SUMMARYA possible method for the production of F, hybrid seed of forage grasses in commercial quantities is described. The method depends on the two-locus incompatibility system postulated by Lundqvist (1961). Inbred lines are made, each deriving from a single genotype, having a high degree of within-line incompatibility so that when two, or more, such lines are interplanted the seed produced will be largely of hybrid origin. The consequences of multi-locus incompatibility mechanisms are examined and a general method for calculating the expected composition, in terms of incompatibility groups, of an inbred line in any generation and for any number of incompatibility loci is given. L INTRODUCTION FOSTER (1971 b) has studied the theoretical expectation of F1 hybridity resulting from bulk interpopulation hybridisation in herbage grasses and, in two other papers (Foster, 1971 a, c), he has reported on the performance of such hybrids under spaced plant and sward conditions. He found that F1 hybrids were not on average superior to the parental varieties but that certain F1's yielded up to 31 per cent more than their higher yielding parent under spaced plant conditions and up to 17 per cent more in swards. Wright (1972) has pointed out that with diploid inheritance, the mean performance of both semi-hybrid varieties of the type proposed by Foster and conventional synthetics is expected to be the same. Burton (1956) has discussed the value of F, hybrids in forage grasses and possible techniques for producing F1 seed in commercial quantities; he concluded that the extra seed cost might well be justified by the increased yields of forage. McWilliam (1962) has shown the value of F1 hybrids between species of Phalaris. It is tempting to consider the possibility of producing F1 hybrids of forage grasses more nearly along the lines used in maize, sorghum and some other crops. Usually, the production of F1 hybrids depends either on a rapid technique of mass emasculation or the use of male-sterility although sporophytic incompatibility has been used in brassicas (Thompson, 1957) and of course F, hybrids between pairs of self-incompatible genotypes are readily made and were used by both Burton (bc. cit.) and McWilliam (bc. cit.). The structure of the flowers of most forage grasses does not permit emasculation on the scale required and the effort involved in developing male-sterile lines, if such could be found, would be unlikely to be worth while. The two-locus selfincompatibility mechanism, proposed by Lundqvist (1961) for Festuca pratensis and stated by Weimark (1968) to operate in some other grasses including Lolium perenne, may provide a basis for the development of varieties consisting largely of F1 hybrid genotypes and may also permit conventional combining ability tests to be made on inbred lines.

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A trial was carried out to find management practices which would permit the best discrimination for winter performance of perennial ryegrass cultivars. Due to the mild winter experienced only a few of the plants were killed outright. The cultivars were therefore assessed for percentage of green herbage and for spring growth.There were four sites: an upland and lowland site at Aberystwyth and one site each at Edinburgh and Cambridge. At all sites two N rates (totals of 125 or 550 kg/ha in the first year after sowing) and 5 autumn cutting treatments were used to give differences in the amounts of herbage remaining at the onset of winter. The last dates of defoliation in the autumn were: C, end of August; Cj end of September; C^ end of October; C^ mid-November; Cj was cut on all these dates. Four cultivars (Grasslands Ruanui, S321, Premo and Argo) which differed in their autumn growth potential, frost susceptibility and degree of winter dormancy were grown.The autumn yield of S321 was, in general, higher than that of the other cultivars but there were interactions with N, management and location. The data on percentage of herbage remaining green in February illustrated differences due to the siting of the trial. In the conditions prevailing the two Aberystwyth sites facilitated greater discrimination between cultivars than those at Edinburgh and Cambridge.In three of the four sites the management which produced most winter 'bum* (including both that due to natural senescence and that due to winter damage) involved accumulation of herbage in the autumn but there were considerable sites x managements interactions. For example at Cambridge management produced no significant effect whereas at the Aberystwyth lowland site frequent defoliation in the autumn had more effect than allowing herbage to remain uncut after the end of August. High N significantly decreased the percentage of green herbage only at the two Aherystwyth sites.Over five-fold differences in spring yield were obtained, the heaviest yields being recorded at Edinburgh and Cambridge. However, there were large interactions between environments and cultivars; for example the higher N rate reduced spring yield at Aberystwyth but increased spring yield at Edinburgh and Cambridge. Premo and S321 had similar yields in spring at the Aberystwyth lowland and Cambridge sites but Premo was higher yielding than S321 at the Aberystwyth upland site and at Edinburgh.Tbe large environmental effects and their interactions with cultivars illustrate the difficulties of cuitivar assessment and the dangers inherent in national recommendations for grass cultivars. INTRODUCnONUnder intensive grassland management, particularly when high rates of N are applied, 315

The theory of expected response to selection between families is extended to allow for the effects of field replication and its interactions with number of plants per plot.

Tho problem of devising suitable progeny assessment techniques for the outbreeding forage grasses is partly that of finding a suitable plant density which, while retaining some of the convenience and ease of management of the wide spacing commonly used by plant breeders, also provides an accurate assessment of sward-yielding ability.Grass breeders have grown plants at wide spacing for purposes of selection; Stapledon (1930Stapledon ( , 1931 and Jenkin (1931Jenkin ( , 1955. Spaced-plant trials retain their importance for assessing certain plant characteristics, particularly maturity type, habit of growth and other characters of high heritability, but many investigators have reported their unsuitability for the assessment of sward yield. Murphy (1952), for example, found simple correlation coefficients ranging from -0-93 to +0-95 for the comparison between spaced and broadcast polycross progenies and Nissen (1961) reported low correlations between hay yields in Timothy grown as spaced plants and in swards. Green & Eyles (1960) found that the order of yields in perennial ryegrass varieties was reversed in spaced plantings compared with that in swards. Lazenby & Rogers (1960, 1962, 1964, 1965a have examined the behaviour of perennial ryegrass varieties over a range of densities, from that of the sward down to a density in which the plants were at 27 in spacing. These authors conclude that densities corresponding to a between-plant spacing of up to 9 in are suitable for assessing swardyielding ability, that such densities are easier to manage than swards and that the lower of the suitable densities (6-9 in spacing) permits the identification of individual plants and might be useful for single-plant selection as well as for progeny assessment.The work of Lazenby & Rogers was based on the detailed examination of the performance of a few cultivars over a range of densities and their evidence for the suitability of certain non-sward densities for the assessment of sward performance depends on the lack or relative lack, of variety x density interactions for yields and various components of yield. Before advocating the widespread adoption of such testing techniques it would be desirable to have some quantitative measure of their excellence and it was therefore thought desirable to conduct an experiment to compare the performance of a relatively large number of populations over a range of densities, including the sward, and to assess the correlations between performance at the different densities. Since most herbage grasses are at present used in mixtures it was thought worthwhile to include some two component mixtures of varieties amongst the populations. These mixtures do not represent anything like the complexity of the average seeds mixture but their performance over a range of non-sward densities, compared to that in the sward, would give some indication of the suitability of non-sward densities for assessing both mixtures and components of mixtures. MATERIALS AND METHODSThe material for this study consisted of seven cu...