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© 2006 Plant Management Network.
Accepted for publication 25 May 2006. Published 18 September 2006.

Nitrogen Fertilization or Legumes in Tall Fescue Pastures Affect Soil and Forage Nitrogen

Karen Absher Vines, The Pennsylvania State University, University Park 16802: Vivien G. Allen, Department of Plant and Soil Science, Texas Tech University, Lubbock 79409-2122; Mark Alley, Department of Crop and Soil Environmental Sciences, and Joseph P. Fontenot, Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg 24061; and David Wester, Department of Range, Wildlife and Fisheries, Texas Tech University, Lubbock 79409

Corresponding author: Karen Absher Vines. kav11@psu.edu

Vines, K. A., Allen, V. G., Alley, M., Fontenot, J. P., and Wester, D. 2006. Nitrogen fertilization or legumes in tall fescue pastures affect soil and forage nitrogen. Online. Forage and Grazinglands doi:10.1094/FG-2006-0918-01-RS.

Abstract

Including legumes in pastures improves forage quality and may reduce nitrate leaching into ground water. In a six-year grazing experiment, ‘Kentucky 31' tall fescue (Festuca arundinacea Schreb.) was fertilized with N at 80 lb/acre each August and April or was grown with either alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.) without N fertilizer with three replicates of each system in a randomized design. Stockpiled forages were winter grazed by stocker cattle and summer forages were harvested as hay. Soils were mixed Typic Hapludults. During years five and six, effects of N fertilization or the legume on soil N, N uptake, and forage yield were investigated. Soil NH₄ and NO₃ were higher in the A than B horizon. Soil NH₄ was higher and NO₃ was lower in fescue-red clover soils than where N-fertilized fescue or fescue-alfalfa was grown. Soil nitrate did not exceed 3 ppm. Annual nitrogen uptake for the two sampling years in fescue-red clover, fescue-alfalfa, and N-fertilized grass was 239, 277, and 162 lb of N per acre, respectively, with annual biomass yields of 4.7, 5.8, and 4.7 tons/acre. After six years, potential for NO₃ leaching beyond the B horizon appeared low but may be influenced by legume species.

Introduction

Tall fescue occupies about 35 million acres in the US (6) as a result of its adaptability to a wide range of environmental conditions (15). Fall stockpiling of tall fescue extends the grazing season (5), produces high-quality forage (2), and is a long-accepted management practice throughout the east-central US (14). Nitrogen fertilization for stockpiling is recommended to obtain maximum yield (14) and improve forage quality (4) for grazing livestock. However, N applications can increase surface water pollution due to runoff and erosion (7) and contamination of ground water from NO₃ leaching (11).

Legumes, frequently included with grasses to improve forage quality (5), fix atmospheric N through associated Rhizobium bacteria. Little is known concerning yield, N accumulation, or impact on soil N of stockpiled tall fescue grown with legumes compared to N fertilization. Our objectives were to determine: (i) effects of N fertilization or legumes grown with tall fescue on forage yield, botanical composition, and N uptake; and (ii) relative effects of legumes and N fertilization on N form and accumulation in soil horizons after six-years management in a stockpiling system.

Long-term Grazing Study

‘Kentucky 31' tall fescue grown alone or with ‘Arc’ alfalfa (Medicago sativa L.) or ‘Kenland’ red clover (Trifolium pretense L.), was established in 1981-1982 for a six-year grazing study (1983-1988) at Virginia Tech’s Northern Virginia Research and Extension Center, Middleburg (38°57'30"N, 77°43'30"W). Four replicates of each forage system (2 acres per replicate except for N-fertilized tall fescue which contained four acres in each per replicate) were used in a completely randomized design described previously (1). Nitrogen-fertilized tall fescue and tall fescue-red clover were stockpiled each year beginning in early August immediately following the final summer grazing (0.8-inch sward height). Stockpiling tall fescue-alfalfa was delayed until early September because previous results showed that earlier stockpiling allowed alfalfa to become over-mature before grazing began (1,3). Yearling Angus steers and heifers (average initial weight 550 lb) began grazing stockpiled forages in early November (stocking rate = one yearling per 0.67 acre). Cattle were removed from tall fescue-alfalfa when forage height was approximately one inch. Cattle remained on tall fescue-N and tall fescue-red clover pastures until mid- to late-March, supplemented as needed with hay cut previously from these pastures. Hay was harvested during spring and summer from all three forage systems. Red clover was overseeded annually each February. Lime, P, and K were applied annually according to soil test recommendations. Nitrogen was applied twice (30% N urea-ammonium nitrate solution; total of 160 lb/acre) annually to the tall fescue monoculture. Applications were early spring when growth of forage began and early August when stockpiling began.

During years five and six, the current study took place within these systems.

Sampling Procedures

Two replicate soil and plant samples were taken within each field replication at each sampling time. Each of these two subsamples was a composite of samples collected from five, 1.64 ft² permanently identified sampling sites. The two composited subsamples were analyzed separately in the laboratory and then averaged prior to statistical analysis for the final mean that represented each field replication. Sampling sites were located randomly to be on uniform soil types (Chester fine-loamy to clayey mixed mesic Typic Hapludult) within the three field replicates of each treatment in autumn 1987. After the initial sampling, soil heterogeneity prompted relocation of some sites resulting in unequal sample numbers at the first sampling date. Soil samples, collected 20 November 1987, 19 May 1988, and 20 December 1988, were taken to a depth of 30 inches, including A and B horizons. The horizon change was identified by color variation. Approximately two inches of transition zone soil was discarded. Composite samples for each horizon were mixed together at sampling and frozen immediately.

Soil NH₄ and NO₃ were extracted (8) using 0.2 oz of wet soil and 0.05 qt of 0.74% w/v KCL. The solution was analyzed colorimetrically for NO₃ and NH₄ using a Quick Chem automated ion analyzer (LaChat Instruments, Zellweger Luwa Group, Uster, Switzerland). Soil NO₃ and NH₄ concentrations were converted to a dry soil basis.

Plant samples were collected 7 November 1987, 18 and 19 May, 1988, 29 June 1988, 8 August 1988, and 2 November 1988. November dates corresponded with beginning of grazing stockpiled forages while spring and summer dates were when forages were cut for hay.

Plants were harvested approximately 0.8 inch from soil surface. Sub-samples of each species present were immediately frozen in liquid N and subsequently freeze-dried for N analysis. Remaining plant material, separated by species, was oven dried to determine botanical composition and dry matter production. Samples for N determination were weighed after freeze-drying for inclusion in yield and botanical composition calculations and ground to pass through a 0.04-inch screen on a stainless steel Thomas-Wiley mill, Model Ed-5 (Arthur H. Thomas Company, Philadelphia, PA.)

Grass, legume, and herbaceous broadleaf weed species were composited into two duplicate samples for each category per pasture replication for nitrogen analysis. Plant samples were analyzed for total N by micro-Kjeldahl (10) and dry matter was determined at 221°F. Results are presented on a dry matter basis.

Statistical Analysis

Effects of stockpiling system, year, and their interaction on forage mass and soil N in November 1987 and 1988 were analyzed as a completely randomized design with respect to stockpiling treatment; year was included as a repeated measurement with PROC MIXED ( version 9.1.3; SAS Institute Inc., Cary, NC,); a type III analysis was used to adjust for unequal sample sizes. Effects of stockpiling system on total annual hay yield and N content were analyzed using GLM procedures (12) with a completely randomized design. Data were further analyzed by orthogonal contrasts to compare (i) N-fertilization versus the mean of legume treatments, and (ii) fescue-red clover versus fescue-alfalfa. Main effects and interactions were considered significant at P < 0.05 unless otherwise noted.

Soil Nitrogen Concentrations

Ammonium. Effects of legumes and N fertilizer on soil NH₄ were consistent for each sampling date within A and B horizons, but differed seasonally. Averaged over forage type and horizon, NH₄ concentrations were higher in spring (May, 9.9 ppm, SE = 0.6) than in autumn [November (5.9 ppm, SE = 0.8) and December (5.7 ppm, SE = 0.6)]. In the A horizon, NH₄ concentrations were 11.9 ppm (SE = 0.5) while NH₄ concentrations in the B horizon were 2.4 ppm (SE = 0.5). Ammonium was not affected by forage type.

Nitrate. Soil nitrates were affected by stockpiling system , soil horizon, and date of sampling. Additionally, effects of stockpiling treatments depended on soil horizon sampled. Nitrates were higher in the A horizon than in the B horizon in each treatment (Fig. 1). Nitrates in the B horizon were not affected by stockpiling treatments but in the A horizon, nitrates were higher in nitrogen-fertilized tall fescue and tall fescue grown with alfalfa than when tall fescue was grown with red clover. Nitrates did not change in the B horizon across dates. In the A horizon, nitrates were lower in autumn than in spring (horizon by date interaction) (Fig. 2).

Concentrations of soil ammonium and nitrate were within ranges reported for grassland (16). Total NH₄-N/acre has generally been found to exceed total NO₃-N/acre in grassland soils (16). In our research, at both the spring and autumn sampling dates, more of the soil N was present as NH₄-N than as NO₃-N.

Fig. 1. Nitrate concentrations in the A and B horizons of a Typic Hapludult soil as influenced by tall fescue fertilized with N or grown with red clover or alfalfa. aSE = standard error of the means. bHorizon means within a stockpiling system followed by the same lower case letter are not significantly different. Stockpiling system means within a soil horizon followed by the same upper case letter are not significantly different.

Fig. 2. Nitrate concentrations in the A and B horizons of a Typic Hapludult soil as influenced by sampling date. aSE = standard error of the means. bHorizon means within a date followed by the same lower case letter are not significantly different. Date means within a soil horizon followed by the same upper case letter are not significantly different.

Nitrates in the A horizon are not as great a concern as at lower soil horizons. This is particularly true in pastures with little bare soil where sod minimizes runoff and erosion. Furthermore, a high proportion of forage plant roots are located in the A horizon, thus, NO₃ is readily removed by forages. In our experiment, the A Horizon contained a maximum of 18 lb/acre NO₃-N. Stewart et al. (13) considered levels of 72 and 66 lb NO₃-N per acre for fertilized native grass and alfalfa, respectively, as safe regarding potential ground water contamination by nitrate leaching. Nitrate levels in both the A and B Horizons were considerably below this level on the dates sampled suggesting this was not a threat in these systems. Additional sampling dates would be needed to ensure that leaching would not occur at other times. Additionally, while both NO₃ and NH₄ were lower in the B than A horizon, there is potential in some soils for accumulation below the 30-inch sampling depth used in the current experiment. Normally, soil N declines with increasing depth, however (17).

Forage Yield and Botanical Composition

More stockpiled N-fertilized tall fescue was available for grazing in early November than when grown with legumes (Table 1). During earlier years of the grazing experiment (1), N-fertilized fescue, fescue-red clover and fescue-alfalfa provided 196, 172, and 94 grazing days/acre, respectively, at similar stocking rate. Stockpiled fescue-red clover was approximately two-thirds grass while stockpiled fescue-alfalfa was composed largely of alfalfa particularly in 1988 (Fig. 3). Percentage weed was relatively small in all forages.

Table 1. Forage mass and botanical composition of stockpiled tall fescue fertilized with nitrogen or grown with red clover or alfalfa sampled in November 1987 and 1988.

 ^a Numbers in parentheses are standard errors of the mean.

 ^b Tall fescue fertilized with N differed from the mean of tall fescue grown with legumes (P < 0.01).

Fig. 3. After 6 years in the grazing study, tall fescue-alfalfa pastures contained more alfalfa than tall fescue.

Total annual forage yield (hay plus forage stockpiled for winter grazing) during 1988 was higher for fescue-alfalfa than for fescue-red clover (Table 2). Percentage grass, legume, and weed components for forage mixtures were similar on a total seasonal basis. Total annual forage yield of N-fertilized fescue was similar to that of fescue-red clover. More N-fertilized tall fescue was harvested as hay in May 1988 than fescue-red clover or fescue-alfalfa combinations (2.8, 2.2, and 2.4 tons/acre, respectively, SE 0.2). Hay harvested from fescue-alfalfa in May included 36% alfalfa, 46% grass, and 18% weed while fescue-red clover hay included 55% clover, 35% grass, and 10% weed (Fig. 4).

Table 2. Total annual yield (hay plus forage stockpiled for winter grazing) and botanical composition of tall fescue fertilized with nitrogen or grown with red clover or alfalfa during 1988.

 ^a Tall fescue-red clover differed from fescue-alfalfa (P < 0.05).

 ^b Tall fescue fertilized with N differed from the mean of fescue grown with legumes (P < 0.01).

Fig. 4. Overseeding tall fescue each February with red clover resulted in about 50% legume during the experimental period.

The spring hay harvest accounted for about 60% of the total seasonal hay yield for N-fertilized fescue but only 40 to 50% of total hay yield for fescue-legume combinations. Inclusion of legumes provided more forage growth later in summer than N-fertilized fescue.

Total Nitrogen Uptake in Above-Ground Biomass

Tall fescue-legume combinations contained more pounds of N per acre than tall fescue fertilized with N (Table 3). If all forage was harvested as hay, N removed in aboveground biomass of the tall fescue monoculture would about equal amounts applied annually as in fertilizer.

Table 3. Total nitrogen in above ground biomass of tall fescue fertilized
with nitrogen or grown with red clover or alfalfa during 1988.

 ^a Tall fescue fertilized with N differed from the mean of fescue grown
with legumes (P < 0.05).

 ^b Tall fescue-red clover differed from fescue-alfalfa (P < 0.05).

 ^c Tall fescue-red clover differed from fescue-alfalfa (P < 0.08).

While spring and summer forage growth was removed as hay, autumn stockpiled growth was grazed. Grazing animals remove relatively small amounts of N (9) as they excrete most N consumed with some N potentially volatilized as ammonia (16). Total N contained in stockpiled forages at the beginning of grazing (mean of 1987 and 1988) was 58, 52, and 51 lb of N per acre for stockpiled N-fertilized tall fescue, fescue-red clover, and fescue-alfalfa, respectively. Much of this N would have been subjected to recycling as discussed above.

Furthermore, hay harvested from tall fescue-red clover and N-fertilized tall fescue was fed back to cattle during grazing, recycling previously removed N to pastures. Total N contained in hay actually harvested was 128, 195, and 242 lb of N per acre, for N-fertilized tall fescue, fescue-red clover, and fescue-alfalfa, respectively. Cattle that grazed stockpiled tall fescue-alfalfa were taken from pastures when hay-feeding began and N contained in hay was not returned to pastures.

After six years of similar management, red clover and alfalfa maintained soil NH₄ at levels similar to that resulting from annual fertilization with 160 lb of N per acre. Because of low nitrate levels found in these pastures, none of these forage management systems appeared to be a threat to groundwater. Soils where red-clover served as the N-source appeared lower in NO₃ than soils where alfalfa was used. Growing tall fescue with alfalfa and red clover provided similar annual forage yield as that achieved with 160 lb of N per acre, but a greater portion of this yield came during mid- and late-summer. This coincides with a time when forage production by monoculture cool season grasses is generally low and would help to fill this mid-summer grazing period with higher-quality forage. Inclusion of these legumes could possibly provide an alternative to use of summer annuals in some locations while still providing for late fall/early winter grazing. Nitrogen fertilization of monoculture tall fescue resulted in higher yields of the first hay cut in spring and more yield of stockpiled forage in autumn. Thus, monoculture, N-fertilized tall fescue was a better choice for extending late fall and winter grazing than inclusion of legumes. Nitrogen present in the above-ground biomass of tall fescue was approximately equal to 160 lb of N per acre applied annually. Nitrogen contained in above-ground biomass was increased with use of legumes and could remove more total N in a hay harvest system than N-fertilized tall fescue.

Literature Cited

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