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The 4th International Symposium of Forage Breeding was held at AgriBio, The Centre for AgriBioscience, Melbourne Australia from 23–25 September 2013. More than 100 delegates from around the world attended the symposium which featured sessions on genetic resources and novel species, breeding and managing symbiosis, overcoming abiotic stresses, forage quality and nutritive value, breeding for new environments and production systems, biotic stresses and disease tolerance, economic impact of genetic improvement of forages, new traits and technologies in forage breeding and whole genome selection in forages.
White clover (Trifolium repens) is adapted to moist, fertile soils in temperate zones. Despite its heterozygous allotetraploid nature, it lacks useful genetic variation for survival and growth in semi-arid, infertile soils. Although white clover is apparently genetically isolated in nature, 11 other taxa have so far been found that can be artificially hybridised into the wider gene pool. These species range from annuals to long-lived, hardy perennials with adaptations to stress environments, and they potentially provide new traits for the breeding of more resilient varieties of white clover. The delineation of the secondary, tertiary and quaternary gene pools is described, along with a review of interspecific hybrids achieved to date. The results of large breeding programs to integrate traits from T. nigrescens and 4x T. ambiguum are reviewed, and schemes introduced for the use of T. uniflorum, T. occidentale, T. pallescens, 2x T. ambiguum and 6x T. ambiguum. Interspecific hybrid breeding of white clover has the potential to enable the development of resilient perennial clovers for seasonally dry, infertile grassland environments in many parts of the world.
Proanthocyanidins (PAs) are polymeric flavonoids derived from the phenylpropanoid pathway, and they bind reversibly to forage proteins within the rumen, providing protection from bloat while enhancing protein utilisation and animal production. The occurrence of PAs varies greatly within forage legume species. Foliar PAs are present in Lotus corniculatus (birdsfoot trefoil) and Onobrychis viciifolia (sainfoin), but such species often show poor persistence under grazing. By contrast, Trifolium repens (white clover) and Medicago sativa (lucerne, or alfalfa) have good persistence but negligible amounts of foliar PAs. We altered the accumulation patterns present in lucerne and white clover by the overexpression of an R2R3-MYB transcription factor (TaMYB14) isolated from T. arvense (rabbit’s foot clover), a species with significant levels of leaf PA. Such plants effectively produced PAs of high degree of polymerisation (DP) in leaf tissue by upregulating genes of the PA pathway. By comparing transcriptome pools, we identified additional MYB transcription factors with putative involvement in PA synthesis in white clover and T. arvense, indicating that these species share a complex PA regulation system. Progress towards producing commercial cultivars of lucerne and white clover containing effective levels of PAs has begun. Such plants will provide a viable option for mitigating bloat in pastoral agriculture-based farming systems.
Tedera (Bituminaria bituminosa C.H. Stirton var. albomarginata and var. crassiuscula) has been identified as one of the most productive and drought-tolerant species of herbaceous perennial legumes based on 6 years of field evaluation in Western Australia in areas with Mediterranean climate and annual rainfall ranging from 200 to 600 mm. Importantly, tedera demonstrated broad adaptation to diverse soils, and some accessions have shown moderate levels of tolerance to waterlogging and salinity. Tedera exhibits minimal leaf shedding during summer and autumn. Economic modelling strongly suggests that giving livestock access to green tedera in summer and autumn will dramatically increase farm profit by reducing supplementary feeding. The breeding program (2006–12) evaluated the available genetic diversity of tedera for its field performance in seven nurseries with 6498 spaced plants in total covering a wide variation in rainfall, soils and seasons. Best overall plants were selected using a multivariate selection index generated with best linear unbiased predictors (BLUPs) of dry matter cuts and leaf retention traits. The breeding program also evaluated tedera for grazing tolerance, grazing preference by livestock, waterlogging tolerance, seed production, cold tolerance, disease susceptibility and presence of secondary compounds. Tedera is a diploid, self-pollinated species. Therefore, 28 elite parents were hand-crossed in several combinations to combine outstanding attributes of parents; F1 hybrids were confirmed with the aid of highly polymorphic, simple sequence repeat markers. The F1s were progressed to F4s by single-seed descent breeding. Elite parent plants were selfed for two generations to be progressed in the breeding program without hybridisation. Over time, selections from the crossing and selfing program will deliver cultivars of three ideotypes: (i) drought-tolerant, (ii) cold- and drought-tolerant, (iii) waterlogging- and drought-tolerant.
Brazil is an agricultural country, with 190 Mha of pastures sustaining 209 million cattle. Fewer than 10% of the cattle are fattened in feedlots, whereas cattle reared on pastures have a competitive advantage for export, eliminating the risks presented by the mad cow disease (bovine spongiform encephalopathy) and considerations related to animal welfare. Brazil has been the world’s largest exporter of beef since 2004 and has the largest commercial herd in the world. In 2011, 16.5% of its production was exported, and the livestock sector contributed 30.4% of the gross national product from agribusiness and 6.73% of the total GNP. Many forage breeding programs, mainly at Embrapa, have contributed to the development of improved pastures, and cultivars of Brachiaria brizantha, B. decumbens, B. humidicola and Panicum maximum are the main pastures used in the country. All have apomictic reproduction, which means there are few cultivars occupying very large, continuous areas, thus suggesting a risk to the productive system. Such is the case of B. brizantha cv. Marandu, which occupies around 50 Mha. The Brazilian tropical forage seed industry is also important, and Brazil is the main seed exporter, supplying all Latin American countries. Due to pasture degradation, around 8 Mha is renovated or recovered each year. Forages are also used and planted each year in integrated crop–livestock and integrated crop–livestock–forest systems. Nowadays, these systems occupy 4 Mha. Improved pastures are thus a major asset in Brazil not only for the beef production chain but also for the dairy industry.
The main limitation of red clover (Trifolium pratense L.) worldwide including in Chile is the lack of persistence related to the high mortality of plants due to a complex of biotic and abiotic factors. We have demonstrated in various trials in Chile that red clover plant population is highly correlated with forage yield once the plant population has dropped to a certain level, from the second or third season onward, depending on the environment of evaluation. We have also found that in the south of Chile, among the biotic and abiotic factors affecting red clover survival, the curculionid Hylastinus obscurus (Marsham) is the main deleterious factor. However, because persistence is a complex trait, we have used a practical approach in our breeding program. We selected for general adaptability under field conditions and used a modified among and within half-family selection methodology, evaluating at the same time families as swards and spaced plants. This breeding methodology and strategy have yielded reasonable genetic gains since we started our breeding program in 1989 at INIA Carillanca, Chile. Since then, we have conducted five cycles of recurrent selection, and two cultivars have been released to replace the old cultivar, Quiñequeli INIA. These are Redqueli INIA and, more recently, Superqueli INIA. Depending on location and trial, average forage yield of the newest cultivar Superqueli INIA has been 23–69% higher than Quiñequeli INIA and 5–36% higher than Redqueli INIA; this difference increases in the third and fourth seasons. Superqueli INIA had four times the yield of Quiñequeli INIA in the fourth season. Therefore, the average realised genetic gain has been 0.4–2.6% per year, depending on location, showing the effectiveness of the breeding methodology and approach used.
Dual-purpose cereals have been important for increasing the flexibility and profitability of mixed farming enterprises in southern Australia, providing winter feed when pasture dry matter production is low, and then recovering to produce grain. A perennial dual-purpose cereal could confer additional economic and environmental benefits. We establish that, at the end of a second growth season, selected perennial cereals were able to achieve up to 10-fold greater below-ground biomass than a resown annual wheat. We review and expand the data on available, diverse, perennial, wheat-derived germplasm, confirming that perenniality is achievable but that further improvements are essential through targeted breeding. Although not yet commercially deployable, the grain yields and dry matter production of the best performing lines approach the benchmarks predicted to achieve profitability. On reviewing the genomic composition of the most promising wheat-derived perennials, we conclude that the best near-term prospect of a productive breeding program for a perennial, wheat-derived cereal will utilise a diploid, perennial donor species, and the most promising one thus far is Thinopyrum elongatum. Furthermore, the breeding should be aimed at complete wheat–Th. elongatum amphiploids, a hybrid synthetic crop analogous to triticale. We advocate the generation of many primary amphiploids involving a diversity of Th. elongatum accessions and a diversity of adapted annual wheat cultivars. Primary perennial amphiploids would be inter-crossed and advanced with heavy, early-generation selection for traits such as semi-dwarf plant height, non-shattering heads, large seed size and good self-fertility, followed by later generation selection for robust perenniality, days to flowering, grain yield, forage yield, stability of grain yield across seasons, and disease resistance.
Persistence is consistently claimed by Australian farmers as a high priority for perennial grasses in long-term pastures. Phalaris (Phalaris aquatica L.) is a productive perennial grass with proven persistence in south-eastern Australia. Nevertheless, factors that determine the persistence of pasture species in southern Australia related to climate (drought), soil (acidity), grazing pressure, and, importantly, their interaction can reduce persistence of phalaris and other species in various situations. These factors and their interactions are discussed in this review, and strategies to improve persistence with emphasis on plant breeding approaches are considered, with the most durable outcomes achieved when breeding and management options are employed concurrently. Two examples of breeding to improve persistence traits in phalaris are described. A program to improve acid-soil tolerance resulted first in the release of cv. Landmaster, and recently Advanced AT, which is the most aluminium (Al)-tolerant cultivar of phalaris to date. It was bred by recurrent selection on acid soils in a population containing genes from a related, more Al-tolerant species, P. arundinacea. The higher Al tolerance of cv. Advanced AT is of most benefit in more assured establishment on acid soils under variable moisture conditions and confers improved flexibility of sowing date. Cultivar Holdfast GT was bred to address complaints of poor persistence under heavy grazing by cultivars of the highly productive, winter-active type, since high grazing tolerance is needed to achieve profitable returns from developed pastureland. Evidence of good persistence under grazing for cv. Holdfast GT and possible tradeoffs with productivity are discussed. Maintaining high productivity under a predicted higher incidence of drought stress (climate change) and increasing areas of acid soils presents ongoing challenges for persistence in pastures.
Genomic selection is now being used at an accelerating pace in many plant species. This review first discusses the factors affecting the accuracy of genomic selection, and then interprets results of existing plant genomic selection studies in light of these factors. Differences between genomic breeding strategies for self-pollinated and open-pollinated species, and between-population level v. within-family design, are highlighted. As expected, more training individuals, higher trait heritability and higher marker density generally lead to better accuracy of genomic breeding values in both self-pollinated and open-pollinated plants. Most published studies to date have artificially limited effective population size by using designs of bi-parental or within-family structure to increase accuracies. The capacity of genomic selection to reduce generation intervals by accurately evaluating traits at an early age makes it an effective tool to deliver more genetic gain from plant breeding in many cases.
Forage legumes and grasses provide the basis for the worldwide ruminant animal livestock industry, and most of these forages come from perennial pasture plants. As animal agriculture has intensified, pest pressures on perennial forages often also become more prevalent. Among these pest plant parasitic nematodes, especially root-knot nematodes (RKN) (Meloidogyne spp.) have been shown to be major forage crop pests. In this paper, we summarise research from the University of Florida related to development of methods for screening germplasm for RKN responses, results from germplasm evaluations across a range of forage legume and grass species, and progress from breeding and selection research including studies of the genetic basis of resistance. We then present prospects for continued genetic improvement in RKN resistance using conventional and modern molecular methods.
Rhizomatous and perennial warm-season C4 grasses such as Miscanthus spp. and switchgrass (Panicum virgatum) are potential bioenergy crops for temperate regions. However, lignin in Miscanthus and switchgrass inhibits the cellulose digestion process during bioethanol production. One of the targets for improvement of forages from feedstocks to bioenergy crops is to develop a cost-efficient biorefinery process through lignin content manipulation. Numerous reports have shown that RNAi suppression of lignin-biosynthesis pathway genes can increase biomass fermentable sugar yields for biofuel production. These studies have also reported that RNAi suppression of cell-wall lignin biosynthesis can decrease biomass yield and resistance to biotic stress in the transgenic plants. Transcriptome and metabolome approaches can be used to clarify the networks and pathways of lignin biosynthesis to facilitate the identification of appropriate target genes for transformation. However, whole-genome sequencing of the forage species, which provides much-needed genomic information, is limited. Germplasm of natural, low-lignin mutants also plays a role in identification of genetic regulation of lignin content and this would be useful breeding material. Molecular markers have been developed and utilised to accelerate identification of quantitative trait loci/genes for traits relating to the biorefinery process. All of these studies will serve as basic information for supporting genetic improvement through classical breeding or genetic transformation, and offer the opportunity to develop cultivars which have enhanced biomass and are cost-efficient for biorefinery process.
Subterranean clover (Trifolium subterraneum L.) is the most widely sown pasture legume in southern Australia and resistance to important diseases and pests has been a major plant-breeding objective. Kabatiella caulivora, the cause of clover scorch, is the most important foliar fungal pathogen, and several cultivars have been developed with resistance to both known races. Screening of advanced breeding lines has been conducted to prevent release of cultivars with high susceptibility to other important fungal foliar disease pathogens, including rust (Uromyces trifolii-repentis), powdery mildew (Oidium sp.), cercospora (Cercospora zebrina) and common leaf spot (Pseudopeziza trifolii). Several oomycete and fungal species cause root rots of subterranean clover, including Phytophthora clandestina, Pythium irregulare, Aphanomyces trifolii, Fusarium avenaceum and Rhizoctonia solani. Most breeding efforts have been devoted to resistance to P. clandestina, but the existence of different races has confounded selection. The most economically important virus diseases in subterranean clover pastures are Subterranean clover mottle virus and Bean yellow mosaic virus, while Subterranean clover stunt virus, Subterranean clover red leaf virus (local synonym for Soybean dwarf virus), Cucumber mosaic virus, Alfalfa mosaic virus, Clover yellow vein virus, Beet western yellows virus and Bean leaf roll virus also cause losses. Genotypic differences for resistance have been found to several of these fungal, oomycete and viral pathogens, highlighting the potential to develop cultivars with improved resistance. The most important pests of subterranean clover are redlegged earth mite (RLEM) (Halotydeus destructor), blue oat mite (Penthaleus major), blue-green aphid (Acyrthosiphon kondoi) and lucerne flea (Sminthurus viridis). New cultivars have been bred with increased RLEM cotyledon resistance, but limited selection has been conducted for resistance to other pests. Screening for disease and pest resistance has largely ceased, but recent molecular biology advances in subterranean clover provide a new platform for development of future cultivars with multiple resistances to important diseases and pests. However, this can only be realised if skills in pasture plant pathology, entomology, pre-breeding and plant breeding are maintained and adequately resourced. In particular, supporting phenotypic disease and pest resistance studies and understanding their significance is critical to enable molecular technology investments achieve practical outcomes and deliver subterranean clover cultivars with sufficient pathogen and pest resistance to ensure productive pastures across southern Australia.
Relatively little is known about the weightings that breeders consciously or subconsciously place on specific traits when selecting individual plants, or the weightings agronomists and producers use when evaluating the relative merits of alternative cultivars and their potential economic value in farm systems. This is despite the many active programs for breeding improved forage plants, and in contrast to most modern animal-breeding programs where the relative merits of novel genetics are assessed against index-based breeding objectives. There are many reasons why breeding objectives based on profit indices are not used when breeding pasture plants. The nature of pasture as an intermediate input to farm output and profit poses unique difficulties in developing breeding objectives based on profit.
In this paper, we review the literature about methods to value genetic gain in perennial grasses. Various methods are canvassed for assessing the value of genetic gain for different pasture species across production systems. In the context of the complexity and cost of estimating the direct economic benefits of superior characteristics of pasture plants in farm systems via bio-economic simulation methods, we outline the use, and usefulness, of discrete choice techniques in the development of weightings for specific traits in forage plant improvement. There is a clear need to estimate the value of new pasture cultivars to producers, and although the differences between individual farms mean that one value or one ‘best’ cultivar is unlikely for any farm, the estimation of potential value of traits and cultivars will allow producers to make choices that are more informed.
Forage species provide the major feed-base for livestock grazing industries supporting production of dairy products, red meat and animal fibres. Because of the complex, multifactorial and highly environmentally sensitive nature of many key breeders’ traits for forage crops, implementation of genomic selection (GS) is a particularly attractive option. Although basic strategies for GS implementation have been devised, forage species display a broad range of biological factors that may influence the precise design of GS-based programs. These factors are described and exemplified by reference to several temperate and warm-season grass and legume species. Current knowledge with respect to such factors, along with the availability of suitable genomic resources and prospects for future activities, is described for several representative species (white clover, tall fescue and phalaris). Generic issues and benefits associated with GS implementation in forage breeding are also assessed.