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© 2005 Plant Management Network.
Accepted for publication 27 July 2005. Published 31 August 2005.

Nitrogen Fertilization Impacts on Stand and Forage Mass of Cool-Season Grass-Legume Pastures

Gregory J. Cuomo, Margaretha V. Rudstrom, and Dennis G. Johnson, West Central Research and Outreach Center, University of Minnesota, Morris 56267; Jon E. Anderson, University of Minnesota, Morris 56267; Avinish Singh, AgriPoint, Truro, Nova Scotia B2N6Z4; and Paul R. Peterson and Craig C. Sheaffer, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul 55108

Corresponding author: Gregory J. Cuomo. cuomogj@morris.umn.edu

Cuomo, G. J., Rudstrom, M. V., Johnson, D. G., Anderson, J. E., Singh, A., Peterson, P. R., and Sheaffer, C. C. 2005. Nitrogen fertilization impacts on stand and forage mass of cool-season grass-legume pastures. Online. Forage and Grazinglands doi:10.1094/FG-2005-0831-01-RS.

Abstract

Combining the benefits of legume N₂ fixation and N fertilization may increase the productivity and profitability of pasture systems. Our objectives were to study the effects of N fertilization on productivity and persistence of legumes in mixtures with cool-season grasses under rotational stocking with short grazing periods. Twelve N fertilization regimes ranging from 0 to 336 kg of N per ha were applied annually to smooth bromegrass and reed canarygrass in monoculture and mixture with alfalfa, birdsfoot trefoil, and kura clover. Alfalfa was the dominant legume in mixtures with cool season grasses in 1999. As kura clover developed, it became the dominant legume species and by the trials end stands averaged over 70% in mixtures with both smooth bromegrass and reed canarygrass and across N treatments. Nitrogen fertilization did not affect alfalfa stands, but reduced kura clover stands by 17%. Smooth bromegrass-legume mixtures with no N fertilization produced more forage (10.5 Mg DM/ha) than any smooth bromegrass monoculture with N treatment (336 kg of N per ha produced 8.0 Mg DM/ha). Cost of forage mass in smooth bromegrass-legume mixtures was less than 50% of smooth brome monocultures. While N fertilization did not increase forage production in treatments with legumes, legumes were able to maintain vigorous stands with up to 336 kg of N per ha.

Introduction

Legumes can increase forage productivity and enhance animal performance in pastures. Poor legume persistence is a major deterrent to more widespread adoption among producers (3,21,22). Increased pasture productivity as the result of including legumes in pastures is related to both N₂ fixation reducing the need for external N inputs (6,17,24) and the seasonal growth pattern of legumes providing a more even forage distribution through the growing season compared to cool-season grass pastures (2).

As land values increase and profit margins decrease, producers who graze forages are faced with challenges in optimizing forage production in an economical manner. If legumes could be combined with N fertilization to optimize both forage productivity and persistence of legumes, highly productive, long-term grazing systems could be developed with minimal external N inputs. Fairey (6) reported that N fertilization of smooth bromegrass-legume pastures increased forage mass production from 2.5 to 3.2 Mg/ha. Other studies have shown that N fertilizer added to grass-legume pastures reduces the portion of legumes in the sward because N fertilization stimulates grass growth and grass competitiveness with the legumes (10,23).

Under rotational stocking systems, questions remain regarding the impact of N on legume persistence and productivity of cool-season grass-legume pastures. The objective of this study was to evaluate the impact of N fertility timing and rate on: (i) persistence of alfalfa, kura clover, and birdsfoot trefoil under rotational stocking in smooth bromegrass and reed canarygrass pastures; (ii) forage mass and disappearance of smooth bromegrass monocultures and smooth bromegrass-legume mixtures; and (iii) the cost of producing forage in smooth bromegrass grass and smooth bromegrass-legume pastures.

Site Description and Treatments

The experimental site was located at the University of Minnesota’s West Central Research and Outreach Center near Morris, MN. Average precipitation is 60 cm/year with about 40 cm falling during the growing season. The experiment was conducted on a pasture of Barnes loam (fine loamy, mixed, Udic Haploboroll) soils which are nearly level, well drained, neutral to slightly alkaline soils formed in uplands on glacial till moraines. The A horizon extends to about 20 cm. At the beginning of the study, soil pH was 7.8, organic matter was 38 g/kg, Olsen P (12) was 31 mg/kg, and soil test K was 328 mg/kg on the experimental site.

In 1997, a randomized complete block experiment with three blocks, and treatments arranged in a split-plot design, was planted into a prepared seedbed on 21 May 1997. Whole plot plant species treatments consisted of (i) monocultures of ‘Lincoln’ smooth bromegrass, (ii) smooth bromegrass with legumes, (iii) monocultures of ‘Palaton’ reed canarygrass, or (iv) reed canarygrass with legumes. Legumes used in mixtures with smooth bromegrass or reed canarygrass were ‘Amerigraze 401’ alfalfa, ‘Norcen’ birdsfoot trefoil, and ‘Endura’ kura clover. There were 12 split-plot treatments consisting of spring or summer applications of N ranging from 0 to 336 kg/ha annually beginning in 1999 (Table 1).

Table 1. Nitrogen fertilization^x regimes applied to smooth bromegrass
and reed canarygrass monocultures and each grass species in mixture
with alfalfa, kura clover, and birdsfoot trefoil near Morris, MN from 1999
through 2002.

 x Spring N was applied in late April or early May, as field conditions
warranted: mid-summer N was applied in July.

Smooth bromegrass was planted at 22.5 kg/ha pure live seed (PLS) in monocultures and 13.4 kg/ha PLS when planted with legumes. Reed canarygrass was planted at 7.8 kg/ha PLS in monocultures and 4.5 kg/ha PLS when planted with legumes. Alfalfa, birdsfoot trefoil, and kura clover were planted at 2.5, 2.9, and 4.7 kg/ha PLS, respectively. Legume planting rates were determined as an attempt to plant 216 seeds of birdsfoot trefoil and kura clover per m² and 108 seeds of alfalfa per m². Lower seeding rates of alfalfa were used in an attempt to keep it from being too competitive with kura clover and birdsfoot trefoil during the establishment phase. Whole plot plant species treatments were 168 by 21 m. Sub-plot N treatments were 14 by 21 m.

Pastures containing the plots were rotationally grazed three times during 1998 and data were collected beginning in 1999. During the trial, lactating dairy cattle grazed flexibly-sized paddocks for 12 h at a stocking rate of about 56,000 kg of lactating Holstein cow per ha. Grazing was initiated when forage height was 25 to 40 cm. Intensive, short duration stocking events were used during this trial to insure as uniform a defoliation as possible and reduce grazing variation as an influence on stand dynamics.

Forage Measurement

Legume persistence data were collected each spring (late May or early June), about 10 days after the first grazing event. Collecting data 10 days after the first grazing event was done to allow legumes to begin active growth in spring and optimize visibility of legumes. Stand data were collected using a 30- by 90-cm frame divided into 27, 10- by 10-cm quadrats. The number of quadrats that contained live rooted plants of the legume species were tallied and divided by 27 to attain a percentage stand value. The frame was randomly placed at 25 locations within each plot. If it is assumed that one plant was present in each 10- by 10-cm quadrat, a 20% stand would be 20 plants per m.

Forage mass was estimated on smooth bromegrass monocultures and smooth bromegrass-legume treatments by clipping three 30- by 60-cm quadrats to a stubble height of 5 cm from each subplot 1 day prior to a replication being grazed. Time and funding did not permit forage mass sampling of reed canarygrass. Forage disappearance was estimated by clipping three 30- by 60-cm quadrats in the same way as done prior to grazing within 2 days of an area being grazed and subtracting forage remaining after grazing from forage available before grazing. All clipped samples were bagged, weighed, dried for at least 48 h at 60°C, and then weighed again.

Economic Analysis

Cost of available forage and forage that disappeared with grazing was estimated for smooth bromegrass with N fertilization and smooth bromegrass-legume with no N fertilization. Establishment costs were calculated by summing machinery and labor costs per hectare planted. Costs associated with fieldwork were derived from farm machinery economic cost estimates (7) and were based on hours of machinery use in establishing the stands. This resulted in a planting cost of $103.07/ha for smooth bromegrass monocultures or $17.18/ha/year when divided by an estimated 6-year pasture life.

The same fieldwork was done to plant smooth bromegrass-legume mixtures. Legumes were planted with a press drill. The added machinery pass resulted in a planting cost of $129.16/ha. Over the 6-year stand life, smooth bromegrass-legume planting costs averaged $21.52/ha per year.

Seed costs were calculated as follows: smooth bromegrass seed cost was $2.44/kg resulting in a total cost per hectare for smooth bromegrass monocultures of $54.90/ha, and an annual cost of $9.15/ha per year for the 6-year stand life.

For the smooth bromegrass-legume mixture only 13.4 kg/ha of smooth bromegrass was included in the mixture. Seed costs were $16.15, $57.01, and $9.92/kg for alfalfa, kura clover, and birdsfoot trefoil, respectively. This resulted in a total seed cost for the smooth bromegrass-legume mixture of $138.08/ha, or a cost of $23.01/ha per year over the 6-year stand life.

Pastures were fertilized using a tractor and a small air spreader. The cost of urea varied from 1999-2001 and was $154, $172, and $258/Mg, respectively. In addition, $123.55/ha per year was assessed as an annual land fee. This reflected an appropriate rental rate for moderately productive pasture.

Statistical Analysis

The analysis used a randomized complete block design with treatments arranged in a split plot design. This design was repeated over years (4 years for stand composition and 3 years for forage mass). Statistical analyses were performed using the MIXED procedure of SAS (15). The whole and split-plot treatments were specified as fixed effects, and block-related terms specified as random effects in the MIXED procedure.

Sward Composition

Alfalfa established and maintained better stands through 2001 when planted into smooth bromegrass (1999-2001 average of 25%) than when planted into reed canarygrass (1999-2001 average of 17%) (Table 2). The opposite was true for kura clover. Kura clover stands averaged 59 and 50% for reed canarygrass and smooth bromegrass, respectively from 2000-2002. Slowly establishing species like kura clover (8,13,16,17) may be able to compete and establish more quickly in reed canarygrass, which also tends to be a relatively slow developing plant (4,19).

Table 2. Species by year after planting interactions for stand dynamics of alfalfa, birdsfoot trefoil, and kura clover, in smooth bromegrass and reed canarygrass pastures near Morris, MN from the spring of 1999 through the spring of 2002 in stands were planted in May 1997. Data presented are averaged across N fertilization regimes. Reed canarygrass only was replanted in May 1998.

 ^x RC = Reed canarygrass; SB = Smooth bromegrass.

 ^y Represents the percentage of quadrats that contained live rooted plants of the legume species.

It is not clear why alfalfa established better in smooth bromegrass than in reed canarygrass. Sheaffer et al. (20) reported that after 2 years under a three-cut system, smooth bromegrass and reed canarygrass competed similarly with alfalfa. In the current study, alfalfa stands declined between the 2001 and 2002 growing season in both smooth bromegrass and reed canarygrass pastures. The year 2002 was the fifth growing season after planting, and stand decline in alfalfa may be expected as stands age.

Total legume, as a percent of stand, was dominated by alfalfa in 1999. As kura clover developed and spread, it became dominant (Table 2). At the conclusion of this study in spring 2002, excellent kura clover stands were evident in both smooth bromegrass and reed canarygrass. These findings are in agreement with Albrecht (1) who reported that from the first to the third full growing season, stands of kura clover increased in grass pastures.

Birdsfoot trefoil stands did not develop well in this trial and were not a significant component of stands, averaging only about 1%. Poor establishment and persistence of birdsfoot trefoil in this study is in contrast to Sheaffer et al. (18) who reported that birdsfoot trefoil was able to compete with smooth bromegrass and reed canarygrass in Minnesota.

N Fertilization Impacts on Legume Persistence

Alfalfa stands, when averaged over the study were generally not impacted by N fertilization and averaged close to 20% across N fertilization regimes (Table 3). Kura clover stands were reduced by some N fertilization regimes. With no N applied, kura clover stands averaged 52% whereas kura clover averaged 43% across treatments where N was applied. Since N fertilization did not impact alfalfa stands, and birdsfoot trefoil was only a minor component of the sward, the trend of N fertilization on total legume component in the sward generally mirrored that of kura clover (Table 3).

Table 3. Nitrogen rate and timing impacts on composition of alfalfa, birdsfoot trefoil, and kura clover, averaged across smooth bromegrass and reed canarygrass pastures near Morris, MN. Data presented are averages of data collected from the spring of 1999 through the spring of 2002 in stands established in 1997. Reed canarygrass only was replanted in May 1998.

 ^x Spring N was applied in late April or early May, as field conditions warranted: mid-summer N was applied in July.

 ^y Represents the number of quadrats that contained live rooted plants of the legume species.

Neither rate nor timing of N fertilization influenced stands of alfalfa or birdsfoot trefoil (Table 3). In contrast, kura clover stands were reduced by N fertilization (52% compared to 43, 45, 45, and 42% for kura clover with no N or with 56, 112, 224, or 336 kg/ha, respectively). Similar stand reductions were found when N was applied to kura clover in spring (45%), summer (43%), or in split application (42%) compared to kura clover with no N applied (52%). However, stands of kura clover were still excellent under all N fertilization regimes used in this study. These results indicate that good stands of alfalfa and kura clover can develop and persist in rotationally-stocked, cool-season grass pastures under high levels of N fertilization. Alfalfa and kura clover persistence under high N fertilization rates may have been the result of short duration and high intensity stocking minimizing animal selectivity and paddocks being grazed before forage reached 40 cm. This minimized the impact of aggressive grass growth and shading of legumes that can be associated with N fertilization.

Forage Mass Available to Animals

No amount of N applied to smooth bromegrass monocultures resulted in greater total forage mass when averaged over the 3 years of this study as did the smooth bromegrass-legume treatment with no N (Table 4). This is in contrast to a Wisconsin report that monocultures of smooth bromegrass fertilized with 336 kg of N per ha produced more forage than smooth bromegrass grown with kura clover or birdsfoot trefoil when clipped (1). However, Albrecht (1) concluded that N replacement value would be expected to be much greater in a grazed pasture where many of the nutrients are returned to the pasture in urine and feces.

Table 4. Nitrogen rate and timing impacts on forage mass of smooth bromegrass and smooth bromegrass-legume pastures near Morris, MN. Data presented are averages of data collected from 1999 through 2001 of stands established in 1997.

 ^x SB = Smooth bromegrass; SB-L = Smooth bromegrass, alfalfa, kura clover, and birdsfoot trefoil.

Season total forage mass for smooth bromegrass monocultures increased with increasing N. The greatest response in total forage mass resulted from the first 56 kg/ha of N applied (2.2 Mg DM/ha increase) (Table 5).

Table 5. Annual nitrogen rate treatment impact on forage mass of smooth bromegrass compared to the smooth bromegrass-legume with no nitrogen fertilization treatment in pastures near Morris, MN. Data presented are averages of nitrogen rate treatments from the 1999 through 2001 growing seasons of stands established in 1997.

  *SB: Smooth bromegrass; SB-L: Smooth bromegrass, alfalfa, kura clover, and birdsfoot trefoil.

**Within columns, means followed by the same letter are not significantly different according to the Bonferroni adjustment procedure at an experimentwise type I error rate of 0.05.

Available forage mass in smooth bromegrass monocultures was increased during August and September with mid-summer and split N applications (3.3 Mg DM/ha) compared to a spring N application (2.6 Mg DM/ha) and no N applied (2.0 Mg DM/ha). One of the goals of a mid-summer application of N would be to increase forage production during late summer when forage is often limiting.

Forage Mass Consumption

Season total forage that disappeared with grazing was greater in smooth bromegrass-legume mixtures (5.4 Mg/ha) than for smooth bromegrass monocultures (2.4 Mg DM/ha) when averaged across N treatments. The fraction of available forage mass that disappeared with grazing was also greater (P < .04) for smooth bromegrass-legume treatments (46% of available forage) than smooth bromegrass monocultures (33% of available forage) when averaged across N fertilization treatments.

Neither rate nor timing of N fertilization influenced forage disappearance in smooth bromegrass monocultures. Forage disappearance was 34, 31, 33, 35, and 29% for smooth bromegrass monocultures with 0, 56, 112, 224, and 336 kg of N per ha, respectively. Forage disappearance for smooth bromegrass-legume mixtures and no N applied was 49%. Timing of N fertilization also did not impact proportion of forage disappearance in smooth bromegrass monocultures. Forage mass disappearance was 33, 32, and 33% for split, spring, and summer N application treatments on smooth bromegrass monocultures, respectively.

Greater forage disappearance when legumes are present in grass pastures has been reported in other studies. Popp et al. (14), Okine et al. (11), and Dougherty et al. (5) reported cattle preferentially selected and consumed more forage when alfalfa was included with grasses compared to grass only pastures.

Economics

Costs of forage mass and forage disappearance are presented in Table 6. Greater forage mass and forage disappearance in smooth bromegrass-legume compared to smooth bromegrass monoculture treatments in this study led to lower costs per Mg of forage. The cost of available forage in the smooth bromegrass-legume mixture with no N fertilization was less than the cost of any of the smooth bromegrass monoculture treatments.

Table 6. Cost per Mg/ha of forage mass available and forage mass that disappeared during grazing for smooth bromegrass N rates and timing treatments compared to the smooth bromegrass-legume with no N fertilization regime for rotationally stocked pastures near Morris, MN. Costs include seed and establishment over a potential six year stand life, annual fertilizer and land costs.

 ^x SB = Smooth bromegrass; SB-L = Smooth bromegrass, alfalfa, kura clover, and birdsfoot trefoil.

 ^y Spring N was applied in late April or early May, as field conditions warranted, mid-summer N was applied in July.

The low cost per Mg of available forage DM, coupled with the higher disappearance rate for the smooth bromegrass-legume mixtures, further emphasizes the utility of legumes in pastures in this region. The cost per Mg of forage DM disappearance for smooth bromegrass-legume with no N applied was at least 50% less than any smooth bromegrass monoculture treatment.

These economic results indicate two general approaches to pasture management; one is a minimal cost (no N fertilization or legumes) and minimal return (low forage production, low stocking rates), and the other is a higher cost approach (inclusion of N fertilization or legumes) with those costs reduced by higher forage production and higher potential stocking rates. Adequacy of an approach depends on an individual producers goals and their ability to effectively manage and use additional forage produced with N or inclusion of legumes.

Summary

Alfalfa and kura clover in rotationally grazed, cool-season grass pastures can develop and persist under N fertilization rates up to 336 kg/ha per year. By the third year after establishment, kura clover became the dominant legume component in both smooth bromegrass and reed canarygrass mixtures. Nitrogen fertilization did not impact alfalfa persistence but reduced stands of kura clover by about 17%. However, kura clover stands were excellent across N fertilization regimes (greater than 55%) by the fifth growing-season after planting in 2002.

In this study, grazing was initiated when the pasture sward was 25 to 40 cm in height. How kura clover would persist in a less intensive defoliation regime, when grasses were given a longer growth period in spring and between harvests, is beyond the scope of this study.

Inclusion of legumes in mixtures with smooth bromegrass produced greater forage mass than any smooth bromegrass monoculture fertilized with as much as 336 kg of N per ha per year. This likely resulted from the combination of additional growth potential from legumes and N supplied to grasses from legumes.

The persistence and vigor of kura clover and the high levels of forage production and disappearance of the smooth bromegrass-legume treatments under the stocking management in this trial resulted in the potential for favorable economic returns from including legumes in these pastures. These results indicate that: (i) robust grass-legume stands that include kura clover can be very productive and produce economical forage over the long-term in cool-season grass pasture systems; and (ii) under the rotational stocking used in this study, although N fertilization did not increase forage mass in grass-legume mixtures, it also did not deteriorate the legume component of the sward.

Acknowledgment

This research was supported by the Minnesota Agricultural Experiment Station.

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