The influence of pastoral fallow on hill country pastures, emphasising white clover growth behaviour : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, New Zealand
Appropriate pre-sowing methods for oversowing new plant germplasm and the presence of productive legume are of key importance to development of New Zealand hill country. A pastoral fallow, which involves not defoliating pasture for a period generally from spring to autumn, has profound influence on plant and soil, and creates a potentially favourable environment for introducing improved germplasm. A series of field and glasshouse experiments examined the response of pasture structure, soil properties and natural reseeding to pastoral fallow, and the post-fallowing effects on white clover (Trifolium repens L.) and pasture growth in moist North Island hill country, New Zealand. Pastoral fallowing effectively reduced the plant population density and altered the structure of a hill sward. A seven-month (October - May) pastoral fallow dramatically decreased the densities of grass tillers by 72%, white clover growing points by 87% and other species by 87%. The decline in tiller density by pastoral fallow was enhanced on a shady, south facing aspect. Root growth and distribution was altered by pastoral fallowing and there was significantly less root biomass at 0 - 50 mm depth of soil in the fallowed than the grazed sward. Decreased plant density during pastoral fallowing was attributed to above-ground biomass accumulation which altered sward structure, leading to inter-plant competition and mortality by self-thinning and completion of the life circle of some matured plants. Pastoral fallowing significantly improved soil physical properties. Compared with the grazed treatment, pastoral fallow increased soil air permeability at 500 mm tension by 38%, saturated hydraulic conductivity by 26%, unsaturated hydraulic conductivity at 20 mm tension by 56% and soil moisture by 10 - 15%, and reduced soil bulk density by 11% at the end of an October - May pastoral fallow. Pastoral fallow had little effect on the concentration of most nutrients in soil both at the end of fallowing and two to three years after fallowing. A spring - autumn pastoral fallow increased the viable grass, legume and weed seed population by 51-160%, compared with the grazed control. The variation in viable seed population during the fallow followed a predictable pattern, which could be used to manipulate natural reseeding in practice. Regression analysis revealed that the patterns of cumulative seedling appearance followed a modified negative exponential function. Partial differentiation of this function derived a germination rate curve on which a two-pool (rapidly germinable pool and base pool) model was developed to quantitatively describe the seed dynamics of soil viable seed reserves. Short-duration (partial) pastoral fallow had a marked effect on plant population density and natural reseeding. Pastoral fallows starting from December, January and February or March with nitrogen addition and ending in June considerably reduced plant population density. Most partial pastoral fallows also considerably reduced viable seed population of all plant species, except for December to June fallow which had a higher viable grass seed population than the grazed sward. Pastoral fallowing increased dispersion of white clover stolons by internode elongation. At the conclusion of a pastoral fallow, the clover stolons initiated branching, and grasses initiated tillering. Their growth and competition resulted in a greater grass growth rate in the first two years after fallowing, and a greater white clover yield and content with an increase in clover patch density and size 3 - 4 years post-fallowing, compared with the grazed pastures.