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© 2004 Plant Management Network.
Accepted for publication 2 June 2004. Published 18 June 2004.

Nitrate Accumulation in Crabgrass as Impacted by Nitrogen Fertilization Rate and Application Timing

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 23824

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

Teutsch, C. D., and Tilson, W. M. 2004. Nitrate accumulation in crabgrass as impacted by nitrogen fertilization rate and application timing. Online. Forage and Grazinglands doi:10.1094/FG-2004-0618-01-RS.

Abstract

Crabgrass (Digitaria species) is a summer annual grass that could provide high quality grazing for ruminant livestock in the southeastern United States. However, the tendency of crabgrass to accumulate nitrate is unknown. This study was designed to evaluate the effect of nitrogen fertilizer rate and application timing on the nitrate concentration of crabgrass forage. Plots were established in late spring of 2001, 2002, and 2003 near Blackstone, VA and were harvested twice in 2001 and three times in 2002 and 2003. The experimental design was a randomized complete block with a factorial treatment arrangement and four replications. Nine N rates ranging from 0 to 400 lb/acre were applied in 50 lb intervals as a single application at seeding or as a split application, one-half at seeding and one-half after the first cutting. Nitrate accumulation in crabgrass increased with nitrogen fertilization rate (P < 0.05) and was greater during periods of moisture stress. First harvest nitrate accumulated to dangerous levels when more than 116 and 148 lb of N per acre were applied at seeding in 2001 and 2002, respectively. Excessive rainfall after seeding in 2003 resulted in lower nitrate concentrations in first and last harvest forage. In 2002, the addition of even small amounts of nitrogen after the first harvest resulted in significant nitrate accumulation and was likely related to below-normal precipitation. Nitrate concentrations were lower by the last harvest, but the higher nitrogen rates still resulted in forage nitrate concentrations of approximately 10000 ppm in 2001 and 2002. Nitrate concentrations in crabgrass forage never exceeded the generally safe range (< 5000 ppm) when N at 50 lb/acre was applied at any single point in time. These findings support current recommendation for summer annual grasses of applying N at 60 to 80 lb/acre at seeding followed by N at 40 to 60 lb/acre after each harvest and suggest that crabgrass could be safely managed in a similar manner.

Introduction

Crabgrass (Digitaria species) is a warm-season annual grass that is commonly considered a weed due to its prolific growth habit, but this species possesses significant potential for supplying high quality summer forage for grazing livestock in the transition zone between subtropical and temperate regions of the United States. Limited research has shown that crabgrass responds well to improved management (8). Crabgrass dry matter yield increased from 2400 to 7100 lb/acre as nitrogen rate increased from 0 to 240 lb/acre (7). Similarly, researchers in Virginia found that crabgrass dry matter production increased from 5000 to 9000 lb/acre as nitrogen rate increased from 0 to 300 lb/acre (18).

Accumulation of nitrate in forage plants can pose serious health problems for ruminant livestock (17). Estimates of the nitrate concentration in forage required to cause acute nitrate toxicity varies due to a number of factors, including rate and amount of forage ingested, forage type, energy level of the diet, adaptation to a high nitrate ration, general health and condition of the animal, and pregnancy (19). As nitrogen fertilization is increased to stimulate yield, the chance of nitrate accumulation in commonly used summer annual grasses also increases, especially when growth is limited by moisture stress (6,14,16,17). Maximum nitrate concentrations in forages reported as safe range from 2500 to 10000 ppm nitrate (2,4,9,12,19). Ball and coworkers (1) have compiled a widely used scale for rating the potential toxicity of nitrate in forage for ruminant livestock (Table 1). This scale identifies forages that have less than 5000 ppm nitrate as being generally safe when fed with a balanced ration (Table 1).

Table 1. Potential toxicity of forages containing varying concentrations of nitrate.

Adapted from Southern Forages, 3rd Ed. (1).

Extension recommendations for other summer annual grasses, such as sorghum-sudangrass, suggest N at 60 to 80 lb/acre can be applied at seeding, followed by N at 40 to 60 lb/acre after each harvest, without increasing tissue nitrate concentrations above the generally safe level. However, the tendency of crabgrass to accumulate nitrates is unknown. The objective of the current study was to determine the effect of nitrogen fertilizer rate and application timing on the nitrate concentration of crabgrass forage.

Trials using Nine Nitrogen Rates over Three Years

‘Red River’ crabgrass [Digitaria ciliaris (Retz.) Koel] was established on 7 May 2001, 16 April 2002, and 15 April of 2003 near Blackstone, VA. The soil series for all three years was a Wedowee sandy loam (fine kaolinitic, thermic Typic Kanhapludults). Initial soil nutrient levels are shown in Table 2. A conventional seedbed was prepared and plots were seeded using a cultipacker type seeder and a seeding rate of 6.0 lb/acre. Nitrogen was applied at 0, 50, 100, 150, 200, 250, 300, 350, and 400 lb/acre (0, 56, 112, 168, 224, 280, 336, 392, and 448 kg/ha) either as a single application of ammonium nitrate at seeding or as a split application, one-half at seeding and one-half after the first cutting. Nitrogen treatments applied at seeding were incorporated into the seedbed by disking once with a finishing disk. All plots also received 100 lb/acre of P₂O₅ and 300 lb/acre of K₂O before seeding. Bentazon [3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] herbicide was applied at a rate of 0.50 lb ai/acre on 31 May 2001 and 0.75 lb ai/acre on 11 June 2001. Bentazon and carfentrazone-ethyl {Ethyl alpha, 2-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-l-yl]-4-fluorobenzenepropanoate} herbicides were applied at a rate of 0.75 and 0.23 lb ai/acre respectively, on 30 May 2002 and 6 June 2003.

Table 2. Soil nutrient levels and pH for 2001-2003.

 ^a Mehlich I extract was utilized.

 ^b Soil Test Recommendations for Virginia (1994).

The experimental design was a randomized complete block with a two-factor (N rate and single versus split application) factorial treatment arrangement and four replications. Plot size was 9 × 20 ft. Plots were harvested on 5 July and 31 August 2001; 11 July, 15 August, and 3 October 2002; and 11 July, 14 August, and 22 October 2003 by clipping a 4 ft wide strip through the center of each plot using a self-propelled sickle bar-type forage harvester. Plots were harvested when the forage reached the late boot stage, except for the last harvest in each year, which was harvested at seed maturity. The clipping height was 4 inches above the soil surface. A subsample of fresh forage was collected for nitrate analysis from each plot and dried in a forced air oven for 5 days at 140°F. The forage was then ground to pass through a #10 (2 mm) and #18 (1 mm) screen using Wiley (Thomas Scientific, Swedesboro, NJ) and Cyclone sample mills (Udy Corporation, Fort Collins, CO), respectively.

Nitrate concentrations in plant tissue were determined colorimetrically using a modified salicylic acid method (3). Nitrates were extracted by placing 0.0352 oz (1 g) of dried forage material in a 8.5 oz (250 ml) Erlenmeyer flask along with 3.38 oz (100 ml) of distilled water. This mixture was shaken for 1 h and then filtered through Whatman #4 filter paper. Sample blanks were necessary due to pigmentation in the extracts.

Data were analyzed using the general linear model procedure from SAS (SAS Institute, Cary, NC). Only the nitrogen rate effect was considered for the first harvest since plots did not receive the split nitrogen application until after the first harvest. Regression analysis was performed on raw data using Sigma Plot 7.0 (Systat, 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. Maximum nitrogen fertilization rates were calculated for risk thresholds of 2500, 5000, and 15000 ppm nitrate using corresponding regression equations.

Effects of N Fertilizer Rate & Timing on Nitrate Concentration

Analysis of variance combined over years indicated significant year × nitrogen rate, year × application timing, and year × nitrogen rate × application timing interactions (P < 0.05). Therefore, data will be presented by year. An application timing × nitrogen rate treatment interaction was found for the second and final harvests in all years except for final harvest in 2003 (P < 0.05). Main effects will be presented for the final harvest in 2003.

Weather data. Severe drought in 2002 and excessive rainfall in 2003 resulted in the driest and wettest years, respectively, ever recorded at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA. Precipitation for the crabgrass growing season (May to September) was near normal in 2001, 6.5 inches below normal in 2002, and 20 inches above normal in 2003 (Table 3). The drier than normal conditions in 2002 were magnified by the extremely dry winter of 2001-2002. For the period of August 2001 to April 2002 rainfall was more than 14 inches below normal resulting in dry soil conditions in the spring of 2002. Temperatures for the growing season were on average normal for the 2001, 1.90°F above normal for 2002 and 2.40°F below normal for 2003.

Table 3. Monthly precipitation data for 2001, 2002, and 2003 growing seasons.

First harvest. Nitrate concentration in the forage removed at the first harvest increased with nitrogen fertilization rate in all years (P < 0.05) (Fig. 1 and Table 4). Although the trend was similar in 2002, crabgrass fertilized at the highest nitrogen rate accumulated less total nitrate (Fig. 1). Nitrate concentrations for the first harvest were in the generally safe range as defined by Ball and coworkers (1) for rates of N up to 116 and 148 lb/acre applied at seeding in 2001 and 2002, respectively (Fig. 1 and Table 5). In 2003, excessive rainfall after seeding likely resulted in nitrogen losses due to leaching and denitrification (Table 3) and only nitrogen rates in excess of 364 lb of N per acre posed a health risk to ruminant livestock (Fig. 1 and Table 5).

Fig. 1. Nitrogen fertilization rate and application timing effects on nitrate concentration in crabgrass forage for the first harvest.		Fig. 2. Nitrogen fertilization rate and application timing effects on nitrate concentration in crabgrass forage for the second harvest.

Fig. 3. Nitrogen fertilization rate and application timing effects on nitrate concentration in crabgrass forage for the final harvest.

Table 4. Equations, r-square values, and probabilities for regression lines in Figs. 1, 2 and 3.

 ^a Dependent variable is nitrate content of the forage in ppm and independent variable is nitrogen fertilization rate in lb of N per acre.

Table 5. Nitrogen fertilization rates (lb of N per acre) resulting in nitrate accumulation for specified risk thresholds.

^aRisk thresholds were calculated using the corresponding equations from
Table 4.

Second harvest. In 2002, nitrate concentrations ranged from 347 to 16530 ppm and increased as nitrogen rate increased (Fig. 2). Accumulation of nitrate above the generally safe level occurred when more than 242 lb of N per acre was applied as a single application at seeding or when N at 109 lb/acre was applied as a split application (Fig. 2 and Table 5). This difference in risk thresholds may have been due to a combination of residual soil nitrogen and dry conditions following the first harvest.

In 2003, significant amounts of nitrate did not accumulate when all of the nitrogen was applied at seeding (Fig. 2). The lower nitrate concentrations were not unexpected since concentrations were also low at the first harvest and no additional nitrogen had been applied (Fig. 1). Residual soil nitrogen was likely leached below the rooting zone from above average rainfall during May, June, and July (Table 3). However, the split application treatment accumulated nitrate to just below the toxic range in 2003 (Fig. 2). Nitrate levels were in the generally safe range for split nitrogen rates below 282 lb of N per acre (Fig. 2 and Table 5). This threshold coincides with the thresholds for the first harvest in 2001 and 2002 (Fig. 1) and may indicate that when the soil contains small amounts of residual nitrogen, up to approximately 150 lb of N per acre could be applied to crabgrass without accumulating dangerous levels of nitrate (Fig. 2 and Table 5).

Final harvest. Nitrate concentrations for the final harvest ranged from just above 0 to 12000 ppm in 2001 and 2002 (Fig. 3). A single application of N at 364 and 309 lb/acre at seeding resulted in accumulation of nitrates in excess of 5000 ppm in 2001 and 2002, respectively. This is approximately 2.5 times the rate that resulted in nitrate accumulations beyond the safe range for the first harvests in 2001 and 2002 (Fig. 1 and Table 5). This indicates that although a considerable amount of nitrogen was used or otherwise lost during the growing season, a significant amount of residual nitrogen was still present just before the final harvest for a single application of N at 300 lb/acre or more. In 2001 and 2002, nitrate concentrations in the split application reached dangerous levels with approximately N at 230 lb/acre (Fig. 3 and Table 5). Nitrate content of the forage increased with nitrogen fertilization rate for the third harvest in 2003 (P < 0.05), but did not differ between the single and split application (Fig. 3). However, none of the nitrogen rates accumulated nitrate in excess of 2000 ppm. Almost 11 inches of rainfall in September 2003 likely moved a large proportion of the nitrogen out of the rooting zone (Table 3).

Nitrogen rate and timing. Nitrogen fertilization in the range of 180 to 265 lb/acre increases dry matter production and crude protein content of summer annual grasses (10). All harvests in all years in the current study showed that nitrate concentration in crabgrass forage increased as nitrogen fertilization increased (P < 0.05). Other researchers have also found similar trends in a number of other cool- and warm-season grasses (5,11,13,15). Although splitting annual nitrogen applications for summer annual forages is widely advocated, little data are available to substantiate this practice (10). In the current study, split application of nitrogen resulted in lower nitrate concentrations in first harvest crabgrass forage, but tended to increase second and third harvest nitrate concentrations. This effect was greatest for the highest nitrogen rates.

Conclusion

The risk of nitrate accumulation in crabgrass increased when high levels of nitrogen fertilizer were applied, especially when coupled with moisture stress. Dry conditions following the first harvest in 2002 resulted in significant nitrate accumulation even when small amounts of nitrogen were applied as the split application. This clearly illustrates the role of moisture stress in nitrate accumulation. The split application strategy used in the current study may not have sufficiently allocated nitrogen over the growing season of crabgrass, especially for the higher nitrogen rates. Nitrate concentrations in crabgrass forage never exceeded the generally safe range when N at 50 lb/acre was applied at one time. These findings support the recommendations of extension educators to apply N at 60 to 80 lb/acre at seeding followed by N at 40 to 60 lb/acre after each harvest or intensive grazing for summer annual grasses and indicate that crabgrass can be safely managed in a similar manner.

Literature Cited

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16. May, M. L., Phillips, J. M., and Cloud, G. L. 1990. Drought induced accumulation of nitrate in grain sorghum. J. Prod. Agric. 3:238-241.

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18. Teutsch, C. D., and Tilson, W. M. 2003. Effect of nitrogen rate and application timing on the yield and nutritive value of crabgrass. Pages 238-242 in: AFGC Annual Meeting Proc. K. Cassida, ed. Lafayette, LA.

19. Wright, M. J., and Davison, K. L. 1964. Nitrate accumulation in crops and nitrate poisoning in animals. Adv. Agron. 14:197-217.