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Weed Management

Nitrogen Fertilizer, Manure, and Compost Effects on Weed Growth and Competition with Spring Wheat

Agric. and Agri-Food Canada, Lethbridge Res. Cent., P.O. Box 3000, Lethbridge, AB Canada T1J 4B1

^* Corresponding author (blackshaw{at}agr.gc.ca)

Received for publication May 20, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Crop fertilization is an important component of integrated weed management systems. A field experiment was conducted to determine the effect of various application timing–tillage intensities and N sources on weed growth and spring wheat (Triticum aestivum L.) yield. Timing–tillage treatments consisted of applying the various N sources in either the previous fall or in spring each under zero-till or tilled conditions. Nitrogen sources consisted of granular ammonium nitrate fertilizer applied either surface broadcast or subsurface banded 10 cm deep between every second wheat row, fresh cattle (Bos taurus) manure, and composted cattle manure. An unfertilized control also was included. Treatments were applied in four consecutive years to determine annual and cumulative effects. Subsurface-banded N compared with broadcast N fertilizer often reduced N uptake by weeds, decreased weed biomass, and increased wheat yield. Weed N uptake and growth with fresh and composted manure tended to be intermediary between banded and broadcast N fertilizer in the initial year but was similar to or greater than that with broadcast N fertilizer in subsequent years. The gradual N release from manure and compost over years appeared to benefit weeds more than spring wheat. The ranking of the weed seedbank at the conclusion of the 4-yr experiment was composted manure = fresh manure broadcast N fertilizer > banded N fertilizer. Information gained in this study will be utilized to develop more efficient fertilization strategies as components of integrated weed management programs in spring wheat production systems.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
HERBICIDES AND FERTILIZER are major input costs in North American cropping systems (Derksen et al., 2002; Grant et al., 2002; Schlegel et al., 2005). Farmers are cognizant of these costs and thus are interested in alternative approaches to managing weeds and improving soil fertility.

Integrated weed management systems have the potential to reduce herbicide use (and associated costs) and to improve weed control over the long term (Buhler, 1999). Manipulation of crop fertilization may be a means of reducing weed interference in crops (DiTomaso, 1995). Nitrogen is the major nutrient added to increase crop yield (Raun and Johnson, 1999; Camara et al., 2003), but it is not always recognized that altered soil N levels can affect crop–weed competitive interactions. Nitrogen fertilizer can affect weed germination and establishment (Egley and Duke, 1985). Many weeds are high-N consumers (Qasem, 1992; Hans and Johnson, 2002), thus limiting N for crop growth. Weeds not only reduce the amount of N available to crops, but the growth of many weed species also is enhanced by higher soil N levels (Supasilapa et al., 1992; Blackshaw et al., 2003).

Crop–weed competitive interactions can be altered by N fertilizer dose (Cathcart and Swanton, 2003), application timing (Angonin et al., 1996; Blackshaw et al., 2004), and application method (Rasmussen et al., 1996; Mesbah and Miller, 1999). However, other studies found that N dose (Satorre and Snaydon, 1992; Gonzalez Ponce, 1998) or application method (Cochran et al., 1990) had little effect on crop–weed competition. Additionally, results may be crop and weed specific. Van Delden et al. (2002) reported that common chickweed [Stellaria media (L.) Vill.] interference with potato (Solanum tuberosum L.) was reduced with higher soil N levels, but the opposite result occurred when it was competing with wheat.

There has been renewed interest in recent years in using livestock manure to improve soil fertility and crop production (Janzen et al., 1999; Mooleki et al., 2004). Manure may be utilized directly or it may be composted before being applied to cropland. Composted manure has several advantages over fresh manure, including reduced numbers of viable weed seeds, reduced volume and associated cheaper transportation costs, and smaller particle size that facilitates more uniform application (Richard and Choi, 1999; Larney and Blackshaw, 2003). Livestock manure may affect crop–weed competitive interactions differently than N fertilizer (Davis and Liebman, 2001), perhaps due to speed of N release or form of N. Nitrogen in fresh manure is >90% ammonia while that of compost is >70% nitrate (Hao et al., 2004).

Greater knowledge of inorganic and organic N effects on weed growth and competitive interactions with crops will allow for a better understanding of why differences occurred among previous studies and would aid the development of fertilization strategies as components of integrated weed management programs. A field experiment was conducted to determine the response of selected grass and broadleaf weed species and spring wheat to N fertilizer, fresh cattle manure, and composted cattle manure applied the previous fall or at the time of planting spring wheat under zero-till or tilled conditions. Treatments were applied in four consecutive years to assess both annual and cumulative effects over time.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
A field experiment was conducted from 1998 through 2001 at Lethbridge, Alberta, Canada. The soil was a Typic Haplustoll with a sandy clay loam texture, pH 7.8, and 3.4% organic matter. The experiment was established on land that had been in spring wheat production in 1995 and 1996 and was fallowed in 1997. The plot site had very low weed populations due to excellent weed control attained during these preceding years. Available soil N (nitrate and ammonia) to a depth of 60 cm at the initiation of the experiment was 32 kg ha^–1. Annual precipitation at the site ranged from 176 to 482 mm over the duration of the study (Table 1). Precipitation received was near or above the long-term mean (387 mm) in 1998 and 1999 but well below average in 2000 and 2001.

View this table: [in this window] [in a new window]   Table 2. Composition and weight of beef cattle feedlot manure and composted feedlot manure applied during the 4-yr experiment.,

Broadleaf weeds were controlled with the commercial mixture of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) plus MCPA [(4-chloro-2-methylphenoxy)acetic acid] at 560 g a.i. ha^–1 in the grass weed plots, grass weeds were controlled with tralkoxydim {2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)-2-cyclohexen-1-one} at 200 g a.i. ha^–1 plus Turbo-charge adjuvant at 0.5% v/v in the broadleaf weed plots, and these herbicides were combined to control weeds in the weed-free plots. Herbicides were applied in June with a small-plot sprayer delivering 100 L ha^–1 at 205 kPa. Additionally, undesired weeds were removed by hand every 2 wk during the growing season.

At wheat anthesis, shoots of six representative plants of each weed species plus wheat were harvested for determination of N concentration. Plant samples were oven-dried at 70°C for 1 wk, finely ground, and analyzed for N concentration using an automated combustion analyzer (CE Instruments, Milan, Italy) coupled with a mass spectrometer (Micromass, Manchester, UK). At maturity, weed shoot biomass was determined by cutting plants at ground level in 1 m² and oven-drying at 30°C for 2 wk. Any weed seed from these biomass samples was returned to their respective 1-m² areas in each plot before snow accumulation each fall. Wheat yield was determined by hand-harvesting 2 m² in each plot; grain was threshed in a stationary thresher and dried at 27°C for 2 wk. Grain protein content was determined using a near-infrared transmission spectrophotometer (Foss Electric Multispec Division, Brampton, ON, Canada).

Weed seed in the soil seedbank was determined at the conclusion of this 4-yr experiment. Nine soil cores (8 cm diam. by 10 cm deep) were randomly collected from each plot, bulked, air-dried, placed in polyethylene bags, and stored for 3 mo at 1°C. Seed determinations were conducted using the greenhouse emergence method (Cardina and Sparrow, 1996). Soil was spread onto plastic trays, placed in a greenhouse with a day/night temperature of 24/17°C, and watered as necessary to keep moist. Weed emergence counts were made twice weekly for 1 mo. Soil was then air-dried, remixed, placed in polyethylene bags, and stored at 1°C for a minimum of 1 mo before conducting the second cycle of emergence counts. The cycle of cool storage/emergence counts was conducted three times, and emergence values were combined.

Data collected over the 4 yr were statistically analyzed as a split-split-split plot design (Steel and Torrie, 1980) with N application timing–tillage intensity as the main plot, N source as the split plot, weed species as the split-split plot, and year as the split-split-split plot factor. All interactions among these four factors also were included in the model. Analyses were performed using the MIXED procedure in SAS (1999), and year was treated as a repeated measure. The UNIVARIATE procedure within SAS was used to examine the residuals for normality and to check for outliers in the data. Any outliers in the data were deleted and the remaining data reanalyzed. A number of variance–covariance structures were modeled for each variable to account for differences in variance and possible correlations among years, and the best model was selected on the basis of the smallest Akaike's Information Criterion (AIC) value (SAS Inst., 1999). Weed seedbank data collected at the end of the study were highly variable and thus were log-transformed to stabilize the variance among treatments before conducting the analysis. Least squares means were generated for significant effects, and treatment means were compared using Fisher's Protected LSD test at the 5% level (Steel and Torrie, 1980).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Weed Nitrogen Concentration
Weed shoot N concentration was affected by N source in all years (P < 0.05). The unfertilized control was always among treatments having the lowest N concentration in all weed species (Fig. 1) , indicating that N uptake by weeds increased with added N. Common lambsquarters and wild mustard shoot N concentrations were greater than those of green foxtail or wild oat (P < 0.05), indicating that broadleaf, as well as grass weeds, potentially are serious competitors with crops for soil N.

View larger version (36K): [in this window] [in a new window]   Fig. 1. Weed shoot N concentration response to N source in four consecutive years. Common lambsquarters data were not collected in 2001 due to insufficient plants being present in the plots. Bars on the graph within a weed species and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Green foxtail shoot N concentration was lower with fresh cattle manure than with broadcast N fertilizer in 2 of 4 yr and was similar to that of banded N fertilizer in 3 of 4 yr (Fig. 1). Wild oat shoot N concentration was lower with fresh cattle manure than with broadcast N fertilizer in all years and was always similar to that of banded N fertilizer. However, shoot N concentration of common lambsquarters and wild mustard was similar with fresh manure and broadcast N fertilizer in all years and was higher than that attained with banded N fertilizer in common lambsquarters in 1 of 3 yr and in wild mustard in 3 of 4 yr. Eghball and Power (1999) found that plant available N is often lower with manure than with inorganic fertilizer in the year of application, and this appeared to be the case with the grass weeds in our study.

In contrast to fresh manure, composted manure resulted in similarly high green foxtail N levels in 2000, and higher N levels in 2001, compared with those attained with broadcast N fertilizer (Fig. 1). Wild oat N levels were similar with composted manure and broadcast N fertilizer in the latter 3 yr. Shoot N concentration of common lambsquarters and wild mustard was at similarly high levels with compost manure compared with surface broadcast N fertilizer in all years and was actually higher a couple of years. Nitrogen release is slower from composted manure than from fresh manure, resulting in lower plant available N in the application year but greater available N in succeeding years (Eghball and Power, 1999; Eghball et al., 2004).

Nitrogen application timing–tillage intensity affected shoot N concentration in some weed species (P < 0.05). Averaged over years, shoot N concentration of green foxtail, wild oat, and wild mustard was lower with spring-applied than with fall-applied N treatments (in zero-till or tilled conditions) (Table 3). Additionally, shoot N concentration was lower with zero-tillage than with tillage in green foxtail and wild oat with fall-applied and spring-applied N treatments, respectively.

Wheat shoot N concentration was greater with banded than with broadcast N fertilizer in 1 of 4 yr under weed-free conditions and in all 4 yr when competing with weeds (Fig. 2) . Rao and Dao (1996) previously reported higher winter wheat leaf N content with banded than with broadcast N fertilizer. It is noteworthy that broadcast N fertilizer did not increase weed-infested wheat N concentration above that of the unfertilized control in 2 of 4 yr, indicating that this is an inefficient method of applying fertilizer when competitive weeds are present. Mason (1987) reported that wheat N uptake was often reduced by competing weeds.

View larger version (56K): [in this window] [in a new window]   Fig. 2. Wheat shoot N concentration response to N source in four consecutive years when grown weed-free or weed infested. Bars on the graph within a weed treatment and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Application timing–tillage intensity affected shoot N concentration of weed-free wheat (P < 0.05) but not weed-infested wheat (P > 0.05). Wheat N levels were greater with spring-applied than with fall-applied N treatments, under either zero-till or tilled conditions, in 3 of 4 yr (Table 4). Additionally, wheat N levels within spring-applied treatments were greater with zero-tillage than with tillage in 1999 and 2001.

View larger version (53K): [in this window] [in a new window]   Fig. 3. Weed biomass response to N source in four consecutive years. Bars on the graph within a weed species and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Grass weed biomass was similarly high with broadcast fertilizer and manure in the first 3 yr but greater with manure in the fourth year. Weed biomass was similar with compost manure and banded N fertilizer in the initial year. However, in subsequent years, grass weed biomass with compost manure was similar to or greater than that attained with broadcast N fertilizer or fresh manure. Eghball and Power (1999) found that weed biomass in corn (Zea mays L.) was usually similar with broadcast fertilizer, manure, and composted manure. Similar to the findings with grass weeds, the combined biomass of common lambsquarters and wild mustard was lower with banded N than with surface-broadcast N fertilizer in all 4 yr and was lower than all other N treatments in the latter 3 yr (Fig. 3). However, unlike the findings with grass weeds, broadleaf weed biomass was lower with fresh and compost manure than with broadcast N fertilizer in the first 2 yr. Broadleaf weed biomass was similar with the two manure treatments and broadcast N fertilizer in 2000 but higher with the manure treatments in 2001.

Application timing–tillage intensity also affected weed biomass (P < 0.05). Spring compared with fall-applied N treatments reduced grass weed biomass in 1999 and 2001 and broadleaf weed biomass in 1999 and 2000 (Fig. 4) . Blackshaw et al. (2004) similarly reported that weed biomass in spring wheat was sometimes lower with spring-applied than with fall-applied N fertilizer. Grass weed biomass was lower with zero-tillage than with tillage with spring-applied N treatments in 2000 and with both spring-applied and fall-applied N treatments in 2001. Similarly, broadleaf weed biomass was lower with zero-tillage than with tillage with fall-applied N treatments in 1998 and 1999. Tillage is known to stimulate germination of some weed species (Roberts, 1981; Mohler, 2001), and this may have been a contributing factor to greater weed biomass in tilled treatments in some instances.

View larger version (46K): [in this window] [in a new window]   Fig. 4. Weed biomass response to N application timing–tillage intensity in four consecutive years. Bars on the graph within a weed species and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

View larger version (45K): [in this window] [in a new window]   Fig. 5. Effect of N source over four consecutive years on the weed seedbank at the conclusion of the 4-yr experiment. The grass weed seedbank was composed of approximately 80% wild oat and 20% green foxtail. The broadleaf weed seedbank was composed of approximately 55% wild mustard, 35% common lambsquarters, and 10% other species such as redroot pigweed (Amaranthus retroflexus L.) and kochia [Kochia scoparia (L.) Schrad.]. Bars on the graph within a weed type with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Fresh and composted manure were among treatments with the greatest seedbank of both grass and broadleaf species (Fig. 5). The broadleaf weed seedbank was much lower with surface-broadcast N fertilizer than with either fresh or composted manure. However, the grass weed seedbank was only lower with broadcast fertilizer than with compost manure (not fresh manure), and the magnitude of the difference was considerably less than that noted with broadleaf weeds.

Application timing–tillage intensity affected the seedbank of grass weeds (P < 0.05) but not broadleaf weeds (P > 0.05). Averaged over both tillage treatments, the grass weed seedbank was lower with the spring-applied than with the fall-applied N treatments (3368 vs. 4570 seeds m^–2). A previous study found that spring-applied compared with fall-applied N fertilizer reduced the seedbank of wild oat and common lambsquarters but not green foxtail or wild mustard (Blackshaw et al., 2004).

Wheat Yield and Protein Content
Nitrogen source and weed treatments interacted to affect spring wheat yield (P < 0.05). Weed-free wheat yield was higher with all fertilizer and manure treatments compared with the unfertilized control in all years (Fig. 6) . This was an expected finding as wheat is known to respond positively to higher soil N levels (Raun and Johnson, 1999; Schlegel et al., 2005).

View larger version (45K): [in this window] [in a new window]   Fig. 6. Wheat yield response to N source in four consecutive years when grown weed-free, with grass weeds (green foxtail and wild oat), or with broadleaf weeds (common lambsquarters and wild mustard). Bars on the graph within a weed treatment and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Weed-free wheat yield was lower with fresh or composted manure than with either of the N fertilizer treatments in the first year (1998). However, wheat yield was greater with compost manure than with either broadcast N fertilizer or fresh manure in 1999. Wheat yield was similar among both manure treatments and the broadcast fertilizer treatment in the latter 2 yr. Reider et al. (2000) documented that corn yields were usually similar with composted and fresh manure, but Loecke et al. (2004) found that corn yields were sometimes greater with composted manure.

Nitrogen source often had a greater effect on yield when wheat was grown in the presence of competing weed species than when grown weed-free (Fig. 6). Wheat infested with green foxtail and wild oat had higher yields with all N treatments compared with the unfertilized control only in the first 2 yr. Neither of the manure treatments yielded more than the unfertilized control in 2000 and 2001. Similarly, broadcast N fertilizer did not result in greater yield compared with the unfertilized control in 2001. Grass-infested wheat yield was greater with banded N fertilizer than with all other N treatments in 3 of 4 yr, indicating that it was the preferred N application method. Kirkland and Beckie (1998) documented that weed-infested spring wheat yield averaged over six experiments was 12% greater with banded than with broadcast N fertilizer.

Unlike the findings with grass weeds, wheat infested with broadleaf weeds had higher yields with all N treatments compared with the unfertilized control in all years (Fig. 6). Banded N fertilizer was always among treatments with the highest wheat yield when competing with broadleaf weeds, but yields were only higher with banded N fertilizer than with broadcast fertilizer or either manure treatment in 2 of 4 yr. These results may indicate that broadleaf weeds are less affected by N source and placement compared with grass weeds. The extensive taproots of common lambsquarters and wild mustard facilitate N extraction from deep portions of the soil profile, and thus they may be less affected by fertilizer or manure applied on or within 10 cm of the soil surface compared with grass weeds that have greater concentration of roots in the upper regions of the soil profile.

Wheat infested with broadleaf weeds had similar or higher yields with fresh and composted manure compared with broadcast N fertilizer in the latter 3 yr (Fig. 6). Indeed, the two manure treatments had similar wheat yields to those of banded N fertilizer in 2 of 4 yr.

Spring compared with fall-applied N resulted in higher yield of weed-free wheat in 1998, grass-infested wheat in 1999 and 2000, and broadleaf-infested wheat in 1998 and 1999 (Fig. 7) . Spring-applied treatments also were better than the fall-tilled N treatment within broadleaf-infested wheat in 2000. A previous study found that weed-infested spring wheat yield was greater with spring-applied than with fall-applied N fertilizer in about 50% of the cases (Blackshaw et al., 2004).

View larger version (43K): [in this window] [in a new window]   Fig. 7. Wheat yield response to N application timing–tillage intensity in four consecutive years when grown weed-free, with grass weeds (green foxtail and wild oat), or with broadleaf weeds (common lambsquarters and wild mustard). Bars on the graph within a weed treatment and year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

Nitrogen source affected wheat grain protein in all years (P < 0.05). In 1998, only surface-broadcast N fertilizer increased wheat protein above that of the unfertilized control (Fig. 8) . However, in the latter 3 yr, all N treatments increased wheat protein above that of the unfertilized control. Wheat protein levels were similar with broadcast N fertilizer, banded N fertilizer, and compost manure (and all were higher than with fresh manure) in the latter 3 yr. Rao and Dao (1996) reported that wheat grain N content often was greater with banded than broadcast N fertilizer, but Karamanos et al. (2003) found no differences in wheat protein between banded and broadcast N fertilizer treatments.

View larger version (54K): [in this window] [in a new window]   Fig. 8. Wheat grain protein response to N source in four consecutive years. Bars on the graph within a year with the same letter are not significantly different according to Fisher's Protected LSD test at the 5% probability level.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Fresh and composted cattle manure exhibited good potential as N sources for spring wheat production. Weed-free wheat yield and grain protein content with manure and composted manure was often similar to N fertilizer. An additional long-term benefit of manure and compost is improved soil quality (DeLuca and DeLuca, 1997; Eghball et al., 2004).

Results support the findings of previous studies indicating that many agricultural weeds are highly responsive to increased soil N levels (Lintell-Smith et al., 1992; Supasilapa et al., 1992; Blackshaw et al., 2003). Thus, indiscriminate N additions have the potential to benefit weeds at the expense of crops. Subsurface-banded N was often better than surface-broadcast N fertilizer in terms of N uptake by wheat vs. weeds, weed biomass production, and wheat yield. Weeds often germinate at or near the soil surface, especially in zero-tillage systems (Yenish et al., 1992; Hoffman et al., 1998), and it is in this situation that the greatest benefits may be realized by physically placing N in an area of the soil profile where crops, but not weeds, are germinating.

Nitrogen uptake and growth of weeds with fresh and composted manure tended to be intermediary between broadcast and banded N fertilizer in the first couple of years. However, with repeated annual applications, weed shoot N concentration and biomass with the manure treatments were similar to or greater than values attained with broadcast N fertilizer. This was likely due to gradual N release from manure and compost over years (Eghball et al., 2004). Weed-infested wheat yield with fresh and composted manure was usually similar to that with broadcast N fertilizer and was often lower than that attained with banded N fertilizer. The ranking of the weed seedbank at the conclusion of this 4-yr experiment was compost manure = fresh manure broadcast N fertilizer > banded N fertilizer, indicating that the two manure treatments were the least desirable N treatments in terms of weed management.

Weed N concentration and weed biomass were sometimes greater with fall-applied than with spring-applied N. Nitrogen is mobile in the soil, and any placement effects of N amendments may be somewhat diminished by high amounts of precipitation. Fall- compared with spring-applied N may have more opportunity for movement within the soil profile because of snowmelt and/or early spring rains.

The present study indicates that fresh and composted manure have the potential to increase spring wheat yield but also the level of weed competition. Menalled et al. (2004) similarly documented that composted swine manure increased the competitive ability of common waterhemp (Amaranthus rudis Sauer) without increasing soybean [Glycine max (L.) Merr.] yield. Alternative application methods may be required to maximize manure benefits to crops and minimize any inadvertent benefits to weeds. For example, Petersen (2003) reported that weed N uptake and weed biomass were 50% lower with subsurface-banded compared with surface-broadcast liquid swine manure. Rasmussen (2002) similarly documented lower weed biomass and higher crop yield with injected than with surface-broadcast liquid swine manure. Integrated weed management programs that emphasize crop rotation, competitive crops, and timely herbicide use also would help achieve the many advantages of manure application to crop production.

Manipulation of soil fertility, whether using organic or inorganic amendments, should be considered an important component of long-term weed management programs. Information gained in the current study will be used to develop more integrated programs for weed management in spring wheat production systems.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

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