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Abstract
The compositional changes of grazed swards of perennial ryegrass (Lolium perenne L. cv. Grasslands Ruanui), cocksfoot (Dactylis glomerata L. cv. Grasslands Wana), and phalaris (Phalaris aquatica L. cv. Grasslands Maru) as affected by annual applications of 2,2-DPA at 1.0 kg/ha and fertiliser (lime and phosphate), were compared from 1985 to 1988. The swards were established on a cultivated, low-fertility, semi-arid hill soil in Marlborough containing a high seed load of the noxious Chilean needle grass (Stipa neesiana Trin. … Rupe.). In the absence of fertiliser or 2,2-DPA, S. neesiana increased to dominance in the ryegrass sward over the three years, remained constant in the cocksfoot, and declined as a proportion of the phalaris sward. The cocksfoot contribution remained constant, but the contributions of both ryegrass and phalaris declined in theirrespecti ve swards. Fertiliser induced temporal stability in the swards in respect of the proportional contributions of S. neesiana and sown grasses, and overall, reduced the growth and seeding of S. neesiana. Annual doses of 2,2-DPA reduced the growth and seeding of S. neesiana by a greater amount than fertiliser, but also reduced the growth of the cocksfoot. The results suggest that the invasion of perennial ryegrass pastures near Lake Grassmere by S. neesiana can be attributed to the interaction of low soil fertility and the divergent adaptive strategies of the two species. Improvement of soil fertility or selective control of invading seedlings with low doses of 2,2-DPA are possible solutions in new perennial ryegrass pastures.