Rapidity and extent of changes in genotypic structure induced by grazing in a ryegrass population

Abstract

An experiment is described in which the rapidity and extent of changes in the genotypic structure of a mixed perennial and Manawa ryegrass popul<l!tion, as induced by different grazing management treatments, were studied. Lax [;fazing treatments applied six months after a period of establishment management resulted in marked changes in genotypic structure to populations containing a high percentage of plants with Manawa ryegrass charaoteristics.Continuous and close grazing treatments applied at the same time also caused marked changes in genotypic structure but resulted in populations containing a high percentage of plants with perennial ryegrass characteristics.A Moderate grazing treatment induced changes more similar to those recorded in the Lax grazing treatment.Thesc changes occurred rapidly, and the major effects were recorded within four months of the application of treatments.From the results obtained, the variety, Manawa ryegrass, was bertter adapted to the range of management treatments applied over a period of three years than the perennial ryegrass variety.The implicat!ons of the results to aspeots of pasture management on farms, and to the interpretation and extrapolation of results from experiments comparing productivity of pastures under different management systems in different environments, are discussed.

Studies ON THE TOLERANCE TO SULPHUR DIOXIDE OF GRASS POPULATIONS IN POLLUTED AREAS

“…All the species examined are outbreeders, and consequently the bred cultivars will contain genetic variability which permits rapid selection in the field according to environmental conditions and agronomic practices (Cooper, 1963;Charles, 1970). The severity of selective pressures on bred cultivars in the field is illustrated by evidence for genetic changes in grass swards within 1 month of sowing (Brougham and Harris, 1967) and the survival to maturity of only 10% of the seeds sown in a pasture of L. perenne (Hayward, 1970). Horsman et al (1978Horsman et al ( , 1979b showed a reduced clonal variability in response to SO2 as tolerance evolved in L. perenne at a polluted site, and this was considered to refiect the elimination of the more sensitive genotypes in the field.…”

Improving the grazing and drought tolerance of temperate perennial grasses

“…Interactions between cutting frequency and morphology do affect production or persistence in a number of perennial grasses (Charles 1961;Rhodes 1971;Jones & Rushton 1975), although there is some phenotypic plasticity within cultivars in response to management (Brock & Fletcher 1993). In ryegrass, set-stocking favours short, dense, later-flowering, plant types, whereas largerleaved, more erect, sparser-tillered, earlier types tend to be more productive in winter and of adequate persistence under other forms of management (Brougham & Harris 1967). Because of these interactions, breeders can either aim for a compromise between grazing tolerance and high productivity, or for more specialised cultivars to which certain management recommendations need to be attached.…”

Rapid population differentiation in a mosaic environment

“…Other studies, such as those by Cooper (1959) have shown that large amounts of potential genetic variation are present even within individual plants of perennial outcrossing species and that this variation is susceptible to recombination and artificial selection. The large amount of genetic variation within populations of grass species in the field is also subject to natural selection, and large changes in the genetic structure of populations can occur within a few years (Charles, 1961(Charles, , 1964Brougham and Harris, 1967;Crossley and Bradshaw, 1968).…”