Introduction of perennial grasses into an existing lucerne sward by overdrilling

Two experiments were carried out to investigate the effects of some management practices on the establishment of perennial grasses (‘Grasslands Matua’ prairie grass, ‘Grasslands Maru’ phalaris and ‘Grasslands Nui’ perennial ryegrass) overdrilled into a five year old lucerne sward and the subsequent impact of the introduced grasses on sward composition and productivity. The first experiment (24 Nov. ’89 to 26 Feb. ’90) evaluated the effects of contrasting levels of lucerne cover (open vs. closed) on initial establishment and seedling growth of perennial grasses overdrilled into lucerne. Lucerne cover had no effect on the start, rate and extent of germination. However, 90 days after sowing (DAS) the closed lucerne canopy reduced seedling survival from 81 to 28% in Nui, from 81 to 37% in Maru, but had no effect on Matua survival. The closed lucerne canopy significantly reduced all attributes of grass seedling growth examined: dry matter (DM) per plant was reduced to 4% (from 1620 to 70 mg), leaf area (LA) per plant was reduced from 98 to 6 cm². Root length density was reduced from 4.8 to 2.2 km m⁻³. The number of tillers per plant of Matua, Maru and under the closed lucerne canopy was reduced to 30, 20 and 13% of the open canopy, respectively. The second experiment (11 Sep.’89 to 24 Oct.’90) examined the effects of some management practices (rainfed vs. irrigation; and untreated vs. 4 g a.i. thiram kg⁻¹ seed) on the establishment of direct drilled grasses and subsequent impact of introduced species on sward composition and productivity. Irrigation increased, though not significantly, seedling emergence from 357 to 424 m⁻². Fungicide had no effect on seedling emergence. Seedling survival 247 DAS was similar under both moisture regimes (~ 11% of maximum emergence). Seedling establishment at 247 DAS was 9% for both Maru and Nui and 17% for Matua. Fungicide seed treatment had no effect on seedling survival. Seedling growth assessment from the open and closed canopies at 40 DAS indicated that canopy closure had more influence on seedling growth than irrigation regime. Under the closed canopy DM per plant was similar whether irrigated or not. Under the open canopy, however, growth increased four to five fold without and with irrigation, respectively. Irrigation doubled leaf area per plant of Maru and Nui but had no effect on Matua. On the other hand there was a 3-, 4-, and 5-fold response in LA plane⁻¹ to canopy closure in Nui, Matua, and Maru, respectively. Four months after sowing, irrigation increased root growth, in the top 20 cm of soil, from 314 to 420 g DM m⁻³. Matua, Maru, and Nui had 442,358, and 303 g DMm⁻³. There was an indication that the performance of the grasses under irrigation during late summer to early autumn was related to the size of their root system. During the establishment year (Sep. ’89 to June ’90) cumulative yield under rainfed conditions was 170,220 and 970 kg DM ha⁻¹ for Nui, Maru, and Matua, respectively. Irrigation increased these yields to 800, 2700, and 4960 kg DM ha⁻¹, respectively. Under dryland conditions, all grasses had no significant effect on lucerne yield. Under irrigation, however, Matua reduced lucerne yield from 15350 to 11210 kg DM ha⁻¹; Maru reduced lucerne yield from 15350 to 13100 kg DM ha⁻¹. Nui had poor persistence and performance and did not affect lucerne yield. During the establishment year total sward yield averaged 12.3 t DM ha⁻¹ and was not affected by the grasses. Under irrigation during late summer to early autumn yield differences were related to the amount of irradiance intercepted by each component of the mixture. During the 1990 winter Matua under both moisture regimes and Maru from the irrigated plots significantly reduced weed and bare ground cover whereas Nui from the rainfed and irrigated plots and Maru from the rainfed plots had no impact on both weed and bare ground cover. The lucerne component of the mixtures was, however, similar in all species-irrigation treatment combinations. In the first spring following the establishment year, total DM yield from the rainfed plots was 16% higher than that from irrigated plots and reduced weed yield to 17% of the pure lucerne plot but it reduced the lucerne yield to 55% of the pure lucerne plot yield. The total yield of the Matua/lucerne mixture was, however, 23% more than the pure lucerne yield. Maru while reducing weed content to 42% of pure lucerne plots, did not affect the yield of the lucerne component. The results suggested that Matua appeared to competitively exclude the lucerne which initially was a vigorous stand. It is thus suggested that overdrilling lucerne, while it maintains complete ground cover may not be beneficial. Maru, however, may be a suitable associate if the lucerne exhibits gapping at the normal time of harvest