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Ley cropping systems benefit from low rates of seed softening of annual pasture legumes in southern Australia.

P.M. Evans, J. Chin, P.A. Riffkin and X. Zhang

Agriculture Victoria, Pastoral and Veterinary Institute, Private Bag 105, Hamilton, Vic.


The rates of seed softening in subterranean clover in cool-temperate environments are about half of those experienced in typical Mediterranean climate. This suggests that a 1:1 crop:pasture system would result in excellent crop yields and regeneration of the pasture legume in the high rainfall zone. To test this hypothesis, an old grazing experiment containing four subterranean clover treatments in mixtures with perennial ryegrass was put into a 1:1 crop:pasture rotation, starting in 1997. The clover treatments were: Leura, Trikkala, Enfield and a mixture containing Karridale, Larisa, Trikkala and Enfield. The highest wheat yields in 1997 and 1999 were achieved with Silverstar, direct-drilled on Leura, with 7 and 7.4 t/ha respectively. The protein levels were 12.6 and 12.8%. Clover seed banks reached 1880 and 1380 kg/ha for Trikkala and Leura respectively in summer 2000, resulting in a seedling regeneration of 2600 and 4640 seedlings/m2 in autumn. It is concluded that in cool-temperate, high rainfall environments longer cropping phases may allow successful regeneration of pasture after cropping.


Subterranean clover, pasture crop rotations, ley system, cool temperate environments.


In cool-temperate environments subterranean clover seed softens at a much slower rate than in lower latitudes with a typical Mediterranean climate (1). This suggests that a 1:1 pasture:crop (P:C) rotation in which all regenerating pasture is killed with herbicide in the crop year should be successful in terms of: a) crop yields- because of the high production of legume in pasture years and, b) pasture regeneration after crop-because of the accumulation of large seed banks. It is now generally regarded that the ley system is in decline (2). This is because in typical Mediterranean climates seed softening rates of pasture legumes are very high, for example more than half the seeds of a subterranean clover seed bank will soften over the first summer and below 20% of the initial seed population persists beyond the third summer (3, 4). A factor affecting the viability of ley systems in southern Australia has been a decline in the legume content of pastures (5, 6) over and above that expected by seasonal fluctuation and reduced legume component as fertility increases (7). This decline was attributed to a decline in the quantity of legume seed in the soil. (5). In cool-temperate areas this need not be so as, after six years, more than 20% of subterranean clover seed initially set still remained in the ground in Tasmania (8).

In this paper we examine the hypothesis that under a cool-temperate regime, a 1:1 crop:pasture rotation is sustainable, both in terms of high crop yields and the accumulation of large banks of subterranean clover seed, leading to dense pasture after the cropping phase.


A grazing experiment comparing different cultivars of subterranean clover in mixtures with perennial ryegrass, sown in 1990, was put into a 1:1 P:C rotation at Hamilton in autumn 1997. The pasture treatments sown in 1990 were Leura, Trikkala and Enfield subterranean clover and a mix containing Enfield, Larisa, Trikkala and Karridale. Subterranean clovers were sown with 5 kg/ha of Ellett perennial ryegrass. Total sowing rate for the subterranean clovers was 10 kg/ha and plots were 0.2 ha. Single superphosphate and muriate of potash were applied at recommended rates to maintain fertility. Nitrogen fertiliser was not applied. Summer and winter seed banks of subterranean clover were determined by taking 30 soil cores from each replicate to a depth of 5 cm, followed by washing off the soil in sieves, threshing and aspirating the samples and finally hand-sorting the seed. Clover regeneration was determined each autumn by counting seedlings in 50 cores of 8 cm diameter from each replicate. Clover content of the pasture was determined during the grazing phase by hand-sorting a sub-sample from 50 quadrats taken at random from each replicate after mixing thoroughly. The grass was killed with selective herbicides in the spring of 1996. In 1997, four wheats ranging widely in maturity were each direct-drilled into each pasture at three dates of sowing, after killing the pasture with glyphosate and dicamba. The pasture was allowed to regenerate in 1998 and in 1999 the same four wheats were sown over the whole area in May.


In September of the first crop year (1997) the average subterranean clover seed bank was 440 kg/ha, the highest seed bank, under Leura, reached 630 kg/ha (Fig. 1). During the pasture years, the highest average content of clover was that of plots sown to Leura - with 31% of dry matter. Silverstar wheat sown after Leura in 1997 yielded 7 t/ha of wheat (12.6% protein). The second crop of Silverstar wheat in 1999 yielded 7.4 t/ha (12.8% protein). The average clover seed bank after 4 seasons under this 1:1 system exceeded 1000 kg/ha. Under continuous pasture with perennial ryegrass, the highest average clover seed bank across all treatments was 580 kg/ha in 1994, which resulted in the greatest seedling regeneration of the pasture phase - 1840 seedlings/m2. After the first crop in 1998, clover regenerated at 2180 seedlings/m2. (Fig. 2). After the second crop the average number of seedlings recorded was 3500/m2. Leura produced 4640 plants/m2.

Figure 1. Seed yield (kg/ha) in the soil of 4 subterranean clover treatments at Hamilton between 1991 and 2000.


The large mass of seed accumulated and dense regeneration of subterranean clover after crops suggest that a 1:1 ley system in a high rainfall cool temperate environment is easily sustainable in terms of fertility and pasture regeneration. Results suggest that longer cropping phases such as P:C:C could be used, while still achieving acceptable pasture regeneration. This latter rotation with increased cropping may reduce the incidence of soil acidification. However, this P:C:C rotation may lead to pasture plant densities below 1000 plants/m2, which, although acceptable from a seed production point of view, would reduce autumn-winter production.

As acceptable wheat yields of more than 3 t/ha can be achieved with spring-sowing in this environment, the problem of herbicide resistance could occasionally be solved in this way. An advantage of spring-sowing is that a knockdown herbicide can be used in August to achieve a complete weed kill and the autumn-winter pasture growth can be grazed in a period when the feed supply is limited.

It is interesting that in this environment the legume seed reserves maintained under a clover-ryegrass mixture are lower that those under a 1:1 pasture:crop regime. Work in Tasmania (8) showed that subterranean clover in a mixture with perennial ryegrass resulted in a 23% reduction in seed yield compared to the clover monoculture. The results from the 1:1 system show that seed yields of clover in summer can exceed 1500 kg/ha after removing grazing animals from mid-October until January.

Figure 2. Seedling regeneration (seedlings/m2) of 4 subterranean clover treatments at Hamilton between 1990 and 2000.

It is clear from the data that since cropping started in 1997, both subterranean clover seed banks and pasture legume regeneration have increased compared to when the clover was grown in a mixture with perennial ryegrass.


This project is supported by the Grains Research and Development Corporation and the Department of Natural Resources and Environment, Victoria. Neil Cameron and Anthony Wright provided technical support.


1. Evans, P.M. and Smith, F.A. 1999. Agronomy Journal 91, 122-127.

2. Howieson, J.G., O’Hara, G.W. and Carr J.S. 2000. Field Crops Research 65, 107-122.

3. Taylor, G.B., Rossiter, R.C. and Palmer, M.J. 1984. Aust. J. of Exp. Agr. Anim. Husb. 24, 200-212.

4. Taylor, G.B. and Ewing, M.A. 1992. Aust. J. of Exp. Agric. 32, 331-337.

5. Carter, E.D. 1982. Proceedings 2nd Australian Agronomy Conference, Wagga Wagga, p 180.

6. Carter, E.D. 1992. In: Pastures in cropping rotations: putting knowledge into practice. Proceedings of Pasture Symposium held at Roseworthy, SA 1992. pp. 49-51.

7. Rossiter, R.C. 1966. Advances in Agronomy 18, 1-57.

8. Evans, P.M. and Hall, E.J. 1995. Aust. J. of Exp. Agric. 35, 1117-1121.

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