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The significance of seed size on survival of some annual clover pasture species in south Australia

F. Squella and E.D. Carter

Plant Science Department,Waite Agricultural Research Institute, University of Adelaide, Glen Osmond SA 5064

Adaptation to grazing is one of the most relevant traits of annual legumes for persistence in pastures.There is clear evidence that under sheep grazing, particularly at high stocking rates, small-seeded legumes are the most persistent (1,2).


In order to achieve a better understanding of the role of seed size on seed survival of annual pasture legumes, a grazing study was conducted on a subterranean clover-balansa clover pasture on a hard-setting, red brown earth soil at Waite Agricultural Research Institute during the summer-autumn period (February-April 1990). To monitor the seed-seedling dynamics, 10 adult Merino wethers each fitted with a faecal-collection harness grazed dry pasture residues in a 0.20 ha paddock continuously for 10 weeks. The following parameters were recognised during this study: the decline in dry pasture, the seed reserves in the pasture-soil complex, the digestibility of the pasture components, the abundance of sheep faeces, the seed transmission through the digestive tract, the seed content and seedling emergence from faecal material, the regeneration of seedlings in the field, and the seeds harvested by ants. Finally, in order to separate the effect of the initial mastication and rumination from the digestion effect on seed survival, four penned rumen-fistulated Merino wethers of 10 months of age were used. The digestibilities of subterranean clover, balansa clover and cluster clover seeds obtained from the grazing study were tested in the rumen by using the nylon bag (in sacco ) technique. The first study considered the effects of seed size and time of seed harvesting during the grazing period on survival of clover seeds exposed during 48 hours in the rumen. The second study involved the effects of seed size and time of incubation in the rumen (12, 24, 48, 72, and 96 h) on survival of seeds collected just before starting the grazing experiment.

Results and discussion

Heads of balansa clover were preferentially selected by sheep early in the grazing period. These represent more accessible and digestible material (57.7% OM base) compared with subterranean clover burrs (48.1% OM base). Only when few heads were left, did sheep graze the subterranean clovers burrs, which at the end of the grazing period practically disappeared, including the buried burrs.

An important seed reserve of balansa clover was maintained in soil during the grazing period, representing a relevant factor on future seedling regeneration and forage production, especially during the early growing season. Comparisons between the disappearance of seed and the intake as shown by sheep faeces, reveals the great impact of chewing and digestion on seed survival, especially in the larger subterranean clover seed. The small and more hard-seeded balansa clover seeds tolerate those processes better. The throughput of total seed was estimated at 1.8% (germinable = 0.6%, hard = 1.2%) and 15.0% (germinable = 1.7%, hard = 13.3%) of the intake for subterranean clover and balansa clover, respectively.

In the first pen study, a significant interaction (species x time of harvesting) was obtained for total viable seed(P<0.005) and germinable seed(P<0.001), ratifying the importance of the level of hard- seededness associated mainly with the small seeded clovers on the survival of the seed. When the clover seeds were exposed in the rumen for different times, only a significant effect for clover species was obtained (viable seed, P<0.001 and germinable seed, P<0.005).

These studies have shown how small, hard-seeded annual pasture species normally have more suitable traits that permit them to better tolerate the impact of grazing and passage through the digestive tract of sheep.


Carter, E.D. and Lake, A. 1985. Proc. XV Int. Grassld Congr., Kyoto, Japan. pp. 654-656.

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

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