The Cook horse paddocks are an ACT Government horse holding facility to the south of Mackellar Crescent, Cook. For equestrians it is a ride of about 800 m from the paddocks to the proposed entry to the Aranda Bushland.
The paddocks are infested with Paterson's curse, and have been for a number of years. During the drought of the early 1980s hay fed to horses in the paddocks is likely to have contained large numbers of Paterson's curse seeds (3), and the infestation may have started then. If it already existed, it probably received a major boost at that time.
The weed is or has been very widely distributed throughout most parts of the paddocks. At times densities have been very high. There are places where the infestation is heavy right up to the fence, but there are only occasional plants on the other side of the fence where horses do not go, and the density diminishes rapidly to zero as one moves away. Drastic measures have been taken against Paterson's curse in the horse paddocks from time to time, but they have not achieved lasting control.
Paterson's curse is also found along the horse trails that radiate from the horse paddocks. In a letter to Canberra Nature Park management in November 1997 the infestation was described in the following terms:
As in previous years there is a worsening problem with Paterson's curse along the horse trail - most of it on and beside the section of trail nearest the Cook horse paddocks mounting yards - and spreading out from this source. I have indicated the worst infestations on the map - but it is spreading further and further into the surrounding grassland as each year goes by without adequate control. There are still large areas of Paterson's curse in the Cook horse paddocks.
The map referred to shows the weed extending along the horse trail for some 350 m to the west of the horse paddocks exit, and a patch (not on a horse trail, but in an area of native grassland that has been used by horses) about 350 m to the east of the exit. On 22 January 1998 only one Paterson's curse plant was found along the well worn horse trail beside the cycle path that leads to the underpass under Bindubi Street, but some concerned Cook residents walk along this trail and hand pull Paterson's curse there whenever they see it.
The following summary of what is known about dispersal of Paterson's curse is due to Parsons and Cuthbertson (2).
Movement of seeds is the only means of dispersal. The seedcoat is roughened allowing the seeds to adhere to the wool and fur of animals. Seeds also pass through the alimentary tracts of grazing animals without losing viability. Movement of seeds in water is important, particularly in hilly country where run-off water carries them to lower areas. The most important means of spread, however, has been as a contaminant of hay or grain; this is most noticeable in times of drought when there is a considerable movement of fodder (as well as livestock) throughout Australia.
Piggin and Sheppard (4) reinforce these statements and list ants and birds as additional vectors.
1. In hay fed to horses in times of drought
This is probably the most important way in which Paterson's curse has been brought into the Cook area, at least since 1980. If it were the sole means of introduction and dispersal within the horse paddocks, the weed would now be found only where hay containing its seeds has been put out. The extensive coverage of the paddocks by the weed indicates that additional means of dispersal have operated.
2. Through the alimentary tract of horses
Because cattle and horses tend to avoid the plant, Paterson's curse becomes more dominant where these animals are grazed. When the plant runs to flower it becomes rank and unattractive to stock (2).
However hay fed to horses during drought may well contain large numbers of Paterson's curse seeds which might be ingested under these circumstances and passed in the faeces in a viable condition.
3. On the coat or hooves of horses
When a horse brushes against a Paterson's curse plant carrying ripe seed, seeds could adhere to the horse's coat and drop off elsewhere.
Soil seed bank densities of 2000-10000 seeds per square metre have been recorded in grazed pastures with a history of Paterson's curse in southern NSW (4). While the density of the seed bank in the horse paddocks is no doubt influenced by factors including the history of weed control measures, it has probably been high for at least some parts of the period since 1980. In the laboratory, seed has been shown to remain viable for at least 6.5 years (4).
Seeds from the surface of the soil could adhere to the coat of a rolling horse and be shed elsewhere.
Compacted soil or mud containing seeds could adhere to horses under or on the hooves or on the coat, and be shed elsewhere.
Paterson's curse is or has been found in nearly all parts of the horse paddocks that are accessible to horses, right up to the fences, and along horse trails outside the paddocks. This pattern of distribution is consistent with dispersal of the seed by horses by the mechanisms outlined above.
4. On vehicles or people
Seeds or soil containing them could adhere to the tyres or other parts of vehicles or to the footwear or clothing of pedestrians, and be shed elsewhere. If vehicles or people were the sole vectors they would be unlikely to give rise to the extensive coverage observed in the horse paddocks.
The Mount Painter unit of Canberra Nature Park adjoins the horse paddocks on the west. Horses have not been in it since it became part of Canberra Nature Park in 1996, and not on a regular basis for many years before that. The mountain paddock was taken out of lease early in the 1970s and used for short term agistment. Whenever horses were allowed in it, local residents complained until they were removed.
There is a gate at the reserve's north eastern corner and another near the reservoirs. Well defined wheel ruts show that both gates have been used as access points to the reserve for vehicles. Both gates are popular entry points for pedestrian visitors, many of whom climb Mount Painter. Many of the vehicles and pedestrians that have visited the reserve would have passed through the Paterson's curse infestation along the horse trails between the reserve and the back of the Cook houses.
The reserve is free of Paterson's curse except for a narrow strip along the horse paddocks fence. Vehicles and pedestrians are capable of spreading Paterson's curse, but at Cook they appear to have played a minor role compared to horses.
5. On kangaroos
The paddock fences in the area are not an effective barrier to kangaroos, which move freely and often between the infested horse paddocks and adjoining areas. If kangaroos were an important vector in the local area, Paterson's curse would be much more widely distributed in the Mount Painter reserve, in the road reserve adjoining the paddocks on the south, and on the verges of Bindubi Street than it actually is.
6. By ants or flowing water
These factors may well be responsible for many of the plants that occur outside and close to the horse paddocks in places where horses do not go.
7. By birds
Birds eat seed of E. plantagineum and may spread it over large distances. The seed forms a large part of the diet of crested pigeons (Ocyphaps lophotes Temminck) in summer and autumn in Australia, especially in drought years (4). The level of activity by birds in and around the horse paddocks is not very different from that in surrounding areas. If birds had played a major role in the dispersal of Paterson's curse in the Cook area one would expect it to be more widely distributed, and less narrowly focussed on the horse paddocks and horse trails.
In summary, the most important means of dispersal of Paterson's curse in and around the Cook horse paddocks appears to be by carriage of seed by horses on the hooves or coat, and in the alimentary tract in times of drought.