Effect of Nitrogen Source and Weed Management Systems on No-Till Corn Yields

Field research was conducted at upstate Missouri to evaluate the impact of weed management systems and pre-plant nitrogen source selection [polymer-coated urea, (PCU); anhydrous ammonia (AA), urea, and ammonium nitrate (AN)] and side dressed urea ammonium nitrate (UAN) at 168 kg N ha on no-till corn grain yield and weed growth. Small-seeded broadleaf weed heights responded differently to PCU and anhydrous ammonia in the two years of study. Corn heights were greater with AN and urea compared to PCU, AA, and side dressed UAN 7 to 9 weeks after planting. Nitrogen fertilizer source selection and weed management system affected total weed biomass (giant foxtail, common waterhemp, and common lambsquarters) at physiological maturity of corn. However, these factors showed no interactive effect on corn grain yields. An early postemergence application of atrazine + dimethenamid-P + glyphosate reduced total weed biomass 86% and 92% compared to atrazine + dimethenamid-P applied preemergence following AA and the non-fertilized control, respectively. A two-pass postemergence system (glyphosate followed by glyphosate) had 74 to 79% greater weed biomass compared to residual systems when following PCU. All weed management systems increased yield 1.5 to 5.09 Mg ha compared to the non-treated control, and no yield difference was observed among weed management systems. PCU, AA, and side dressed UAN are preferred over broadcast urea for integrated weed management of no-till corn production in this region.


Introduction
Early in the growing season, weeds can accumulate N rapidly, which can contribute to early-season interference and subsequent yield loss in corn (Zea mays L.) (Teyker et al., 1991;Davis & Liebman, 2001;Evans et al., 2003aEvans et al., , 2013b;;Cathcart & Swanton, 2004;Harbur & Owen, 2004;Lindquist et al., 2010), wheat (Triticum aestivum L.) (Blackshaw et al., 2002;Blackshaw et al., 2004), rice (Oryza sativa L.) (Ampong-Nyarko & De Datta, 1993a, 1993b), and canola (Brassica napus L.) (Blackshaw et al., 2011).Weeds can reduce soil NO 3 -N up to 50% in corn (Lindquist et al., 2010).Several integrated weed management studies (Walker & Buchanan, 1982;Di Tomaso, 1995) have investigated nitrogen because direct uptake by weed species may affect control (Kim et al., 2006) and grain yields depending on fertilizer rate (Evans et al., 2003a(Evans et al., , 2003b;;Cathcart & Swanton, 2004;Lindquist et al., 2010), placement (Blackshaw et al., 2002;Blackshaw et al., 2004), timing (Blackshaw et al., 2004;Harbur & Owen, 2004), and source (Teyker et al., 1991;Davis & Liebman, 2001;Blackshaw et al., 2011).In corn, nitrogen fertilizer recommendations and the impact on weed interference may depend on the weed species (Harbur & Owen, 2004).Weed management has been more critical as rates of N were reduced (Evans et al., 2003b;Cathcart & Swanton, 2004).However, with the introduction of enhanced efficiency fertilizers such as polymer-coated urea (PCU), N source selection (Teyker et al., 1991) may be an important component of integrated weed management.Kissel, 1986;Rochette et al., 2009) and yield loss (0.42 to 0.81 Mg ha -1 ) compared to ammonium nitrate (AN) (Stecker et al., 1993).However, recent regulations have decreased the availability of AN to farmers, a situation that has prompted industry to develop technology that increases efficiency of urea fertilizers.Polymer-coated urea is a controlled-release urea fertilizer that allows farmers to broadcast apply preplant N and reduce gaseous fertilizer loss such as N 2 O up to 49% in no-till compared to non-coated urea (Rochette et al., 2009;Halvorson et al., 2010).In canola, a preplant application of controlled-release N fertilizers and/or deep placement of N reduced overall weed growth and N uptake in the biomass of weeds, which could mitigate crop-weed competition for NO 3 -N in soil (Blackshaw et al., 2011).However, minimizing weed growth by limiting soil N availability may reduce the effectiveness of herbicide applications (Evans et al., 2003b) due to reduced interception and retention of herbicides on weeds (Kim et al., 2006).Changes in N management might also affect the critical period for weed control (Evans et al., 2003a).Therefore, N management practices in no-till corn that promote controlled-release of available N early in a growing season may require more intensive weed management systems.Such systems could be similar to those used in research evaluating low N rates (Evans et al., 2003a(Evans et al., , 2003b;;Cathcart & Swanton, 2004;Lindquist et al., 2010) to obtain the potential yield benefits derived from reduced weed-crop competition and the subsequent increase in available soil N for crop uptake.
The Midwestern U.S. contains more than 4 million ha of claypan soils (Anderson et al., 1990).Low hydraulic conductivity in the claypan subsoil layer minimizes the potential for N loss through leaching, but it increases the potential for denitrification loss due to its high propensity for extended periods of soil saturation.Also, the potential can be large for volatilization loss from surface-applied urea-based fertilizers directly after application due to warm, moist soil conditions in the spring (Ferguson & Kissel, 1986).Maximizing no-till corn yields in a claypan soil requires N fertilizer with the lowest potential for denitrification and volatilization loss (Nash et al., 2012a).Injecting anhydrous ammonia (AA) at depth reduces the potential for volatilization loss and generally presents the lowest risk of yield-limiting denitrification loss compared to other conventional N fertilizers (Scharf & Lory, 2006;Nash et al., 2012a).Addition of a polymer coating around urea prills with PCU results in a slow release of available N over time.This reduces volatilization and denitrification loss compared to conventional urea fertilizers (Rochette et al., 2009;Halvorson et al., 2010), and increases grain yields in high-risk areas of fields compared to non-coated urea (Noellsch et al., 2009;Motavalli et al., 2012).Reduced gaseous N loss with AA and PCU might result in greater N availability throughout the growing season and increase overall weed growth by the time corn reaches physiological maturity.
Weed control in no-till corn with weed management systems is expected to vary, depending on the N source.This is due to aggressive weed growth that might result from readily available N sources such as non-coated urea or AN.The availability of N to the crop or weed in a no-till production system might depend on the N source selection because some sources are placed below the soil surface (AA) or are controlled-release (PCU) (Blackshaw et al., 2004;Blackshaw et al., 2011).Polymer-coated urea is a controlled-release N source that might limit early weed growth due to this technology's slow N release properties.Similarly, AA is banded 15 to 20 cm below the soil surface, and root growth is necessary to access this N source.In other research, AA and PCU yields were similar in high-risk N loss areas of a field (Noellsch et al., 2009;Motavalli et al., 2012).It may be imperative to have better early season weed control with an N source that is placed below the soil surface in a no-till production system.
Research conducted on how N sources and weed management systems affect crop production is very scanty.The handful of studies includes primarily an N source study on sweet corn (Davis & Liebman, 2001), greenhouse experiment with corn (Teyker et al., 1991), and no-till canola (Blackshaw et al., 2011).Most field corn research looks at conventional tillage systems (Davis & Liebman, 2001;Evans et al., 2003aEvans et al., , 2003b;;Cathcart & Swanton, 2004;Lindquist et al., 2010).However, no research has evaluated how AA or new controlled-release PCU fertilizer affects no-till corn production with common weed management systems.We hypothesized that broadcast preemergence-applied PCU, deep placement of AA, and side dressed urea ammonium nitrate (UAN) would have shorter weeds than faster release N sources such as AN and non-coated urea.This would subsequently affect no-till corn grain yields, depending on which weed management systems were implemented.This research sought to determine how weed management systems and preplant N source selection affects no-till corn grain yield, weed heights, and weed control.
Differences in dry weights of individual weed species were due primarily to an interaction between year and weed management system (Table 3).Common waterhemp and giant foxtail dry weights were greater in 2007 than in 2006, probably because of more favorable overall growing conditions.However, common lambsquarters dry weights were greater in 2006 than in 2007 (data not presented).Individual weed control differences among management systems were due mainly to differences in application timing of the residual herbicide as a preemergence only (atrazine+dimethenamid-P) treatment compared to a sequential application of herbicides.Overall weed control differences were evident within a weed management system depending on the N source at physiological maturity (Table 2).The N source significantly affected common waterhemp dry weights (P = 0.0098) (Table 3).In a greenhouse experiment, redroot pigweed (Amaranthus retroflexus L.) was more competitive than corn under high NO 3 -N levels (Teyker et al., 1991).Anhydrous ammonia, PCU, and side dressed UAN had common waterhemp dry weights that were 40 to 75% greater than the non-fertilized control or urea (data not presented).This indicated that PCU, deep placement of AA, and side dressed UAN also provided N to late germinating weeds such as common waterhemp.In other corn research, preplant and topdressed AN had similar common waterhemp seed production (Harbur & Owen, 2004).

Yield
Although an interaction between N source and weed management system was detected for final weed biomass, there was no such interaction for yield (P = 0.62); therefore, main effects are presented.Similarly, no interaction was observed between green foxtail [Setaria viridis (L.) Beauv.]density and N rates (Cathcart & Swanton, 2004), but in other research the critical period for weed control was affected by management of N rate (Evans et al., 2003a).A significant (P < 0.0001) two-way interaction (year* weed management system) for grain yield was observed.Grain yields averaged 6.14 Mg ha -1 greater in 2007 than in 2006 (Table 2), probably due to differences in precipitation distribution during the summer between years (Figure 1).Grain yields for all weed management treatments were similar to the weed-free control (Table 2).Weed management systems increased grain yield 1.65 to 5.09 Mg ha -1 in 2006 and 2007 compared to the non-treated control.Effective weed management systems that are based on recommended rates and timings (Table 1) provide no-till farmers with flexible management options regardless of the N fertilizer source.
All N sources increased yield compared to the non-fertilized control (Figure 5).Side dressed UAN had the highest overall yield (10.5 Mg ha -1 ), which was similar to AA, AN, and PCU.A preemergence application of PCU and side dressed UAN increased yields 1.33 and 2.21 Mg ha -1 greater than urea, respectively.This was similar to other research evaluating anhydrous ammonia and PCU on claypan soils (Noellsch et al., 2009;Nash et al., 2013), but it differed from studies showing greater yield loss with a delayed application of AN compared to a preemergence application when weeds were allowed to compete with corn (Harbur & Owen, 2004).In Missouri, Stecker et al. (1993) reported greater N fertilizer use efficiency and yields (0.42 Mg ha -1 ) with AN for no-till corn than with surface-applied urea.Since in both study years common waterhemp and lambsquarters heights were among the tallest with PCU 6 to 7 WAP, it may be assumed that greater corn yields with PCU than with urea did not stem from reduced weed growth and N uptake.This yield increase may have come from reduced environmental N loss and greater available N for corn uptake.This is counter to a four-year, no-till canola study on well drained soils in the semiarid Canadian prairies.The study found that PCU was generally effective in reducing weed biomass, but it only increased yields in 4 of 20 site-years compared to urea (Blackshaw et al., 2011).Contrasting responses among the studies presumably are due to differences in crops, soils, and climate.

Conclusions
Small-seeded broadleaf weed heights responded differently to PCU and AA during the two years of this research.This indicated that N source might affect the critical period for weed control in a no-till production system.Corn heights were greater with AN and urea compared to PCU, AA, and side dressed UAN 7 to 9 WAP, indicating that N source could affect canopy development of no-till corn.The N fertilizer source and weed management system affected total weed biomass (giant foxtail, common waterhemp, and common lambsquarters) at physiological maturity of corn; however, these factors showed no interactive effect on corn grain yields.An early postemergence application of atrazine + dimethenamid-P + glyphosate reduced total weed biomass 86% and 92% compared to atrazine + dimethenamid-P applied preemergence following AA and the non-fertilized control, respectively.A two-pass postemergence system (glyphosate followed by glyphosate) had 74 to 79% greater weed biomass compared to residual systems when following a PCU application.All weed management systems increased yield 1.5 to 5.09 Mg ha -1 compared to the non-treated control, and no differences appeared among weed management systems.In this region, AA, broadcast PCU, and side dressed UAN are recommended over broadcast urea for no-till corn production.This study indicates that no-till corn farmers have several flexible options for N management and effective weed management systems that are based on label recommendations for weed heights and effective rates.

Figure 5 .
Figure 5. Corn grain yield response to nitrogen sources applied at 168 kg N ha -1 .Data were combined over years (2006 and 2007) and weed management systems.LSD (P = 0.01) is 1.30 Mg ha -1

Table 1 .
Soil test values and corn management practices

Table 2 .
methoxy-1-methylethyl)acetamide.Total weed dry weight interaction between weed management system and nitrogen source at physiological maturity, and corn grain yield response to weed management systems

Table 3 .
ANOVA table of giant foxtail, common lambsquarters, and common waterhemp dry weights when corn