Published March 27, 2017
Research
Grain Yield and Nitrogen Accumulation in Maize Hybrids Released during 1934 to 2013 in the US Midwest Jason L. DeBruin,* Jeffrey R. Schussler, Hua Mo, and Mark Cooper
ABSTRACT Nitrogen (N) application in maize (Zea mays L.) reached a maximum of 145 kg N ha−1 in the US Midwest in 1975. Grain yield has continued to increase at a rate of 111 kg ha−1 yr−1, implying an improvement in N efficiency. Our objective was to measure the rate of genetic gain and the traits that contributed to the observed N efficiency for a set of DuPont Pioneer hybrids released between the era decades (ERA) of 1934 to 2013. These hybrids represent the most widely sold hybrids (by volume) in each ERA. A randomized complete block experiment in a split-plot arrangement was conducted at Sciota, IL, and Marion, IA, during 2013 and 2014, with plant densities of 39,500 and 79,000 plants ha−1 as the whole plot, respectively, and 47 ERA hybrids as the split plot. This experiment was grown in a low-N (56 kg N ha−1) block and in a high-N (>200 kg N ha−1) block at each location. Grain yield increased at an average rate of 109 kg ha−1 yr−1 from 1934 to 2013. Partial factor productivity increased from 13.8 in 1934 to 55 kg grain kg applied N−1 in 2013 under high-N conditions and 79,000 plants ha−1. Traits associated with yield improvement without increasing N application were (i) greater synchrony in floral development, (ii) reduced concentration of grain N, (iii) increased specific leaf nitrogen, (iv) increased kernel number per ear (KPE), and (v) increased kernel mass. Breeding efforts that select for increased KPE under increased plant density should increase yield, and this yield increase could partially be supported through greater postanthesis N remobilization from vegetative tissue without requiring greater N application.
J.L. DeBruin, DuPont Pioneer, 21888 N 950th Rd., Adair, IL, 61411; J.R. Schussler, DuPont Pioneer, 3261 N Alburnett Rd., Marion, IA, 52302; H. Mo and M. Cooper, DuPont Pioneer, 7200 NW Ave., Johnston, IA, 50310. Received 25 Aug. 2016. Accepted 27 Jan. 2017. *Corresponding author (
[email protected]). Assigned to Associate Editor Jeff Melkonian. Abbreviations: ASI, anthesis–silking interval; BLUP, best linear unbiased predictor; DAP, days after planting; ERA, era decades; HN, high nitrogen; KPE, kernel number per ear; KWT, kernel weight; LN, low nitrogen; PD, plant density; PFP, partial factor productivity; S/D, supply/demand; SLN, specific leaf nitrogen; UAN, urea ammonia nitrate.
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mproved genetics, combined with optimized agronomic practices, have increased maize (Zea mays L.) grain yield by an average of 111 kg ha−1 yr−1 between 1965 and 2012 (USDA, 2015). Beginning in 1965, when nitrogen (N) application data were initially recorded, N application rates increased at a pace of >4 kg ha−1 yr−1 and eventually reached a plateau of ~145 kg N ha−1 (USDA-ERS, 2016). Between 1965 and 1975, grain yield did not increase at the same pace as N applications. Thus, partial factor productivity (PFP, kg grain kg−1 applied N) declined between 1965 and 1975 by 1.15 kg grain kg N ha−1 yr−1. However, after N application rates plateaued in 1975, grain yield and PFP increased at a rate of 0.69 kg grain kg N ha−1 yr−1. Yield improvement under constant N application rates demonstrates the enhanced N use efficiency of modern hybrids. The most recently published estimate for high-yield maize production indicates that a yield of 12.0 Mg ha−1 (23.0 Mg biomass ha−1) requires total N uptake of 286 kg N ha−1, with 166 kg N ha−1 contained in the grain at maturity (Bender et al., 2013a). Nitrogen uptake occurs throughout the season, with maximal uptake periods between V10 and V14, where N uptake averages 8.9 kg ha−1 d−1. Approximately 65% of the total seasonal N accumulation Published in Crop Sci. 57:1–16 (2017). doi: 10.2135/cropsci2016.08.0704 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved.
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(vegetative + grain) is taken up by flowering. In a highyield production system (³12.0 Mg ha−1), ~100 kg N ha−1 must be taken up from the soil after flowering. Recent work by Bender et al. (2013b) provides evidence that the addition of corn root worm (Diabrotica spp.) protection in roots has improved both the amount of N uptake and the duration of N uptake postanthesis. Effects of limited N availability are well understood in maize. Early-season N stress results in a smaller crop canopy (leaf area) with a lower leaf N concentration (Boomsma et al., 2009; Massignam et al., 2009; Ciampitti and Vyn, 2010) that ultimately reduces radiation use efficiency (Massignam et al., 2009) and crop biomass (Boomsma et al., 2009; Ciampitti and Vyn, 2010). Reduced plant growth results in reduced kernel number per ear, kernel mass, and ultimately lower grain yield (Boomsma et al., 2009; Ciampitti and Vyn, 2010; Haegele and Below, 2013). Maize breeding over the last 85 yr has led to tremendous gains in crop yield. Yield gains are projected to continue to increase at 125 kg ha−1 yr−1 (Smith et al., 2014). DuPont Pioneer maintains the inbred parents needed to produce previously sold hybrids from each era-decade (ERA). Hybrids included in this ERA set are the most widely sold hybrids from each decade, from the 1930s to the present. These hybrids have been studied intensively (Duvick and Cassman, 1999; Campos et al., 2006; Reyes et al., 2015; York et al., 2015), and the composition of this sequence of hybrids is continually being updated as improved hybrids are released for commercial production (Duvick et al., 2004; Smith et al., 2014). Initial investigations with the ERA hybrid set were conducted in Midwestern US environments over multiple years, where it was demonstrated that increased grain yield was associated with reductions in grain protein and tassel size, and with increases in leaf angle and staygreen (Duvick and Cassman, 1999). Additional evaluation of the ERA hybrids was conducted under conditions where water was withheld during the flowering and grain-filling periods. Positive yield gains for modern hybrids were observed, and traits such as anthesis–silking interval (ASI, the time between pollen dehiscence and silk exertion) and barrenness were both reduced (Campos et al., 2006; Reyes et al., 2015). A recent summary by Smith et al. (2014) showed that maize yields increased 79.6 to 90.8 kg ha−1 yr−1 in well-watered environments and 49.9 to 59.1 kg ha−1 yr−1 in droughtstress environments when evaluating hybrids released between 1930 and 2011. Similar to Duvick and Cassman (1999), these experiments documented a reduction in tassel size for modern hybrids. In contrast, however, leaf angle, staygreen, and plant resilience to barrenness had not continued to change in recent decades. Although the ERA hybrids have been studied extensively in well-watered, drought-stress, and typical 2
Midwestern US production systems with sufficient N, there have been limited evaluations in conditions where N has purposely been limited. When the ERA hybrids were evaluated under low N (LN, 22 kg applied N ha−1) and high N (HN, >200 kg applied N ha−1) conditions in Woodland, CA, yield declined by >50% under LN compared with HN, and there was extremely low genetic gain (3.8 kg ha−1 yr−1; Smith et al., 2014). Recently, a subset of four ERA hybrids from each of the periods 1900 to 1941, 1946 to 1954, 1982 to 2002, and 2006 to 2011 was grown in LN (0 kg N ha−1) and HN (145 kg N ha−1 applied as urea) conditions, and a linear, positive yield improvement with time of hybrid release was documented in both N conditions (York et al., 2015). Recent work by Haegele et al. (2013) measured the genetic rate of gain to be 56 kg ha−1 yr−1 under LN (0 kg N ha−1) and 86 kg ha−1 yr−1 under HN (252 kg N ha−1) for a selection of Dekalb hybrids released between 1967 and 2006. In both N conditions, the rate of genetic gain was positive. Improved grain yield was mostly attributed to improved kernel number and a reduced number of barren plants. Other factors contributing to these consistent yield gains were increased N use (kg of grain kg N plant−1) and greater N uptake postanthesis. Our objective was to measure the yield response for the set of DuPont Pioneer ERA hybrids in LN and HN conditions in the US Midwest to determine how genetic gain is affected by reduced soil N status. A second objective was to document any associations between leaf N and grain N traits with yield to provide insight into how N uptake and partitioning have changed over time.
Materials and Methods A field experiment was conducted in 2013 and 2014 at locations near Sciota, IL, (soil type: Sable [silty clay loam, poorly drained, mixed superactive mesic, Typic Endoaquolls]) and Marion, IA, (soil type: Franklin [silt loam, somewhat poorly drained, finesilty, mixed superactive mesic, Udollic Endoaqualfs]). At each location, there are 16-ha fields that have been further divided into an 8-ha section that has been managed as a LN area for multiple years (200 kg N ha−1 yr−1) area. Within each of these sections (LN and HN), an experiment was established as a randomized complete block design in a split-plot arrangement with plant density (PD, 39,500 and 79,000 plants ha−1, referred to as 39K and 79K, respectively) as the whole-plot treatment and 47 hybrids released between 1934 and 2013 (Table 1) as the split-plot treatment. There were two replications of each hybrid and density combination in each of the N sections (LN and HN). The N treatment (LN and HN) was not replicated at a particular location; rather, the N treatment was replicated by years (2013 and 2014) and locations (Sciota and Marion). Plots were four rows with 76.2-cm row spacing and 4.54-m row length. At both locations in both years, the experiment was grown after maize production in the previous year. Plots in the LN section received 56 kg N ha−1 as liquid urea ammonia nitrate
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Table 1. Names and year of release for a set of DuPont Pioneer hybrids tested during 2013 and 2014 near Sciota, IL, and Marion, IA, under low and high nitrogen conditions at two planting densities. Hybrid name
Year of release
Hybrid name
Year of release
Hybrid name
Year of release
351OLD 322HYB 330HYB 339HYB 352HYB 347HYB 301B 354HYB 329HYB 354A 3618
1934 1936 1939 1942 1946 1950 1952 1953 1954 1958 1961
3541 3382 3377 3475 3379 3394 3563 3489 3335 33A14# 34G81
1975 1976 1982 1984 1988 1991 1991 1994 1995 1997 1997
34H31 34N42† 34A16† 34P88 34R67‡ 33D14§ 33T59¶ 35K04‡ 33D49‡ 33W84§ P1162XR§, ††
2002 2003 2005 2005 2007 2007 2007 2008 2008 2008 2009
3376 3390 3388 3517 3366
1965 1967 1970 1971 1972
33G26 34B23 33B51# 33P67# 34M95#
1998 1999 1999 1999 2001
P1395XR§ P1151HR‡, ‡‡ P0987HR‡ P1221AMXT§§
2009 2011 2012 2013
† Hybrids with HX1 (Cry1F) and resistance to glufosinate (LL) herbicide. ‡ Hybrids with HX1 (Cry1F) and resistance to glufosinate (LL) and glyphosate herbicides (RR2). § Hybrids with HXX (Cry1F and Cry34/35Ab1) and resistance to glufosinate (LL) and glyphosate herbicides (RR2). ¶ Hybrid with HXX (Cry1F and Cry34/35Ab1) and resistance to glufosinate (LL) herbicide. # Hybrids with YGCB (Cry1Ab). †† XR indicates that the hybrid contains HXX (Cry1F and Cry34/35Ab1) and resistance to glufosinate and glyphosate herbicides. ‡‡ HR indicates that the hybrid contains HX1 (Cry1F) and resistance to glufosinate and glyphosate herbicides. §§ AMXT indicates that the hybrid is a blend of 95% seed that contains Cry1F, Cry1Ab, mCry3A and Cry34/35Ab1 for insect protection and resistance to glufosinate and glyphosate herbicides. A 5% refuge seed (resistant only to glyphosate) is blended in the bag.
(UAN, with an N–P–K analysis of 28–0–0, 1.2 kg N L−1) prior to planting at both locations and in both years. Plots in the HN section received 224 kg N ha−1 as UAN prior to planting in 2013 at Sciota, whereas in 2014, 201 kg N ha−1 was applied at planting and 45 kg N ha−1 was applied as sidedress at the V4 (four collared leaves) growth stage. At Marion, 246 kg N ha−1 was applied as UAN prior to planting, with an additional 56 kg N ha−1 applied as sidedress at the V4 growth stage both years. When N was applied prior to planting, it was incorporated to a 5-cm depth. Sidedress N was injected into the soil through a fertilizer knife attached to a coulter to a depth of 5 cm. Planting occurred on 18 May 2013 in LN and HN locations at both Marion and Sciota. In 2014, planting occurred in HN and LN on 6 May at Sciota. At Marion, HN planting occurred on 9 May and LN planting on 20 May. During planting, the soil insecticide Force (tefluthrin) was applied as a broadcast application of 8.4 g a.i. ha−1 on top of the row to provide control of northern corn rootworm (Diabrotica barberi). Preemergent and postemergent herbicide applications were made at both locations during the season as needed to control weeds. An insecticide (Hero, a.i.: zeta-cypermethrin and bifenthrin) was applied at a rate of crop science, vol. 57, may– june 2017
56.1 g a.i. ha−1 via aerial application at Sciota in 2013, and the insecticide Warrior (lambda-cyhalothrin) was applied at a rate of 33.6 g a.i. ha−1 in 2014 to control Japanese beetles (Popillia japonica). An aerial application of the fungicide Headline AMP (pyraclostrobin + metconazole) was applied 2 wk after flowering at Sciota during 2013 and 2014 and at Marion in 2014 at a rate of 110.9 g a.i. ha−1. Data collected included thermal time (base 10°C) from planting to the time when 50% of the plants in the center two rows had anthers visibly dehiscing and releasing pollen and thermal time from planting to when 50% of the plants in the center two rows had silks present (R1 growth stage). The difference between thermal time to shed and thermal time to silk is the ASI. Two weeks after plots had reached 50% silk, leaf punches were collected to determine specific leaf N (SLN, g N m−2 leaf area). Four consecutive plants were selected in the interior of rows two or three and tagged prior to flowering to ensure that leaf punches were taken from the same plants at 2 wk postflowering and at physiological maturity. A leaf punch with a diameter of 2.22 cm (surface area of 3.8 ´ 10−4 m 2) was used to collect two punches from the leaf one nodal position above the ear from each plant. In total, eight punches were collected for N determination. The punches were collected at the midpoint of the leaf lengthwise and taken from either side of the midrib. This method of phenotyping for SLN was developed and tested over several years. A study was conducted in 2010 using three commercial hybrids under non-N-limiting conditions. Two of the hybrids used in this study share a common parent, but the third hybrid is not related, and together the three hybrids are representative of the germplasm. The results across hybrids documented that the leaves that accounted for >85% of the captured solar radiation had an average SLN of 1.58 g N m−2 that was approximately equal to the ear leaf SLN of 1.54 g N m−2 (Fig. 2 in DeBruin et al., 2013). Given the relative unrelatedness of the hybrids, the finding that the ear leaf represented the canopy SLN provided evidence that a relatively simple measurement of a single leaf within the canopy could assess SLN of the entire canopy. Nitrogen content of a single leaf provides a measurement of the potential of a hybrid to concentrate N in the leaves similar to point measurements or indirect measurements of leaf tissue with a SPAD meter. In the past, SPAD meters have been used to assess canopy N status for a limited number of hybrids in response to N treatments and PD (Boomsma et al., 2009; Ciampitti et al., 2012) and among a set of hybrids that differed in year of release (Woli et al., 2016). Additionally, SPAD meters have been used to nondestructively measured SLN (Chapman and Barreto, 1997) and have recently been proposed as the initial step in building a Nitrogen Nutrition Index value that has been reported to relate well to increase in crop yield (Ciampitti et al., 2012). A unique difference in our method of calculating leaf N, along with SLN, is that it relates to a trait well understood to influence photosynthesis and radiation use efficiency (Sinclair and Horie, 1989; Muchow and Sinclair, 1994; Vos et al., 2005), along with grain yield, leaf area index, and kernel mass (DeBruin et al., 2013). Leaf punches were placed in a paper envelope and dried at 62°C for several days until the dry mass did not change. After recording mass of the punches for each sample, samples were sent to a laboratory for N determination by combustion. The
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analysis was conducted at a DuPont Pioneer laboratory with the following methods. Sample preparation consisted of grinding samples with a 2000 Geno Grinder to reduce the size of the material. An oxidation reactor chamber (FlashES 1112 N/ Protein Analyzer, Thermo Electron Corporation) was used to create a mixture of N2, CO2, H 2O, and SO2 gases. The gas mixture was passed through a filter to remove CO2, SO2, and water. The remaining N2 gas was further separated through a chromatographic column, and the electrical signal was converted to N percentage via instrument software. Using the leaf mass and N concentration, total leaf N (g) contained in the punches was determined. The total leaf area of the punches was determined by the surface area of one punch multiplied by eight. Specific leaf N was calculated by the division of total N by leaf surface area in the sample. Leaf punches were collected again at physiological maturity using the same method as described for sample collection 2 wk post-flowering. The punches were taken in the same area of the leaf on either side of the midrib. The leaf discs were dried and submitted to the same laboratory for N analysis via combustion. Once leaf N content was calculated, leaf N remobilization was calculated by subtracting leaf N at maturity from leaf N 2 wk postflowering, dividing by leaf N 2 wk postflowering, and multiplying by 100. The leaf punch method presented here is a nonsubjective method to assess canopy staygreen, as reported by previous work with the ERA hybrids (Duvick and Cassman, 1999; Duvick et al., 2004; Smith et al., 2014). The leaf punch method of a single leaf used to measure SLN and remobilization at maturity is supported through review of the literature and internal studies. Valentinuz and Tollenaar (2004) provide an assessment of canopy senescence patterns and indicate that the pattern was predictable and similar for both old and new hybrids. In their work, leaf senescence began at the base of the plant and continued upward toward the ear leaf. At approximately the mid-grain-fill period, senescence began at the top of the canopy and progressed downward toward the ear leaf, indicating that the central part of the canopy, around the ear leaf, was the last section of the canopy to remobilize N. Thus, the middle of the canopy is a logical sampling point to ensure N content and remobilization are not under- or overestimated. In addition, we conducted a separate study during 2013 and 2014 using four hybrids and four N rates. Leaves were sampled above and below the ear leaf at both 2 wk after flowering and again at maturity and were used to calculate remobilization. The method could measure differences among leaf layers, although the pattern did not exactly follow that described by Valentinuz and Tollenaar (2004). Our results were similar in that SLN was the lowest in leaves below the ear but were different in that we measured a greater amount of leaf N in the upper part of the canopy than in the ear leaf. The ear leaf (leaf 12) SLN at maturity, across hybrids and N rates, was 0.10 g N m−2 lower than the average of leaves 10, 14, and 16. This resulted in 5.9% greater (p < 0.01) remobilization from the ear leaf than from leaves above and below. Because the method was consistent across N rates and hybrids (lack of a N ´ hybrid interaction) during the 2 yr of testing, our assessment was that the method was a rapid way to assess canopy N remobilization as long as the same position in the canopy was measured for each hybrid. 4
Prior to combine harvest, five plants were tagged in each of the center two rows from areas with equidistant plant spacing. Both primary and secondary ears were removed from the plant and placed in separate mesh bags. Ears were oven dried at 32°C for 72 h. Grain was removed from the cobs of primary and secondary ears using an automated bulk sheller (ALMACO). The samples from primary and secondary ears were weighed, and a subsample (~400 kernels) of grain was removed from the primary ear sample and weighed. The subsample was then dried at 62°C for 5 d to remove all moisture and weighed a second time, and then initial moisture content of the shelled sample could be calculated. Finally, the kernels in the subsample were counted and kernel mass was determined. Grain biomass per plant was calculated as the weight of the shelled samples (primary + secondary) divided by 10 plants and was adjusted to 155 g kg−1 grain moisture. Kernel number per plant was determined by total grain weight of the primary ear sample divided by wet weight of the subsample, multiplied by the number of kernels in the wet subsample, and divided by the number of plants included in the harvest sample. Grain N concentration was measured in the dried grain subsample by combustion following the same methods as described above for both sample preparation and N determination. Grain N content per plant was determined by the shelled grain weight and the N concentration expressed on a dry basis. Grain N content on an area basis (g N m−2) was calculated from grain N content per plant multiplied by PD. The center two rows of the four-row plot were harvested to obtain harvest weight and grain moisture. Harvest weight was adjusted to 155 g kg−1 moisture content and expressed as kilograms per hectare. Grain biomass weight from the 10 plants harvested to determine yield components was added back to the combine harvest weight to accurately calculate crop yield on an area basis. Partial factor productivity was calculated as the division of grain yield (kg ha−1) at 155 g kg−1 moisture content by N application amount (kg ha−1) and is presented as kilograms of grain yield per kilogram of applied N. The N treatment was not replicated at a particular environment (year ´ location). Rather, 2 yr and two locations, each with a HN and LN treatment, provide replication of the N treatment (n = 4). The growing conditions of these four environments were quite different from each other; however, at every environment, the LN treatment resulted in a yield reduction compared with the HN treatment, averaging 5559 kg ha−1 (49.9%) across plant densities, years, and locations (Table 2). Air temperatures were normal during the month of May 2013 and were slightly below normal during the flowering period (early to mid-July) at both locations during 2013 (−1.5°C) and 2014 (−2.4°C at Marion and −4.0°C at Sciota). During 2013, precipitation was 112 mm above average at Sciota in May and was normal during the same period at the Marion location (Table 3). During the remainder of the growing season, precipitation was below average at both locations. During 2014, both locations experienced above-normal rainfall during the month of June, followed by below-average rainfall in July and August. No additional irrigation was applied at Marion or Sciota in either year. A supply/demand (S/D) ratio was used to assess the availability of soil moisture to meet crop demand. The supply function
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Table 2. Grain yield at 155 g kg−1 moisture averaged across hybrids grown under high (>200 kg N ha−1) and low (£56 kg N ha−1) N and 39,500 and 79,000 plants ha−1 at Marion, IA, and Sciota, IL, during 2013 and 2014. Year
Location
2013 2013 2014 2014
Marion Sciota Marion Sciota
High nitrogen 39,500 plants ha−1 79,000 plants ha−1
Low nitrogen 39,500 plants ha−1 79,000 plants ha−1
————————————————————————————— kg ha−1 ————————————————————————————— 10,938 (1,857)† 12,121 (3,776) 5,735 (1,119) 5,102 (1,484) 10,405 (1,316) 11,988 (3,091) 5,499 (1,066) 4,604 (1,400) 9,892 (1,887) 10,695 (2,781) 5,701 (1,259) 4,546 (1,214) 11,350 (2,162) 11,386 (2,963) 6,919 (1,038) 6,194 (1,493)
† Standard deviation presented in parenthesis.
Table 3. Average monthly weather data for the months May through September at Marion, IA, and Sciota, IL, in 2013 and 2014. Temperature is the average daily temperature recorded each month, and precipitation is the sum of all rain events during the month. Values in parentheses are the deviations from the 40-yr average. Temperature Location Marion Marion Marion Marion Marion Sciota Sciota Sciota Sciota Sciota
Month May June July August September May June July August September
2013
Precipitation 2014
————————————– °C ————————————– 15.9 (0.01) 15.9 (−0.03) 20.8 (−0.16) 21.7 (0.75) 21.6 (−1.55) 20.7 (−2.41) 21.8 (−0.01) 22.0 (0.18) 19.0 (1.68) 16.9 (−0.47) 17.1 (0.25) 18.0 (1.17) 21.5 (−0.34) 22.4 (0.53) 22.6 (−1.55) 20.1 (−4.03) 22.4 (−0.43) 22.3 (−0.51) 19.8 (1.37) 17.3 (−1.12)
is based on soil moisture at planting, rainfall events during the season, and root depth to determine the total available soil water the crop can access. The demand function is based on crop growth and biomass accumulation, transpiration efficiency, and vapor pressure deficit. The S/D ratio was scaled between 0 and 1, where 1 indicates that water supply fully meets the crop canopy demand, and 0 indicates no water available to meet the demand. Gaffney et al. (2015) defined water-limited environments as locations with a supply/demand ratio of 0.66 for ³1 d during flowering or grain fill. The method has been used by Cooper et al. (2016) and Gaffney et al. (2015) to quantify total soil moisture available to the crop during the season. In 2013 at Sciota, the S/D ratio was >0.66 throughout the season (Fig. 1). Similarly, the Marion location did not experience water stress during flowering (73 d after planting [DAP]) but did experience water stress during the grain-fill period beginning ~120 DAP. Rainfall was sufficient at Sciota during 2014, and the S/D ratio was 1.0 during all of the growing season (Fig. 2). Water availability was limiting
2013
2014
———————————– mm ———————————– 101.6 (−7.4) 69.6 (−41.9) 67.2 (−56.5) 292.4 (166.9) 18.8 (−86.30) 84.8 (−20.7) 16.1 (−93.9) 62.0 (−48.4) 1.4 (−85.9) 136.7 (49.3) 231.8 (112.2) 41.9 (−82.4) 45.8 (−72.6) 173.7 (57.4) 35.9 (−77.0) 67.9 (−43.7) 3.0 (−96.5) 85.3 (−12.2) 36.1 (−59.7) 143.9 (51.0)
at Marion during 2014, as the S/D ratio approached 0.66 at ~82 DAP. This water stress occurred after flowering (76 DAP) but persisted through ~121 DAP. According to the water S/D ratio, the 2014 Marion location can be classified as a water-limited environment beginning in the grain-fill period, but not in the flowering time period. Analyses of variance by mixed-model methodology followed the recommendations of Gilmour et al. (1997) for within-environment spatial analysis of experimental error and van Eeuwijk et al. (2001) for hybrid ´ environment interaction analysis. As the first step in the analysis, combinations of year (2013 and 2014) ´ location (Marion and Sciota) ´ N treatment (HN and LN) were considered as eight environments (siteyears). The model used included hybrid, PD, environment, and replication, as well as the interactions among those terms. This model allowed for assessment of the hybrid ´ environment interaction (which included year, location, and N treatment effects). Results from this analysis indicated that the hybrid ´ Fig. 1. Daily modeled water supply/demand ratio for Sciota, IL, and Marion, IA, during 2013. The predicted flowering date at Sciota was 19 July (64 d after planting) and 29 July (73 d after planting) at Marion. A supply/demand of 1.0 indicates sufficient soil water to meet evapotranspiration demand. The dashed line is placed at 0.66 and represents the supply/demand ratio where water limitation begins.
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Fig. 2. Daily modeled water supply/demand ratio for Sciota, IL, and Marion, IA, during 2014. The predicted flowering date at Sciota was 19 July (74 d after planting) and 23 July (76 d after planting) at Marion. A supply/demand of 1.0 indicates sufficient soil water to meet evapotranspiration demand. The dashed line is placed at 0.66 and represents the supply/demand ratio where water limitation begins.
environment and PD ´ environment interactions were small and were not a significant source of variation to consider. Additional evaluation of the year ´ hybrid, N treatment ´ hybrid, and year ´ hybrid ´ N treatment interactions was considered and confirmed the results of the original analysis, assuming eight environments, that these sources of hybrid ´ environment interactions were small and not significant. Because the environment ´ hybrid and N treatment ´ hybrid interactions were not significant, data from the 2 yr and two locations were combined to provide a source of replication for the N effect. Due to nonsignificant interactions, the next step in the analysis was performed jointly across all locations and N levels. Data were analyzed by a linear mixed model with N level, PD, location, and the interactions of density ´ N-level and density ´ location as fixed effects. The mixed model, with spatial adjustment of within-environment residuals, was designed to reduce the contribution of field variability to the amount of experimental noise in the data while preserving the genetic signal (Gilmour et al., 1997). Random effects included hybrid ´ density, hybrid ´ location, hybrid ´ density ´ location, row ´ location, column ´ location, replication (location), and density ´ replication (location). The N treatment ´ hybrid interaction was modeled, allowing for differences in magnitude of hybrid variation within N treatments. Following the recommendations of Gilmour et al. (1997), for the combined ANOVA across N treatments, the experimental error variance was modeled to allow for heterogeneous residual variance among locations, and a separable autoregressive structure for both row and column directions within the experimental layout was fitted for each location. Best linear unbiased predictors (BLUPs) for each hybrid were generated for each trait from this analysis. The primary objective of our work was to measure the change in hybrids over time within N treatments and planting densities. For this reason, the year of hybrid release was converted to a value starting at 0 (1934 release date) and continuing through time 80 (2013 release) and is denoted as year of hybrid release. The linear regression of hybrid prediction against year of hybrid release was performed. In the regression model, intercept, slope and quadratic term of year of hybrid release, density ´ year of hybrid release, N level ´ year of hybrid release, and 6
density ´ N level ´ year of hybrid release were included. All analyses were implemented using the ASReml_R software (Gilmour et al., 2009). The main effect of year of hybrid release indicates whether the trait has changed over year of hybrid release. The main effects of N treatment and PD indicated whether LN is different from HN and whether the 39K density is different from the 79K density. The year ´ N treatment and the year ´ PD terms indicate whether the response for a particular trait over the year of hybrid release changes depending on N treatment or PD. The year ´ N treatment ´ PD term indicates whether the response over year of hybrid release is unique depending on the interaction of N treatment ´ PD. The N treatment ´ PD interaction is not included as an individual effect in the model because it is not of primary interest. Year of hybrid release in relation to N treatment, PD, and N treatment ´ PD are effects of primary interest. Finally, year2, year2 ´ N treatment, and year2 ´ PD indicate whether the quadratic term(s) are significant and document whether there is a change in the linear trend over time for a particular trait. Due to three-way interactions for 9 of the 11 traits measured, the response curves are presented separately for each N treatment ´ PD for all traits except SLN 2 wk after flowering and kernel mass at maturity. Following analysis of data across years and locations, the hybrid BLUP values for the traits were used to calculate Pearson correlation coefficients in SAS (SAS Institute, 2011). Correlation coefficients were calculated between grain yield and ASI, leaf N traits, grain N concentration, and yield component traits. Correlation coefficients were not calculated between grain yield and traits using grain yield in their calculation (i.e., PFP and grain N m−2). Additional correlation coefficients were calculated for grain N concentration and leaf N traits, for grain N m−2 and leaf N traits and grain N concentration, and finally for leaf N traits.
Results and Discussion Yield and Yield Components Grain yield increased rapidly after release of the first hybrids in the 1930s. The rate of gain is influenced by the N level and PD through the year ´ N treatment ´ PD
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crop science, vol. 57, may– june 2017
interaction (Table 4). Under HN and 79K conditions, yield increased from 5449 kg ha−1 in 1934 to 14,934 kg ha−1 in 2013 (Fig. 3), representing a linear improvement of 143 kg ha−1 yr−1 across all years. Due to the significant year2 effect (Table 4), there is evidence that yield improvement is still increasing in the later years, but at a reduced rate. The estimated rate of gain for each 10-yr period beginning in 1934 (1 = 1930s and 7 = 2010s) was 1 = 120, 2 = 117, 3 = 113, 4 = 109, 5 = 105, 6 = 101, and 7 = 98 kg ha−1. This represents an average 18.3% decline in the rate of genetic gain in modern hybrids. Modern hybrids benefited greatly from increased PD, as the addition of 40,500 plants ha−1 increased yield by 2944 kg ha−1 (Fig. 3), whereas greater density decreased the yield of older hybrids by ~54% (2148 kg ha−1). There is a noticeable crossover in the 1960s, when greater PD (79K) increased yield compared with the lower PD (39K) in conditions when N was not limiting.
A similar crossover is noted by Duvick et al. (2004) using the PDs of 30,000 and 79,000 plants ha−1. Year of release and PD have smaller effects under limited-N conditions. Overall, yield increased at a linear rate of 87 kg ha−1 under LN and 79K between 1934 and 2013, but there is evidence of an approaching yield plateau due to the significant year2 term. When broken down by 10-yr periods beginning in 1934 (1 = 1930s and 7 = 2010s), the rate of gain under LN and 79K has been 1 = 72, 2 = 65, 3 = 50, 4 = 43, 5 = 36, 6 = 28, and 7 = 21 kg ha−1 yr (70% decline in the rate of genetic gain, when comparing the most recent decade with the 1930s). Experiments conducted under nonlimiting N conditions estimate genetic gains at 87.6 (Smith et al., 2014), 86 (Haegele et al., 2013) and 90.2 kg ha−1 yr−1 (Castleberry et al., 1984). The reported rates of genetic gain under limited-N conditions are 3.8 kg ha−1 yr−1 (Smith et
Table 4. Analysis of variance for traits measured on 47 hybrids grown in high (>200 kg N ha−1) and low ( F ————————————————————————————————————
