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Published online 1 September 1999
Published in Agron J 91:787-795 (1999)
© 1999 American Society of Agronomy
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Agronomy Journal 91:787-795 (1999)
© 1999 American Society of Agronomy

FORAGE & GRAZING MANAGEMENT

Response of Selected and Unselected Bahiagrass Populations to Defoliation

Roger N. Gatesa, Gary M. Hilla and Glenn W. Burtona

a Dep. of Crop and Soil Sciences, Univ. of Georgia, Coastal Plain Exp. Stn., Tifton, GA 31793 USA

rngates{at}tifton.cpes.peachnet.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Recurrent restricted phenotypic selection (RRPS) has increased spaced-plant yield of `Pensacola' bahiagrass (Paspalum notatum Flügge var. saurae Parodi) and led to higher-yielding, more erect genotypes. Response to defoliation was examined in three entries: selection Cycle 0, Pensacola; Cycle 9, `Tifton 9', and Cycle 14. In a 3-yr plot study, two cutting heights (15 or 100 mm) in combination with three regrowth intervals (2, 4, or 8 wk) were imposed throughout 24 wk. A split-split plot arrangement of regrowth interval (whole plot), cutting height (subplot) and entry (sub-subplot) was used, with six replications. In a 2-yr grazing study, response of the same entries to defoliation from continuous stocking was evaluated. In the plot study, yield (7030 kg ha-1) was maximized by low cutting height and 8-wk regrowth interval during the first year. In Year 2, a 4-wk regrowth interval and low cutting height produced the highest yield (11220 kg ha-1). Mean yields of Tifton 9 (7650 kg ha-1) and RRPS cycle 14 (7320 kg ha-1) were greater than Pensacola (6500 kg ha-1), but response of entries to cutting height and regrowth interval varied among years. In the grazing study, herbage mass was greater for Tifton 9 (1670 kg ha-1) and RRPS Cycle 14 (2000 kg ha-1) than for Pensacola (1420 kg ha-1). Carrying capacity of Tifton 9 (935 d ha-1) was greater than Pensacola (855 d ha-1). Daily gains were similar (0.34 kg) for all germplasms. Bahiagrass stand declined rapidly for RRPS Cycle 14. Continuous stocking at high grazing pressure was not suitable for RRPS Cycle 14.

Abbreviations: DM, dry matter • IVDMD, in vitro dry matter disappearance • RRPS, recurrent restricted phenotypic selection


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
PENSACOLA BAHIAGRASS, a warm-season perennial introduced from South America (Burton, 1967), is an important forage in the lower southeastern USA and is grown on more than 2 million ha of land (Beaty and Powell, 1978). Bahiagrass is widely adapted, especially to sandy soils, and tolerates low fertility, drought, intermittent flooding, and heavy, continuous grazing. Pensacola, the earliest introduction recognized as a cultivar, is most widespread. Additional plant introductions, leading to release of `Argentine' and `Paraguay-22,' have historically provided germplasm alternatives.

Beginning in 1960, Burton (1974, 1982) used modified mass selection, termed recurrent restricted phenotypic selection, to develop improved bahiagrass populations from Pensacola. Werner and Burton (1991) documented the progressive morphological modifications that accompanied selection for increased aboveground yield: spaced plants of advanced populations were taller, with longer and wider leaves, but had decreased plant diameter. Morphological observations suggested that the large investment of biomass in surface energy storage structures [referred to as either rhizomes (Hitchcock, 1935; Pedreira and Brown, 1996a, 1996b; Ball et al., 1996) or stolons (Beaty et al., 1977; Sampaio et al., 1976)] that is typical of Pensacola might have been reduced by selection for more upright growth. Pedreira and Brown (1996a) confirmed that individual plants from selected populations had a higher ratio of height to diameter and fewer rhizomes. Placement at the soil surface protects stolons, which accumulate carbohydrate, from removal by cutting or grazing (Adjei et al., 1988). Pedreira and Brown (1996a) found no differences in growth rate or leaf photosynthesis among bahiagrass populations. In additional experiments, Pedreira and Brown (1996b) found lower yields of Pensacola compared with Tifton 9 and RRPS Cycle 14 in the final 2 yr of a 3-yr clipping study. Cutting at 3.5 cm resulted in harvest of a greater fraction of the total biomass than cutting at 10 cm for all populations. Response to simultaneous variation in cutting height and duration of regrowth among bahiagrass populations has not been reported. Given the dissimilar effects of mechanical harvest and grazing on plant yield and persistence (Trlica and Rittenhouse, 1993), information about the response of selected populations to grazing is needed. Our objectives were to compare the productivity and persistence of two bahiagrass populations selected for greater harvestable herbage production (Tifton 9 and RRPS Cycle 14) with the unselected population, Pensacola, under mechanical harvests of variable frequency and intensity. Response of the same populations to heavy, continuous stocking was evaluated concurrently.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Clipping Experiment
Three bahiagrass entries were seeded (20 kg ha-1) 12 June 1989 with a cultipacker seeder on Tifton loamy sand soil (fine-loamy, siliceous, thermic Plinthic Kandiudults) at the Coastal Plain Experiment Station, Tifton, GA (31°26' N, 83°35' W). The plot area had been planted with annual crops for several years and was free of bahiagrass. After emergence, 112 kg N ha-1 was applied. A substantial stand of weedy annual grasses developed with the bahiagrass and was controlled by mowing periodically through the first growing season. Oryzalin [4-(dipropylamino)-3,5- dinitrobenzenesulfonamide] was applied at 5 L ha-1 in March 1990 to control germinating weeds. Residue from fall and winter growth was removed by mowing in early spring each year. Identical fertilizer applications were made each year from 1990 through 1992. Three equal applications providing a total of 168 N kg ha-1, 18 kg P ha-1, and 70 kg K ha-1 were made in early spring and following the harvests made after 8 and 16 wk of regrowth. A randomized complete block of split-split plot experimental design was used with six replications. Regrowth interval (2, 4, or 8 wk) was the whole-plot treatment, within which two subplot cutting height treatments (15 or 100 mm; Table 1) and sub-subplots (1.8 by 3.4 m) of bahiagrass entries (Pensacola, Tifton 9, RRPS Cycle 14) were randomly assigned.


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  		Table 1 Cutting height and cutting interval treatments applied to Pensacola, Tifton 9, and recurrent restricted phenotypic selection (RRPS) Cycle 14 bahiagrass for three years at Tifton, GA

 
A self-propelled sickle-bar mower cut a strip 0.6 m wide from the center of each plot each time it was harvested. A cutting height (stubble height) of about 15 mm resulted from operating the mower with the sickle bar resting on the ground surface. Skids were used to elevate the sicklebar and obtain the 100-mm cutting height. Following harvests, plot edges were cut to the same stubble height as the harvested area. Harvests were made during a 24-wk growing season beginning in April (Table 1). After the final harvest, residual biomass remaining on plots harvested using a 100-mm cutting height was harvested to 15 mm. This yield was added to the seasonal total in an effort to account for the substantial growth of bahiagrass close to the soil surface, which might otherwise inflate differences attributed to cutting height.

Stand ratings were made after the first harvest in the first two seasons. A 1.0 m2 frame divided into 100 square sections (100 by 100 mm) was placed randomly within each plot and the presence or absence of bahiagrass in each section was recorded. Stand ratings were expressed as the percentage of sections containing any bahiagrass.

Nutrient reserves were evaluated by measuring initial growth in the spring using the techniques of Burton (1995) and Cuomo et al. (1998). Residual dead growth was removed by clipping all plots in February. Two galvanized steel cans (150 mm in diameter, 170 mm deep) were inverted to eliminate light and placed in each plot on 2 Mar. 1992. Cans were placed at random on either edge of the plot, avoiding the plot center where yield harvests were made. Etiolated growth was harvested to ground level on 30 March, 21 April, and 6 May. Cans were replaced after the first two harvests. Total harvested dry weight estimated nutrient reserves.

All plots were cut to ground level 4 Aug. 1993. Following 8 wk of regrowth, with three intervening harvests on 2-wk interval plots and one on 4-wk interval plots, two quadrats (0.1 m2) in each plot of three replicates were cut to ground level. Samples were laid flat and cut into segments from ground level to 76, 76 to 152, 152 to 229, 229 to 305, and >305 mm. Segments were dried and weighed so that vertical distribution of biomass within the canopy could be evaluated. A soil core (75 mm in diameter by 75 mm deep) was removed from the soil from each harvested quadrat. After drying, soil and root material were separated by mechanical agitation. Root samples from each core were then redried and weighed.

The IVDMD was determined on all whole-plant samples harvested from 1990 to 1992. Procedures followed the direct acidification technique of Moore and Mott (1974), except that samples were not ashed and values were calculated on a dry matter rather than organic matter basis.

Analysis of variance was implemented using SAS (1987). Main effects for which only two treatments were involved were evaluated by the F-test. For remaining effects, Fisher's protected LSD was used to separate means.

Grazing Experiment
Two pastures (0.81 ha) each of Pensacola, foundation Tifton 9, or RRPS Cycle 14 bahiagrass were planted 13 Apr. 1988 near Alapaha, GA (31°23' N, 83°13' W). Seed of each entry (22 kg ha-1) were planted with a cultipacker seeder onto a seedbed prepared by plowing and disking. Flatwoods soils of the experimental pastures, Leefield loamy sand (loamy, siliceous, subactive, thermic Arenic Plinthaquic Paleudults) and Alapaha loamy sand (loamy, siliceous, thermic Arenic Plinthic Paleaqdults), are typical of this area and characterized by a frequently high water table.

The summer of 1988 was extremely dry, and poor fall stands resulted following spring planting. Supply of RRPS Cycle 14 seed was very limited. In an effort to preserve existing plants and surviving dormant seed, pastures were overseeded (22 kg ha-1) with the same entries that were planted in 1988 using a no-till drill on 24 Apr. 1989. Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) was applied (0.56 kg ha-1) immediately after seeding to provide control of existing vegetation. Pastures were mowed periodically and sprayed with 2,4-D amine (2,4-dichlorophenoxyacetic acid dimethylamine) to control broadleaf weeds during the establishment year.

Pastures were burned in February 1990. Total fertilization rates were 140 kg N ha-1 split into three applications during 1990 and two applications in 1991. Annual applications of 35 P2O5 kg ha-1 and 70 kg K2O ha-1 were made each spring. Summer applications of triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid; 0.84 kg a.i. ha-1} plus 2,4-D (1.68 kg a.i. ha-1) were made in both years to control broadleaf weeds, particularly pigweed (Amaranthus spp.) and horsenettle (Solanum carolinense L.). Three yearling crossbred (Bos taurus x indicus) heifers averaging 282 kg were assigned randomly to pastures. These heifers, designated as testers, grazed respective pastures continuously during the season. Similar heifers were added and removed from pastures in an effort to maintain comparable forage availability. Available forage was estimated biweekly by cutting all forage within six randomly located quadrats (0.1 m2) per pasture to ground level. Grazing started on 17 Apr. 1990 and 15 Apr. 1991 and was discontinued on 18 Sept. 1990 and 27 Aug. 1991. The mean of two full weights of tester heifers taken on consecutive days at the beginning and end of each grazing season was used to estimate starting and ending weight.

Botanical composition of each pasture was monitored using a paced-transect method in the spring and fall of each year. An observer walked over equally spaced transects, classifying vegetative cover approximately every 1.5 m. A minimum of 300 observations were made in each pasture on each sampling date.

Heifer daily gain, grazing days, and gain per hectare were analyzed using ANOVA with a model including year and bahiagrass entry. A similar model used to analyze herbage mass included sample date. Botanical composition of pastures was examined for changes over time using multivariate analysis of variance to detect interactions between bahiagrass entry and date (Stroup and Stubbendieck, 1983).


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Clipping Experiment
Establishment and Persistence
Stands at the end of the establishment year were poor, as assessed by visual ratings made in October 1989 (Table 2) . However, some advantage (P < 0.05) for Tifton 9 and RRPS Cycle 14 compared with Pensacola was apparent. This advantage relative to Pensacola was also apparent for Tifton 9, but not RRPS Cycle 14, in plant counts obtained the following spring. Similarly, stand frequency of Tifton 9 was higher (P < 0.05) than Pensacola at the same date. By October of the first harvest year, Pensacola plots had filled in sufficiently that stand frequencies were greater (P < 0.05) than for Tifton 9 or RRPS Cycle 14. Stands were not influenced (P > 0.05) by any treatment after the first year. Cutting interval had no influence (P > 0.05) on stands throughout the experiment.


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  		Table 2 Stand rating, plant counts, and stand frequency as influenced by cutting height and entry for Pensacola, Tifton 9, and recurrent restricted phenotypic selection (RRPS) Cycle 14 bahiagrass grown for three years at Tifton, GA

 
Spring Reserves
Cumulative effects of two successive years of cutting treatments were evaluated by examining reserves available for new growth in spring of the third year (Table 3) . Reserves were greater (P < 0.05) in plots which had been cut on an 8-wk frequency than in those cut every 2 or 4 wk during the previous seasons. Cutting height and bahiagrass entry did not influence spring reserves (P > 0.05).


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  		Table 3 Spring reserves estimated as the total etiolated growth produced within a light-excluding cylinder for Pensacola, Tifton 9, and recurrent restricted phenotypic selection (RRPS) Cycle 14 bahiagrass grown for three years at Tifton, GA, and cut at two stubble heights and three regrowth intervals.{dagger}

 
Seasonal Growth Pattern
Biweekly harvest data reflected most clearly the shortterm responses of growth rate to environmental fluctuations (Fig. 1) . Growth rates tended to be highest in mid and late summer, after 156 d, in 1990 and 1992. In 1991, growth rates were highest at the four initial harvest dates, peaking on Day 156, perhaps due to abundant moisture availability (Fig. 2) , and were lower throughout the remainder of the growing season. With the exception of harvests made on Day 170 and Day 198 in 1990 and the first two harvests in 1991, when herbage accumulation of Tifton 9 and RRPS Cycle 14 was greater (P < 0.05) than Pensacola (Fig. 1), differences in 2-wk forage accumulation due to bahiagrass entry were relatively small during the 3-yr experiment. Growth rate declined in late season every year in response to declining temperature and day length.



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  		Fig. 1 Yield of three bahiagrass entries harvested biweekly during three growing seasons at Tifton, GA. Stars and solid line, Pensacola; diamonds and long dashes, Tifton 9; circles and short dashes, RRPS (recurrent restricted phenotypic selection) Cycle 14

 


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  		Fig. 2 Monthly cumulative departure from normal precipitation during three growing seasons at Tifton, GA

 
Total Dry Matter Yield
All three-way interactions involving year were significant (P < 0.05) sources of variation for total yield. Interaction of cutting interval and cutting height with year resulted from differences in the magnitude of the advantage of low as compared with high cutting height (Fig. 3) . With the exception of the 2-wk cutting interval treatment in 1991, higher yields resulted from low cutting height.



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  		Fig. 3 Interaction among year, cutting interval, and cutting height on mean total dry matter yield of three bahiagrass entries harvested during three growing seasons at Tifton, GA

 
As the interval between cuttings increased from 2 to 8 wk, total yield tended to increase in 1990 and 1992 (Fig. 4) , although the pattern of increase varied with entry. The response in 1991 was different and resulted in year x cutting interval x entry interaction (P < 0.01). In 1991, an atypically wet growing season, total yields were lower for the 8-wk cutting interval than for 2- or 4-wk intervals.



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  		Fig. 4 Interaction among year, regrowth interval, and bahiagrass entry on total dry matter yield averaged across clipping heights during three growing seasons at Tifton, GA. RRPS, recurrent restricted phenotypic selection

 
The magnitude of the advantage for low cutting height varied according to year and entry (Fig. 5) , as evidenced by the year x cutting heightx entry interaction (P < 0.05). However, total yield was generally enhanced by cutting at the lower height.



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  		Fig. 5 Interaction among year, cutting height, and bahiagrass entry on total dry matter yield averaged across regrowth interval during three growing seasons at Tifton, GA. RRPS, recurrent restricted phenotypic selection

 
In Vitro Dry Matter Disappearance
Laboratory estimates of forage digestibility ranged from more than 600 g kg-1 early in the season to 400 g kg-1 or less by the end of the growing season (data not shown). Other than sampling date (P < 0.05), no effects of entry, cutting interval, or height were identified. Regression of IVDMD on day of the year resulted in linear prediction equations that were similar each year. Predicted decline in IVDMD ranged from 0.40 g kg-1 d-1 in 1990 to 0.65 g kg-1 d-1 in 1991; predicted decline in 1992 was intermediate.

Dry Matter Distribution among Canopy Strata
Canopy structure of the three bahiagrass entries after 3 yr of treatments was examined by dividing accumulated biomass among vertical strata following 2, 4, or 8 wk of regrowth. As cutting interval was delayed from 2 to 8 wk, dry matter increased in all strata for each entry (Fig. 6) and the accumulation was greatest in the lower strata. Differences among entries were evident after 8 wk of growth, but were minimal after 2 or 4 wk of accumulation. After 8 wk, RRPS Cycle 14 had 1.72 g less DM in the lowest stratum, but 2.13 g more DM in the highest stratum than Pensacola. Distribution of DM among strata of Tifton 9 followed a pattern intermediate to Pensacola or RRPS Cycle 14. Cutting interval influenced (P < 0.01) the quantity of DM contained in the three middle strata, but DM content of the bottom and top strata was not influenced by cutting interval (P > 0.05).



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  		Fig. 6 Influence of regrowth interval (2, 4, or 8 wk) on distribution of dry matter among canopy strata of three bahiagrass entries harvested for 3 yr and averaged across two cutting heights at Tifton, GA. RRPS, recurrent restricted phenotypic selection

 
Cutting height influenced the DM content of each of the strata except the highest (P < 0.05; Fig. 7) . It is not surprising that greater accumulation would occur in strata below the height of cut of the higher cutting treatment. Greater residual leaf area remaining after harvest with a higher stubble might promote greater accumulation throughout the canopy. This is in contrast to the consistent observation of higher yields resulting from lower cutting height.



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  		Fig. 7 Influence of cutting height (low = 15 mm; high = 100 mm) on distribution of dry matter among canopy strata of three bahiagrass entries harvested for 3 yr and averaged across three regrowth intervals at Tifton, GA

 
Root Mass
Pensacola produced a greater root mass (P < 0.05) than Tifton 9 (Table 4) . Neither cutting interval nor cutting height influenced the quantity of root harvested to a depth of 75 mm (P > 0.05); average root mass was 27 g.


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  		Table 4 Mass of roots removed from soil core (75 by 75 mm) taken from Pensacola, Tifton 9, or recurrent restricted phenotypic selection (RRPS) Cycle 14 bahiagrass grown at Tifton, GA

 
Grazing Experiment
Herbage Mass
Herbage mass varied with sampling date. Average estimates for individual dates ranged from 990 to 3060 kg ha-1 (Fig. 8) . In spite of efforts to maintain equal forage availability, mean dry matter available for grazing was highest (P < 0.01) for RRPS Cycle 14 and lowest for Pensacola.



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  		Fig. 8 Herbage mass of standing forage in pastures planted with Pensacola, Tifton 9 or RRPS (recurrent restricted phenotypic selection) Cycle 14 bahiagrass and continuously grazed by yearling heifers during two growing seasons near Alapaha, GA

 
Persistence
Persistence was evaluated from changes in estimates of botanical composition made on three occasions (Fig. 9) . Bahiagrass cover was nearly 70% in April 1990; the major remaining component was bare ground. By the end of the first year of grazing, RRPS Cycle 14 stands were less than 50% bahiagrass. Although good stands of bahiagrass were present for Pensacola and Tifton 9 in the spring of 1991, considerable contamination of common bermudagrass [Cynodon dactylon (L.) Pers.] had developed. Following the second year of continuous grazing (October 1991), persistence of Tifton 9 was comparable to Pensacola.



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  		Fig. 9 Botanical composition (foliar cover) of pastures planted with Pensacola, Tifton 9 or RRPS Cycle 14 bahiagrass and continuously grazed by yearling heifers during the 1990 and 1991 growing seasons near Alapaha, GA

 
Animal Production
Main effects of year, but no interactions, were detected for the grazing trial. Although heifer daily gains were higher (P < 0.01) in 1990 (0.55 kg d-1) than in 1991 (0.12 kg d-1), daily gains of individual heifers were not influenced by bahiagrass entry (Table 5) . Carrying capacity was lower for all entries in 1990 (835 d ha-1) than in 1991 (955 d ha-1), but Tifton 9 provided more grazing days both years than Pensacola (P < 0.14; Table 5), while RRPS Cycle 14 (928 d ha-1) tended to exceed (P < 0.21) Pensacola (742 d ha-1) only during the first year. No difference in animal gain per hectare could be attributed to bahiagrass entry and averaged 287 kg ha-1.


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  		Table 5 Mean performance of heifers grazing three bahiagrass entries for two years

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Establishment
Stand development of both plots and pastures was slow, as is typical of bahiagrass. Fall stand ratings the year of planting and subsequent spring plant counts were higher for Tifton 9 than the other two entries and may reflect greater seedling vigor reported for Tifton 9 (Burton, 1989). Before clipping treatments or grazing were initiated, comparable stands were established for each of the entries (Table 2; Fig. 9). One full growing season was required to establish plots; two seasons were required for pastures. Gates and Burton (1998) demonstrated better early germination of RRPS Cycle 18 (advanced four cycles beyond RRPS Cycle 14) compared with Tifton 9 or Pensacola. However, this advantage was not maintained 35 d after planting, and seed treatments that increase early germination have not been effective in improving the rate of stand establishment of Tifton 9 (Gates and Mullahey, 1997; Gates and Dewald, 1998).

Forage Production
Except for rapid early growth in 1991, the most forage was produced by all three bahiagrass entries during midseason. Forage production that is concentrated in midseason has been documented previously with Pensacola bahiagrass (Beaty et al., 1960) and is related to both leaf and tiller production (Beaty et al., 1977). Incremental differences in forage yield among entries were generally small and primarily nonsignificant (P > 0.05; Fig. 1). Total yield, however, was higher for Tifton 9 and RRPS Cycle 14 than for Pensacola (Fig. 4 and 5). This increased growth potential accounted for greater average herbage mass (Fig. 8) and carrying capacity (Table 5) of Tifton 9 and RRPS Cycle 14 pastures compared with Pensacola.

Higher yield of populations selected from Pensacola has been demonstrated in several environments under varying harvesting schemes (Mislevy et al., 1991; Pedreira and Brown, 1996b; Burton et al., 1997). In contrast, in two comparisons including Pensacola and Tifton 9, no difference in total yield was detected (Cuomo et al., 1996; Morrison et al., 1994).

Increasing regrowth interval from 2 to 8 wk increased total yield for each entry (Fig. 5) at both cutting heights. Surprisingly, decreasing the harvest frequency from 4 to 8 wk depressed total yield in 1991. Rapid initial growth, resulting from abundant moisture in 1991, may have led to subsequent DM losses due to canopy respiration and senescence that exceeded new growth between 4 and 8 wk after harvest. Stanley (1994) recorded a depression in total yield of Tifton 9 when harvest interval increased from 8 to 16 wk. The rank of populations for total yield was reversed for the 8-wk cutting frequency in 1991, with Pensacola producing more forage than the other populations. Other research indicates that bahiagrass yield increases as intervals between harvests become longer (Beaty et al., 1963; Adjei et al., 1989; Cuomo et al., 1996). Beaty et al. (1970) reported very little variation in forage yield of Pensacola bahiagrass due to clipping frequencies varying from 1 to 6 wk. In that study, harvest was made to soil level, capturing substantial biomass that might escape harvest even with a short stubble height.

Regardless of cutting interval (Fig. 3) or bahiagrass entry (Fig. 4), total yields generally increased when plots were harvested to 15-mm rather than 100-mm stubble. This was similar to the cutting height response reported by Pedreira and Brown (1996b) with the same entries. With all three entries we found that, regardless of cutting interval, more than half of the forage was below 152 mm (Fig. 6). Beaty et al. (1968) found that less than 16% of Pensacola forage was produced above 130 mm; more than half was produced below 51 mm.

We speculated that morphological changes accompanying 14 cycles of RRPS selection might modify the response to frequency or intensity of defoliation. Selection has modified individual plants so that they are taller, but with fewer stolons and a reduced diameter (Werner and Burton, 1991; Pedreira and Brown, 1996a, 1996b). Corresponding changes were evident in the vertical distribution of biomass when those same populations were planted as swards (Fig. 6). Swards of each population, including RRPS Cycle 14, maintained a large proportion of total biomass in the portion of the canopy closest to the soil surface. Beaty et al. (1974) recognized the capacity of Pensacola bahiagrass to withstand extreme environmental conditions and close, frequent defoliation. The partitioning of large amounts of photosynthate to stolons at the soil surface and the capacity to produce new leaves very rapidly (using a minimum of stored carbon) were identified as factors contributing to the hardiness of Pensacola bahiagrass. Apparently, enough of these characteristics have been retained in selected populations that, compared with Pensacola, no dramatic changes in their response to biweekly mechanical harvest have occurred. In contrast, persistence of RRPS Cycle 14 was reduced when exposed to continuous stocking where daily defoliation is possible.

In this study, we found a decline in the nutritional value of these bahiagrass populations throughout the growing season. This is consistent with previous evaluations of bahiagrass (Cuomo et al., 1996; Stanley et al., 1977). Cutting height, frequency, and bahiagrass entry did not influence IVDMD. Stanley et al. (1977) reported that varying cutting height did not influence cell wall concentration of the resulting harvest and concluded that cell wall content of lower portions of the canopy was similar to upper components. Such a forage quality response for bahiagrass is in contrast to cool-season grasses and bermudagrass for which forage quality consistently declines with increasing regrowth interval (Ball et al., 1996).

Cuomo et al. (1996) also found that harvest frequency delayed from 20 to 40 d had little impact on fiber content or digestibility; time of harvest (early or late in the season) had much greater influence. Montgomery et al. (1972) also reported no decline in nutritive value when regrowth interval increased from 4 to 10 wk until late in the season. Varying harvest interval from 1 to 6 wk did not change the proportion of leaf and stem in harvested forage (Beaty et al., 1963). The propensity of bahiagrass to initiate new tillers continuously during the growing season and the increase in rate of tillering from spring to midseason (Beaty et al., 1977) tends to compensate for aging tillers and minimizes the effects of cutting height or frequency on nutritive value. Other research has detected either very small (Mislevy et al., 1991; Cuomo et al., 1996) or no differences (Gates and Burton, 1990) in digestibility among bahiagrass cultivars.

Previous grazing comparisons have not documented any clear superiority of other bahiagrass cultivars to Pensacola in supporting animal production (Chambliss and Jones, 1980). No differences in daily gains of heifers were detected in our 2-yr experiment (Table 5), providing further evidence that these bahiagrass populations produce forage of comparable nutritive value. Tifton 9 provided greater carrying capacity than Pensacola, reflecting the additional yield. Although the 2-yr average daily gains (0.34 kg d-1) of heifers observed in our experiment were comparable to the 0.4 kg d-1 reported for steers grazing Pensacola (Stephens and Marchant, 1960; Chapman et al., 1972; Utley et al., 1974), this average was lowered due to poor gains in 1991. High rainfall and wet conditions in 1991 probably contributed to poorer performance. Leaching of soil nutrients, particularly N, would have reduced forage protein content, which may have limited intake. Additionally, standing water and low forage dry matter concentration have also been reported to reduce intake (Marsh, 1975; Butris and Phillips, 1987; John and Ulyatt, 1987). The progressive decline in growth rates observed with clipped plots in 1991 is atypical, as midseason production is generally greater than spring or fall production.

Persistence
Uniformly excellent persistence of all bahiagrass entries in the clipping trial (Table 2) indicate that reserves and residual leaf area were sufficient to maintain stands, even with the most frequent and intense defoliation. Some indication of depletion of reserves available for growth was evident in the reduction of etiolated spring growth of plots harvested biweekly for 2 yr (Table 3) in comparison with 4- or 8-wk harvest intervals. Cutting height and bahiagrass entry had no effect on reserves. These results are similar to those reported by Beaty et al. (1970), who found that weekly cutting of Pensacola to soil level over a 2-yr period had no effect on yield. Stolon and root weights were reduced, however, and Beaty et al. suggested that continuation of such defoliation might eventually reduce yields.

In contrast to excellent stand maintenance observed in all plots harvested mechanically, persistence of pastures grazed continuously was influenced by bahiagrass entry. Open areas in original stands were invaded by common bermudagrass (Fig. 9). Rapid encroachment of common bermudagrass and other weedy species in this experiment resulted in part from slow stand establishment and replanting. During 1988, although bahiagrass did not become well established because of extended dry soil conditions, weedy species expanded and produced seed and vegetative propagules. Paraquat treatment at planting and spot spraying with glyphosate [N-(phosphonomethyl)glycine] was not adequate to control weeds. Thus, aggressive weed competition prevented establishment of a complete bahiagrass sod in this grazing study.

While the persistence of Pensacola bahiagrass is well documented in many experiments, two grazing seasons is insufficient time to verify the persistence of new germplasm. However, it is clear that RRPS Cycle 14 cannot be expected to persist under conditions of continuous close grazing (Fig. 9). Rapid stand loss of RRPS Cycle 14 suggests that RRPS Cycle 14 will require management other than close continuous grazing in order to maintain productive stands. Recent observation of more advanced selection cycles, RRPS Cycle 18 and RRPS Cycle 23, revealed substantial stand loss following a single season of close continuous grazing (R.N. Gates, unpublished data). In contrast, Tifton 9 stands were comparable to Pensacola. Repeated demonstration of the increased yield potential of Tifton 9 when compared with Pensacola and the higher carrying capacity observed in this research indicate that Tifton 9 can be expected to support higher stocking rates than Pensacola. However, care should be exercised in stocking Tifton 9 continuously until its long-term persistence is established.SAS Institute 1987


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Joint contribution of the USDA-ARS and the Univ. of Georgia Coastal Plain Exp. Stn.

Received for publication September 9, 1998.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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