Agronomy Journal 94:450-454 (2002)
© 2002 American Society of Agronomy
SEED
Crop Residue Removal and Nitrogen Fertilization Affects Seed Production in Meadow Bromegrass
Heather A. Loeppky*,a and
Bruce E. Coulmanb
a Alberta Agric., Food, and Rural Dev. (AAFRD), 6000 C&E Trail, Lacombe, AB, Canada T4L 1V5
b Agric. and Agri-Food Can. (AAFC), 107 Science Place, Saskatoon, SK, Canada S7N 0X2
* Corresponding author (heather.loeppky{at}gov.ab.ca)
Received for publication May 8, 2000.
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ABSTRACT
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Seed yield in meadow bromegrass (Bromus riparius Rehm.) declines rapidly after two to three seed crops. This is a critical limitation to economic seed production. Field experiments were conducted at Saskatoon and Outlook, SK, Canada, to determine the influence of residue removal and N fertilization on seed yield. Three N treatments (0, 50, and 100 kg ha-1) were applied in September each year for the first three seed production years, and four residue removal treatments (none, after harvest, October, and after harvest + October) were applied in the second and third seed production years. Residue removal after harvest and N application (100 kg ha-1) increased yield 0 to 572 kg ha-1 in the second seed crop compared with the untreated control. In the third-year seed crop, residue removal increased seed yield 30 to 90 kg ha-1. Application of N fertilizer increased third-year seed yield 90 kg ha-1 at Outlook only. Mean seed yield was reduced in the third compared with the second crop year, regardless of treatment. Residue removal after harvest combined with the application of 100 kg N ha-1 increased the cumulative 2-yr seed yield by 390 to 490 kg ha-1 compared with the untreated control. At the current seed price (Can$2.50 kg-1) and N fertilizer cost (Can$0.66 kg-1) of meadow bromegrass, the additional seed yield from residue removal and 100 kg N ha-1 would provide a net return of Can$975 to Can$1225 ha-1 on an additional investment of <Can$100 ha-1.
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INTRODUCTION
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MEADOW BROMEGRASS is a temperate-zone bunchgrass that has rapid vegetative regrowth, which makes it desirable for grazing. As a result, it has become widely accepted as a pasture species, particularly in areas of the Canadian Prairies that receive between 350 to 500 mm of annual precipitation (Kruger, 1997). Due to its desirability, demand for meadow bromegrass seed is high. Seed production of meadow bromegrass is less reliable than that of smooth bromegrass (B. inermis Leyss.), and a significant decline in seed yield is frequently observed after the second seed crop (Upton, 1983; Knowles et al., 1993). Because the cost of establishment, approximately Can$360 ha-1 (Kruger, 1997), must be amortized over the life of the stand, prolonging the productive life of a stand could significantly improve the economic return on investment for meadow bromegrass seed production.
Two important management practices to improve grass seed production include N fertilization and crop residue removal. Both practices affect tiller density and size and flower and seed production in perennial grass species (Canode and Law, 1978; Nordestgaard, 1980; Thompson and Clark, 1989; Thompson and Clarke, 1993). Meadow bromegrass fertilized with 84 kg N ha-1 2 wk after emergence in the spring and again in the spring of the first seed production year produced significantly less seed than a single application of 84 kg N ha-1 in the fall before the first seed crop (Upton, 1983). In Holland, delaying spring N application from the end of February (before growth is initiated) until the end of March or April resulted in fewer fertile tillers in red fescue (Festuca rubra L.), orchardgrass (Dactylis glomerata L.), meadow fescue (F. pratensis Huds.), and perennial ryegrass (Lolium perenne L.) (Meijer and Vreeke 1988b). In all but orchardgrass, however, seed number tiller-1 and seed weight compensated so that seed yield was not affected by the delay in N application. Floral initiation in orchardgrass occurs very early (January or February) in Holland; thus, late application of N stimulated vegetative growth at the expense of reproductive growth.
Under dry conditions in the northern USA, increased N rate decreased seed yield of smooth bromegrass and crested wheatgrass [Agropyron cristatum (L.) Gaertn.]. However, N fertilization increased seed yield in both crops when precipitation was adequate (Canode and Law, 1978).
Soil mineral N influences N fertilizer response. Based on soil mineral N, response to N fertilizer rate has been calculated for perennial ryegrass, Kentucky bluegrass (Poa pratensis L.), and red fescue under Dutch conditions and for smooth bromegrass, crested wheatgrass, intermediate wheatgrass (A. intermedium (Host.) Beauv.], and timothy (Phleum pratense L.) in Saskatchewan (Meijer and Vreeke, 1988b; Loeppky et al., 1999). In soils with intermediate levels of available N and P (2 mg N kg-1 and 8 mg P kg-1 at 0–60 cm and 0–15 cm, respectively), the estimated seed yield response of smooth bromegrass was 540 kg ha-1 to application of 50 kg N ha-1 plus 9 kg P ha-1 (Loeppky et al., 1999).
Removing crop residue after seed harvest has been shown to increase seed production in several perennial grass crops (Canode, 1965; Canode and Law, 1978; Nordestgaard, 1980; Upton, 1983; Thompson and Clarke, 1989). Residue removal improves light penetration through the canopy (Meijer and Vreeke, 1988a) and changes light quality (Silvertown, 1980). Residue management is frequently studied in conjunction with N fertilization although interactions between these two factors are reported infrequently (Nordestgaard, 1980; Upton, 1983). In smooth bromegrass and crested wheatgrass, tiller density and size and seed yield were greater following burning than following mechanical removal (Canode and Law, 1978). Residue removal increased panicle production and seed yield in smooth bromegrass in southern Ontario (Fulkerson, 1980). Time of residue removal was important in this case; late-fall (15 Oct.) removal produced the highest panicle density and yield compared with nonremoval or early removal (15 Aug., 15 Sept., or both). On the other hand, Knowles (1966) found that while residue removal by burning or mowing increased smooth bromegrass seed yield, time of removal was not important. In Montana, under irrigation, seed yield response to residue removal in a second-year stand of meadow bromegrass depended on fall application of N (Upton, 1983). This combination of residue removal and N fertilization increased seed yield more than either residue removal or N fertilization alone; however, seed yield remained much lower in the second seed year than the first. The Montana study was discontinued after the second seed crop was harvested. No information is available on the effect of N fertilization and crop residue removal on meadow bromegrass seed production under dryland conditions.
The objectives of this study were to determine whether crop residue removal and N fertilization could increase meadow bromegrass seed yield and longevity.
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MATERIALS AND METHODS
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Field trials were conducted under rainfed conditions at Saskatoon, SK (52°07' N, 106°38' W), and under irrigation at Outlook, SK (51°30' N, 107°03' W), during 1994 through 1997. Both sites are located on Typic Haploboroll soils. At Saskatoon, growing season precipitation (April to October) was 307, 282, 366, and 247 mm in 1994, 1995, 1996, and 1997, respectively. At Outlook, growing season precipitation (April to October) was 216 mm with an additional 150 mm of irrigation applied in 1994, 272 mm of rainfall plus 225 mm of irrigation in 1995, 298 mm of rainfall plus 106 mm of irrigation in 1996, and 197 mm of rainfall plus 319 mm of irrigation in 1997. Tensiometers were used to determine soil moisture, and irrigation was applied to maintain soil moisture content above 50% of the available water-holding capacity (Sonmor, 1963). The irrigation system used was a low-pressure linear system, which applied from 1.9 to 2.5 cm of water at each irrigation.
‘Paddock’ meadow bromegrass was seeded with bulk certified seed on 10 June 1994 at 12 kg ha-1 in rows spaced 30 cm apart. A disc seeder was used to place and pack seed at 1.25-cm depth. Plots were mowed in the establishment year to control annual weeds and remained weed-free throughout the study. Fertilizer (11–51–0) was incorporated during seedbed preparation to supply 22 and 11 kg ha -1 P and N, respectively. Each fall after establishment (1994–1996), all plots received an additional 22 and 11 kg ha-1 P and N (11–51–0), respectively. Treatments consisted of three rates of additional N (0, 50, and 100 kg ha-1), applied each fall from 1994 to 1996, and four residue removal treatments (none, after harvest, October, or after harvest + October). In the fall of 1995, before applying fertilizer to the plot areas, soil cores were taken from each plot at sampling depths of 0 to 15, 15 to 30, and 30 to 60 cm. These samples were air-dried and analyzed for NO3–N and sodium bicarbonate (NaHCO3)–soluble P by methods of Gentry and Willis (1988) and Hamm et al. (1972), respectively. Available N levels were 1.8, 0.9, and 1.8 kg ha-1 at 0 to 15, 15 to 30, and 30 to 60 cm, respectively, at Outlook. At Saskatoon, available N was 3.6, 0.9, and 6.2 kg ha-1 at 0 to 15, 15 to 30, and 30 to 60 cm, respectively. Seed was harvested in the last week of July (1995–1997), and then postharvest treatments were applied within a week each year. The October time of removal represents the end of the growing season on the Canadian Prairies. Treatments were replicated four times. The design was a split plot with N rate as the main plot and residue removal as the subplot. Subplots were 1.5 by 6.0 m. Ammonium nitrate [NH4NO3] (34–0–0) was broadcast at the prescribed N treatment rates in early to mid-September each year from 1994 to 1996. Residue treatments consisted of removal with a forage harvester and then scalping with a flail mower to a height of 2.5 cm in the fall of 1995 and 1996. Tiller density was determined in two 0.25-m2 quadrats per plot each fall and spring starting in the fall of 1995. Tiller survival (%) was calculated as follows: (spring tiller number/fall tiller number) x 100. Biomass was clipped to a height of 5 cm from the two 0.25-m2 quadrats at seed maturity in 1996 and 1997. These samples were dried overnight at 70°C and weighed when dry. Biomass yield was then expressed as dry matter yield. Seed was harvested from the whole plot annually, dried to 8 to 10% moisture, and weighed.
Data were subjected to an ANOVA using the general linear model (GLM) of SAS (SAS Inst., 1985). All effects, except replicates, were considered fixed. Data from each year were analyzed separately. The data for the two sites were analyzed separately as site x treatment interactions were significant in both years. The effects of N rates were partitioned into linear and quadratic components, and the residue removal treatment means were evaluated through a series of a priori contrasts. Orthogonal contrasts were also used to determine the nature of interactions between N rate and residue removal treatments. Differences were considered statistically significant at P 
0.05. Main effects are presented only when interactions were not significant. Least significant differences (LSD; P < 0.05) are provided to show differences among treatment means.
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RESULTS AND DISCUSSION
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There was no response to N treatments in the first seed production year (1995) at either Saskatoon or Outlook. Mean first-year seed yields were 447 and 782 kg ha-1 at Outlook and Saskatoon, respectively. In general, crop residue removal and N application increased seed production of the second seed crop (1996) (Table 1). However, seed yield response differed at the two locations. At Saskatoon, seed yield increased linearly with N rate (Table 1). Crop residue removal increased seed yield if it was removed after harvest or both after harvest and in October. The greatest seed yield response to N occurred when residue was removed twice. In contrast, there was no response to N fertilization when residue was not removed, which resulted in an interaction of the linear effect of N fertilization and no removal vs. residue removal. At Outlook in 1996, N fertility did not influence seed yield, and a single residue-removal treatment was as effective as double removal to increase seed yield. In contrast, Upton (1983) found that N application was required following residue removal to enhance seed yield of second-crop meadow bromegrass under irrigation. This difference in response may be related to winter injury.
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Table 1. The effects of residue removal and N fertilization on second-year seed yield for Paddock meadow bromegrass at Saskatoon and Outlook in 1996.
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Late-October residue removal treatments (both single and double) resulted in a decrease in tiller density from fall to spring at Outlook in 1996 (Table 2). Tiller survival is important for meadow bromegrass seed production because flowering is stimulated by cool temperatures (Loeppky, 1999); consequently, tillers that survive the winter are more likely to flower than those that emerge in the spring. In this study, tiller survival was not directly correlated with seed yield (Loeppky and Coulman, 2001). However, stage of tiller development early in the spring was correlated with seed yield in three of four site-years, and spring development was generally greater in tillers that survived the winter. Late clipping has been shown to reduce total soluble carbohydrates and significantly reduce winter survival in meadow fescue (Huokona, 1974). High rates of N, especially if applied late in the season, generally increase incidence of winter injury in perennial grasses (Smith, 1987). In this study, N application did not affect tiller survival, but panicle density was reduced by the combination of 100 kg N ha-1 application and double residue removal in 1996 (Loeppky and Coulman, 2001). Winter injury was not uniform within the plots at Outlook in 1996; consequently, variability within plots was high. The Outlook site generally has less consistent snow cover than Saskatoon; hence, it is more prone to winter injury. Under Canadian Prairie conditions, uncut residue traps snow, which provides insulation and reduces winter injury, but residue removal often increases seed yield in perennial grasses. In spite of the risk of winter injury, residue removal generally increased meadow bromegrass seed yield in this case (Table 1).
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Table 2. The effects of residue removal across N treatments on the change in tiller density for Paddock meadow bromegrass at Saskatoon and Outlook, 1996–1997.
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At Saskatoon, very dry conditions in the fall of 1996 suppressed fall tiller growth. Good tiller survival and subsequent early tiller growth at both sites in the spring of 1997 resulted in an increase in tiller density, particularly when residue was removed (Table 2). Because tillers arising in the spring are not generally induced to flower, they remain vegetative, potentially competing with reproductive tillers for resources and reducing seed production.
Third production year yields (1997) were low at both sites (Tables 3 and 4). Nitrogen fertilization did not affect third-year seed production at Saskatoon, but the 100 kg ha-1 rate increased seed yield compared with other N rates at Outlook (Table 3). Growing season precipitation (247 mm) at Saskatoon in 1997 was below what is considered adequate for meadow bromegrass seed production (250–350 mm during the growing season) (Coulman et al., 1997). The drought affected treatment response and also increased the variability within plots, resulting in unusually high coefficients of variation for seed yield at the Saskatoon site (Tables 3 and 4).
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Table 3. Mean third-year seed yield for N fertilization in Paddock meadow bromegrass at Saskatoon and Outlook, 1997.
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Table 4. Mean third-year seed yield for residue removal in Paddock meadow bromegrass at Saskatoon and Outlook, 1997.
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Crop residue removal improved third-year seed yield at both sites, regardless of timing or frequency (Table 4). In a study conducted in Saskatoon from 1984 through 1989, seeding meadow bromegrass in 90-cm-wide rows compared with 30-cm rows resulted in lower seed production in the first year; however, overall yields were higher with the wider rows. In the fourth year, the wide rows produced 427 kg ha-1 compared with 98 kg ha-1 for the 30-cm rows (Knowles et al., 1993). In the present study, 30-cm row spacing was chosen to determine whether residue removal and N fertilization could improve and prolong seed production in this more intensive system. While these treatments did improve seed production, they did not stop the decline in yield over time. Ninety percent of the cumulative 2-yr seed production occurred in 1996 (Table 5).
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Table 5. The effects of residue removal and N fertilization on cumulative second- and third-year seed yield of Paddock meadow bromegrass at Saskatoon and Outlook during 1996 and 1997.
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Seed yield response to N fertilization was inconsistent even though soil NO3–N levels were very low in the fall before the second seed crop. In contrast, there was a consistent linear increase in biomass as N rates increased (Table 6). Competition between vegetative and reproductive tillers is often cited as a reason for poor seed production, especially in perennial grasses such as meadow bromegrass. For example, Clemence (1982) found that vegetative tillers of perennial ryegrass received assimilate from stems and leaves of reproductive tillers. In this study, biomass production was consistent even though there was winter injury with certain residue removal treatments at Outlook in 1996 and drought at Saskatoon in 1997, which affected seed production. In both cases, new spring tillers were not vernalized and remained vegetative.
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Table 6. Mean second- and third-year dry matter yield for N fertilization in Paddock meadow bromegrass during 1996 and 1997 (sites combined).
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At current meadow bromegrass seed prices of Can$2.50 kg-1, the cost of N fertilizer at Can$0.66 kg-1, and the cost of residue removal at approximately Can$25.00 ha-1 (Kruger, 1997), residue removal after harvest and application of 100 kg N ha-1 would have been profitable at Outlook, but not at Saskatoon, in the third year of seed production. Seed yield declined steadily at both sites, and it is unlikely that economic seed production could be sustained beyond 3 yr, even with these treatments.
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SUMMARY AND CONCLUSIONS
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Residue removal and N fertilization generally improved seed yield. In terms of total seed yield, there was a linear increase in seed yield as N rate increased, and residue removal enhanced this response. However, winter injury and dry conditions influenced this response. Perennial cool-season grasses like meadow bromegrass are particularly vulnerable to weather fluctuations because they require vernalization to promote flowering and seed production. Regardless of whether it occurs as a result of tiller death during winter or delayed development because of drought, if tillers are not present in fall when vernalization conditions are prevalent, flowering and seed production will be reduced or eliminated. For this reason, fall management is one of the most critical and unpredictable steps in producing meadow bromegrass seed.
At Saskatoon, total seed yield increased with N rate and residue removal, regardless of timing and frequency of removal. At Outlook, winter injury (indicated by reductions in tiller density over winter) before the second crop influenced this response. Under these conditions, total seed yield increased in response to N rate if residue was removed once but decreased if it was removed twice. Therefore, if there is a risk of winter injury, producers should either avoid residue removal more than once or reduce the rate of N.
Under dry conditions (Saskatoon, 1997), N fertilization increased vegetative growth at the expense of seed production. Because dry matter yield was not reduced, haying or grazing under these conditions could partially compensate for the loss in seed yield and minimize the risk associated with N fertilization.
In spite of the positive effects of residue removal and N fertilization on seed production, yield declined to unprofitable levels in the third production year. Economic seed production depends on producing sufficient seed in the first two seed crops to cover both establishment and maintenance costs. This is possible only if residue is removed after harvest.
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ACKNOWLEDGMENTS
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The authors would like to thank Bruce Hesselink, Don Penner, and Cheryl Duncan for their technical assistance on this study.
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REFERENCES
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ABSTRACT
INTRODUCTION
