Recurrent Restricted Phenotypic Selection Research Articles

Switchgrass as a bioenergy feedstock: advances in breeding and genomics research

Switchgrass (Panicum virgatum L), a native perennial of the North American prairie, possesses high biomass yield potential in marginal environments with limited input. It is an outcrossing tetraploid (2n = 4x = 36) with disomic inheritance. Previous research on cultivar improvement was focused primarily on herbage yield and forage digestibility. The decision of the U.S. Department of Energy Biomass Feedstock Development Program (NFDP) to develop switchgrass as a lignocellulosic bioenergy feedstock in the USA in the 1990s prompted a growing motivation for breeding and genomics research. The species is in early stages of domestication and current cultivars include mostly early releases for forage use that were selected directly from collected strains. Recent releases specifically for biomass feedstock have undergone one or two cycles of selection. As an outcrossing self-incompatible species, switchgrass possesses ample genetic diversity both between and within native populations. Conventional population improvement approaches such as recurrent restricted phenotypic selection (RRPS) are effective in improving forage yield and digestibility. Hybrids between different populations also demonstrated heterosis for key feedstock traits. However, genetic gains per year from selection using conventional approaches are low due to perennial growth habit and low heritability of important traits. Genomic approaches could be helpful in improving selection gain. Significant efforts have been placed in developing genomics resources. Genetic linkage maps were published and a large number of DNA-based markers were developed. Whole-genome sequencing is near completion, and the genetic bases of inheritance of key feedstock traits are being investigated. New insights into the molecular mechanisms will enable tailoring more efficient cultivar breeding approaches in the future.

Registration of ‘TifQuik’ Bahiagrass

TifQuik' (Reg. No. CV‐4, PI 648303, PVP No 200700209) is a fast emerging forage bahiagrass (Paspalum notatum Flügge) released by the University of Georgia–Tifton Campus and the USDA‐ARS in 2008. The current forage bahiagrass cultivars have a considerable amount of hard seed and thus require 2 or 3 wk to establish a full stand, during which time weeds may infest the pasture or moisture for germination may be restricted. The purpose of the breeding program that produced TifQuik was to select for an increased rate of germination and emergence. TifQuik was produced by employing four cycles of recurrent restricted phenotypic selection for improving the emergence and establishment rates originating from ‘Tifton 9’ bahiagrass. TifQuik had a fourfold improvement of emergence compared with Tifton 9 after 1 wk in greenhouse trials. Emergence rates, plant heights after 3 wk from planting, and first harvest yields were significantly greater for TifQuik than for other cultivars in replicated field trials. The accelerated emergence and establishment of TifQuik will be useful in sod‐based rotation systems with cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.).

Recurrent Restricted Phenotypic Selection for Improving Stand Establishment of Bahiagrass

Bahiagrass (Paspalum notatum Flügge) is an important perennial forage grass for the southeastern United States, being well adapted to conditions such as sandy or poorly drained soils, drought, and heavy continuous grazing. Current bahiagrass cultivars germinate over an extended period of time and the slow emergence produces incomplete stands, allowing weeds to compete and delaying grazing or first cutting for hay. A recurrent restricted phenotypic selection (RRPS) program began in 1996 and continued for four cycles for improving the emergence and establishment rates within Tifton 9 bahiagrass (‘Pensacola’ type). The resulting cycle‐4 (C4) rapid‐emergence ‘TifQuik’ was then increased and tested for seedling emergence, vigor, and establishment. TifQuik had a fourfold improvement of emergence compared to Tifton 9 after 1 wk in greenhouse trials. Emergence rates after 1 and 2 wk were significantly greater for TifQuik compared to Tifton 9 in replicated field trials. Plant heights after 3 wk from planting were 0.12 m for TifQuik versus 0.08 m for Tifton 9. The first‐harvest dry‐matter yield of TifQuik was 3128 kg ha−1 compared with 1539 kg ha−1 for Tifton 9 and the total establishment year dry‐matter yields were 9230 kg ha−1 and 7466 kg ha−1 for TifQuik and Tifton 9, respectively. TifQuik had greater first‐clipping yields than Tifton 9 at Ona, FL, in 2005. The accelerated emergence and establishment of TifQuik will be useful in sod‐based rotation systems with cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.).

Early Plant Selection Effects on Crown Traits in Pensacola Bahiagrass with Selection Cycle

A positive relationship between crown traits of greenhouse‐grown plants and crown development in the field would allow for the efficient evaluation of large populations of bahiagrass (Paspalum notatum Flügge) for rapid stand establishment. The objective of this study was to determine if young plants exhibiting greater crown vigor (expressed as rapid crown area growth and profuse tillering) in the greenhouse would result in larger, more vigorous plants when grown in the field. Fifty young bahiagrass plants, from each of four cycles (C0, C4, C9, and C23) resulting from recurrent restricted phenotypic selection (RRPS), were evaluated in the greenhouse for plant vigor, crown width, crown wet weight, and tiller number. Plants were transplanted to the field and crown area was determined at 6 and 15 mo after establishment. Crown vigor of greenhouse plants was greatest for C4 and C9 plants. Vigor ratings for C9 plants were smaller (P < 0.05) than C4 in Exp. 2, but larger than C0 and C23. Crown area, crown wet weight, and tillering of the greenhouse plants favored plants from C4 populations. Greenhouse plants of C23 generally had smaller crowns and less tillering. Crown area of individual plants from greenhouse plants, for all cycles, was positively correlated with crown area in the field after 6 mo following transplanting (r = 0.74 to 0.87 for Exp. 1 and 0.75 to 0.93 for Exp. 2). At 15 mo after transplanting to the field, bahiagrass crowns had filled the area between spaced plants. Selecting young superior bahiagrass plants in the greenhouse should aid breeders in developing germplasm that could hasten field establishment.

Herbage Accumulation of Three Bahiagrass Populations during the Cool Season

Fall and winter are periods of critical forage shortage in subtropical regions of the world, including the extreme southeastern USA. Cool‐season herbage accumulation has not been documented for bahiagrass [Paspalum notatum Flügge var. saurae Parodi] that has been selected using recurrent restricted phenotypic selection (RRPS) for increased warm‐season yield. Regrowth of ‘Pensacola’, ‘Tifton 9’, and RRPS Cycle 18 that accumulated for 15 or 30 d was harvested from mid‐September through mid‐April at Tifton, GA and Ona, FL for 2 yr. Herbage accumulation was essentially parallel for the three populations; minimums were coincident in midwinter, and maxima generally occurred at the first autumn harvest or last spring harvest. Fall and spring yields of Tifton 9 and RRPS Cycle 18 were generally higher than those of Pensacola and often more than double. This advantage was more pronounced at Ona than Tifton. Digestibility (600 g kg−1) and crude protein (CP) concentration (150 g kg−1) at Ona remained high throughout the cool season. However, forage quality of bahiagrass at Tifton tended to decrease with increased frosts. Cool‐season growth of three bahiagrass populations was slow, but RRPS selection has led to populations with greater growth potential, except when cold temperatures prohibit growth entirely.

Response of Selected and Unselected Bahiagrass Populations to Defoliation

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.

Yield Distributions of Spaced Plants within Pensacola Bahiagrass Populations Developed by Recurrent Restricted Phenotypic Selection

Pensacola bahiagrass (Pb), Paspalum notatum Flugge var. saure Parodi, is a warm season perennial sward grass grazed by livestock on more than a million hectares in the southeastern USA. This research was designed to evaluate the 5-yr effect of recurrent restricted phenotypic selection (RRPS) on the average yield and the distribution of spaced plant yield (SPY) components within broad-based Population A and narrow-based Populations B and E. RRPS Cycles 0, 9, and 18 in Population A, and Cycles 0, 5, and 10 in Population B produced 5-yr average spaced-plant-population progress (SPPP) yields of 364, 1014, 1767, 823, 1158, and 1427 g plant>−1, respectively. The average SPY increase in Population A proceeded at the same rate from RRPS Cycles 16 to 22 as from 0 to 16 previously reported. The average SPY increase proceeded at a faster rate at RRPS Cycles 5, 6, and 7 for very narrow based Population E than for Populations A and B. Comparison of SPYs from different cycles in the 5-yr SPPP tests revealed that RRPS increased the number of high yielding plants and reduced the number of low yielding plants in successive cycles. RRPS Cycle 18 of Population A that produced two plants yielding 4540 g in the SPPP test had 27 plants weighing only 454 g. Seven, 14, and 22 cycles of RRPS were required to create plants with SPYs yielding 4086 g in 100-plant populations of E, B, and A, respectively.

Seed Production on Detached Culms of Pearl Millet × Elephantgrass Hexaploid Hybrids

Five nutrient solutions were evaluated in the greenhouse to determine which solutions would allow detached culms of Pennisetum to produce seed. The genotypes tested originated from the hybridization of Pennisetum glaucum L. (Pearl millet) × P. pennisetum Schum. (elephantgrass). The solutions were water, Hoagland's, sucrose, sucrose + hydroxyquinoline sulfate (HQ), and Hoagland's + sucrose + HQ. Neither the water nor the Hoagland's solution supported high seed set. Although the sucrose solution enhanced seed production, the seeds were low in weight and did not germinate well. The best nutrient solutions were 2% sucrose + 0.02% HQ or Hoagland's + 2% sucrose + 0.02% HQ. The four genotypes used differed substantially in seed production, but all produced seed, with germination >25%. This result indicates that the cut-culm technique is a possible way of getting recurrent restricted phenotypic selection seed in Pennisetum hexaploid hybrids.

Switchgrass as a sustainable bioenergy crop

Switchgrass ( Panicum virgatum L.) shows potential as a sustainable herbaceous energy crop from which a renewable source of transportation fuel and/or biomass-generated electricity could be derived. In 1992, a new research program focused on developing switchgrass as a biomass energy feedstock was initiated by the U.S. Department of Energy in five of the southern United States. The multifaceted, multi-institution research addresses breeding for improved biomass yields, regional field tests, cultural practices, physiology and tissue culture. Recent progress is highlighted in this paper. Preliminary results from the breeding program indicate that recurrent restricted phenotypic selection could lead to development of new cultivars. A technique for regenerating switchgrass plants via tissue culture has been proven and new populations of regenerated plants have been established in the field. Performance trials at three regional cultivar testing centers in Virginia, Alabama and Texas have shown that ‘Alamo’ switchgrass has higher biomass yield and broader adaptability than other cultivars tested. Research on management practices designed to maximize biomass yield has shown that multiple harvests of switchgrass may reduce total seasonal yields in some instances and that responses to fertilizer inputs vary with the environment. Seed dormancy often retards rapid establishment of competitive stands of switchgrass. Our research has indicated that seed dormancy can be modified, resulting in increased seed germination and a greater number of switchgrass plants. Research on the physiology of switchgrass has shown that lowland and upland ecotypes differ in photosynthetic rate but not in respiration rate. Findings in each of these areas can contribute to development of switchgrass as a sustainable bioenergy crop. Future research will address molecular biology techniques for exploiting genetic variation, explore canopy architecture and carbon allocation patterns affecting biomass yield, elucidate key factors in successful establishment of switchgrass and provide technology transfer that facilitates scale-up of switchgrass production for commercial energy production.

Recurrent restricted phenotypic selection for forage yield in timothy and orchardgrass

Three cycles of recurrent restricted phenotypic selection for forage yield were carried out in orchardgrass (Dactylis glomerata L. 'Avon' and 'Pennlate') and timothy (Phleum pratense L. 'Salvo' and 'Drummond'). The objectives of this study were to measure the effectiveness of this selection and to assess whether the selection produced any correlated responses on other characters. The three selected cycles and the original populations (cycle 0) were evaluated in both sward and space-planted experiments over a period of 4 yr. For the majority of the years of evaluation, there were no significant (P < 0.05) differences in forage yield among the different selection cycles. Under space-planted evaluation, cycle 3 of Drummond timothy was lower in both forage and seed yield than the other populations. In general, there was a trend to taller plants with greater circumferences in orchardgrass, and shorter, smaller plants in timothy over selection cycles. Selection had little effect on plant persistence and maturity. In conclusion, three cycles of recurrent restricted phenotypic selection were not effective in improving the forage yield of timothy or orchardgrass. Key words: Timothy, Phleum pratense, orchardgrass, Dactylis glomerata, recurrent selection

Predicted and Realized Gains from Selection for In Vitro Dry Matter Digestibility and Forage Yield in Switchgrass

Improved forage yield and quality, which can lead to more efficient livestock production, are important goals in switchgrass (Panicum virgatum L.) breeding. Objectives of this study were to determine the effectiveness of multiple cycles of recurrent restricted phenotypic selection (RRPS) in improving forage yield in switchgrass and in improving in vitro dry matter digestibility (IVDMD), and to compare predicted and realized gains from selection. Six switchgrass populations were developed by selecting a base population for one cycle of low and three cycles of high IVDMD and two cycles for high yield. Half‐sib progenies of parents selected from the base population for high and low IVDMD were used to estimate heritability for IVDMD by parent‐progeny regression. Realized gains from selection were determined in a seeded sward trial that included the base and six selected populations. Estimated and realized heritabilities for IVDMD were 0.40 and 0.31, respectively. Predicted gains and (in parenthesis) realized gains from selection for IVDMD in grams per kilogram per cycle were −21 (−15) for the Low IVDMD Cycle 1, 16 (15) for High IVDMD Cycle 1 (‘Trailblazer’), 14 (4) for the High IVDMDCycle 2, and 19 (18) for the High IVDMD Cycle 3 populations. There was a low but positive genetic correlation between forage yield and IVDMD of 0.10. Forage yield was not improved by RRPS, probably because of lack of genetic variability for forage yield in the base population. It is concluded that RRPS is effective for improving IVDMD in switchgrass, but was not effective in improving forage yield in the populations evaluated, and that predicted gains exceeded realized gains from selection for IVDMD due to limited selection for traits other than IVDMD.

Recurrent Restricted Phenotypic Selection for Yield Alters Morphology and Yield of Pensacola Bahiagrass

Sixteen cycles of recurrent restricted phenotypic selection (RRPS) have resulted in consistent forage yield increases of space‐planted Pensacola bahiagrass (Paspalum notatum Függe var. saurae Pexodi) plants. The purpose of this study was to assess morphological changes occurring as a result of RRPS for yield. Tests were established in the field in 1988 and 1989. Approximately 100 spaced 1st‐yr plants of each of commercial Pensacola and Cycles 9 and 16 were compared for culm number per plant, racemes per culm, height, leaf length and width, raceme length, plant diameter, plant weight, culm weight, and leaf weight. Differences existed among cycles for all traits. Cycle 16 plants had the highest values and Pensacola plants had the lowest values, except for plant diameter, which decreased as cycles increased. Increases in culm number per plant, plant height, and leaf length increased from Pensacola bahia to Cycle 16 by 32, 44, and 79%, respectively. Mean leaf, culm, and whole plant weights increased by 423, 308, and 382%, respectively. Plant diameter decreased by 15%. Multiple linear regression analysis showed that, in general, plant weight could be closely predicted with two variables, leaf length and number of culms per plant. It was concluded that RRPS for yield has resulted in an increase in size or number of all measured morphological traits while reducing plant diameter. Cycle 16 contained enough variability that progress is expected to continue in future cycles.

Responses to Selection for Root Dimension in a South European Variety of Perennial Ryegrass)*

AbstractThe objective of the study was to determine the effect of selection for large root dimension on root and shoot characteristics of an Italian variety of Lolium perenne L. (cv. ‘Vejo’). Two cycles of recurrent restricted phenotypic selection (RRPS) were carried out in 1983 and 1984 at Perugia (Italy). In each cycle, 20 out of 1,500 plants were selected and intercrossed under isolation. In 1985, 120 plants from‘Vejo selected’ and an equal number of plants from‘Vejo’ initial seed‐lot were grown hydroponically for 92 days; at the end of the experiment, the plants were measured for their number of leaves and stems, leaf length, length of the longest root, root volume and shoot and root dry weight.Sufficient genetic variability existed among plants of cv. ‘Vejo’ to allow highly significant responses to selection for root dimension (+25 % in root length, +39 % in root volume, + 67 % in root dry weight and +31 % in root: shoot dry weight ratio). Furthermore, two cycles of RRPS for root dimension did not show negative responses of the above‐ground parts of the plant.

Loss of Half‐Sib Families in Mass Selection1

To study the loss of half‐sib families in mass selection, progenies of seven populations of Pensacola bahiagrass (Paspalum notatum var. saure Parodi) being improved by recurrent restricted phenotypic selection (RRPS) were studied in detail. Accession numbers assigned to progenies of selected plants recorded in spaced‐plant field plans of randomized populations of 990 to 1200 plants per cycle permitted a determination of half‐sib families lost with each cycle of grid selection. Loss of half‐sib families averaged 31.2% per cycle for five populations when the top yielding five plants in 25‐plant blocks were selected in a planting with six randomly placed plants of each family in a population. With only five randomly placed plants of each family, the loss of half‐sib families increased to 38%. Loss of these low yielding half‐sib families permitted more rapid progress than a plant‐to‐row breeding system that would have retained them. In 35 computer‐generated populations with six randomly placed offspring from each of 42 selections, reducing the percent of plants selected from 25.0 to 8.3, increased the percent of half‐sib families lost from 20.3 to 61.2. In four populations that lost 31% of their half‐sib families, 51% of the remaining families were recovered only once whereas 3.3 and 0.8% were recovered four and five times, respectively. Because half‐sib families recovered most frequently were the most productive, mass selection as practiced here permitted more rapid advance than a breeding system requiring equal representation from each family in the polycross for the next cycle.

Spaced‐Plant‐Population‐Progress Test1

This paper describes a precise and efficient method of measuring the progress realized and expected in a population improvement breeding program. The spaced‐plant‐population‐progress (SPPP) test described here included 100 spaced plant progenies of each entry arranged in 100 blocks with one plant of each entry per block. Analysis of variance of the first‐year spaced‐plant yield data from two tests of Pensacola bahiagrass (Paspalum notatum var. saure Parodi) populations being improved by recurrent restricted phenotypic selection (RRPS) gave significant (P = 0.05) mean differences among selection cycles for all but one entry comparison. The SPPP test also supplied variances and coefficients of variation (CVs) for each population that served as relative indices of the variability left in each population and the progress that might be expected from the next cycle of selection. In a precise seeded plot test, relative first‐year mean yields of the F1 hybrid of clones 2 ✕ 4 (from Cycle 4) and Cycle 9 compared with the commercial check were 167 and 175%, respectively, and were not different (P = 0.05). In the SPPP test, their respective relative mean yields, 226 and 267%, were different (P = 0.01). Likewise, the difference between mean yields of Cycles 8 and 9 that was significant (P = 0.05) with the SPPP test was not significant in the replicated seeded plot test. The SPPP test gives valid relative yields but over estimates the yield potential of a population that will be grown in solid stands established from seed. Samples of 50 or 25 plants compared in SPPP tests gave more variable mean yields and larger variances than the 100‐plant populations.

Interrelationships between Rates of Leaf Appearance and Tillering in Selected Tall Fescue Populations1

Tiller density and yield/tiller are yield components associated with dry matter productivity of grasses. Breeding programs may benefit by exploiting the mechanisms that control growth and development of these components. Our objective was to evaluate the relationship between leaf appearance and tillering in seedlings of tall fescue (Festuca arundinacea Schreb.). Plants were fourth‐cycle progeny from a recurrent restricted phenotypic selection program for high (H) and low (L) rates of leaf area pansion. Four half‐sib populations were used to represent each selection direction. Seedlings were grown in a controlled‐environment chamber for 12 weeks at 25/20°C and a 14‐h photoperiod. Photosynthetic photon flux density was 500 μmol m−2 s−1. Plants remained vegetative during the growth period. In general, there were few differences among L or among H populations for the characters measured. Herbage yield of the H populations averaged 12% greater than that for the L populations. Relative tillering rate (tillers/tiller/day) for the L populations was 32% greater than for the H populations. The L populations had a 12% greater appearance rate of fully‐collared leaves on each tiller, and consequently, more potential sites available for tillering. They also had 11% greater utilization of tillering sites than did the H populations. Collectively, data showed differential genetic control of tillering rate in these contrasting populations. Tillering rate was not dependent on concentration of water‐soluble carbohydrates in leaf, root, and especially stem base tissues as concentrations were less in L than in H populations. Leaf elongation rate and yield/tiller were positively associated with herbage yield. The H populations averaged 28% greater in specific leaf weight than the L populations. Specific leaf weight was positively associated with yield/tiller and herbage yield. Carbon dioxide exchange rate of single leaves was not altered by selection. Genetic improvement of both tiller weight and tillering rate may be difficult using simultaneous selection.

Improved Recurrent Restricted Phenotypic Selection Increases Bahiagrass Forage Yields1

This paper describes the restrictions that make improved recurrent restricted phenotypic selection (RRPS) more efficient than mass selection for increasing Pensacola bahiagrass forage yields. Eight cycles of RRPS have given a consistent first‐year space‐plant forage yield increase of 16.4% per cycle. Cycle 6, compared with commercial Pensacola bahiagrass as the control, yielded 91% more in the spaced‐plant‐progress test and 84% more in a seeded small‐plot test. Cycle 4 yielded 16% more liveweight gain than the control in a 3‐year replicated grazing trial which was expected based on forage yields from a small plot clipping test. In a seeded test, Cycle 6 yielded two‐thirds as much as F1 hybrid 2 × 3 in the first year and almost as much as the hybrid in the unusually dry second year. In vitro dry matter digestibility of the forage was not reduced by six cycles of RRPS. Improved RRPS allows one cycle per year and is four times more efficient than ordinary mass selection in increasing bahiagrass yields.

Recurrent Restricted Phenotypic Selection Increases Forage Yields of Pensacola Bahiagrass1

Recurrent restricted phenotypic selection (RRPS), modified form of mass selection, was evaluated as a breeding technique for improving the forage yield of Pensacola bahiagrass (Paspalum notatum var. saure ‘Parodi’), a sexual diploid. Selection was based on single green weights of spaced plants at the end of the first season and spring yield the following year. Four cycles of RRPS increased forage yield of this grass 17.7% over that of the commercial check. The rate of progress per 2‐year cycle in a previously selected narrow‐gene population (NGP) was about 2%/cycle. In an unselected wide‐gene population (WGP), rate of progress was 6%/cycle. Variances and coefficients of variability (CV's) for yield spaced plants of the original and fourth‐cycle populations compared in the same test were similar for the NGP but were reduced substantially to the NGP level in the WGP. RRPS increased the frequency and yield of high‐yielding plants. Increasing seeding rates from 3 to 27 kg/ha in methyl bromide treated soil increased firstyear forage yields 76%, but had no effect on the relative yields of the commercial check and the fourth‐cycle populations. The data suggest that rate of progress could be doubled by use of a 1‐year cycle and the saving of 3 to 4 rather than 5 of the best plants in each 25‐plant block.