White clover dynamics in New Zealand pastures

The study presented in this thesis is based on the overall target of increased herbage production in cooler times of the year, through the use of fertiliser N, while maintaining the benefits of white clover ( Trifolium repens L.) in New Zealand pastures. To help identify management strategies and plant breeding goals to attain this target, seasonal dynamics in cattle grazed pastures of clover population and plant structure and stolon dynamics were studied, including the effects of fertiliser N applications, grazing management and their interactions. The grazing treatments applied were continuous stocking with 4.9 Friesian bulls/ha and rotational grazing with either 4.9 or 7.4 bulls/ha. Fertiliser N (urea) was applied in mid autumn and late winter, at 50 kg N/ha per dressing.Average total herbage dry matter accumulation was about 16 tonnes/ha/year. The average efficiency of the applied fertiliser N was 17.5 kg DM/kg N in 1991 and 12.9 kg DM/kg N in 1992. The grazing treatments applied did not result in great differences in herbage accumulation or composition, and clover was maintained under all grazing treatments. Fertiliser N tended to decrease clover DM accumulation by 15% and clover content by 3.6%.Stolon growth dynamics and structures of populations and plants showed great seasonal variation. Growth was related positively to average soil temperature at 10 cm depth, which explained the greatest part of the variation for most growth parameters. Rooting was not related to temperature, rainfall, distribution of rainfall nor radiation.Continuous stocking during the cooler periods of the year, shifting to rotational grazing when temperatures rise, may help to maintain clover in the sward. Moderate fertiliser N applications in the cooler times of the year increase herbage accumulation and do not compromise clover permanently. However, it remains to be confirmed that the inhibited root production in spring, when N is applied, makes clover plants or branches in grazed swards more susceptible to local or temporal stress, such as drought. Plant breeding should be directed to improved rooting, and maintenance or improvement of N fixation.<strong>Key words</strong> : biomass allocation; branching; cattle grazing; climate; continuous stocking; fertiliser nitrogen; grassland; population and plant structure; radiation; rainfall; rainfall distribution; rooting; rotational grazing; stolon growth dynamics; temperature; Trifolium repens L.; white clover