CHANGES IN PLANT-POPULATION DENSITY, COMPOSITION AND SWARD STRUCTURE OF A HILL PASTURE DURING A PASTORAL FALLOW

Appropriate pre-sowing methods for the introduction of improved forage legume and grass germplasm are an important issue for hill pasture im provement in New Zealand. A pastoral fallow, which involves not defoli ating pasture for a period generally from late spring/early summer to autumn, could create a potentially favourable environment for introduc ing improved germplasm. A field study was conducted on two aspects (sh ady and sunny) of moist, low-fertility hill country with or without ad ded fertilizer (phosphorus and sulphur) in the southern North Island o f New Zealand, to investigate the changes in plant population density and sward structure during a full or partial pastoral fallow, compared with a rotationally grazed pasture. A 7-month (October to May) pastor al fallow dramatically decreased the densities of grass tillers by 72% (P<0.01), white clover (Trifolium repens L.) growing points by 87% (P <0.01) and other species by 87% (P<0.05). The decline in tiller densit y by pastoral fallow was enhanced on the shady aspect. Fertilizer appl ication increased white clover growing-point density on the shady aspe ct (P<0.05) and grass tiller density on the sunny aspect (P<0.05). Dec reased plant density during pastoral fallowing was attributed to above ground biomass accumulation, which altered sward structure, leading to interplant competition and mortality by self-thinning and completion of the life cycle of some matured plants. The plant size-density relat ionship during pastoral fallowing in this mixed-species sward followed the self-thinning rule, particularly when the calculation was based o n all plant species rather than grass alone. There was no significant (P>0.05) difference in final plant population density between the 7-mo nth pastoral fallow and a shorter term (October to December) pastoral fallow. It is concluded that pastoral fallowing effectively reduced th e plant population density and altered sward structure of a hill pastu re. Such changes create a more favourable environment for the introduc tion of improved forage species.