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FORAGES

Timing, Defoliation Management, and Nitrogen Effects on Seed Yield of `Argentine' Bahiagrass

^a Univ. of Florida, Range Cattle Research & Education Ctr., Ona, FL 33865 USA
^b Dep. Of Agric. & Consumer Serv., Mayo Bldg., Tallahassee, FL 32304 USA

mba{at}gnv.ifas.ufl.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Seed yield of bahiagrass (Paspalum notatum Flugge) can be increased by burning the sward and applying N fertilizer at the proper time during the growing season. Our objective was to determine the optimum calendar month (photoperiod) for managing `Argentine' bahiagrass pasture for seed production. Triplicate paddocks of a 10-yr-old Argentine bahiagrass pasture established on a Pomona fine sand (sandy, siliceous, hyperthermic, Ultic Alaquods) were grazed or not grazed (main plots) from October–February, 1988–89 and 1989–90, in a randomized complete-block design. Cattle were removed from the area on 25 February. Then, on the 25th of February, March, April, and May (subplots) each year, portions of residue (sub-subplots) were burned or mowed in both grazed and ungrazed fields. The residue removal dates corresponded with day lengths of 11.5, 12.3, 13.1, and 13.7 h, respectively. Immediately after residue removal, sub-sub-subplots (2.4 by 6.1 m) were fertilized with 0, 100, or 200 kg N ha^-1. Mature raceme density, seed yield, and seed germination (1989 only) were determined in 1989 and 1990. Grazing did not affect any measured trait. Burning increased seed yield only for the 25 May 1990 treatment. Date of residue removal and N rate influenced (P < 0.0001) the number of mature racemes and seed yield interactively (P < 0.05). Nitrogen rate increased (P < 0.05) all measured traits in a quadratic manner when applied only during late-April to late-May. Results suggest that seed production from Argentine bahiagrass pasture in the subtropics could be enhanced with management implemented at day lengths >13.0 h.

Abbreviations: GD, growing degree days • DOY, day of year • DL, day length • SY, seed yield • RD, raceme density

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
BAHIAGRASS seed frequently is produced in subtropical regions by commercial beef cattle producers to generate secondary income. Seed companies harvest, process, and market the seed. Typical management of bahiagrass pasture for seed includes fertilization in early spring followed by grazing until June or the first sign of inflorescence. Cattle are removed then to allow for raceme development and seed set. Earlier studies conducted on `Pensacola' bahiagrass (Adjei et al., 1992) indicated that removing cattle in June reduced seed yield. For example, delaying the cattle removal date from mid-April to early-June reduced seasonal seed yield by about 70%, although the number of racemes m^-2 was reduced by only 14%. The decrease in seed yield was attributed to the adverse effect of water-saturated soils in summer on grain fill and seed size. Although bahiagrass seed yield from sod-bound (old, rank) pastures was increased by burning and N fertilization (Burton, 1943, 1944), the response was month-dependent and varied between years (Adjei and Mislevy, 1987; Adjei et al., 1992).

The development and longevity of perennial grass tillers vary according to genetic factors (Langer and Lambert, 1959; Saxby, 1956), date of origin (Langer, 1963; Lambert and Jewiss, 1970), and environmental conditions (Langer, 1963). Environmental factors such as photoperiod, seasonal temperature ranges, and rainfall patterns affect floral initiation, floret development, and seed yield, and determine where, when and how seed crops of particular grass cultivars may be successfully produced (Loch, 1980).

The particular management system used to produce pasture grass seed must integrate plant requirements with environmental factors to maximize seed yield. Tillering in perennial warm-season grasses was stimulated by residue removal (Burton, 1943) and N fertilization (Humphreys and Davidson, 1967; Cameron and Mullaly, 1969), which formed the basis of a synchronized crop for seed production. The timing of residue removal and N fertilization relative to physiological maturity of new tillers was vital to improving grass seed production (George et al., 1990; Adjei et al., 1992).

The objectives of this study were to determine the influence of (i) grazed (October–February) vs. ungrazed sward, (ii) month of residue removal, (iii) method of residue removal (burning vs. mowing), and (iv) rate of nitrogen fertilization following residue removal on yield and germination of Argentine bahiagrass seed.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
The experiment was conducted on a 10-yr-old Argentine bahiagrass pasture located at the Range Cattle Research and Education Center in southcentral Florida (82°55' W, 27°26' N). The soil was Pomona fine sand (sandy, siliceous, hyperthermic, Ultic Haplaquods). The experimental design was a split-split-splitplot with three replicates. Main plots (different for 1989 and 1990) were either grazed or ungrazed from fall through winter (October–February). Subplots consisted of date of residue removal: 25 February, 25 March, 25 April, or 25 May 1989 and 1990. Sub-subplots consisted of method of residue removal (burn vs. mow) on each date of residue removal. Plots were fertilized immediately after residue removal with 0, 100, or 200 kg ha^-1 of N (sub-sub-subplots) as ammonium nitrate. All plots received a uniform application of 0–50–190 kg N–P–K ha^-1 at the time of N fertilization. Each sub-sub-subplot measured 2.4 by 6.1 m.

The amount of residue and soil moisture (0–15 cm depth) were determined before residue removal in 1989 and 1990. Residue was measured from three random harvests in each subplot. Gravimetric soil moisture was estimated from 15 core soil samples randomly removed from a subplot and dried to constant weight. In addition, burning temperatures were monitored in 1990 by placing a copper/constantan thermocouple at the soil surface and at 25 mm below the soil surface in each burned sub-subplot. Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) was applied to the April and May burned treatments in 1990 at the rate of 2.3 L a.i. ha^-1 in 280 L water ha^-1 4 d before burning to desiccate the canopy and facilitate burning. Treatment dates were characterized as to day length and number of growing degree days (GD) taken to reach seed maturity: , where T_min and T_max were daily minimum and maximum ambient temperatures, (T_min + T_max) / 2 30°C, and a base temperature assumption of 10°C (Russell and Webb 1976).

Seed was harvested by clipping all racemes in a 1-m² quadrat in each plot when about 5% of the seed could easily be detached by rubbing in the hand. There was no floral development for the late-February treatment in 1989. Harvest dates in 1989 were 26 August for plots treated in late-March and late-April and 30 August for plots treated in late-May. All treatments were harvested between 7 and 12 August, 1990. Before harvesting a plot, mature racemes were counted in a 0.5-m² quadrat. Mature racemes had undergone a slight change in color from green to a light green or yellow.

Harvested racemes were dried at 35°C in a forced-air oven to approximately 10% moisture, threshed with a hammer mill (Seedburo Equip., Chicago, IL) and cleaned through a series of screens and an aspirator to approximately 95% purity. Cleaned seed was weighed. Samples of cleaned seed from the 1989 harvest were sent to the Florida State Seed Testing Laboratory for germination analysis. Four replicates of a 100-seed sample were tested for germination on blotters in clear plastic petri dishes at alternating temperatures of 20°C for 16 h and 35°C for 8 h in a daylight/fluorescent-lighted germinator (Assoc. Off. Seed Analysis, 1988). Petri dishes were arranged in a randomized complete-block design. Seedlings were counted and removed from petri dishes every third day during a 28-d test period. Germination percentage was determined as (total number of normal seedlings / total number of seeds) times 100.

Due to occurrence of two-way and three-way interactions among years, date, and N rate, analysis of variance (ANOVA) was performed on mature raceme density and seed yield for separate years and seed germination for 1989. Also, grazing treatment had no effect on any seed traits, did not interact with any experimental variable, and was consolidated into the error term. Therefore, ANOVA for each year was reduced to a split-splitplot design, with date, method of residue removal, and N rate. Means for date, method of residue removal, and N rate were separated by Fisher's protected LSD test (P 0.05). Additionally, sum of squares for N rate was partitioned into linear and quadratic effects by orthogonal contrasts with actual P levels reported. Stepwise regression analysis was used to identify significant (P < 0.05) relationships between seed traits and environmental factors. Based on the outcome, forward regression analysis was used to establish polynomial equations separately for date of residue removal expressed as day of year (DOY), day length at residue removal (DL), and GD, each with N rate. All statistical analyses were performed using either the GLM or REG procedures of Statistical Analysis System (SAS, 1988).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Environment
Residue
Mean residual biomass before removal in 1989 was greater (P < 0.0001) on ungrazed (10.8 Mg ha^-1) than on grazed fields (2.0 Mg ha^-1), and there were no differences among dates of removal. Residual biomass in 1990 averaged 9.7 and 1.5 Mg ha^-1 for ungrazed and grazed pastures, respectively. This implied that more fuel was available for burning on ungrazed than grazed plots. However, there was no effect of grazing and no interaction (P > 0.65) between method of residue removal and grazing treatments on any seed trait.

Soil Moisture
In 1989, soil moisture content at 0 to 15 cm before residue removal from ungrazed fields in February and April declined (P 0.05) from 214 g kg^-1 to 156 g kg^-1, respectively. These soil moisture values were approximately 30% greater (P < 0.0001) for ungrazed than for corresponding grazed field values between February (157 g kg^-1) and April (105 g kg^-1). For the 25 May 1989 residue removal, soil moisture content was 59 and 38 g kg^-1 for the ungrazed and grazed treatments, respectively. In 1990, soil moisture content before residue removal declined from 286 g kg^-1 in February to 145 g kg^-1 in April, and there were no differences between grazed and ungrazed treatments. Soil moisture increased to 258 g kg^-1 on all fields just before the May 1990 residue removal because of a heavy rainfall (62.5 mm) 2 d earlier. The soil was wet but grass burned well on the grazed plots because of paraquat application 4 d earlier. The ungrazed plots also burned well, but a 5 to 10 cm unburned stubble remained due to wet, dead vegetation on the soil surface. Soil moisture at residue removal did not affect (P > 0.45) any seed trait.

Burning Temperatures
Before burning in 1990, temperatures at the soil surface and 25 mm below the soil surface averaged 30 and 24°C, respectively. The amount of residual fuel and soil moisture tended to influence burning temperatures. Maximum temperatures recorded at the soil surface briefly exceeded the 400°C capacity of the thermocouple on ungrazed plots, except for the late-May 1990 burning when maximum temperature was 200°C. Maximum surface temperatures during burning on grazed plots averaged 260°C. Maximum temperatures 25 mm below the soil surface ranged from 23 to 81°C on grazed plots and 30 to 169°C on ungrazed plots. Maximum temperatures were reached within 2 s of flame passage, then decreased substantially within 30 s of flame passage. Generally, temperatures recovered to ambient levels within 30 min after burning. Hopkins (1965) showed that ground-level temperatures in both late and early dry-season burns invariably exceeded 538°C, but dropped quite sharply at a height of 10 cm. Pitot and Masson (1951) observed that temperatures at soil level rose very steeply to between 100 and 850°C depending on wind and height and density of sward, but usually returned to ambient temperatures within a few minutes. The soil temperature at a depth of 2 cm changed little, varying at most by 14°C, but more often as little as 3 to 4°C or less (Pitot and Masson, 1951). The effect on subterranean portions of grasses was thus probably slight and may be the reason why no clear relationships between burning conditions and seed traits were obtained.

Day of Year, Day Length, Growing Degree Days and Nitrogen Regression on Seed Traits
The DOY and day lengths corresponding with the 25 February, 25 March, 25 April, and 25 May dates of residue removal were 56, 84, 115, and 145 d, and 11.5, 12.3, 13.1, and 13.7 h, respectively (Fig. 1a) . The 4-d minimum and maximum ambient temperatures and 4-d total rainfall for 1989 and 1990 are shown in Fig. 1b and 1c. Calculated GD were 4791, 4334, 3688, and 2954 in 1989, and 4226, 3760, 3123, and 2318 in 1990, respectively, for the 25 February, 25 March, 25 April, and 25 May treatment dates. A summary of statistics for the regression of environmental variables on seed traits is shown in Table 1 . Since DL and GD were both related to DOY, separate regression analyses were made with each and N as the independent variable. In 1989, the quadratic and cubic effects of DOY on raceme density and seed yield were significant (P < 0.0001), however, the linear DOY partial R² in both raceme density and seed yield predictive equations accounted for >70% of the complete model R². The N variable did not meet the 0.05 significance level for entry into the seed yield model and its interaction with DOY accounted for less than 1% of raceme density model R² in 1989. In 1990, the linear DOY partial R² was 79% of seed yield model R² and the X³N term provided 77% of the raceme density model R² (Table 1). Therefore, Argentine bahia-grass floral development and seed yield depended on date of residue removal to a greater extent than the rate of N application. Billbug (Sphenophorus spp) insect damage to some caryopses was observed during seed harvest in 1990, which probably reduced the seed yield R² in 1990. Seed trait predictive equations developed with DL or GD and N as independent variables provided similar equations and R² as developed with DOY and N (Table 1).

View larger version (37K): [in this window] [in a new window]   Fig. 1 (a) Mean 4-d duration of daylight at 27°30' N from February to August (U.S. Naval Observatory, 1946) with arrows indicating dates of residue removal; (b) and (c) Mean 4-d maximum and minimum temperatures, and mean 4-d rainfall at experimental site for 1989 and 1990, respectively

Seed Yield
A positive linear relationship existed between seed yield (SY) and raceme density (RD) in 1989 similar to the relationship reported for an earlier trial on Argentine bahiagrass (Adjei et al., 1989). In 1990, the relationship was curvilinear, with a lower predictability , probably because of insect damage to caryopses in the field.

In 1989, method of residue removal had no effect on seed production, nor were there any interactions between it and other variables. However, date of residue removal and N rate significantly affected seed yield (Tables 4 and 5) . Seed yield showed very little response to N fertilizer application in late-February or late-March but responded positively to N in late-April to late May, which resulted in an interaction between date and N rate in 1989 (Table 4). In 1989, highest seed yields were obtained following residue removal in late-April or late-May and a 100 kg N ha^-1 fertilization (Table 4), in line with the dynamics of raceme density (Table 2). Seed yield response to N for the late-April application was quadratic (P < 0.003), which reinforces the need for moderate N rate towards increased seed development (Adjei and Mislevy, 1989; Adjei et al., 1992; Gates and Burton, 1998). From late-March through late-May in 1990, seed yield response to N fertilization was greater where residue was removed by burning instead of mowing. The three-way interaction between date, method of residue removal and N rate (Table 5) was due to a total absence of yield response to N for the late-February treatment. For the late-May burning and fertilization, where highest seed yield was obtained at harvest, the response was also quadratic, with a peak yield with 100 kg N ha^-1. Yearly variation in the effect of burning on Argentine bahiagrass seed yield has been a problem (Adjei and Mislevy, 1989) and may be related to the intensity of defoliation. Although the canopy was completely burned, the water-saturated soil during late-May 1990 prevented burning to ground level, as obtained in 1989. The lower seed yield obtained in 1990 compared with 1989 could have resulted from other factors. First, the 1990 ambient temperatures and consequently the GD during the late-April and late-May post-treatment period were lower than in 1989. Despite similar day lengths, post-treatment GD for the 25 April and the 25 May treatments, respectively, were 3688 and 2954 in 1989, and 3123 and 2318 in 1990. Secondly, in 1989, there was continuous rainfall during the late-April and the late-May post-treatment, tiller production period (Fig 1b, ); and good but scattered rainfall during the seed maturation period (Fig 1b, ). Comparatively, in 1990, there were fewer scattered rain showers during the tiller production phase, but good continuous rain during the seed maturation phase (Fig 1c). The 1989 rainfall distribution supported a higher tiller and raceme density, a better response to N, and higher seed yield for the late-April and late-May residue removal treatments (Tables 2 and 4). The 1990 rainfall pattern supported a lower raceme density and seed yield (Tables 3 and 5), except for the late-May treatment, which had the DOY 180 to 220 rains. Tillers from burned sward were always the slowest to mature and benefitted most in seed yield from the late 1990 rains (Table 5), another plausible explanation for yearly variations in seed yield response to burning.

Seed Germination
Calculated percentage seed germination ranged from 51 to 82%. Nitrogen fertilizer was the only factor that affected (P < 0.02) Argentine seed germination, and the N effect was independent of other variables. Mean seed germination percentage for the 0 kg N ha^-1 (71%) was similar to the 100 kg N ha^-1 (73%), but greater than the 200 kg N ha^-1 (58%). This was probably due to production of light-weight seed that resulted from suppression of floret development by excessive vegetative growth at the highest N rate. Bahiagrass seed germination has been found to be positively correlated with seed weight (Adjei et al., 1992). Seed weight and germination of Pensacola bahiagrass were depressed when residue removal was delayed until emergence of first-raceme, visible in early June (Adjei et al., 1992). Seed germination of Argentine bahiagrass was not affected by date of residue removal, suggesting that Argentine bahiagrass floral development may be more tolerant of the water-saturated soil in summer than Pensacola bahiagrass.

	Summary and conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Argentine bahiagrass is day-sensitive and will flower only at DL > 13.8 h in the subtropics. This study investigated the best date, relative to time at plant maturation, for residue removal and N fertilization. Highest Argentine bahiagrass seed yields were obtained when sward residue from previous growing season was removed between late April and late May and immediately fertilized with 100 kg N ha^-1 using a base 0–50–190 N–P–K kg ha^-1. Removal of residue at earlier dates or the application of 200 kg N ha^-1 caused greater herbage accumulation, which inhibited raceme development. The optimum schedule for Argentine residue removal will be about a month later than required for optimum Pensacola bahiagrass seed production. Since Argentine bahiagrass has a later maturation date, the optimum time schedule for Argentine residue removal coincides with the standard period for cattle removal from pasture to initiate bahiagrass seed production, but additional N fertilization may be required at that time to promote seed yield.Association of Official Seed Analysts 1988; SAS Institute 1988

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Contribution from the UF-IFAS, Florida Agric. Exp. Stn. Journal Series no. R-06640.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 

• Adjei, M.B., and P. Mislevy. 1987. Bahiagrass seed production as influenced by mechanical, cultural, and chemical treatments. p. 103. In Agronomy Abstracts. ASA, Madison, WI.
• Adjei, M.B., and P. Mislevy. 1989. Cultural and fertilizer practices for bahiagrass seed production. p. 62–63. In Proc. Beef Cattle Short Course, 18th. 3–5 May 1989. Inst. Food and Agric. Sci., Univ. of Florida, Gainesville.
• Adjei M.B., Mislevy P., Chason W. Seed yield of bahiagrass in response to sward management by phenology. Agron. J. 1992;84:599-603.[Abstract/Free Full Text]
• Association of Official Seed Analysts. Rules for testing seeds. (1990 rev.) J. Seed Technol. 1988;12(3):1-109.
• Burton G.W. Factors influencing seed setting in several southern grasses. J. Am Soc. Agron. 1943;35:465-474.
• Burton G.W. Seed production of several southern grasses as influenced by burning and fertilization. J. Am Soc. Agron. 1944;36:523-529.
• Cameron D.G., Mullaly J.D. Effect of N fertilization and limited irrigation on seed production of Molpo buffelgrass. Queensl. J. Agric. Anim. Sci. 1969;26:41-47.
• Gates R.N., Burton G.W. Seed yield and seed quality response of Pensacola and improved bahiagrasses to fertilization. Agron. J. 1998;90:607-611.[Abstract/Free Full Text]
• George J.R., Reigh G.S., Mullen R.E., Hunczack J.J. Switchgrass herbage and seed yield and quality with partial spring defoliation. Crop Sci. 1990;30:845-849.[Abstract/Free Full Text]
• Hopkins B. Observations of savanna burning in the Olokemeji Forest Reserve, Nigeria. J. App. Ecol. 1965;2:367-381.
• Humphreys L.R., Davidson D.E. Some aspects of pasture seed production. Trop. Grassl. 1967;1:84-86.
• Lambert D.A., Jewiss O.R. The position in the plant and the date of origin in tillers which produce inflorescences. J. Br. Grassl. Soc. 1970;25:107-112.
• Langer R.H.M. Tillering in herbage grasses. Herb Abstr. 1963;33:141-148.
• Langer R.H.M., Lambert D.A. Ear-bearing capacity of tillers arising at different times in herbage grasses grown for seed. J. Br. Grassl. Soc. 1959;14:137-140.
• Loch D.S. Selection of environment and cropping system for tropical grass seed production. Trop. Grassl. 1980;14:159-168.
• Pitot A., Masson H. Some data on temperature changes during brush fires near Dakar. Bull. Inst. French W. Africa. 1951;13:711-732.
• Russell J.S., Webb H.R. Climatic range of grasses and legumes used in pastures. Result of a survey conducted at the 11th International Grassland Congress. J. Aust. Inst. Agric. Sci. 1976;42:156-163.
• SAS Institute, Inc. SAS user's guide: Statistics, 6th ed Cary, NC: SAS Inst, 1988.
• Saxby, S.H. 1956. Pasture production in New Zealand. N.Z. Dep. Agric. Bull. 250.
• U.S. Naval Observatory. 1946. Tables of sunrise, sunset and twilight. Supplement to the American ephemeris, Nautical Almanac Office, Washington, DC. p. 507–508. In R.L. List (ed.) Smithsonian meteorological tables. 6th ed. Smithsonian Institution, Washington D.C., 1945.

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Adjei, M. B. Articles by Chason, W. Search for Related Content PubMed Articles by Adjei, M. B. Articles by Chason, W. Agricola Articles by Adjei, M. B. Articles by Chason, W. Related Collections Forage Management Other Forage Crops Seed Production Soil Fertility and Productivity