplanting soil testing to the time the plant needs it due to the unpredictable Illinois weather (Fernandez et al., 2012). Soybean was the previous crop in both years in both locations. Weather values for CU were obtained from the National Weather Service Forecast Office for Central Illinois (National Oceanic and Atmospheric Administration Urbana weather station 118740, 40°05′ N, 88°14′ W, elevation: 220 m above sea level); reported departures from average are compared to the 30‐yr monthly averages (1981–2010). Dixon Springs weather values were obtained from the Illinois Climate Network (Dixon Springs weather station, 37°44′ N, 88°67′ W, elevation: 50 m above sea level); departures from average reflect the 20‐yr monthly averages (1990–2010) available from that station.

The study was designed as a randomized complete block with six replications of each treatment. Plots were 5.3 m long by 3.0 m wide and consisted of four rows spaced 0.76 m apart. Plots were planted with an ALMACO SeedPro 360 research plot planter (Nevada, IA) with variable seeding rate capacity. Tillage included a chisel plow in fall with two field cultivations in spring for seedbed preparation. Planting occurred on 26 May 2009 and 24 May 2010 in CU and 8 June 2009 and 18 May 2010 in DS. The soil insecticide tefluthrin [2,3,5,6‐tetrafluoro‐4‐methylbenzyl (1RS)‐cis‐3‐([Z]‐2‐chloro‐3,3,3‐trifluoroprop‐1‐enyl)‐2,2‐dimethylcyclopropanecarboxylate] was applied with seed at planting at a rate of 0.11 kg a.i. ha−1. Weeds were managed with a pre‐emergent application of S‐metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐([1S]‐2‐methoxy‐1‐methylethyl)acetamide], atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine], and mesotrione (2‐[4‐(methylsulfonyl)‐2‐nitrobenzoyl]‐1,3‐cyclohexanedione) at a rate of 3.32 kg a.i. ha−1.

Crop grain yield and moisture were determined by harvesting the center two rows of each four‐row plot with a research plot combine along the entire length of each plot. Yield was calculated based on 0% moisture content. Average individual kernel weight was estimated by weighing 300 randomly selected kernels from each plot and expressed at 0% moisture. Kernel number was estimated by dividing grain yield by the average individual kernel weight of each plot.

Treatments The five agronomic management factors considered were: (i) plant population, (ii) transgenic insect resistance conferred by the Bt trait, (iii) strobilurin‐containing fungicide, (iv) P–S–Zn fertility, and (v) N fertility (Table 1). Each factor consisted of two levels representing either the current or lesser agronomic practice (referred to as ST) or a supplemental level (referred to as HT). The population levels used were 79,000 and 111,000 plants ha−1, representing an average and high population, and denoted as –Pop or +Pop, respectively. For determination of the effect of transgenic insect resistance, a non‐Bt (refuge) (DeKalb hybrid DKC61‐22 with glyphosate resistance) and its near isoline containing Bt (DeKalb hybrid DKC61‐19 with resistance to European corn borer and corn rootworm) were used, denoted as –Bt or +Bt, respectively. Both hybrids had a 111‐d relative maturity rating and possessed transgenic tolerance to the herbicide glyphosate. For determination of the influence of strobilurin fungicide application on yield, the treatment levels were either none or with fungicide, denoted as –Fung or +Fung, respectively. Headline (BASF, Florham Park, NJ), a product containing pyraclostrobin (a foliar fungicide in the strobilurin chemical class) was the fungicide used in this study and was applied at the tasseling stage (VT) at the maximum‐labeled rate of 0.21 kg a.i. ha−1. The two levels comprising the fourth management factor, P–S–Zn nutrition, were none or with added P, S, and Zn, denoted as –P–S–Zn or +P–S–Zn, respectively. The intensified level of the fourth factor consisted of P, S, and Zn application using MicroEssentials SZ [12–40–0–10 (S) –1 (Zn)] at a rate of 280 kg ha−1 (The Mosaic Company, Plymouth, MN). The MicroEssentials SZ was broadcast immediately before planting and incorporated with a cultivator‐harrow. The –P–S–Zn level would be the normal practice in the fields of this study, since soil test results for P and K were above the critical threshold determined by Vitosh et al. (2007) for corn production. The two levels of the fifth management factor, N, were application at the base rate and base application plus side‐dressing, denoted as –N or +N, respectively. For the –N rate, N was applied at the V1 growth stage as 28% urea‐ammonium nitrate solution at a rate of 202 kg N ha−1. The +N rate consisted of a supplemental broadcast application at the V5 growth stage of 112 kg N ha−1 of SuperU (Koch Agronomic Services, Wichita, KS), a stabilized urea N fertilizer (46–0–0) containing a urease inhibitor (N‐(n‐butyl) thiophosphoric triamide) and a nitrification inhibitor (dicyandiamide) to decrease losses due to volatilization, leaching, and denitrification. Table 1. Supplemented and withheld treatment structure: The treatment exceptions are either supplemented (+ factor) to the standard technology control, or withheld (–factor) from the high technology control. Controls are indicated by exception none. Treatment† Factor Primary technology Exception‡§ Pop Bt trait Fungicide P–S–Zn N Standard None Average Refuge None None Base Standard +Pop High Refuge None None Base Standard +Bt Average Bt None None Base Standard + Fung Average Refuge With None Base Standard +P–S–Zn Average Refuge None P–S–Zn Base Standard +N Average Refuge None None Base + side‐dress High None High Bt With P–S–Zn Base + side‐dress High –Pop Average Bt With P–S–Zn Base + side‐dress High –Bt High Refuge With P–S–Zn Base + side‐dress High –Fung High Bt None P–S–Zn Base + side‐dress High –P–S–Zn High Bt With None Base + side‐dress High –N High Bt With P–S–Zn Base

Supplemented Versus Withheld Treatment Structure The supplemented versus withheld treatment structure used in this study assessed the individual and combined effects of the five management factors, resulting in 12 treatments (Table 1). Five supplemented treatments (+Pop, +Bt, +Fung, +P–S–Zn, and +N) were established by individually substituting the HT level of each management factor while all other management factors were maintained at the ST level (Table 1). For example, the +Pop treatment was created by substituting the higher plant population (111,000 plants ha−1) for the standard level (79,000 plants ha−1) while all other management factors were maintained at the lower, standard level. Similarly, five withheld treatments (–Pop, –Bt, –Fung, –P–S–Zn, and –N) were established by individually substituting the lower level of the factor while maintaining all other factors at the HT level. Thus, the –Pop treatment was created by substituting the lower plant population (79,000 plants ha−1) for the higher plant population (111,000 plants ha−1) while all other management factors were maintained at the advanced level. In this way, the value of each management factor was tested at the ST level of agronomic management in an intensified management system (HT).