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a Agron. Dep., Univ. of Florida, P.O. Box 110300, Gainesville, FL 32611-0300
b 5920 W. 53rd St., Stillwater, OK 74074
* Corresponding author (les{at}gnv.ifas.ufl.edu)
Received for publication January 5, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: DM, dry matter • TNC, total nonstructural carbohydrate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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While attempting to identify higher-quality forage germplasm, researchers selected Mott dwarf elephantgrass, a tropical forage that was tested and released in Florida (Sollenberger et al., 1989). In a 3-yr study, steers (Bos taurus) grazing Mott pastures gained an average of 0.97 kg d-1 compared with 0.38 kg d-1 for those grazing bahiagrass pastures, with no summer slump in gain for the animals grazing Mott (Sollenberger and Jones, 1989). During late July to late September or early October, these authors reported that gain with bahiagrass was 0.22 kg d-1 but was 1.09 kg d-1 with Mott. Yield of Mott was greater than or comparable to that of other tropical forages presently utilized in grazing systems in Florida (Kalmbacher et al., 1987). Mott has demonstrated excellent persistence, having survived for more than 10 yr under grazing near Gainesville, FL (Sollenberger et al., 1988), and nondefoliated plants have survived at Tifton, GA, for more than 10 yr (Hanna and Monson, 1988). Despite its outstanding forage quality, production, and persistence, costly and labor-intensive establishment has limited use of Mott by producers (Sollenberger et al., 1988).
Genetic improvement of Pennisetum spp. forages in Florida, involving the interspecific hybridization of pearl millet with elephantgrass, has produced a number of dwarf- to intermediate-height genotypes that demonstrate similar leafiness and nutritive value to Mott but have the potential for higher herbage production. These hybrids are reportedly easier to establish than Mott (Schank et al., 1989). Similar hybridization with elephantgrass and pearl millet was ongoing in Dr. Wayne Hanna's program at Tifton, GA. Hybrids from both programs have been evaluated for their forage production potential and persistence (e.g., Hanna and Monson, 1980; Schank et al., 1989; Sotomayor-Rios et al., 1989; Williams, 1990). These studies have established that the hybrids are productive and have high forage quality. In the Williams (1990) study, the hybrids that were evaluated did not perennate. Sotomayor-Rios et al. (1989) concluded that the perennial nature of the interspecific hybrids made them potentially valuable grasses for the tropics, but they did not report any persistence data. Hanna and Monson (1980) indicated that the hybrids are perennial where minimum temperature is above 0°C, but their study was replanted every year, so no data on persistence were reported. Subsequent studies reported lack of persistence (Adjei et al., 1994; Spitaleri et al., 1994) or failure of hybrids to perennate (Cuomo et al., 1996). For any of the hybrids to successfully become an alternative to Mott as a high quality tropical grass suitable for permanent pastures, their cold tolerance and persistence under defoliation need to be determined.
Three hybrids were chosen for evaluation from a population that originated from a series of pearl millet x elephantgrass crosses involving the cytoplasmic male-sterile inbred ‘Tift 23DA’ (dwarf) pearl millet and Mott dwarf elephantgrass. Hybrids S4 and S41 are sexually sterile F1 triploids 
. Hybrid S360 is a hexaploid 
created by chromosome doubling of the triploid to restore fertility. Planting materials for these hybrids came from the breeding program of Dr. Stan Schank (now deceased) at the University of Florida. The objectives were to quantify herbage production and persistence of Mott elephantgrass and the three hybrids harvested at two defoliation frequencies (6- and 12-wk intervals) and two stubble heights (20 and 40 cm) and to examine the relationships among persistence, tillering ability of the plants, and total nonstructural carbohydrate (TNC) concentration in storage organs.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Treatments were all combinations of four genotypes (Mott, S4, S41, and S360), two defoliation frequencies (6 and 12 wk), and two stubble heights (20 and 40 cm). Previous studies with Mott (Sollenberger et al., 1988) and with other pearl millet–elephantgrass interspecific hybrids (Hanna and Monson, 1980) suggest that optimal forage production can be obtained when plants are harvested at approximately 6-wk intervals (or up to four times per season) to a 20-cm stubble height. The longer defoliation interval and higher stubble height treatments were selected to represent more lenient defoliation management. Treatments were replicated three times in a randomized block design.
Plots were three rows, each 4-m long with 1 m between rows. There were 3-m spaces between plots and blocks. Plots were planted using stem cuttings in November 1988. Shoots emerged in 1989, and any open areas were filled that summer. The target plant spacing within rows was 50 cm (or nine plants per row). Plants were not defoliated except to remove dead, frosted material during the 1989–1990 winter. The plants were allowed to grow unharvested during this establishment year because of the need to replant areas where there was no shoot emergence from the previous fall planting (considered problematic with Mott; Sollenberger et al., 1990). Thus, plants newly emerged after the summer planting needed to become established before defoliation could be imposed.
Fertilizer N was broadcast-applied as ammonium nitrate (NH4NO3). In1989, N was applied at the rate of 120 kg ha-1 yr-1 in three equal applications beginning in April and subsequently at 8-wk intervals. During 1990 and 1991, N was applied at the rate of 160 kg ha-1 yr-1 in three applications (40 kg ha-1 in April and 60 kg ha-1 after staging and 12 wk later). Phosphorus and K were applied at 18 and 66 kg ha-1 in April each year.
Plots were cut to a uniform 20-cm height on 16 May 1990 to ensure all treatments started at a common stage. Plots were subsequently harvested every 6 or 12 wk to treatment stubble height. The center row of each plot, minus 50-cm borders at both ends of the row (i.e., a 3- by 1-m area), was clipped to determine yield. Fresh weight was quantified, and a subsample of about 1000 g was dried in a forced-air oven at 60°C until constant weight was achieved for dry matter (DM) determination. A second subsample was taken from each plot and hand-separated into leaf blade, leaf sheath, and stem fractions (subsequently used to determine their nutritive value for a companion study). Fractions were dried at 60°C to determine their proportion in the harvested material, which enabled calculation of leaf-blade DM yield.
The first two 6-wk harvests and the first 12-wk harvest are described as early season and the third and fourth 6-wk harvests and the second 12-wk harvest as late season. In 1991, harvesting of plots was discontinued after the first 12 wk due to loss of plant stands in the plots of the interspecific hybrids.
Samples for analysis of rhizome TNC and N concentration were taken from the outer rows. The first samples for the 6- and 12-wk treatments were taken 1 d before the first harvest. A second sample was taken the day before the final harvest of the year. No samples were taken in 1991 because there were so few plants remaining in some hybrid plots that destructive sampling might jeopardize yield measurement. One-half of the bunch (crown area) from each of two plants from each plot and the associated rhizomes were harvested, washed, and subsampled. Rhizomes were cut into pieces and heated at 100°C for 1 h to stop respiration. Subsequently, they were dried at 60°C until constant weight was achieved and ground in a Wiley mill to pass a 1-mm screen. Rhizome TNC concentration was determined using techniques described by Chaparro et al. (1996). The procedure uses amyloglucosidase and invertase to degrade carbohydrates to reducing sugars and measures the resulting monomers spectrophotometrically. Samples for N analysis were digested using a modified aluminum block digestion technique (Gallaher et al., 1975). Ammonia in the digestate was determined using semiautomated colorimetry (Hambleton, 1977).
Plant persistence was quantified by counting every plant in all plots at the beginning and end of each growing season in 1990 and 1991 and at the beginning of the 1992 growing season. Survival is reported as the percentage of plants surviving from one spring to the next. Four plants in the center row of each plot were permanently identified and used for tiller counts in 1990. Because of plant stand loss, all remaining plants in hybrid plots were used for tiller counts during 1991. Tiller counts were taken at 12-wk intervals from the staging cut through the end of the growing season.
Data were analyzed by fitting mixed-effects models (Littell et al., 1996) using the PROC MIXED procedure in SAS (SAS Inst., 1992). For total and leaf-blade DM yield data, season (early and late 1990 and early 1991) was considered as a repeated measure in time. For the tiller count data, both year and season were considered repeated measures in time. Year was the repeated measure for the plant survival data. Subject for all statistical models was each replication x genotype x defoliation frequency x stubble height combination. Treatment differences were separated by pairwise comparisons using probability of difference (PDIFF of SAS; SAS Inst., 1992) for sources of variation determined to be significant (P < 0.05) in the model. When interactions occurred, comparisons between treatments were made at fixed levels within a factor to avoid inflating the risk of a type I error. Means are reported as different at the P < 0.05 level unless otherwise indicated.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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To quantify the distribution of forage production during the growing season, comparisons were made between the early and late-season data of 1990. Because data were not obtained for the late season in 1991, comparisons between 1990 and 1991 were limited to the early season data. Total herbage DM yield was not different between early and late-season 1990 for Mott at any defoliation frequency (Table 1). Yields of S4 and S41 also did not change with season when harvested at the 6-wk defoliation frequency, but at the 12-wk defoliation frequency, S4 had increased yield and S41 a trend for increased yield in the late season. Yield of S360 increased in the late season regardless of defoliation frequency. The yield increases observed in the late season for the interspecific hybrids were associated with increased stem percentage, resulting from stem elongation when plants entered the reproductive phase. This stem elongation phenomena was not observed in Mott. Over the whole season, Mott ranged from 2 to 6% stem and had less (P < 0.05) stem than the interspecific hybrids (7–29%), with lower numbers occurring at the 6-wk defoliation frequency. Lower leaf percentage was observed during the late season than during the early season for the interspecific hybrids (Macoon et al., 2001). Williams (1990) observed that pearl millet x elephantgrass hybrids tended to accumulate a higher percentage of stem than Mott, especially when they were not defoliated frequently.
Early season DM yield of Mott was greater in 1991 than in 1990, but yields of the interspecific hybrids, with the exception of S41, were generally lower in 1991 regardless of defoliation frequency. Yields of S41 were not different between years at the 6-wk defoliation frequency. The general reduction in yields of the interspecific hybrids in 1991 was a result of poor plant persistence. Early season DM yield of Mott in 1991 was 83% of its total annual yield in 1990. The greater yield of Mott in 1991 was likely due to moisture availability. Annual rainfall in 1990 was 70% of the 70-yr average (1388 mm) while in 1991, it was 90%. In a study conducted in an adjacent field, Chaparro et al. (1995) reported increased yield of Mott from 1990 to 1991, which they attributed to annual rainfall. Other researchers have associated variation in yield of Pennisetum spp. with variation in moisture availability (Hanna and Monson, 1980; Williams, 1990).
The season x genotype x stubble height interaction occurred because stubble height effects were not the same for all genotypes during each season (Table 2). In early season 1990, total herbage DM yield of all genotypes was greater at the 20- than at the 40-cm stubble height, but in late-season 1990, there was no effect of stubble height. In late-season 1990, there was a trend (P = 0.10) for greater DM yield of Mott at the 20- than at the 40-cm stubble height. In early season 1991, yield of Mott was greater at the 20- than at the 40-cm stubble height, but yield of S360 was greater at the 40- than at the 20-cm stubble height while yields of S4 and S41 were not different between stubble heights. The early season 1990 response occurred because at the 40-cm stubble height, compared with the 20-cm stubble height, a smaller proportion of the plant canopy was harvested. Lack of stubble height effects in the late season may be explained in part by increased plant vigor observed in plants defoliated at the less intense (taller) defoliation height. This effect was not as pronounced in Mott likely because it has a compact, dense canopy, resulting in self shading of leaves; thus, it may not benefit from lenient defoliation management because growth rates may plateau earlier compared with the interspecific hybrids. The same reason would explain why early and late-season DM yields of the interspecific hybrids in 1990 were not different when defoliated at the 20-cm stubble height, but late-season yield was greater than early season yield at the 40-cm stubble height. In early season 1991, yield responses of Mott to stubble height treatment were similar to early season 1990.
Considering differences among genotypes at fixed levels of stubble height in each season, the highlight of these results indicated that, despite the interactions, S41 yield was as high as or higher than other genotypes regardless of defoliation treatment when plant stands were not depleted. Also, yields of the interspecific hybrids generally were similar to or less than yield of Mott.
Mott yield generally was lower than the range of 10.9 to 15.2 Mg ha-1 reported from other studies in Florida (Kalmbacher et al., 1987; Knettle et al., 1991; Woodard and Prine, 1991) likely because rainfall in 1990 was 70% of average. When rainfall was closer to average in 1991, early season DM yield was 83% of that reported for all of 1990. In a previous study, yields of S41, S4, and S360 were superior to those of Mott, and S41 had highest yields (24.8 Mg ha-1) of 20 lines tested when harvested once at the end of the growing season in December (Schank et al., 1989). Plants in the Schank et al. (1989) study were utilized for characterization of genetic traits, and ideal field conditions were maintained by irrigation and fertilizing at regular intervals, which may, in addition to a single harvest, explain the high yields obtained. Hybrid S41 also yielded as much or more than other genotypes in the first year of the current study. Greater yields of hybrids compared with Mott were attributed to accumulation of stem by the hybrids (Schank et al., 1989).
These results suggest that Mott yield was less likely to benefit from lenient defoliation management than the hybrid yields. Higher yields of Mott at lower stubble are consistent with reports of Knettle et al. (1991) and Chaparro et al. (1995). Mott has shorter internodes and is lower growing than the hybrids (Schank et al., 1989); thus, a large amount of herbage and leaf area are not harvested when it is cut to 40 cm. As a result, new growth occurs rapidly because of available leaf area and growing points (Richards, 1993), and it is likely that relatively early growth rates plateau and begin to decrease due to self shading. In contrast, the hybrids have longer internodes and are taller growing than Mott (Schank et al., 1989), so most leaf lamina is removed with a 12-wk harvest, even to 40 cm. Ability of tall Pennisetum spp. to elongate stems and distribute leaf lamina throughout a greater vertical distance allows for prolonged periods of high growth rates (Woodard et al., 1993).
Leaf Blade
Leaf-blade DM yields were affected by a season x genotype x defoliation frequency interaction (P < 0.001; Table 3) and a season x genotype x stubble height interaction (P = 0.015; Table 4). In early season 1990, Mott and S4 had greater leaf-blade DM yield at the 6- than at the 12-wk defoliation frequency while there was no difference between defoliation frequencies for S360 and S41. In late-season 1990, there was no defoliation frequency effect on leaf-blade yield of any of the genotypes (Table 3). In early season 1991, leaf-blade DM of Mott tended to be less when harvested at the 12- compared with the 6-wk defoliation frequency; the interspecific hybrids S360 and S4 were not, but S41 was affected by defoliation frequency (Table 3). At the 6-wk defoliation frequency, leaf-blade yields of Mott, S4, and S41 were greater in early season than late-season 1990, but leaf-blade yield of S360 did not change. At the 12-wk defoliation frequency, on the other hand, leaf-blade yields of interspecific hybrids were greater during early season compared with late-season 1990, but leaf-blade yield of Mott was not different between seasons. These results reflect the stem elongation phenomena and increase in stem weight in the total harvest of the interspecific hybrids compared with little change in proportion of leaf blade in harvested material of Mott as the season progressed (Macoon et al., 2001). Leaf-blade yields of interspecific hybrids actually decreased when total herbage yields increased in the latter half of the growing season, more so at the longer defoliation interval, further indicating the role of proportion of leaf blade in the total harvested material. At fixed levels of defoliation frequency within season, patterns of differences in leaf-blade yield among genotypes were similar to those of total herbage yield in the early season 1990. In the late season, on the other hand, leaf-blade yield of Mott was greater compared with S360 and S4 while S41 was intermediate. When compared with the results for total herbage yield (Table 1), these data provide further evidence to demonstrate the role of stem elongation during flowering in determining leaf proportion of the total harvested material of the interspecific hybrids. Differences in leaf-blade yield among genotypes and defoliation frequency in early season 1991 followed a similar pattern as total herbage yield.
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Plant Persistence
Percent Survival
The percentage of plants that survived from one growing season to the next was influenced by a year x genotype x defoliation frequency interaction (P = 0.002) and a year x stubble height interaction (P = 0.026). Mott had the best survival of all of the genotypes tested, with all Mott plants surviving in both years regardless of defoliation treatment (Table 5). When percent survival was estimated at the beginning of the growing season in 1991, S41 had better survival when plants were defoliated every 6 wk than every 12 wk, but the other genotypes were not affected by defoliation frequency (Table 5). Among the interspecific hybrids, S41 had the best survival, followed by S360, and S4 had the least when plants were defoliated every 6 wk, but there were no differences among interspecific hybrids when plants were defoliated at 12-wk intervals.
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Omitting results for Mott, percent survival of the interspecific hybrids at the beginning of 1991 was greater for plants defoliated at a 40-cm stubble height (25) than for those clipped at 20 cm (12). Field observations indicated that very little leaf area remained for interspecific hybrids after plots were harvested at the 20-cm stubble height, a consequence of greater stem elongation of the interspecific hybrids compared with Mott. This likely resulted in mobilization of assimilate supplies for regrowth (Richards, 1993), thereby causing reduction of stored reserves for overwintering. Percent survival estimated in spring of 1992 cannot be related to defoliation management, given the total survival of Mott and complete loss of the interspecific hybrids.
Lack of persistence under defoliation by other pearl millet x elephantgrass hybrids and some tall elephantgrass genotypes has been reported in other studies conducted in Florida (Calhoun and Prine, 1985; Schank et al., 1991; Woodard and Prine, 1991) and in subsequent studies elsewhere (Adjei et al., 1994; Cuomo et al., 1996). Schank et al. (1991) suggested that poor persistence of pearl millet x elephantgrass hybrids is due in large part to pearl millet being an annual. Evidently, there is weak perennality in the F1 hybrids of the annual x perennial cross. At the initiation of the current study, this evidence had not been established, and earlier reports (e.g., Hanna and Monson, 1980; Sotomayor-Rios et al., 1989) had alluded to the potential value of interspecific hybrids based on their expected ability to perennate.
Tillering Responses
Number of tillers per plant was influenced by a year x season x genotype interaction (P = 0.035; Table 6). In 1990, tillers per plant of Mott increased, but for the interspecific hybrids, there was no change from the early to the late season. Mott had greater number of tillers per plant than all of the interspecific hybrids at the end of the growing season. In 1991, number of tillers per plant of Mott was practically the same as for 1990. Tiller data of the interspecific hybrids from 1991 are difficult to explain because large differences in plant survival among genotypes and defoliation treatments led to inconsistent stand density and degree of interplant competition. Additionally, loss of entire plant stands in some plots led to a large number of missing data, which further led to questionable value of these data. The results indicate generally, however, that the interspecific hybrids produced more tillers per plant in 1991 than in 1990. Also, the hybrids produced more tillers in late season than in early season, unlike the trend observed for 1990. The vigorous tiller production of the interspecific hybrids in 1991 likely occurred because of a decrease in interplant competition resulting from reduced stand density.
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Defoliation management also influenced tiller production, but these responses could not consistently explain subsequent plant persistence, so these results are not presented in detail. There was a year x genotype x defoliation frequency interaction (P = 0.009) and a year x defoliation x stubble height interaction (P = 0.027) on number of tillers per plant. Generally, in 1990, there was no effect of defoliation frequency among genotypes, but in 1991, S360 defoliated every 12 wk produced a greater number of tillers per plant than when defoliated every 6 wk, S4 produced more tillers when defoliated every 6 wk compared with every 12 wk, and Mott and S41 were not affected by defoliation frequency. During 1990, there was also no defoliation frequency effect at either level of stubble height. At the 6-wk defoliation frequency, there was no difference in number of tillers between stubble heights, but at the 12-wk defoliation frequency, plants harvested at 20 cm produced about 38% more tillers than those at 40 cm. In 1991, there was no effect of defoliation frequency at the 20-cm stubble height, but at 40 cm, plants harvested every 6 wk produced 25% more tillers per plant than those harvested every 12 wk. Also, plants harvested every 6 wk produced more tillers at the 40- than at the 20-cm stubble height while the reverse occurred when plants were harvested every 12 wk.
The 1991 responses were likely affected by inconsistent plant density among treatments, so they are difficult to explain. The highlight of the 1990 responses was that all genotypes appeared to produce the least tillers when defoliated at the most lenient defoliation treatment (harvested at a 40-cm stubble height and 12-wk regrowth interval). This is consistent with reports that grasses tend to increase tiller production as individual plants adapt to maintain an equilibrium between assimilate resource allocation patterns and resource availability in condition where defoliation pressure is increased (Chapman and Lemaire, 1993). The lack of a consistent relationship between tiller response and plant survival suggests that other mechanisms, such as storage of organic reserves, should be evaluated because this is closely associated with plant responses to defoliation and subsequent regrowth (Richards, 1993).
Carbon and Nitrogen Reserves
There was a genotype effect (P < 0.001) on rhizome TNC at season-end 1990 while rhizome N concentration was influenced by a genotype x defoliation frequency interaction (P < 0.010). The highest TNC concentration was in S360 (243 g kg-1) while the other genotypes were similar to each other (201, 190, and 188 g kg-1 for Mott, S41, and S4, respectively). Rhizome N concentration was similar among all genotypes within a defoliation frequency, averaging 18.3 and 9.5 g kg-1 for 6 and 12 wk, respectively. Genotype x defoliation frequency interaction may have occurred because N concentration was greater at 6 wk than at 12 wk for all genotypes except S4.
Concentration of TNC in rhizomes at season end was not closely related to subsequent plant survival. Concentrations were greatest in S360, which demonstrated the poorest plant persistence after the 1990–1991 overwintering period. Mott achieved 100% plant survival with TNC concentrations similar to S4 and S41, which had <50% survival after the first year of defoliation. There is evidence to show that storage organ mass is more closely related to plant persistence than is organ TNC or N concentration in Pennisetum spp. (Spitaleri et al., 1994; Chaparro et al., 1996). In the current study, defoliation frequency and genotype may have influenced the growth and spread of rhizomes more than they affected reserve storage concentrations in tissues. Although not quantified, field observations suggest that rhizome mass and crown diameter of interspecific hybrids were less than those of Mott.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Despite their yield potential, winter survival of these hybrids was <50% after 1 yr of multiple defoliations, and no plants survived the second winter. This poor persistence is in contrast to Mott, which had 100% plant survival throughout the experiment. At the end of the first year of defoliation, Mott had nearly twice as many tillers per plant as the hybrids, but rhizome TNC and N concentration were not related to subsequent persistence. Data from other studies and observations from this study suggest that rhizome mass is more closely related to persistence of Pennisetum spp. genotypes than is TNC concentration. Evidence reported subsequent to this study suggests that there is weak perennality in the F1 hybrids of the annual x perennial cross (Schank et al., 1991). Additionally, lack of survival of pearl millet x elephantgrass hybrids was subsequently reported (Cuomo et al., 1996), even in areas where cold temperatures do not inhibit plant growth (Adjei et al., 1994). The lack of variation among the hybrids, which might provide the basis for selection for better persistence, raises the question of whether or not future breeding efforts with interspecific hybrids should be continued.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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