Factors influencing the establishment of ryegrass under different cultivation systems
Establishment of grassland by direct re-seeding into old swards sprayed with a herbicide is an attractive technique for establishing grassland on the sandy soils of North-east Scotland, because it can reduce the risk of soil erosion by wind or water that accompanies conventional cultivation. However, the generally poor establishment of direct reseeds is preventing its adoption in this region. Series of experiments carried out in the laboratory, glasshouse and in small field plots investigated several factors as possible causes of poor establishment on these soil types and identified some potential strategies for its improvement. Beneficial effects of direct drilling were confirmed in field sowings in mid-summer, possibly due to a greater retention of soil moisture under grass trash. However, conventional cultivation gave better emergence in cooler and in wetter sowing conditions. In glasshouse experiments poor establishment was associated with the presence of grass trash on the soil surface, removal of which generally improved emergence. This trash was observed to affect ryegrass establishment in several ways. Laboratory experiments showed that the physiological effects of grass trash on seed germination could be attributed to both retention of herbicide residues and more importantly the leaching of phytotoxins inhibitory to grass seed germination. However although leachates prepared from grass trash inhibited ryegrass germination on laboratory germination papers, this effect was not seen when experiments were performed on seeds on soil. Furthermore, the toxicity of these leachates was removed when leachates were passed through a column of loamy sandy soil. The rapidity with which these toxins could be leached and the characteristic low moisture content of these sandy soils indicate that leachates are unlikely to be effective in inhibiting germination in the field. The failure of calcium peroxide seed coatings to improve emergence further supports this suggestion. Part of the effects of these leachates was thought to be that of herbicide residues. Although glyphosate, the herbicide normally used to desiccate the old sward was rapidly deactivated on contact with Soil, grass trash removed after the conventional 14 day interval between spraying and sowing, inhibited seed germination. Direct physical effects of grass trash pressed into the drill slit were observed. Germinating grass seeds were susceptible to poor seed/soil contact particularly at the low moisture tensions experienced in these sandy soils. The marrow particle size range and greater number of large pores predispose such soils to rapid drying and consequently a low soil moisture content frequently occurs. A relatively small drop in soil moisture content was shown to stop or prevent the gemination of ryegrass seeds. The rapidity with which moisture cam fee lost from these soils emphasises the importance of ensuring closure of drill slits in order to conserve soil moisture and to maximise seed/soil contact. Poor seed/soil contact may result in imbibed but not germinated seeds being left in the soil. Although ryegrass seeds could survive dehydration provided no radicle protrusion had occurred, dehydration of germinated seeds caused their death. Ryegrass seed lots with acceptable laboratory germination were seen to differ in field performance (vigour). Rapid tests such as mean seed weight and conductivity of the seed soak water were not indicative of field performance . Tests which deteriorated seeds at high moistures and temperatures before a germination test could identify seed lots which had a high respiration rate which performed well under moisture stress and which also did well in direct drilled sowings. The use of high vigour seed was seen as a potential strategy to improve establishment in adverse sowing conditions. These results are considered in terms of the necessity of any cultivation system to place seeds in sites on or within the soil where they can safely germinate. Those factors which operate to reduce or improve establishment on these sandy soils are discussed in terms of the ability of the cultivation to create and to fill with seeds sufficient safe sites to produce a good sward.