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© 2005 Plant Management Network.
Accepted for publication 27 July 2005. Published 30 August 2005.

Nitrate Accumulation in Crabgrass as Impacted by Nitrogen Fertilization Rate and Source

Chris D. Teutsch, Assistant Professor, and William M. Tilson, Research Associate, Virginia Polytechnic Institute and State University, Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA 23824

Corresponding author: Chris D. Teutsch. cteutsch@vt.edu

Teutsch, C. D., and Tilson, W. M. 2005. Nitrate accumulation in crabgrass as impacted by nitrogen fertilization rate and source. Online. Forage and Grazinglands doi:10.1094/FG-2005-0830-01-RS.

Abstract

Crabgrass (Digitaria spp.) could provide high quality summer grazing in the southeastern U.S. However, little information is available on the tendency of crabgrass to accumulate nitrate. This study evaluated the effect of N fertilizer rate and source on the nitrate concentration of crabgrass forage. Plots were established in late spring of 2001-2003 near Blackstone, VA. Seven N rates ranging from 0 to 300 lb/acre were applied in 50 lb intervals at seeding as ammonium nitrate or broiler litter. Nitrate concentrations in the forage increased with N rate (P < 0.01) and were greater during periods of moisture stress. First harvest nitrate accumulated to dangerous levels (> 5000 ppm) when more than 84 and 143 lb of N per acre was applied as ammonium nitrate in 2001 and 2002, respectively. In 2002, nitrate concentrations in second harvest forage fertilized with ammonium nitrate ranged from 1000 to 20000 ppm and increased with N rate (P < 0.001). In contrast, nitrate concentrations in first and second harvest forage fertilized with broiler litter never exceeded the generally safe range (P < 0.01). Final harvest nitrate concentrations never exceeded the generally safe range regardless of N source.

Introduction

Crabgrass is a warm-season annual grass that is commonly considered a weed due to its prolific growth habit (Fig. 1), but this species possesses significant potential for supplying high quality summer forage for grazing livestock in the mid-Atlantic region of the U.S. Limited research has shown that crabgrass responds well to nitrogen fertilization (5,13), but could accumulate nitrates (14).

Fig. 1. Crabgrass can commonly be found in row crop fields, pastures, and lawns. In this picture crabgrass has volunteered between tobacco rows at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA.

Accumulation of nitrate in forages can pose serious health problems for ruminant livestock (10,17). Ball et al. (1) have compiled a widely used scale for rating the potential toxicity of nitrate in forage for ruminant livestock. This scale identifies forages as being generally safe, dangerous, and toxic when they contain < 5000, 5000 to 15000, and > 15000 ppm nitrate, respectively.

In the southeastern U.S., annual production of more than 6.6 billion broilers (11) generates approximately 8 million tons of manure containing more than 500 million pounds of N (15). Broiler litter could provide a readily available and economical N source for crabgrass. However, the tendency of crabgrass to accumulate nitrates and the effect of supplying N as broiler litter on nitrate accumulation in crabgrass are unknown. The objective of the current study was to determine the effect of N fertilizer rate and source on the nitrate concentration of crabgrass forage.

Trials Using Seven Nitrogen Rates and Two Sources over Three Years

‘Red River’ crabgrass was established on 7 May 2001, 16 April 2002, and 15 April 2003 near Blackstone, VA. The same plot area was used for all three years of this study. The soil series was a Wedowee sandy loam (fine kaolinitic, thermic Typic Kanhapludults). Initial soil nutrient levels are shown in Table 1. Plots were seeded using a cultipacker type seeder and a seeding rate of 6.0 lb/acre (Fig. 2). Nitrogen was applied at 0, 50, 100, 150, 200, 250, and 300 lb plant available N per acre as a single application of either ammonium nitrate or broiler litter (Fig. 3). Treatments were immediately incorporated by disking once with a finishing disk (Fig. 4). Nutrient content of the broiler litter is shown in Table 2. In 2003, broiler litter application rates were adjusted to account for mineralization of organic N applied in 2001 and 2002 (16). Phosphorus and K were applied in 2001 and 2002 so that all plots received the same amount contained in the broiler litter applied at the highest N rate. No supplemental P and K were applied in 2003 since soil test levels were in the high range (Table 1). Bentazon and carfentrazone-ethyl herbicides were applied to control broadleaf weeds.

Table 1. Extractable soil nutrient levels (lb/acre) and pH for 2001, 2002, and 2003.

 ^a Mehlich I extraction was utilized.

 ^b Soil Test Recommendations for Virginia, 1994.

Table 2. Nutrient content (ppm) of broiler litter used in 2001, 2002, and 2003 on a dry matter basis.

 ^a TKN = Total Kjeldahl Nitrogen.

 ^b PAN = Plant Available Nitrogen = NH₄-N × 0.90 + Organic N × 0.60 (broadcast and immediately incorporated) (16).

	Fig. 2. Crabgrass was seeded using a cultipack type seeder in late spring of 2001, 2002, and 2003.		Fig. 3. Broiler litter and ammonium nitrate treatments were weighed out and broadcast onto each plot.

Fig. 4. Ammonium nitrate and broiler litter treatments were immediately incorporated by disking onetime.

The experimental design was a randomized complete block with factorial treatment arrangement (N rate and N source) and four blocks. Plots were 9 × 20 ft and were harvested on 5 July and 31 August 2001; 11 July, 15 August, and 3 October 2002; 11 July, 14 August, and 22 October 2003. Harvest was initiated when the forage had reached the late boot stage, except for the last harvest in each year, which was harvested at seed maturity (Fig. 5). A subsample of fresh forage was collected for nitrate analysis. Nitrate concentration in plant tissue was determined colorimetrically using a modified salicylic acid method (3).

Fig. 5. The final harvest in each year occurred when the crabgrass plant had reached seed maturity.

Data from each harvest were analyzed using the general linear model procedure from SAS (SAS Institute, Inc., Cary, NC). Nitrogen rate and source variables and year were considered fixed. The mean square value for block within year was used to test for differences between years (6). Regression analysis was performed on raw data using Sigma Plot 9.0 (Systat Software, Inc., Point Richmond, CA). In 2001, only two harvests were made and the second harvest was similar in maturity to the third harvest in 2002 and 2003. Therefore, the final harvest from each year was grouped for analysis and discussion.

Effects of Nitrogen Fertilizer Rate and Source on Nitrate Concentration

Analysis of variance combined over years indicated significant year × N rate, year × N source, and year × N rate × N source interactions for each harvest (P < 0.05). Therefore, data will be presented for each harvest-year combination. A N source × rate treatment interaction was found for all harvests in 2001 and for the first and second harvests in 2002 (P < 0.05). Main effects will be presented for the final harvest in 2002 and all harvests in 2003.

Weather data. Severe drought in 2002 and excessive rainfall in 2003 resulted in the driest and wettest growing seasons ever recorded at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA. Precipitation for the crabgrass growing season (May to September) was approximately normal in 2001 (31 inches), 6.5 inches below normal in 2002, and 20 inches above normal in 2003 (Fig 6). The drier than normal conditions in 2002 were magnified by an extremely dry winter (2001-2002). For the period of August 2001 to April 2002 rainfall was more than 14.0 inches below normal resulting in dry soil conditions in the spring of 2002. Temperatures for the growing season were normal for 2001 (72.2°F), 2.4°F above normal for 2002, and 1.9°F below normal for 2003.

Fig. 6. Monthly precipitation data for 2001, 2002, and 2003 growing seasons.

First harvest. Nitrate concentration in the forage removed at the first harvest increased with N fertilization rate in all years except for broiler litter in 2001 (P < 0.001) (Fig. 7). This increase was greater for ammonium nitrate than broiler litter (Fig. 7). In 2001 and 2002, application of N at 300 lb/acre as ammonium nitrate resulted in the accumulation of nitrate in the toxic range, but broiler litter applied at the same rate was never associated with nitrate beyond the generally safe range (Table 3). Nitrate concentrations for the first harvest were in the generally safe range as defined by Ball et al. (1) for N rates up to 84 and 143 lb/acre applied as ammonium nitrate in 2001 and 2002, respectively (Table 3). In 2003, none of the N rates resulted in nitrate levels that posed a health risk to ruminant livestock (Table 3). Although supplying N as ammonium nitrate resulted in higher concentrations of nitrate in the forage tissue than broiler litter for the first harvest in 2003 (P < 0.05), the small magnitude of the differences and the fact that both sources were well within the safe range make these differences biologically insignificant (Fig. 8).

Fig. 7. Nitrogen fertilization rate and source effects on nitrate concentration in crabgrass forage for first harvest (5 July 2001, 11 July 2002, 11 July 2003).

Table 3. Calculated N fertilization rates resulting in nitrate accumulation for safe to generally safe, generally safe to dangerous, and dangerous to toxic risk thresholds.

 ^a Risk thresholds were calculated using the corresponding regression equations found in Figures 7, 8, and 9.

Fig. 8. Nitrogen source effects on nitrate concentration in crabgrass forage for the final harvest in 2002 (3 October 2002) and all harvests in 2003 (11 July 2003, 14 August 2003, 22 October 2003). Bars within a harvest-year combination with the same letter are not significantly different according to Fisher’s protected least significant difference (P < 0.05).

Second harvest. In 2002, nitrate concentrations in second harvest forage tissue fertilized with ammonium nitrate ranged from 1000 to 20000 ppm and increased with N rate (Fig. 9). In contrast, nitrate concentrations in second harvest forage fertilized with broiler litter ranged from 1000 to 5000 ppm (Fig. 9). Accumulation of nitrate above the generally safe level occurred when more than 165 lb of N per acre was applied as ammonium nitrate at seeding and did not occur for broiler litter (Table 3). In 2003, significant amounts of nitrate for the second harvest did not accumulate even at the highest N rate (Fig. 9).

Final harvest. Nitrate concentrations for the final harvest were below 5000 ppm in 2001, 2002, and 2003 (Fig. 9). A single application of N at 300 lb/acre approached, but did not reach the dangerous threshold in 2001 and 2002 (Table 3). Although N source had varying effects on nitrate concentrations for the final harvest, differences were small in magnitude and likely had little biological significance (Fig. 8 and 10).

Fig. 9. Nitrogen fertilization rate and source effects on nitrate concentration in crabgrass forage for second harvest (15 August 2002, 14 August 2003).

Fig. 10. Nitrogen fertilization rate and source effects on nitrate concentration in crabgrass forage for final harvest (31 August 2001, 3 October 2002, 22 October 2003).

Influence of Rainfall on Nitrate Accumulation

Dry conditions prior to and after the first harvest in 2001 and 2002 resulted in significant nitrate accumulation for the higher N rates applied as ammonium nitrate. This confirms the role of moisture stress in nitrate accumulation and agrees with the results of other researchers (4,8,17). However, in the current experiment first harvest yield was greatest in 2002 (data not shown), when moisture was most limiting (Fig. 6). This along with higher N concentrations in the forage tissue resulted in high rates of N removal indicating that more N was available for plant growth in 2002 (Table 4). In contrast, excessive rainfall in the spring of 2003 decreased N concentrations in the rooting zone of crabgrass resulting in N removal that was 65% lower than the previous year for the first harvest (Table 4). Nitrate concentration in the forage tissue and N removal was lower for all harvests in 2003. These data indicate that nitrate accumulation in crabgrass is sensitive to not only slowed plant growth, but also by the concentration of soil N in the rooting zone of the plant.

Table 4. Nitrogen removal of crabgrass for the 2001, 2002, and 2003 growing seasons.

 ^a Nitrogen removal was calculated by multiplying TKN × DM Yield.

 ^b Standard error was calculated using the standard error option with LSMEANS (SAS Institute, Inc., Cary, NC).

Nitrate Accumulation from Manure Applications

Manure applications to forages have been long implicated in nitrate poisonings (7,9,17). Mayo (9) documented several cases of extremely high levels of nitrate in corn (Zea mays L.) fodder that resulted in livestock losses. In these cases corn had been grown in areas that were formerly barn lots and had presumably very high rates of manure applied for multiple years. Since manure was applied in an unregulated and undocumented manner it is difficult to draw conclusions about either N rate or source in the terms of their ability to stimulate nitrate accumulation in forages. Due to this difficulty, the general assumption has been that the source of N has little effect on nitrate accumulation (7,17).

Results from the current study are in disagreement with past research and observations in that our results clearly illustrate that when N was applied on a plant available basis as an organic source (broiler litter), nitrate levels in the crabgrass forage tissue were always lower than when the equivalent amount of N was supplied as a commercial inorganic fertilizer, with the exception of the final harvest in 2002 and 2003. First harvest N removal was higher in 2001 and 2002 for the ammonium nitrate treatment (Table 4) indicating that more N was available for plant uptake early in the growing season. This difference was due to higher concentrations of N in the forage tissue in both years and higher yield in 2001 only (data not shown). This observation was likely due to the fact that not all N from the broiler litter was immediately available at application. Past research indicates that approximately one-half of the plant available N in poultry litter is available early in the growing season (2,12).

Conclusion

The risk of nitrate accumulation in crabgrass increased when high rates of N fertilizer were applied. Nitrate concentrations in forage tissue were almost always lower when N was supplied as broiler litter versus ammonium nitrate. This indicates that readily available organic N sources, such as broiler litter, could be safely utilized for crabgrass production. However, in order to safely and efficiently utilize these sources, additional research is needed to better understand the nature of their N release characteristics when applied to grasslands. This will be especially important as nutrient management strategies evolve that allow organic N sources to be shipped longer distances to exploit larger acreages of low fertility grassland in the southeastern United States.

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