Environmental selection of pasture species at Lake Tekapo

Abstract

Thirty environments resulting from the combination of five soil fertility regimes, three stocking rates and two grazing methods were imposed on multi-species mixtures overdrilled into hawkweed-dominated short tussock grassland at Lake Tekapo.

Legume species dominated during the three year trial period covered by this study. The most important legume from 0 to 250 kg fertiliser ha-1 yr-1 was Russell lupin (Lupinus polyphyllus ), used here as a pasture species. The second most important species was alsike clover, followed by red and white clovers, the latter only under irrigation and high fertilisation.

The most important experimental factor affecting the relative ranking of species during the experimental period was soil fertility regime; secondly, stocking rate and last, grazing methods.

Introduced and resident species were significantly related to environments defined as follows:

• Fescue tussocks, mouse-ear hawkweed and adventive grasses with unimproved soils and light grazing; • Russell lupin with little-amended soil conditions and moderate grazing pressure; • White clover with fully developed conditions; • Red and alsike clover with increasing grazing pressure and lower soil fertility regimes.

Hieracium did not disappear as a consequence of soil improvement and grazing but appeared to be favoured by hard grazing.

Tussocks within dense legume swards did not seem to play any role of importance; if the canopy was open they became more conspicuous, but always at the lower range of relative abundance. A supplementary experiment showed that in the absence of other vegetation shelter, the shelter provided by silver tussocks to shorter grasses resulted in increased relative growth rates in swards, providing that a rather sparse tussock distribution (plants 120 cm apart) was maintained. A closer tussock canopy could produce an opposite effect.

Sheep grazing preferences were detected at all situations, but actual total utilisation of pasture did not present important differences between introduced species. True rejection occurred only for mouse-ear hawkweed at soil improved treatments.