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The significance of cluster clover (trifolium glomeratum l.) In a subterranean clover-perennial ryegrass pasture under two constrasting management systems.

F.Squella and E.D. Carter

The University of Adelaide Waite Agricultural Research Institute. Glen Osmond, South Australia 5064

Summary. Pasture survey data were collected from two nearby pasture paddocks carrying a mixed stand of subterranean clover, Trifolium subterraneum, and perennial ryegrass, Lolium perenne on a sandy loam soil under two contrasting management systems. Paddock I had been set-stocked during the past 15 years with about 20 crossbred ewes/ha plus lambs. Paddock II had been cut for hay over the past twelve years and stocked at c. 13.8 crossbred ewes/ha plus lambs. Subterranean clover cultivars were affected more severely by heavy set-stocking than by regular hay cutting: however. adequate seed was set in both paddocks. Cluster clover, Trifolium glomeratum, was far more abundant on the hard-grazed area than on the area cut for hay.

Introduction

In all Mediterranean-type environments of the world, sheep grazing pressure has a dramatic impact on yield and botanical composition of annual legume-grass pastures. Furthermore, under continuous, heavy grazing pressure small-seeded clovers such as cluster clover increase as a percentage of botanical composition and often in total quantity (I, 2, 3). Under conditions of severe grazing pressure there is a critical balance between seed production and seed consumption during summer and autumn which determines seed survival and subsequent additions to the seedbank or seedling density and consequent productivity of legume pastures.

Recent in vivo feeding experiments have shown the importance of hard-seededness and seed size in the survival of pasture legumes following ingestion by sheep (4). Furthermore, in sacco digestibility studies have shown clearly that legume seed is mainly destroyed by chewing by sheep rather than digestion: small seed escape chewing and hard seed escapes digestion (6, 7). This paper summarises some of the results of a farm survey to study the influence of two contrasting pasture management systems on botanical composition.

Methods

The pasture survey was conducted at Parawa on Fleurieu Peninsula, South Australia. Two paddocks (hereafter referred to as Paddock I and Paddock II) containing a subterranean clover, Trifolium subterraneum, and perennial ryegrass, Lolium perenne, pasture mixture established on a lateritic podzol soil with sandy loam surface with contrasting management histories were selected. Paddock I had been heavily set-stocked during the past fifteen years with about 20 crossbred ewes/ha plus their lambs. Paddock II had been cut for hay over the past twelve years (1978-1989) and stocked with about 13.8 crossbred ewes/ha plus lambs. During the period of the study (14 September 1990 - 16 May 1991) the paddocks were grazed continuously by sheep and stocked according to the rates indicated above.

To monitor the seed-seedling dynamics in the soil-pasture -animal complex, the seed reserves in the soil, pasture and faeces as well as some sward characteristics (plant density, herbage availability, botanical composition and percentage bare ground) were estimated, before the legume species commenced flowering (14 September 1990), before the subterranean clover cultivars and cluster clover had set seed (5 November 1990), when the seed of both clover species was mature (9 January 1991), before the opening of the normal rainfall season (18 March 1991) and after 48.5 mm of rainfall (16 May 1991) to examine the re-establishment of the pasture sward species. Furthermore, data were collected from grazed and ungrazed areas. However, data for the grazed areas only are presented in this paper.

Results and discussion

Data on plant density, herbage availability and legume hard seed reserves (Table I) show the impact of management practices on the decline of subterranean clover (hereafter referred to as sub clover) and the increase of cluster clover and winter grass, Poa annua, on the hard-grazed area as found previously (1,2,3). Barley grass, Hordeutn leporinum and capeweed, Arctotheca calendula, were also prominent on the hard-grazed area.

Table I. Plant density, herbage availabilty and legume hard seed reserves in a grazed subterranean clover-perennial ryegrass pasture at Parawa in September. 1990.

The data on seed reserves (Tables 2, 3) clearly show that the heavy grazing reduces the percentage of sub clover seed that is buried : in part this is due to greater soil compaction. However, on both paddocks sub clover seed production is satisfactory which contrasts with many of the dairy pastures in the Adelaide Hills (5). It should be noted that cluster clover seed levels in Paddock I were significantly higher in January 1991 than in September 1990. Despite this, a large amount of seed disappeared from the grazing area. At least some of this loss of seed is ascribed to ants (6).

Table 2. Total legume seed reserve in grazed subterranean clover-perennial ryegrass pasture at Parawa in January 1991.

Table 3. Total seed reserve under grazed conditions in a subterranean clover -perennial ryegrass pasture at Parawa (Paddocks 1 and II; March, 1991).

Table 4. Seedling density, herbage availability and legume hard seed reserves under grazed conditions in a subterranean clover-perennial ryegrass pasture at Parawa in May, 1991.

Table 5. Changes in the legume seed reserve in the soil-plant complex of a grazed subterranean clover- perennial ryegrass pasture at Parawa.

Following the opening rains the data for May 1991 (Table 4) again highlight the adaptation of cluster clover to heavy grazing pressure and the relative reduction of productivity of sub clover. However, as shown in Table 5, the Parawa environment with 900mm annual rainfall has ideal soil and climate for growing excellent pastures of subterranean clover ( a range of cultivars) and perennial ryegrass. In conclusion, this survey has shown that despite the potential for growth of subterranean clover in the Parawa environment, this species gives way to cluster clover under conditions of sustained heavy grazing inter alia because a high percentage of cluster clover seed (c. 76%) can survive ingestion by sheep (6) whereas only c. 2% of sub clover seed survives ingestion by sheep (4).

Acknowledgements

We thank the Chilean Government for providing a scholarship to allow Fernando Squella to undertake this survey as part of his Ph. D studies. Miss Patricia Gianquitto, Mr. Richard Porter and Mr. Rick Llewellyn greatly assisted with field and laboratory work and preparation of the paper.

References

1. Carter. E.D. 1968. In: Biennial Report, Waite Agric. Res. Inst. 1966-67, p.43.

2. Carter. E.D. 1987. In: Temperate Pastures: Their Production, Use and Management (Eds. J.L Wheeler, C.J. Pearson and G.E. Robards) Australian Wool Corporation/CSIRO pp 35-51.

3. Carter. E.D. 1990. Proc. 9th Aust. Weeds Conf. Adelaide SA. pp. 239-242.

4. Carter. E.D, Challis, S., Knowles, R. C. and Bahrani. J. 1989. Proc. 5th Aust. Agron. Conf.. Perth. WA. p.437.

5. Carter. E.D. and Cochrane, M.J. 1985. Proc. 3rd Aust. Agron. Conf. Hobart.Tas. p.217.

6. Squella, F. 1992. The Ecological Significance of Seed Size in Mediterranean Annual Pasture Legumes. Ph. D. Thesis. The University of Adelaide, 466p.

7. Squella. F. and Carter, E.D. 1993. The significance of seed size on survival of some annual clover seeds in sheep pastures of South Australia. Proc. XVII Int. Grassi. Cong. Palmerston North. New Zealand. (In Press).

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