A summary of the objectives of the present studies has been presented in a previous paper (Lazenby & Rogers, 1962): traditional methods used in the selection of improved grass varieties were reviewed together with a history of varietal assessment and some recent research developments of importance to both grass breeder and grassland agronomist. For full evaluation of selection criteria, the performance of the grass plant should be studied when the variables potentially affecting growth and development are controlled individually and in combination. Results were reported of an experiment designed to study the behaviour of one variety of Lolium perenne, S.24, grown in different densities and under two moisture regimes. Data presented were total yield per unit area and the botanical components of such yield-number of plants, yield por plant, number and weight of sterile and fertile tillers-together with a statistical appraisal of their interrelationships (Campbell, 1962).The many examples which exist of yield interactions between grass varieties and growing technique (e.g. England, 1961;Lazenby & Rogers, 1962) show clearly the lack of reliability of widely spaced plant performance as an indicator of sward yield. There have been attempts to explain such results (e.g. Lazenby, 1957; Proudfoot, 1957;Fejer, 1959), whilst some workers (e.g. Knight, 1961) have tried to correlate yields of different varieties in the sward with their yield components, in order to obtain reliable selection criteria.Inconsistency in relative varietal behaviour in swards and as widely spaced plants is not fully understood; satisfactory criteria cannot yet be given to the grass breeder seeking to isolate individual plants giving predictable sward performance. This paper reports an experiment designed to help achieve such objectives; the growth and development of four varieties of L. perenne grown in a series of related densities were studied. Half the plots were irrigated during the final year of the investigation. MATERIALS AND METHODSThe experiment was conducted at the Plant Breeding Institute, Cambridge, on a sandy loam, similar in physical and chemical characters to the soil on which Exp. 1 of the present series was sited (Lazenby & Rogers, 1962). The four varieties of L. perenne used in the investigation-S.24, Irish, Kent and S. 23-were chosen because of differences in their characteristics when grown as widely spaced plants. In particular, there were differences in their growth rhythms (S.24 and Irish start growth earlier in spring than the other two and their anthesis in Cambridge is some 2 weeks earlier than Kent and 3 weeks earlier than S.23), their habit of growth (S.24 is erect, Irish semi-erect, Kent semi-prostrate and S.23 prostrate, Hawkins, 1958) and their vigour (in order of tiller production S.23 > Kent > S.24 > Irish).To achieve a trial of manageable proportions, the varieties could only be grown in four densities: all were square planted at 27 in., 9 in., and 3 in. and also broadcast in sward plots, due consideration being t...

1. To evaluate selection criteria in grass breeding, a series of investigations was conducted measuring plant performance in a wide range of field environments.2. This paper reports on the behaviour of one variety of Lolium perenne, S. 24, grown in a logarithmically related series of densities and frequently defoliated for 2 years. Half the plots received only natural rainfall, the rest were irrigated so that the calculated soil moisture deficit did not exceed 2 in.3. In 1958, with little effect of irrigation, swards yielded most per unit area, widest spacing (24 in.) least; there were no significant differences in yields from the other densities (3, 6 and 12 in.). In 1959, when yields were lower, irrigation increased production in all plots, the higher the density the greater the increase. In contrast to 1958, over the range of spaced densities, the higher the density the greater the yield. Individual harvests did not always conform to the annual pattern.4. Lowest mean production per plant was from broadcast plots in both 1958 and 1959, yields of plants from 3, 6 and 12 in. plots showed a linear increase with a reduction in density, those from 24 in., though heaviest, yielded less than linear expectation. In 1959 irrigation increased yields per plant.5. Of the components of yield, number of sterile tillers gave the best estimate of total fresh weight (95·3% reliability). However, total number of tillers (sterile plus fertile) was almost as efficient (94%). Tiller weight was a much less reliable index.6. The results and their value in establishing efficient criteria for selection in grass breeding are discussed.

1. The yields and yield components are recorded for selected clones of Lolium perenne, grown at different levels of applied nitrogen (0, 200, 400, 600 and 800 lb. of nitrogen per acre per annum) and at two spacings (widely spaced plants and simulated monoculture swards). Four harvests were taken in each of the two years 1962 and 1963.2. In both years there were significant and consistent differences between clones in their drymatter production per unit area, detectable at both spacings. A genotype x spacing interaction was detected only in 1962. A genotype x nitrogen interaction at some harvests was not found on annual totals.