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© 2008 Plant Management Network. Morphological Development and Winter Survival of Switchgrass and Big Bluestem Seedlings Timothy R. O’Brien, Pioneer Hi-Bred Intl. Inc., DuPont Agriculture and Nutrition, 8100 South 15th Street, Lincoln, NE 68512; Lowell E. Moser, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 279 Plant Science, P.O. Box 830915, Lincoln, NE 68583; Robert A. Masters, Dow AgroSciences, Indianapolis, IN 46268; and Alexander J. Smart, Department of Animal and Range Sciences, South Dakota State University, Box 2170, Brookings, SD 57007-0392 Corresponding author: Timothy O’Brien. tim.o’brien@pioneer.com O’Brien, T. R., Moser, L. E., Masters, R. A., and Smart, A. J. 2008. Morphological development and winter survival of switchgrass and big bluestem seedlings. Online. Forage and Grazinglands doi:10.1094/FG-2008-1103-01-RS. Abstract The stage of morphological development required for winter survival of switchgrass and big bluestem seedlings has not been established. A study at Lincoln, NE determined how autumn developmental status influenced winter survival of switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii var. gerardii Vitman) seedlings. Seedings were made on a Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludoll) from early August through November in 1997 and 1998. Although these seeding dates generally would not be recommended, irrigation following seeding produced a wide range of seedling growth stages at the onset of autumn dormancy. In October of both years, Mean Root Stage (MRS), Mean Shoot Stage (MSS), leaf area, root weight, and shoot weight of seedlings were determined. Autumn and spring seedling densities were used to measure stand survival. In addition, individually marked seedlings were staged and winter survival determined. Winter survival of switchgrass and big bluestem populations was ≥ 50% and seedling density in the spring was ≥ 20 plants/m² at a MSS of 5 (four to six collared leaves) and MRS of 5 (four to six adventitious roots) for switchgrass, and a MRS of 4.4 (three adventitious roots) for big bluestem. Seedlings with MSS > 7 (≥ two or more additional tillers) had high winter survival. In both grasses individual seedlings with shoot stage < 3 (one collared leaf) at the end of the first growing season did not survive winter and individual seedlings with shoot stages 4, 5, and 6 from earlier seedings had higher winter survival than same shoot stages seeded later. Both root stage and shoot stage reliably predicted winter survival. Introduction Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii var. gerardii Vitman) are perennial warm-season grasses native to tallgrass prairies of North America. Seedling establishment is often difficult and can limit use of these species. Switchgrass and big bluestem are usually seeded in the spring even though weed competition is a major problem (3,8). Even with good weed control, stand establishment of warm-season prairie grasses can be risky (13). Most investigations evaluating seedling density requirements for satisfactory stands recommend a range from 10 to 20 plants/m² (5,9). Information about grass seedling growth stage that must be achieved prior to the first winter to assure establishment success is limited. Late summer plantings of blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.] seedlings did not form adventitious roots because of dry conditions and did not survive the following winter (13). Later plantings of cool-season grasses resulted in smaller shoot height and less root penetration (4). Blue grama seedlings are considered established when they have six leaves, two adventitious roots at a depth of 10 cm, and two tillers (7). Seedlings of warm-season prairie grasses at the two to three-leaf stage in the autumn had a winter survival of 80 to 100% in unbroken prairie soils (2). Seedling morphological development has been used successfully to predict survival of cool-season grasses (1,11). The stage of morphological development required for winter survival of switchgrass and big bluestem seedlings has not been established. The objective of these experiments was to determine the stage of morphological development at which switchgrass and big bluestem grass populations and individual seedlings will survive winter. Site Preparation and Treatments Two experiments were conducted on adjacent sites at Lincoln, NE, on Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludoll). Temperature and rainfall were obtained within 1 km of the experiments from monitoring stations at the University of Nebraska. Experiments were designed as randomized complete blocks with four replicates. Treatments were arranged in a split-plot with the main plot factor as seeding date and species as the subplot factor. In both experiments, subplots were 1.6 m wide by 4 m long. For the autumn 1997 seeding (Exp. 1) glyphosate [N-(phosphonomethyl) glycine] was applied on 23 July 1997 to kill existing plant cover. Remaining dead and live plant material was removed by flail chopping from the site on 1 August and grasses were seeded into the dead stubble. The autumn 1998 seeding (Exp. 2) site was fallowed in spring and summer 1998 and grasses were seeded into a clean seed bed. On each seeding date, 'Pathfinder' switchgrass or 'Pawnee' big bluestem were seeded at 330 pure live seeds/m² in separate subplots with a seven-row cone drill with an 18-cm row spacing (Hege Inc., Waldenberg, Germany) at a depth of 1.5 cm. The experiments were not designed to evaluate seeding date but to produce a wide range of seedling stages at autumn dormancy. Seeding dates were 3, 8, 14, and 21 August, 5 and 18 September, 3 and 16 October, and 21 November 1997; and 8, 14, 20, and 30 August, 4, 17, and 29 September, 14 October, and 9 December 1998. Plots were irrigated with 12 to 13 mm of water immediately after each of the first seven seedings in each experiment and every 2 to 3 days during periods of low rainfall during the initial growing season. The last two seedings in each experiment were not irrigated because there was adequate soil moisture from late fall precipitation. Although normal seeding dates for Nebraska would be in April and May, late summer and autumn seeding dates in combination with irrigation provided a wide range of seedling growth stages for evaluation at the end of the growing season. With Exp. 1 weed control in the summer and autumn consisted of periodic hand weeding. In the spring of 1998 a single post-establishment weed control herbicide application of atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine] at 2.2 kg ai/ha was made. Weed control in Exp. 2 consisted of an autumn 1998 pre-emergence application of atrazine at 2.2 kg ai/ha and periodic hand weeding. In spring 1999 there was a post-establishment weed control application mixture of atrazine at 2.2 kg ai/ha and picloram [4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid] at 0.28 kg ai/ha. Data Collection Three subsamples were taken from each subplot on 9 and 10 October, 1997 and 1998. Seedings made after this date were not sampled in the autumn. Average frost date (0°C) at the National Weather Service Lincoln, NE, is 18 October. After frost, most development would be stopped in warm-season species. For each subsample, 25 consecutive seedlings from a randomly selected point were excavated to a depth of 20 cm, maintaining the shoot and root intact (Fig. 1). Most soil was removed from the roots in the field, plants were placed in plastic bags, and frozen. In the laboratory seedlings were thawed, roots and shoots were rinsed with tap water, and morphological data recorded. Seedling root and shoot systems were staged using a seedling staging system modified from Moser et al. (6) (Table 1). Individual seedling identity was maintained so shoot stage could be related to root stage. Mean Root Stage (MRS), Mean Shoot Stage (MSS), leaf area, root dry weight, and shoot dry weight were recorded. Leaf area was measured with a LI-3000 portable area meter (LI-COR Inc., Lincoln, NE). Roots and shoots were oven dried at 60°C to a constant weight before weighing.
Table 1. Growth stages, numerical indices, and descriptions for staging grass seedling shoots and roots.*
* Modified from Moser et al. 1993 (6). To determine stand survival, seedling densities were taken in permanently marked random sections of the drill rows 28 May 1998 and compared to those taken 13 and 20 November 1997 for Exp. 1 and plant densities on 4 June 1999 were compared with those taken 14 and 17 November 1998. Seedings made after this date did not emerge and were only sampled in the spring for seedling density data. Three 1-m subsamples were taken per subplot from randomly selected 1-m sections of row in the center 3 m of the middle five drill rows of the subplots. Areas that had previously had seedlings removed by excavation were excluded. Randomly selected dormant plants were marked with metal tags and shoot stage recorded in late October and early November of both years to estimate individual plant survival the following spring. In each subplot 15 plants of each stage were tagged if plants of desired stage were available. Survival of marked plants was recorded the following spring on 28 May 1998 (Exp. 1) and 3 June 1999 (Exp. 2). Maintaining individual plant integrity was not a problem. Statistical Analysis Analysis of variance was used to compare seeding date for population stand survival, MSS, MRS, shoot weight, leaf area, and root weight using PROC MIXED (SAS Institute Inc., Cary, NC). Data from each species were analyzed separately. Data from experiments were analyzed separately because of differences in seeding dates for the two experiments. The focus of the experiment was on seedling stages rather than seeding date. However, data are reported by seeding date because of potential differences in similar stages at different seeding dates. The MSS and MSR were calculated from an average of 75 seedlings and shoot weight, root weight, leaf area were calculated from an average of the 3 subsamples that included 25 seedlings per subsample. The MSS, MRS, shoot weight, root weight, and leaf area were correlated to winter survival. Mean morphological stages for shoots and roots of destructively sampled populations were used to predict autumn MRS of field populations. Pearson correlation coefficients between winter survival and all the other variables were calculated using PROC CORR (SAS). Climate Data Average weekly winter temperatures were generally above normal for both experiments. The first time minimum temperatures reached 0°C in the autumn was 17 October 1997 and 22 October 1998. Snow occurred earlier in 1997 than in 1998. There were 47 days of snow cover for the December through March period for Exp. 1 (1998) and 36 days for Exp. 2 (1999) (Table 2). Late summer and autumn plantings in both years had adequate soil water from rainfall and supplemental irrigation. Table 2. Number of days of winter snow cover greater than
Winter Survival of Grass Seedling Populations Seedling densities in November 1997 were high for both big bluestem and switchgrass for the 3 August to 18 September seeded plots, although few grass seedlings survived the winter in plantings made after 5 September (Table 3). No switchgrass seedlings emerged from the 21 November planting as observed by weekly observations in November and December, but limited seedling emergence occurred (presumably from dormant seed) in the spring. Evaluations made in November 1998, showed excellent emergence of big bluestem and switchgrass planted between 8 August and 14 October. Seedling survival was acceptable (10 to 20 plants/m²) from plots planted 8 August to 4 September. This was within the range recommended by Launchbaugh and Owensby (5) and Vogel (9) for establishing a satisfactory stand. There were acceptable big bluestem stands in the spring of 1999 from the 9 December dormant planted plots, because seed germination was delayed until the spring. Seedlings that had emerged by November of 1998 from the 14 October plots did not survive winter. Acceptable big bluestem stands were recorded in the spring of 1999 from the 14 October planted plots, because some seeds germinated in the spring 1999. Results were similar for switchgrass, but only the 9 December planting had acceptable seedling establishment after seed germinated in the spring. Table 3. Stand density of big bluestem and switchgrass seedlings in the autumn and spring of 1997-1998 and 1998-1999 as affected by 1997 and 1998 seeding date.
w Data collected 13 to 20 November 1997 in Exp. 1 and 13 to 20 November 1998 in Exp. 2. x Means in each row followed by the same letter within a species are not significantly different at P = 0.05 (F-protected LSD). y Not yet seeded or no seedlings emerged at time of November sampling. z Data collected on 28 May 1998 and 4 June 1999. Autumn seedling densities recorded at dormancy for the September seeding dates were generally higher than those from earlier seeding dates. These higher densities likely were due to more moderate temperatures and better water relations. The seedling populations from earlier seeding dates had a higher survival, MSS, MRS, shoot weight, root weight, and leaf area than later seeding dates (Tables 4 and 5). In Exp. 1 (Table 4) winter survival of big bluestem was over 70% and for switchgrass was over 60%, in plots planted on 3, 8, and 14 August 1997, which was higher than for later seeding dates. Survival of seedlings in plots planted on 21 August 1997 was acceptable but survival was negligible for seedlings from later planting dates. Plants sampled in October from these populations had a MSS or MRS of below 5 was associated with poor winter seedling survival. A MSS or MRS of 5 appeared to be a threshold value, above which acceptable establishment occurred and below which there was establishment failure. Table 4. Winter survival of big bluestem and switchgrass seedling populations during 1997-1998 and autumn 1997 Mean Shoot Stage (MSS), Mean Root Stage (MRS), dry shoot weight, dry root weight, and leaf area at Lincoln, NE, as affected by seeding date.
u Based on data collected between 13 and 20 November 1997 and 28 May 1998. v Means in each row followed by the same letter are not significantly different at P = 0.05 (F-protected LSD). w Data collected on 9 and 10 October 1997. x Mean for 75 seedlings. y Plants not emerged at time of data collection. z Total for 25 seedlings. Table 5. Winter survival of big bluestem and switchgrass seedling populations during 1998-1999 and autumn 1999 Mean Shoot Stage (MSS), Mean Root Stage (MRS), dry shoot weight, dry root weight, and leaf area at Lincoln, NE, as affected by seeding date.
u Based on data collected between 14 and 17 November 1998 and 4 June 1999. v Means in each row followed by the same letter are not significantly different at P = 0.05 (F-protected LSD). w Data collected on 9 and 10 October 1998. x Mean for 75 seedlings. y Plants not emerged at time of data collection. z Total for 25 seedlings. Results of Exp. 2 (Table 5) were similar and stand survival for both species from the 4 September plantings were acceptable. The MSS and MRS obtained from destructively sampled plants in October at the end of the growing season were about 4.5, although the MRS for big bluestem was 3.5 (about one to two adventitious roots). Differences in seedbed preparation, timing of temperature extremes, and timing and length of snow cover may have played a role in seedling development differences between the two experiments. It appears that switchgrass seedlings require more adventitious roots to have the same root mass as big bluestem. This could be one of the reasons for the apparently greater survival of big bluestem compared to switchgrass from later seedings. The mean stage shoot (MSS), shoot weight, leaf area, mean stage count root (MRS), and root weight taken at the end of the first growing season all were highly correlated with seedling survival (Table 6). The MSS and MRS had especially high correlation coefficients with seedling population survival. Because of this high correlation, MRS and MSS are very important visual indicators of establishment and winter survival potential. The MSS of a seedling population is relatively quick and easy to determine when compared to the other parameters measured. This makes MSS a valuable tool in predicting population winter survival of grass seedlings. The autumn relationship between MSS and MRS was determined for big bluestem and switchgrass seedling populations. The prediction equations developed from these correlations for big bluestem was MRS = -1.98 + 1.20 × MSS with r² = 0.93 and RMSE = 0.39. The prediction equations developed from these correlations for switchgrass was MRS = -1.09 + 1.11 * MSS with r² = 0.91 and RMSE = 0.43. Table 6. Pearson correlation coefficients of seedling morphological parameters with winter survival of big bluestem and switchgrass seedlings in 1997-1998 and 1998-1999 at Lincoln, NE.
* Indicates significance at the P = 0.01 level. Winter Survival of Individual Plants Within seedling populations individual plants with low shoot and root stages of development likely did not survive the winter, lowering overall population survival. Individual plant winter survival and stand establishment can be related to seedling morphological development at the end of the first growing season. None of the plants marked at shoot stages 2 and 3 survived winter (Table 7). Individual plant survival was very low for shoot stage 4, regardless of seeding date. Survival of individually marked big bluestem and switchgrass seedlings at shoot stages of 5 or 6 was greater than 50% except for switchgrass in 1999. These seedlings would have four to six collared leaves and one additional tiller prior to frost. Winter survival was greater than 90% when the seedlings had reached a shoot stage of 7 or 8 before the first frost except for switchgrass in 1998 (Table 7). Seedlings at stages 7 and 8 had developed a minimum of two to three additional tillers. Table 7. Winter survival of individual big bluestem and switchgrass seedlings as affected by autumn shoot stage in 1997-1998 and 1998-1999 at Lincoln, NE.
x Data collected between 23 October and 8 November 1997 and data collected on 28 May 1998. y Means in each row followed by the same letter are not significantly different within a date and species at P = 0.05 (F-protected LSD). z Data collected between 23 October and 14 November 1998 and data collected on 3 June 1999. The shoot stage of seedlings may have been the same at several seeding dates, but winter survival was not. This was especially apparent for grasses that achieved shoot stages 4, 5, and 6. Within the same shoot stages, seedlings from earlier seeding dates generally had a higher predicted MRS, based on destructive harvest of similar plants, and a higher rate of winter survival. For high winter survival, big bluestem seedlings required a MRS of about 5. Big bluestem seedlings at shoot stage 5 from seedings in August of both years had a predicted MRS of 5 to 5.5 (data not shown). However, seedlings at shoot stage 5 from early September plantings had a predicted MRS of only 4 to 4.2 and there was essentially no winter survival. Even at shoot stage 6 in Exp 1 there was no survival for 5 September-seeded plots when predicted MRS was 4.4. There were similar results for switchgrass. Individual plants at shoot stage 4 from August-seeded plots had 27 to 35% survival, although the predicted MRS was only 4.1 to 4.3. Like big bluestem, survival of shoot stage 5 to 6 switchgrass plants was low from September-planted plots and in each case predicted MRS was below 5.0. Apparently root developmental stage is more important than shoot stage for predicting survival of individual seedlings. Other factors besides little root development may have contributed to the high mortality of late-planted shoot stage 5 or 6 seedlings. Total nonstructural carbohydrates (TNC) and vegetative storage proteins (VSP) are also important in seedling survival (10,12). There may have been insufficient cold hardening of seedlings before the onset of winter, storage of TNC, or VSP accumulation in seedlings in late-planted plots. Conclusions The MSS and MRS at the end of the first growing season accurately predict seedling establishment and subsequent winter survival. Winter survival, which indicates full establishment, of over 50% for populations of big bluestem and switchgrass required a MSS of about 5 (four to six collared leaves) and a MRS of 5 (four to six adventitious roots) at the end of the growing season of the seeding year. Seedlings at these mean stages of development produced seedling densities that were greater than 20 plants/m². Although closely related, root stage of development appears to be more important than shoot development in determination of establishment and winter survival of individual plants. However, in the field, measuring an above ground metric such as MSS, is easy, and provides a reliable predictor of the seedling root stage of development and seedling winter survival. Literature Cited 1. Arakeri, H. R., and Schmid, A. R. 1949. Cold resistance of various legumes and grasses in early stages of growth. Agron. J. 41:182-185. 2. Blake, A. 1935. Viability and germination of seeds and early life history of prairie plants. Ecol. Monogr. 5:405-460. 3. Hsu, F. H., and Nelson, C. J. 1986. Planting date effects on seedling development of perennial warm-season forage grasses II. Seedling growth. Agron. J. 78:38-42. 4. Keim, F. D., and Beadle, G. W. 1927. Relation of time of seeding to root development and winter survival of fall seeded grasses and legumes. Ecology 8:251-264. 5. Launchbaugh, J. L., and Owensby, C. E. 1970. Seeding rate and first-year stand relationships for six native grasses. J. Range. Manage. 23:414-417. 6. Moser, L. E., Moore, K. J., Miller, M. S., Waller, S. S., Vogel, K. P., Hendrickson, J. R., and Maddux, L. A. 1993. A quantitative system for describing the developmental morphology of grass seedling populations. Pages 317-318 in: Proc. XVII Int. Grass. Congr., Palmerston North, New Zealand. 8-21 Feb. 1993. M. J. Baker, ed. New Zealand Grassland Soc. Inc., Gardens, NZ. 7. Ries, R. E., and Svejcar, T. J. 1991. The grass seedling: When is it established? J. Range Manage. 44:574-576. 8. Vassey, T. L., George, J. R., and Mullen, R. E. 1985. Early-, mid-, and late-spring establishment of switchgrass at several seeding rates. Agron. J. 77:253-257. 9. Vogel, K. P. 1987. Seeding rates for establishing big bluestem and switchgrass with preemergence atrazine applications. Agron. J. 79:509-512. 10. Volenec, J. J., Ourry, A., and Joern, B. C. 1996. A role for nitrogen reserves in forage regrowth and stress tolerance. Physiol. Plant. 97:185-193. 11. White, W. J., and Horner W. H. 1943. The winter survival of grass and legume plants in fall sown plots. Sci. Agric. 23:399-408. 12. Wilson, A. M. 1984. Nonstructural carbohydrates and root development in blue grama seedlings. J. Range Manage. 37:28-31. 13. Wilson, A. M., and Briske, D. D. 1979. Seminal and adventitious root growth of blue grama seedlings on the central plains. J. Range Manage. 32:209-21. |
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