The Influence of Nitrogen Application Timing and Rate on ... - BioOne

Weed Science 2012 60:510–515

The Influence of Nitrogen Application Timing and Rate on Volunteer Corn Interference in Hybrid Corn Ryan M. Terry, Paul T. Marquardt, James J. Camberato, and William G. Johnson* Volunteer corn (VC) in hybrid corn has become more prevalent in recent years and can reduce grain yield. Nitrogen (N) management can influence VC interference in corn. Field experiments were established to determine the effects of N fertilizer management and VC interference on hybrid corn growth and grain yield. Treatments consisted of three VC densities (control, 0 plants m22; low density, 1 plant m22; high density, 4 plants m22) and six N fertilizer treatments (0 kg N ha21, 67 kg N ha21 at planting, 67 kg N ha21 at planting + 133 kg N ha21 at V5 corn growth stage, 67 kg N ha21 at planting + 133 kg N ha21 at V10 corn growth stage, 200 kg N ha21 at V5 corn growth stage, and 200 kg N ha21 at V10 corn growth stage). The effect of VC on hybrid corn was dependent on N rate. When 200 kg N ha21 was applied, regardless of application timing, hybrid corn dry weight, hybrid corn N content, and hybrid corn grain yield were reduced by the high VC density. However, when VC grain yield was added to hybrid corn grain yield, VC density did not affect total grain yield. When 0 and 67 kg N ha21 were applied, neither hybrid corn dry weight nor hybrid corn N content was affected by either VC density, but the high VC density reduced hybrid corn grain yield for both N rates by 19% and total grain yield by 9 and 10%, respectively. Application timing of N fertilizer had no effect on hybrid corn dry weight, N content, or grain yield. However, late N fertilizer applications (200 kg N ha21 at V10 and 67 kg N ha21 at planting +133 kg N ha21 at V10) resulted in greater VC N content, VC grain yield, and total yield. Assuming the harvestability of VC, the ability of a late N treatment (V10) to maximize total grain yield allows growers to use a late N application to reduce the competitive effects of VC in hybrid corn. Nomenclature: Corn, Zea mays L. Key words: Competition, glyphosate-resistant volunteer corn, interference, nitrogen.

Nitrogen (N) is a major constituent in various biological compounds that affect photosynthesis and ultimately corn grain yield (Anderson et al. 1984; Swank et al. 1982). N fertilizer universally is acknowledged as a necessary input to maximize corn grain yield (Below et al. 1981), but N fertilizer continues to be one of the costliest inputs. Prices for anhydrous ammonia, urea, and nitrogen solutions nearly tripled from 2000 to 2008 (USDA–ERS 2008). Along with higher N costs, environmental concerns regarding N pollution of surface and ground water (Turner and Rabalais 2003) could lead to environmental regulation that requires the implementation of more efficient N fertilizer management practices. In-season N fertilizer applications (sidedressing) have received interest as a more efficient N management practice. Sidedressing optimizes the N uptake in corn by applying N (Andraski et al. 2000) closer to the time of maximum plant uptake, which in turn can increase N-use efficiency (Fox et al. 1986; Russelle et al. 1981). Opposition to widespread adoption of sidedressing has revolved around the fear of delayed or prevented N applications due to weather conditions and time constraints (J. J. Camberato, personal communication). Research has shown that delayed N application and visual N deficiency symptoms do not always result in yield loss. For example, no yield reduction was measured when the N application was applied at the V11 growth stage (Scharf et al. 2002), as described by the collar method (Ritchie et al. 1992). Scharf et al. (2002) recorded a 3% yield reduction when N was applied to V12 to V16 growth stage (even when corn showed signs of N deficiency), compared to plots where N was applied at planting. DOI: 10.1614/WS-D-11-00197.1 * First, second, and fourth authors: Graduate Research Assistant, Research Associate, Professor, Department of Botany and Plant Pathology, 915 W. State Street, Purdue University, West Lafayette, IN 47907; third author: Associate Professor, Department of Agronomy, 915 W. State Street, Purdue University, West Lafayette, IN 47907. Corresponding author’s E-mail: [email protected]

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Weed Science 60, July–September 2012

Corn grain demand is projected to increase due to federal mandates that require greater biofuel production and also because of a growing world population. As a result, experts estimated that by 2016, approximately 38 million corn hectares will be planted with continuous corn, comprising 30% of the total corn hectares planted (Malcolm and Aillery 2009). VC emerging in the succeeding corn crop can affect yield in continuous corn. Shauck and Smeda (2011) reported 6.2 to 99 kernels m22 remained in the field after harvest as a seed source for VC. The VC problem has been compounded with the increased adoption of herbicide-resistant corn hybrids. As of 2011, herbicide-resistant corn was planted on 72% of the corn hectares in the United States (USDA-ERS 2011), and the majority of these were glyphosate-resistant (GR). The GR gene is a single dominant gene and is present in 75% of GR hybrid progeny when the hybrid is heterozygous (Magulama 2009). Results from a greenhouse study showed no difference in tolerance to glyphosate between GR hybrid corn and the GR hybrid progeny (W. G. Johnson, personal communication). VC interference in hybrid corn likely will be season-long, unless herbicide-resistant genetics are rotated or interrow cultivation is performed, yet there are no data in the literature quantifying the effect that VC interference has on hybrid corn. Any potential hybrid corn yield loss due to the presence of VC is presumed to be result from direct competition for available resources (i.e., light and nutrients). Grass and broadleaf weed species accumulate N, reduce N availability, and consequently decrease grain yield when growing in competition with corn (Cathcart and Swanton 2003; Hans and Johnson 2002; Hellwig et al. 2002; Johnson et al. 2007). When mixed-grass weed species emerged with corn at densities of 300 plants m22, the mixed-grass species accumulated up to 71 kg N ha21 by the time the weeds reached 31 cm in height (Hellwig et al. 2002). Season-long interference of shattercane [Sorghum bicolor (L.) Moench subsp. arundinaceum (Desv.) de Wet and Harlan] at a density of 200 plants m22 reduced N content in corn up to 88% and

Table 1. Mean monthly air temperature and total precipitation at the Throckmorton Purdue Agricultural Center (TPAC) and the Pinney Purdue Agricultural Center (PPAC) in 2010 and 2011.a Temperature

Precipitation

TPAC Month

2010

PPAC 2011

2010

TPAC 2011

2010

PPAC 2011

2010

2011

----------------------------------------------------------------------C ------------------------------------------------------------------------------------------------------------------------------------------cm -------------------------------------------------------------------April 20–30 May June July August

13.3 18.1 23.6 24.3 24.2

11.8 16.8 22.7 25.8 22.8

10.1 16.6 21.5 23.3 22.6

9.2 14.6 21.1 24.3 21.2

4.6 6.2 10.6 6.5 4.4

15.2 11.4 9.4 4.8 2.6

3.3 9.8 16.3 10.7 4.8

10.4 11.9 10.4 13.0 6.1

September Mean Total

19.7 20.5 —

17.8 19.6 —

17.8 18.6 —

16.1 17.7 —

2.4 — 34.6

7.1 — 50.6

6.3 — 51.2

8.5 — 60.3

a

Indiana State Climate Office Web site. http://climate.agry.purdue.edu/climate/index.asp. Accessed: January 3, 2011.

reduced grain yield up to 85%. Yield loss was prevented when mixed-grass weed species and shattercane were controlled before weed heights reached 15 and 23 cm, respectively (Hans and Johnson 2002; Hellwig et al. 2002). Johnson et al. (2007) reported that giant ragweed (Ambrosia trifida L.), growing in densities of 0.5 plants m22, reduced yield by 19% and accumulated up to 104 kg N ha21 when present throughout the growing season. N content in giant ragweed at 0.5 plants m22 was similar to the N uptake in shattercane for 200 plants m22 (Hans and Johnson 2002). Removal of the weedy species during the growing season can reduce the severity of weed interference and N content reduction in hybrid corn. However, removal of VC in corn is difficult, and VC interference in corn often is season-long as a result. The extent of N accumulation by VC in corn and the effect of N timing on VC interference in corn is unknown. Therefore, the objectives of this research were to evaluate the influence N application timing has on VC interference in hybrid corn, and to quantify the dry weight, N content, and grain yield of VC growing in hybrid corn. Materials and Methods

Field experiments were established in the spring of 2010 and 2011 at the Throckmorton Purdue Agricultural Center (TPAC) in Lafayette, IN and the Pinney Purdue Agricultural Center (PPAC) in Wanatah, IN. The soil type at TPAC was a Toronto–Milbrook silty loam (fine-silty, mixed, superactive, mesic Udollic Endoaqualfs) with a pH of 6.2 and 2.9% organic matter. The soil type at PPAC was a Hanna sandy loam (coarseloamy, mixed, active, mesic Aquultic Hapludalfs) with a pH of 6.5 and 2% organic matter. All sites were planted to soybean the preceding year. The sites were fall chisel-plowed and fieldcultivated in the spring prior to planting, and fertilized according to recommendations (Vitosh et al. 1995), with the exception of N. Just after planting, a premixture of 1.46 kg ai ha21 of s-metolachlor, 1.46 kg ai ha21 of atrazine, and 0.188 kg ai ha21 of mesotrione (Lexar, Syngenta Crop Protection, Inc., Greensboro, NC 27419) was applied at all site-years for weed control followed by POST applications of glyphosate (Roundup Powermax, Monsanto Company, St. Louis, MO 63167) at 0.84 kg ae ha21 as needed. Temperature and weather data for all site-years are shown in Table 1. The experimental design was a randomized complete block with a total of 18 treatments with four replications. Treatments

consisted of a control (0 plants m22), a low VC density (1 plant m22), and a high VC density (4 plants m22), assigned to one of six N treatments (Table 2). Corn growth stages were determined by the collar method (Ritchie et al. 1992). Plot size was 4.5 m wide by 9.1 m long at both locations. A GR corn hybrid, DeKalb 61-19 in 2010 and 62-54 in 2011 (Monsanto Company) was planted in 76-cm rows at 79,000 seeds ha21 (7.9 plants m22) on April 20, 2010 and May 10, 2011 at TPAC and on May 4, 2010 and May 9, 2011 at PPAC. VC seeds were hand-planted between rows with jab planters (Almaco, Nevada, IA 50201) on the same day the hybrid corn was planted. Corn seed was collected from GR hybrid corn in the fall of 2009 (DeKalb 60-18) and 2010 (DeKalb 61-19) and used as the VC seed the following spring. For all N fertilizer applications, urea-ammonium nitrate (28% N) was banded on the soil surface next to each corn row with a CO2-pressurized backpack sprayer equipped with a hooded single-nozzle sprayer. TeeJet StreamJet nozzles (TeeJet Technologies, Wheaton, IL 60187) SS0006, SS0010, and SS0015, at a pressure of 124 kPa, were used to apply 67, 133, and 200 kg N ha21, respectively. Single photon avalanche diode (SPAD) meter (Spectrum Technologies, Inc., East Plainfield, IL 60585) values were measured at the V5, V10, and R3 hybrid corn growth stages on 20 hybrid plants in each plot. SPAD measurements were taken from the highest leaf with a visible leaf collar at the V5 and V10 growth stages and from the uppermost ear leaf at the R3 growth stage. The SPAD measurement was taken approximately halfway up the leaf, and approximately halfway between the leaf margin and the midvein of the leaf (Scharf et al. 2006). At physiological maturity, 10 plants of both hybrid and volunteer corn were removed from each plot and dried, Table 2. Experimental design and treatment numbers for nitrogen (N) application timing and rate within each volunteer corn (VC) density. Treatment numbers VC density (plants m22) N timing At planting V5 SPLITa V10 SPLITb V5 single V10 single a b

N rate kg N ha21

0

1

4

0 67 67 + 133 67 + 133 200 200

1 4 7 10 13 16

2 5 8 11 14 17

3 6 9 12 15 18

67 kg N ha21 at planting + 133 kg N ha21 at V5 corn growth stage. 67 kg N ha21 at planting + 133 kg N ha21 at V10 corn growth stage.

Terry et al.: The influence of nitrogen and volunteer corn on hybrid corn

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Table 3. Dates of corn planting, nitrogen (N) applications, single photon avalanche diode (SPAD) measurements, biomass harvest and grain harvest at Throckmorton Purdue Agricultural Center (TPAC) and Pinney Purdue Agricultural Center (PPAC) in 2010 and 2011. TPAC

Planted Nitrogen at planting V5 nitrogen application and SPAD V10 nitrogen application and SPAD R3 SPAD Corn biomass harvest Grain harvest a

PPAC

2010

2011

2010

2011

April 20 April 20 June 1 June 22 July 23 September 1 October 1

May 10 May 10 June 8 June 29 August 1 September 6 October 4

May 4 May 4 June 9 July 1 August 2 September 15 October 13

May 9 May 9 June 14 July 5 August 11 September 20 October 7

Abbreviations: V5, V5 corn growth stage; V10, V10 corn growth stage; R3, R3 corn growth stage.

weighed, and ground to determine N concentration. Plant N concentration was determined by a commercial analytical laboratory (A & L Great Lakes Laboratories Inc., Ft. Wayne, IN 46608). Hybrid corn grain yield was measured by harvesting the center two rows with a plot combine, and VC grain yield was determined by hand-harvesting ears from plants between the center two hybrid rows prior to machine harvest. Yield was adjusted to 15.5% moisture. Dates of all the above operations are listed in Table 3. Data were analyzed with the PROC MIXED procedure in SAS 9.2 (SAS Institute, Inc., Cary, NC 27513). For data analysis, N treatments were separated according to total N rate (0 kg N ha21, 67 kg N ha21, and 200 kg N ha21) because N rate was significant when tested with ANOVA for all fixed effects (P # 0.0001). The 200 kg N ha21 treatments were then further separated by the four N application timings (V5 SPLIT, V10 SPLIT, V5 SINGLE, and V10 SINGLE). VC density, N application timing, and their interactions were considered fixed effects. None of the interactions were significant (P . 0.05) and thus were not included in the Table 4. Hybrid corn single photon avalanche diode (SPAD) values as affected by volunteer corn (VC) density and nitrogen (N) treatment. Data were pooled over both years and locations. Linear contrasts were used to compare the low (1 plant m22) and high (4 plants m22) VC density treatment means to the control (0 plants m22) for each N treatment. The dashes represent where no SPAD values were taken due to the 200 kg N ha21 application timing at V5 and V10 hybrid corn. P values in bold are significant (a # 0.05). SPAD measurement timing VC density plants m22

V5 hybrid corn

V10 hybrid corn

P.F

R3 hybrid corn

P.F

P.F

----------------------------------------------------- 0 kg N ha21 ---------------------------------------------------0 1 4

42.0 40.8 38.5 40.7 0.92a 37.7 0.85a 41.8 0.64a b b 41.0 0.02 37.6 0.005 35.1 ,0.001b ----------------------------------------------------67 kg N ha21 ---------------------------------------------------

0 1 4

44.1 43.7 43.6

0.39a 0.33b

46.1 46.8 44.8

0.21a ,0.001b

45.9 46.5 41.2

0.23a 0.02b

-------------------------------------------- 200 kg N ha21 at V5 ------------------------------------------0 1 4

— — 50.0 53.9 54.4 0.11a — — 49.7 0.59a — — 48.0 ,0.001b 52.6 ,0.001b -------------------------------------------200 kg N ha21 at V10 ------------------------------------------

0 1 4

— — —

— — —

— — —

— — —

a

54.0 53.8 52.4

0.98a 0.04b

P value from the comparison of the control to the low volunteer corn density. P value from the comparison of the control to the high volunteer corn density. b

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final analysis. Year and location were considered random effects. When analyzing the effect of N application timing on all VC growth parameters, means of the VC were averaged over low and high VC densities. Mean comparisons of treatment combinations of interest were made using linear contrasts with an alpha value of 0.05. PROC CORR in SAS was used to correlate SPAD values with hybrid corn dry weight and hybrid corn N content at maturity. Results and Discussion

SPAD Values. SPAD values did not indicate a difference in hybrid corn leaf greenness (chlorophyll content) at the low VC density, compared to the no VC control, across all N rates and application timings (Table 4). Without fertilizer N, the high VC treatment decreased leaf greenness, in comparison to the no VC control, at V5, V10, and R3. When 67 kg N ha21 was applied at planting, differences in hybrid corn leaf greenness caused by the high VC density were not detected until the V10 growth stage of the hybrid corn (Table 4). Differences in hybrid corn leaf greenness due to the high VC density were observed at V10 when 200 kg N ha21 was applied at V5 and at R3 when 200 kg N ha21 was applied at V5 or V10. SPAD values can detect differences in corn leaf greenness, which often is directly correlated to N stress in corn (Scharf et al. 2006). Previous research has used SPAD values to assess N stress in hybrid corn due to weed interference with variable success. Cordes et al. (2004) detected differences in greenness in corn growing in a high density (369 to 445 plants m22) of common waterhemp (Amaranthus rudis Sauer), whereas no difference in corn leaf greenness was detected from giant ragweed at a density of 0.5 plants m22 (Johnson et al. 2007). The use of hybrid corn leaf greenness values as an indicator of N stress caused by weed interference and the corresponding correlation of the values to hybrid corn N content and biomass at hybrid corn maturity might not have utility in measuring hybrid corn N competition with all weed species. In the case of VC growing in hybrid corn, the ability to measure differences in hybrid corn leaf greenness relatively early in the growing season might allow for in-season N rate adjustments to reduce hybrid corn N stress and potentially reduce the effects of VC interference on hybrid corn. Corn Dry Weight and Nitrogen Content. Hybrid corn dry weight and hybrid corn N content at maturity were unaffected by VC interference at either density when 0 or 67 kg N ha21 were applied. However, hybrid corn dry weight and N content were reduced by high VC density when 200 kg N ha21 was applied (Table 5). As stated previously, SPAD values

Table 5. The effect of volunteer corn (VC) density and nitrogen rate on dry weight and nitrogen (N) content for hybrid and VC. Data were pooled over both years and locations. Linear contrasts were used to compare the low (1 plant m22) and high (4 plants m22) VC density means to the control (0 plants m22) for each N treatment when comparing hybrid corn. Comparisons for the VC were made between the low and high VC densities. P values in bold are significant (a # 0.05). VC density plants m22

Hybrid corn dry weight kg ha21

P.F

VC dry seight kg ha21

Hybrid corn N content kg ha21

P.F

P.F

VC N content kg ha21

P.F

------------------------------------------------------------------------------------------------------------------------------------- 0 kg N ha21 -----------------------------------------------------------------------------------------------------------------------------------0 1 4

12,213 12,379 11,053

0.84a 0.12b

— 696 2492

106 110 87

,0.001c 21

------------------------------------------------------------------------------------------------------------------------------------ 67 kg N ha 0 1 4

15,021 15,433 13,614

0.65a 0.08b

— 739 2541

139 144 120

,0.001

21

0 1 4

18,240 18,223 15,813 a b c

0.98a ,0.001c

— 841 3035

c

,0.001

— 6.1 22.2

,0.001c

-----------------------------------------------------------------------------------------------------------------------------------

c

-----------------------------------------------------------------------------------------------------------------------------------200 kg N ha

0.81a 0.08b

0.79a 0.10b

7.2 25.9

,0.001c

---------------------------------------------------------------------------------------------------------------------------------200 198 172

0.95a ,0.001b

10.3 37.2

,0.001c

P value from the comparison of the control to the low volunteer corn density. P value from the comparison of the control to the high volunteer corn density. P value from the comparison of the low to the high volunteer corn density.

indicated a reduction in corn leaf greenness for all N rates when in the presence of a high VC density (Table 4). SPAD values at R3 when compared with hybrid corn N content at maturity and with hybrid corn dry weight were correlated (R2 5 0.72 and 0.68, respectively). Therefore, the lack of differences (a # 0.05) observed in hybrid corn dry weight and hybrid corn N content with lower N rate treatments (0 or 67 kg N ha21) might be the result of experimental variability due to fewer low N treatments and to sampling error associated with the destructive plant samples at corn maturity. VC dry weight and VC N content were nearly four-fold greater for the high VC density across all N rates, indicating that the increased VC dry weight and VC N content were a function of the higher VC density, and that VC dry weight and N content on a per plant basis were similar regardless of N rate (Table 5). This last point is illustrated by the fact that the high VC density accumulated 22.2 kg N ha21 with no N fertilizer and as much as 37.2 kg N ha21 with the 200 kg N ha21 treatments (Table 5). When comparing early N applications (V5 SINGLE and V5 SPLIT) to late N applications (V10 SINGLE and V10 SPLIT) at the 200 kg N ha21 rate hybrid corn, dry weight was reduced by late N applications, but hybrid corn N content was unaffected (Table 6). Conversely, VC dry weight and VC

N content were greater with late N applications (Table 6). Neither dry weight nor N content of hybrid corn or VC was affected by the SPLIT N application vs. the SINGLE application, at either V5 or V10. Johnson et al. (2007) showed a similar response where hybrid corn N content, growing in competition with giant ragweed, was unaffected by N fertilizer application timing. Grain Yield. For all N rates, low VC density did not decrease hybrid corn grain yield or total grain yield (hybrid corn grain yield + VC yield) when compared to the VC-free control (Table 7). Hybrid corn grain yield was decreased by 19, 19, and 12% at the 0, 67, and 200 kg N ha21 rates, respectively when the VC density was 4 plants m22. The greater yield loss observed when lower N rates were applied shows the impact that N has on corn grain production. VC grain yield in both of the 0 and 67 kg N ha21 rates did not compensate for the lost hybrid corn grain yield, which resulted in total grain yield being decreased by 8% and 10%, respectively (Table 7). At the 200 kg N ha21 rate, VC grain yield compensated for the lost hybrid corn grain yield (due to VC competition) with the end result being no total yield loss. The observed loss in hybrid corn grain yield due to VC interference, regardless of N rate, is similar to previous research addressing season-long

Table 6. The effect of nitrogen (N) timing applied at a total rate of 200 kg ha21 on dry weight and N content at maturity for hybrid and volunteer corn (VC). Data were pooled over both years and locations. Linear contrasts were used to compare treatment means. P-values in bold are significant (a # 0.05). Hybrid corn dry weight N timing

21

kg ha

P.F

VC dry weight kg ha

21

P.F

Hybrid corn N content kg ha

21

P.F

VC N content kg ha

21

P.F

------------------------------------------------------------------------------------------- Early (V5 SINGLE + V5 SPLIT) vs. Late (V10 SINGLE + V10 SPLIT) ------------------------------------------------------------------------------------------Early 17,538 1,876 188 22.7 Late 16,817 0.02 2,117 0.01 192 0.45 26.1 0.009 ----------------------------------------------------------------------------------------------------------------------------------------- V5 SPLIT vs. V5 SINGLE ----------------------------------------------------------------------------------------------------------------------------------------V5 SPLIT 17,653 1,944 190 23.4 V5 SINGLE 17,420 0.48 1,807 0.34 186 0.49 22.0 0.47 ---------------------------------------------------------------------------------------------------------------------------------------V10 SPLIT vs. V10 SINGLE -------------------------------------------------------------------------------------------------------------------------------------V10 SPLIT 16,846 2,048 196 24.9 V10 SINGLE 16,788 0.82 2,186 0.65 188 0.24 27.2 0.21 a N timing abbreviations: V5 SPLIT 5 67 kg N ha21 at planting + 133 kg N ha21 at V5; V10 SPLIT 5 67 kg N ha21 at planting + 133 kg N ha21 at V10; V5 SINGLE 5 200 kg N ha21 at V5; V10 SINGLE 5 200 kg N ha21 at V10.


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Table 7. The effect of volunteer corn (VC) interference and nitrogen (N) rate on grain yield of hybrid and VC. Data were pooled over years and locations. Linear contrasts were used to compare the low (1 plant m22) and high (4 plants m22) VC density means to the control (0 plants m22) for each N rate when comparing hybrid corn. Comparisons for the VC were made between the low and high VC densities. P values in bold are significant (a # 0.05). Hybrid corn yield

VC density plants m22

kg ha

21

P.F

Total yielda

VC yield kg ha

21

kg ha21

P.F 21

----------------------------------------------------- 0 kg N ha

P.F

----------------------------------------------------

0 1 4

7,466 — 7,466 358 7,498 0.93b 7,140 0.39b c d 6,048 ,0.001 784 ,0.001 6,832 0.04c ----------------------------------------------------67 kg N ha21 ---------------------------------------------------

0 1 4

10,584 — 10,584 339 10,866 0.43b 10,527 0.91b c d 8,614 ,0.001 872 ,0.001 9,486 0.005c ---------------------------------------------------200 kg N ha21 --------------------------------------------------

0 1 4

12,699 12,454 11,174

0.63b ,0.001c

— 408 1,280

12,699 12,862 12,454

,0.001d

0.32b 0.18c

a

Total yield 5 hybrid corn yield + VC yield. P value from the comparison of the control to the low volunteer corn density. P value from the comparison of the control to the high volunteer corn density. d P value from the comparison of the low to the high volunteer corn density. b c

grass weed interference (Hans and Johnson 2002; Hellwig et al. 2002). However, the overall effect of VC interference is not as dramatic because of the ability of VC to produce grain to compensate for lost hybrid corn yield, when full N fertilizer rates (i.e., 200 kg N ha21) are applied. Hybrid corn grain yield was unaffected by N timing (Table 8). However, late N applications increased VC grain yield when compared to early N applications, which resulted in increased total grain yield (Table 8). The increased VC grain yield due to late N applications was the same response observed with VC dry weight and VC N content. It is possible that late N applications were more efficient due to the relatively high rainfall in June for all site-years, which was equal or greater than the 30-yr average of 10.9 and 11.0 cm at TPAC and PPAC, respectively, which likely resulted in N leaching with the early N applications. This allowed for more available N from the late N applications, at the time of maximum plant uptake, resulting in higher total grain yield (Table 1). During an average rainfall year we can expect the early applications of N to remain available to the corn plants for a longer period of time, which would potentially eliminate the difference in grain yield between the early and late N applications.

VC interference results in reduced hybrid corn grain yields, but VC grain yield can compensate for lost hybrid corn grain yield. However, the ability of VC grain yield to compensate for lost hybrid corn grain yield is based on the assumption that VC ears will be harvested efficiently by commercial combines. Harvest inefficiencies (i.e., corn grain left in the field due to failure to enter the combine) as a result of VC interference might be caused by a number of potential factors, such as smaller ear size, VC plants not being in the planted row, and increased corn (hybrid and volunteer) stalk lodging due to localized high plant density. Increasing corn plant density consistently results in higher incidences of corn stalk lodging, ultimately increasing in-field grain loss at harvest (Pedersen and Lauer 2002; Stanger and Lauer 2006). Also, aspergillis (Aspergillis flavis), diplodia (Stenocarpella maydis), fusarium (Fusarium spp.), and penicillium (Penicillium spp.) ear rots were noted on the VC ears (P. T. Marquardt, personal observation), which at high levels can reduce grain quality. Blandino et al. (2008) showed that as corn plant density (the presence of VC increases corn plant density) increased, the percentage and severity of diseased corn kernels increased. Another potential issue with VC growing in hybrid corn relates to the expression of Bt (Bacillus thuringiensis) toxins by many of the VC plants specifically targeting corn rootworm (Diabrotica spp.). Expression of Bt by VC plants is reduced in N-deficient environments (Marquardt et al. 2012). VC expressing less Bt can expose corn rootworm to sublethal doses of the toxin, potentially increasing the speed of resistance evolution to Bt in rootworm populations. Despite the ability of VC grain yield to compensate for lost hybrid corn grain yield due to VC interference at 200 kg N ha21, the aforementioned issues with VC strongly support management practices to prevent VC in corn. However, future studies on the efficient and effective harvest of VC growing with hybrid corn by commercial combines might be necessary to fully assess the ability of VC grain yield to compensate for lost hybrid corn yield. As N fertilizer prices increase, and the pressure to reduce the environmental impact of unused fertilizer N increase, N fertilizer rates might decrease. A decrease in N could increase the effect of VC interference on hybrid corn. This research showed the yield effect that low N rates (0 or 67 kg N ha21) had on hybrid corn grain yield when growing in competition with VC and the ability of VC to compensate for lost hybrid corn yield. The increased total grain yield from VC interference in hybrid corn when a late N application was

Table 8. The effect of nitrogen (N) timing applied at a total rate of 200 kg ha21 on dry weight and N content at maturity for hybrid and volunteer corn (VC). Data were pooled over both years and locations. Linear contrasts were used to compare treatment means. P values in bold are significant (a # 0.05). Hybrid corn yield a

kg ha

N timing

21

VC yield

P.F

kg ha

21

Total yield P.F

kg ha

21

P.F

------------------------------------------------------------------------------------------- Early (V5 SINGLE + V5 SPLIT) vs. Late (V10 SINGLE + V10 SPLIT) ------------------------------------------------------------------------------------------Early 12,049 691 12,507 ,0.001 13,009 0.002 Late 12,344 0.06 1000 ----------------------------------------------------------------------------------------------------------------------------------------- V5 SPLIT vs. V5 SINGLE ----------------------------------------------------------------------------------------------------------------------------------------V5 SPLIT 12,096 634 12,516 V5 SINGLE 12,002 0.49 747 0.38 12,498 0.76 ---------------------------------------------------------------------------------------------------------------------------------------V10 SPLIT vs. V10 SINGLE -------------------------------------------------------------------------------------------------------------------------------------V10 SPLIT V10 SINGLE

12,435 12,253

0.32

916 1085

0.13

13,037 12,981

0.72

a N timing abbreviations: V5 SPLIT 5 67 kg N ha21 at planting + 133 kg N ha21 at V5; V10 SPLIT 5 67 kg N ha21 at planting + 133 kg N ha21 at V10; V5 SINGLE 5 200 kg N ha21 at V5; V10 SINGLE 5 200 kg N ha21 at V10.

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made suggests that late N applications can serve as a ‘‘rescue’’ treatment to eliminate the competitive effect of VC. However, the potential agronomic problems and unknown harvest capabilites associated with VC in a commercial operation should encourage management practices to reduce the presence of VC in corn.

Acknowledgments The authors would like to thank the graduate students in the Purdue Integrated Weed Management lab for their help and support in making this project a success.

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Received November 22, 2011, and approved March 21, 2012.


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