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© 2008 Plant Management Network.
Accepted for publication 9 June 2008. Published 18 August 2008.

Bermudagrass Yield Response to Nitrogen and Potassium Fertilization in Northwest Arkansas

Robert Seay, Extension Agent, Benton County, Division of Agriculture Cooperative Extension Service, University of Arkansas, Bentonville 72712; and Nathan A. Slaton, Associate Professor, Crop, Soil, and Environmental Sciences Department, Division of Agriculture, University of Arkansas, Fayetteville 72704

Corresponding author: Nathan Slaton. nslaton@uark.edu

Seay, R., and Slaton, N. A. 2008. Bermudagrass yield response to nitrogen and potassium fertilization in northwest Arkansas. Online. Forage and Grazinglands doi:10.1094/FG-2008-0818-01-RS.

Abstract

Potassium performs a vital role in maintaining yield potential and stand persistence of bermudagrass (Cynodon dactylon L.) making proper fertilization critical for sustainable hay production. Our primary objective was to demonstrate how N and K fertilizer rates influence bermudagrass yield and soil-test K across a four-year period. Each year, N was applied at 180 and 300 lb/acre in three equal split applications in combination with K₂O at 0 and 180 lb/acre. Potassium fertilization maintained soil-test K between 157 to 160 ppm (optimum). When no K was applied, soil-test K declined to 62 ppm (low) in fall 2007 following the fourth year of study. On average, forage yields were increased 25% by increasing N rate from 180 to 300 lb of N per acre/year and 12% by increasing K rate from 0 to 180 lb of K₂O per acre/year, respectively. Forage yield per unit of N fertilizer also decreased from 42 to 36 lb of forage per pound of N when no K was applied. Proper K fertilization is essential for maximizing forage utilization of N fertilizer and maintaining soil K fertility on low cation exchange capacity soils in northwestern Arkansas.

Changes in and Challenges of Nutrient Management for Forages

Bermudagrass (Cynodon dactylon L.) is the predominant warm-season grass produced for pasture and hay in northwestern Arkansas. Poultry litter has been used for decades as the primary nutrient source sustaining forage production in this region, but overuse has resulted in high soil-test P and water quality concerns. Regulations now prohibit or limit the amount of poultry litter that can be applied to fields in certain watersheds making the use of soil testing and inorganic fertilizer critical to sustain soil productivity and high forage yields (2). Slaton et al. (10) concluded K removal by harvested crops, primarily hay, in western Arkansas was equal to or greater than K additions to soil resulting in a net K balance that was near zero or negative, but the balance for P was positive. Potassium is essential for high yields (9), stand persistence (13), and winter-survival of bermudagrass (6), especially in northern Arkansas, which approaches the northernmost limit of the bermudagrass adaption zone.

Using inorganic fertilizers for forage production is foreign to many growers accustomed to applying poultry litter as their exclusive nutrient source. The costs of purchasing and applying the rates of inorganic fertilizer needed to produce high forage yields is an economic hardship to many growers. Based on 2006 fertilizer prices (12) for the south-central USA, the cost of fertilizer materials needed to supply 300 lb of N and 180 lb of K₂O per acre applied as urea ($443/ton) and muriate of potash ($271/ton) exceeds $180.00/acre. Applied research demonstrating the benefits of sound nutrient management practices that sustain forage yields is needed to educate growers and prevent decreased forage production. Decreased forage production could negatively influence the beef cattle industry of northwest Arkansas and/or increase erosion, via loss of ground cover, leading to further degradation of surface water quality. The primary objective of this project was to demonstrate how inorganic N and K fertilizer rates influence bermudagrass yields and soil-test K across a 4-year period. A secondary objective was to summarize previous research to calibrate the inorganic N-fertilizer rate needed to produce near optimum bermudagrass yields.

Calibration of Nitrogen Rate

Published N-fertilizer rate data were used to calibrate the inorganic-N fertilizer rate required to produce near optimum yields of non-irrigated bermudagrass in northwestern Arkansas. Yield data were from a 5-year trial with ‘Tifton 44' bermudagrass receiving N at 0 to 600 lb/acre as NH₄NO₃ in split applications (5) and a 2-year trial (7, 11) conducted with common bermudagrass receiving N at 0 to 450 lb/acre in split applications and averaged across inorganic-N fertilizers (urea, urea + Agrotain, and NH₄NO₃). Both trials were conducted on Captina silt loams (fine-silty, siliceous, active, mesic Typic Fragiudults) at the Arkansas Agricultural Research and Extension Center in Fayetteville, AR and provided a total of seven site-years of N response data. Although the yield potential of hybrid bermudagrass (i.e., Tifton 44) is greater than common bermudagrass, we assumed the relative yield response to N rate would be similar. Therefore, actual yield data for each annual trial were converted to percent relative yield by dividing the yield of each N rate by the highest yielding treatment and multiplying by 100. Regression analysis was performed using the PROC REG procedure in SAS (SAS Institute Inc., Cary, NC). A quadratic model was fit to the data with intercept, linear and quadratic coefficients being interpreted as significant if P < 0.05.

Nitrogen and Potassium Trials

An experiment was established in spring 2004 on a soil mapped as a Nixa silt loam (loamy-skeletal, siliceous, active, mesic Glossic Fragiudult) having an established stand of ‘Greenfield’ bermudagrass (3) near Highfill, AR. Prior to 2004, the research area had been grazed periodically and mowed annually with little or no fertilization. Farm records showed the most recent application of poultry litter was in 1994 at a rate of 2 ton litter per acre. The experiment was a randomized complete block design with a 2 × 2 factorial treatment arrangement of N and K rates and three replicates per treatment. An unfertilized control (0 lb N and K₂O per acre) was included in each replicate as a reference treatment, but was not included in statistical analysis. Treatments were applied to 5 × 20 ft plots annually for four years beginning in 2004. For the duration of the study, each plot received the N and K rates that were assigned at the onset of the study. Nitrogen was applied as NH₄NO₃ at total rates of 180 and 300 lb of N per acre/year in three equal split applications of N at 60 and 100 lb/acre per application made at spring greenup and following the first and second hay harvests. Muriate of potash (60% K₂O) was applied in three equal, split applications for an annual cumulative rate of 180 lb of K₂O per acre/year as described for N; an additional treatment that included no K fertilization was included in the factorial treatment structure. Hay was harvested three times each year with harvest dates of 3 June, 19 July, and 6 September in 2004; 7 June, 11 July, and 1 September in 2005; 10 June, 15 July, and 1 September in 2006; and 6 June, 16 July, and 14 September 2007 resulting in an average of 38, 39, and 50 days between harvests. Precipitation data (Table 1) was obtained from a weather station located < 1 mile from the research area at the Northwest Arkansas Regional Airport (8).

Table 1. Annual precipitation amounts by month between 2004 and 2007 (6).

The center 39 inches of each 20-ft long plot was hand harvested with a cutting height of 2.5 inches. Fresh weight was recorded and a 1-lb sub-sample was dried to a constant weight and used to adjust total plot fresh weight to dry weight. Agronomic efficiency of N-fertilization rate was evaluated by subtracting the unfertilized control yield from all other treatment yields and dividing net yield by the applied N rate (i.e., dry matter produced per pound of N).

A composite soil sample (0 to 6 inches) was taken from the entire plot area in September 2003 and again in April 2004 (Table 2) before the first year of treatments were applied. Beginning in 2005, a composite soil sample was collected from each plot annually between December and February. Soil samples were analyzed for soil water pH in a 1:2 soil:water mixture and Mehlich-3 extractable nutrients including K.

Table 2. Selected soil-test data by year for soil samples collected from the unfertilized control treatment.

 ^x Single composite sample collected from entire plot area in September 2003.

 ^y Single composite sample collected from entire plot area in late April 2004.

 ^z Mean values of composite samples collected from the unfertilized control in each replicate.

Analysis of variance procedures were conducted on yield (2004-2007) and soil-test K data (2005-2007) with the PROC GLM procedure in SAS (SAS Institute Inc., Cary, NC). For each plot, yield for each harvest was summed to calculate the season-total annual yield which was used for statistical analysis. Yield and soil-test K data were analyzed as a split plot, 2 × 2 factorial treatment structure where the whole plot was each N and K rate combination and the subplot was year. Fisher’s protected least significant difference procedure was used to separate means for significant effects at P = 0.05.

Optimum N-Fertilizer Rate

The mean actual yield of bermudagrass receiving N at 0 lb/acre, averaged across seven site-years, was 3251 lb DM/acre (range 1520 to 5920 lb/acre) and represented 24% (range 9 to 45%) of maximum yield. The mean maximum yield was 15124 lb DM/acre. Bermudagrass relative yield response to N-fertilizer rate was significant and nonlinear (Fig. 1). The equation predicted the maximum relative yield of 98% would be produced with 550 lb of N per acre. Relative yields that were 80 and 90% of the maximum were predicted with 274 and 364 lb of N per acre, respectively. 

Fig. 1. Relative bermudagrass yield response to inorganic N fertilizer rate.

Soil-Test K Response

The initial soil-test K from the entire plot area (Table 2) was considered ‘Optimum’ (130175 ppm) by University of Arkansas recommendations. Soils having an optimum soil-test K level and cropped to warm-season grasses receive yield-goal specific recommendations ranging from 50 to 250 lb of  K₂O per acre/year to replace 50 to 70% of the K removed by harvested hay to reduce the rate of soil-test K decline between soil sample collection times (i.e., most fields sampled once every 3 years).

Soil-test K was affected significantly by the main effects of K-fertilizer rate and year as well as the K rate × year interaction (P < 0.0070, Table 3). Application of 180 lb of K₂O per acre/year maintained soil-test K for samples collected between 2005 and 2007, whereas soil receiving no K fertilizer declined during the same period. When K was applied annually the mean soil-test K was initially interpreted as ‘Optimum’ and after 4-years of hay harvest the interpretation had not changed (Table 2). When K was not applied, the soil-test K level (4) was ‘Medium’ (91 to 130 ppm) in 2005, ‘Low’ (61 to 90 ppm) in 2006, and ‘Low’ in 2007, but near the boundary of ‘Very Low’ (< 61 ppm). 

Table 3. Soil-test K as affected by the year by K-fertilizer rate interaction, averaged across N rates.

Forage Yield Response

Bermudagrass receiving no N or K fertilizers averaged 5087 lb/acre in 2004, 1083 lb/acre in 2005, 1118 lb/acre in 2006, and 1800 lb/acre in 2007. The high yields in 2004 for forage receiving no N and K fertilizers were likely due to residual soil N plus sufficient rainfall to sustain forage growth (Table 1). Although N fertilizer was not applied to this site prior to 2004, manure deposition from grazing cattle and recycling of nutrients from mowing likely contributed to greater plant-available N in the soil profile during the first year of hay production. Coblentz et al. (1) also reported relatively high yields for bermudagrass receiving no N in the first year followed by a substantial decline in yield during the subsequent year for an N-rate trial established on a soil that had previously received high rates of poultry litter. The high initial yields were likely from residual inorganic N in the soil profile plus organic N from poultry litter that continued to mineralize.

Bermudagrass forage yields were significantly affected only by the main effects of year (P < 0.0001), N rate (P = 0.0005), and K rate (P = 0.0121). Forage dry matter yield, averaged across N and K rates, in 2004 (13276 lb/acre) was greater than all other years, which were statistically similar and ranged from 10300 to 10899 lb/acre. The high yields produced in 2004 were likely due to adequate moisture from April through July (Table 1) and residual soil N.

Forage yield, averaged across K rates and years, increased by 25% (2499 lb/acre) as N rate increased from 180 to 300 lb/acre/year (Table 4). Likewise, yield, averaged across N rates and years, increased by 12% (1289 lb/acre) from application of  K₂O at 180 lb/acre/year (Table 5). These data suggest that maximal bermudagrass yields can only be produced when sufficient amounts of both N and K are applied to soils in northwestern Arkansas. 

Table 4. Bermudagrass forage yield and agronomic efficiency as
affected by N rate, averaged across years and K rates.

Production Efficiency Response

The agronomic efficiency of N applied to bermudagrass was affected by only the main effects of year (P = 0.0055) , N rate (P < 0.0001) , and K rate (P < 0.0001). Unlike yield, the lowest numerical agronomic efficiency, averaged across N and K rates, occurred in 2004 (36 lb of  forage per pound of N), but was significantly lower than only 2005 (42 lb of  forage per pound of N). The low yield per unit of N applied in 2004 was likely due to the relatively high yield (5087 lb of forage per acre) of bermudagrass receiving no N during the first year of study and residual soil N. Agronomic efficiency, averaged across years and K rates, decreased as N rate increased from 180 to 300 lb/acre/year (Table 4). In contrast, averaged across years and N rates, forage produced per pound of N increased from application of  K₂O at 180 lb/acre/year (Table 5). These data indicate that inadequate K nutrition reduces bermudagrass response to N which could eventually contribute to increased N losses via run-off and leaching in addition to reduced yields.

Table 5. Bermudagrass forage yield and agronomic efficiency as affected by K rate, averaged across years and N rates.

Discussion and Summary

Woodhouse (13) reported that near maximal bermudagrass yields were supported by 50 lb of K₂O per acre/year for five years on an Eustis sand (siliceous, thermic Psammentic Paleudults) before yields benefited from higher K-fertilization rates (100 to 200 lb of K₂O per acre/year). Withholding K for eight years resulted in yield reductions exceeding 90% and after ten years severe stand loss. Most soils in northwestern Arkansas have sandy- to silt-loam textures making it reasonable to assume that similar yield and stand losses could occur from years of K mismanagement. In our study, soil-test K declined annually when K was withheld from a soil that initially had an optimum soil-test K level. The average yield loss attributed to inadequate K fertilization was 8% in 2004 and 2005 and increased to 13 and 16% in 2006 and 2007, respectively. Across the southern USA, low cation exchange capacity soils are commonly used for production of bermudagrass, which has the potential to produce high yields and remove large amounts of K. For these reasons soil samples should be collected every year or every other year to monitor soil-test K, especially when minimal rates of K-fertilizer are applied. Slaton et al. (11) reported bermudagrass removes the equivalent of 34 to 51 lb of  K₂O per ton depending on the level of K fertilization with removal rates < 44 lb of  K₂O per ton occurring when inadequate K fertilization limited yield.

Forage growers should routinely collect and submit soil samples to monitor soil nutrient availability indices and levels across time, especially when forage is managed for moderate to high yield. Closely monitoring soil-nutrient availability is especially important as growers begin using inorganic fertilizers as a replacement for or in combination with poultry litter to fertilize forages and try to reduce high soil-test P via phytoremediation. Forage receiving sufficient N to produce moderate to high forage yields can remove significant amounts of K causing rapid annual changes in soil-test K that influence K-fertilization recommendations and forage yield. Proper K fertilization also aids in efficient plant use of applied N fertilizer and should be recognized as a best nutrient management practice for maintaining proper ground cover.

Literature Cited

1. Coblentz, W. K., Daniels, M. B., Gonsaulis, J. L., Turner, J. E., Scarbrough, D. A., Humphrey, J. B., Coffey, K. P., Moore, P. A., Jr., Teague, K. A., and Speight, J. D. 2004. Effects of nitrogen fertilization on phosphorus uptake in bermudagrass forage grown on high soil-test phosphorus soils. Prof. Anim. Sci. 20:146-154.

2. Delaune, P. B., Haggard, B. E., Daniel, T. C., Chaubey, I., and Cochran, M. J. 2006. The Echa/Spavinaw phosphorus index: A court mandated index for litter management. J. Soil Water Cons. 61:96-105.

3. Elder, W. C. 1955. Greenfield bermudagrass. Bull. No. B-455. Oklahoma Agric. Exp. Stn., Stillwater, OK.

4. Espinoza, L., Mozaffari, M., and Slaton, N. A. 2006. Soil testing, lime and fertilizer recommendations handbook. MP463-11-06N. Coop. Ext. Serv., Div. of Agric., Univ. of Arkansas, Little Rock, AR.

5. Honeycutt, H. J., West, C. P., and Phillips, J. M. 1988. Responses of bermudagrass, tall fescue and tall fescue-clover to broiler litter and commercial fertilizer. Ark. Agric. Exp. Stn. Bull. 913. Fayetteville, AR.

6. Keisling, T. C., Rouquette, F. M., and Matocha, J. E. 1979. Potassium fertilization influence on Coastal bermudagrass rhizomes, roots and stands. Agron. J. 71:892-894.

7. Massey, C. G., Slaton, N. A., DeLong, R. E., Golden, B. R., and Maschmann, E. T. 2008. Bermudagrass forage response to nitrogen fertilization. Pages 7-17 in: Wayne E. Sabbe Arkansas Soil Fertility Studies 2007. Ark. Agric. Exp. Stn. Res. Ser. 558. N. A. Slaton, ed. Fayetteville, AR.

8. National Climatic Data Center (NCDC). 2008. On-line climate weather inventories. Online. NCDC, Asheville, NC.

9. Nelson, L. R., Keisling, T. C., and Rouquette, F. M., Jr. 1983. Potassium rates and sources for Coastal bermudagrass. Soil Sci. Soc. Am. J. 47:963-966.

10. Slaton, N. A., Brye, K. R., Daniels, M. B., Daniel, T. C., Norman, R. J., and Miller, D. M. 2004. Balance between nutrient inputs and removals for nine geographic regions in Arkansas. J. Envir. Qual. 33:1606-1615.

11. Slaton, N. A., DeLong, R. E., Golden, B. R., Massey, C. G., and Roberts, T. L. 2007. Bermudagrass forage response to nitrogen, phosphorus and potassium fertilization rate. Pages 52-57 in: Wayne E. Sabbe Arkansas Soil Fertility Studies 2006. Ark. Agric. Exp. Stn. Res. Ser. 548. N.A. Slaton, ed. Fayetteville, AR.

12. United States Department of Agriculture-National Agricultural Statistics Service (USDANASS). 2007. Agricultural prices. Online. USDA, Washington, DC.

13. Woodhouse, W. W., Jr. 1968. Long-term fertility requirements of Coastal bermudagrass, I. Potassium. Agron. J. 60:508-512.