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The white clover ( Trifolium repens) nuclear genome (n = 2x = 16) is an important yet under-characterised genetic environment. We have developed simple sequence repeat (SSR) genetic markers for the white clover genome by mining an expressed sequence tag (EST) database and by isolation from enriched genomic libraries. A total of 2,086 EST-derived SSRs (EST-SSRs) were identified among 26,480 database accessions. Evaluation of 792 EST-SSR primer pairs resulted in 566 usable EST-SSRs. Of these, 335 polymorphic EST-SSRs, used in concert with 30 genomic SSRs, detected 493 loci in the white clover genome using 92 F1 progeny from a pair cross between two highly heterozygous genotypes--364/7 and 6525/5. Map length, as estimated using the joinmap algorithm, was 1,144 cM and spanned all 16 homologues. The R (red leaf) locus was mapped to linkage group B1 and is tightly linked to the microsatellite locus prs318c. The eight homoeologous pairs of linkage groups within the white clover genome were identified using 96 homoeologous loci. Segregation distortion was detected in four areas (groups A1, D1, D2 and H2). Marker locus density varied among and within linkage groups. This is the first time EST-SSRs have been used to build a whole-genome functional map and to describe subgenome organisation in an allopolyploid species, and T. repens is the only Trifolieae species to date to be mapped exclusively with SSRs. This gene-based microsatellite map will enable the resolution of quantitative traits into Mendelian characters, the characterisation of syntenic relationships with other genomes and acceleration of white clover improvement programmes.

Improvements in white clover (Trifolium repens L.) performance during the past 60 yr can be attributed to increased use of fertilizer, better management practices, and development of genetically superior cultivars. The rate of genetic improvement during the past six decades was evaluated in New Zealand using a world collection of 110 white clover ecotypes and cultivars. Genetic differences were assessed by evaluating all cultivars in the same environment. Field trials conducted from 1985 to 1989 were managed using rotational grazing by sheep (Ovis aries L.) with an average of nine grazings per year. Cultivars were grouped according to decade of release, the climate in which they originated (cold, cool, or warm), and plant type (small‐leaved, medium‐leaved, large‐leaved non‐ladino, and large‐leaved ladino). Regression analyses were used to determine the rate of genetic improvement. In absolute units, white clover yield has increased at an annual rate of 1.44 g m−2, whereas the percentage clover by weight in the sward has increased at an annual rate of 0.14%. This equates to a genetic gain of 6% per decade for both traits, which is higher than rates reported for other forage crops. Genetic gains for clover yield and percentage clover in the sward have been greatest in small‐leaved (7.5 and 8.7% per decade) and large‐leaved, non‐ladino types (6.6 and 8.3% per decade), whereas cultivars originating in cool climates (8.3 and 8.1% per decade) have improved at a faster rate than those originating in either cold (3.5 and 3.2% per decade) or warm climates (1.5 and 2.6% per decade).

Greater complementary gene interaction in autotetraploid alfalfa (Medicago sativa L., 2n = 4x = 32) may explain differences in vigor and breeding behavior between diploids and autotetraploids. Complementary gene interaction is nonallelic gene interaction or epistasis where dominant alleles at heterozygous loci may complement each other by masking recessive alleles at respective loci. This paper describes how tetrasomic segregations of linkage blocks in linkage disequilibrium produce tetraploid individuals and populations with greater complementary gene interaction than is possible at the diploid level. This finding helps explain autotetraploid superiority and unique breeding behavior. Research on gene action in autotetraploid alfalfa has demonstrated that favorable alleles in linkage blocks underpin population improvement and increased heterosis. The individual favorable alleles with additive effects also contribute to non‐additive complementary gene interactions in linkage blocks. Apparent multiple allelic interaction (overdominance) effects discussed in earlier studies inbreeding depression and progressive heterosis in alfalfa are due mainly to linkage disequilibrium, which agrees with findings in maize. The severe inbreeding depression in autotetraploids is due mainly to the rapid loss of complementary gene interactions in the first few generations of inbreeding. Correspondingly, the progressive heterosis of autotetraploids is due mainly to a progressive increase in complementary gene interactions. Greater complementary gene interactions in tetraploid alfalfa also helps explain recent DNA research indicating that yield in tetraploids is more responsive to genetic diversity than in diploids. Many differences between diploid and autotetraploid alfalfa reported in earlier studies now may be explained by inherent differences in the levels of complementary gene interactions.

Morphology, flowering, cyanogenesis and leaf markings of 109 white clover (Trifolium repens L .) cultivars, grown as spaced plants and in small plots, were measured . Principal component analysis and cluster analysis were used to compare cultivars . Cultivars were classified into four broad groups . The most important criteria for distinguishing between groups were leaf size, cyanogenesis and combinations of these . Group I, termed small, included small-leaved, prostrate cultivars ; Group II, termed intermediate, included the majority of the cultivars which were characterised by medium sized leaves and relatively low cyanogenesis levels ; Group III, termed large, included the large-leaved highly cyanogenic cultivars ; and Group IV, termed ladino, included large-leaved acyanogenic cultivars .

Methane production has been measured from lambs fed contrasting forages. This work has been driven by the need to reduce greenhouse gas emissions from agriculture and to determine energy losses to methane from contrasting diets. Young ram lambs were fed either fresh ryegrass/white clover pasture, lucerne (also pelleted lucerne), sulla, chicory, red clover, Lotus pedunculatus (lotus) and mixtures of sulla and lucerne, sulla and chicory and chicory with red clover. The effects of condensed tannin (CT) in lotus on methane production were also measured. The trials were carried out indoors with sheep held in metabolism crates to enable an accurate measurement of intake and digestibility as well as methane production. Principal findings were a two-fold range in emissions from 11.5g CH4/kg dry matter intake (DMI) with lotus to 25.7g CH4/kg DMI with pasture and a 16% reduction in methane production due to the CT in lotus. This range in emissions from good quality forages represents a loss of about 7-11% of metabolisable energy and presents a clear direction for future research to better utilise the feeding value of pastures and reduce greenhouse gas (GHG) emissions from agriculture. High quality perennial forages should be used where practical, and researchers need to identify plant parameters responsible for the variation in methane emissions. Research must focus on rapid passage of digesta through the rumen of grazing animals and will involve manipulation of the fibre content of grasses. Introduction of CT into diets is a likely target to reduce methane production. Improving the rapidly digestible constituents of forages is another opportunity, but difficult to target. Keywords: condensed tannins, forage quality, forages, greenhouse gases, methane emissions, sheep

The checklist presents a summary of information published since 1985 on the origin, breeding, and characteristics of white clover (Trifolium repens L.) varieties. This complements the previous checklist (Caradus 1986). Origin and breeding history, agronomic potential, disease susceptibility, and the maintenance of a cultivar are listed where known.

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