Summary
Current practice relating to live sheep export feedlotting
Live sheep export feedlotting is an important part of live sheep export. Its purpose is two-fold. Firstly, it enables large numbers of sheep to be assembled over a short period prior to export. Secondly, during this period sheep have an opportunity to acclimatise to ship-like conditions of feeding and general management. In paddock-based feedlots, which are located in Victoria, South Australia and Western Australia, sheep are held at high stocking density in a series of small paddocks. In shed-based feedlots, which are located only in Western Australia, sheep are held in large, raised sheds, each holding approximately 7,500 animals. , The standard of management in all large Australian live sheep export feedlots, including the paddock - and shed-based systems, is very high. Regardless of whether sheep are owned by the feedlot operator or managed under contract, there are very strong incentives for operators to minimise sheep losses during feedlotting.
Salmonellosis during feedlotting
There are two different syndromes of salmonellosis in the live sheep export trade. The current problems during feedlotting are due to feedlot-related salmonellosis, which has become a significant problem for many animal based industries following intensification. The second syndrome, the persistent inappetence-salmonellosis-inanition (PSI) complex, is the main cause of death of sheep during shipping. The PSI complex was the focus of detailed research by scientists from Agriculture WA during the 1980s and early 1990s.
Outbreaks are sporadic and unpredictable, but may be more common between the autumn break and early summer. All outbreaks are related to high throughput, and are believed to involve Salmonella typhimurium and/or S. bovis-morbificans. In all outbreaks of salmonellosis during feedlotting, deaths have been highly clustered in a small number of non-contiguous paddocks. However, 'problem' paddocks did not persist from one consignment to the next. Though not consistently affected, high-risk lines include young animals, animals in poor conditions and long-haul and pastoral sheep. Risk of disease is substantially higher if animals arrive when the weather is cold, windy and wet. Goats are highly susceptible to salmonellosis. Similar outbreaks of salmonellosis have occurred in goats, which are highly susceptible to the disease, but not in cattle. In feedlots, cattle and sheep are generally managed separately.
Why the problem is occurring
A number of factors are critical to the development of feedlot-related salmonellosis, including the size of the Salmonella challenge, the level of host resistance and the virulence of the serovar involved. There is a dose-response relationship between Salmonella challenge and clinical disease, and as host resistance is reduced, a much smaller challenge of Salmonella organisms is needed to produce clinical disease.
Host resistance and Salmonella challenge are both important causal factors for outbreaks during feedlotting, although there is good evidence suggesting that host resistance is the more important. Time-off-feed is a key risk factor for outbreaks.
Long-term control of feedlot-related salmonellosis
The development of a constructive and sustainable partnership between industry and relevant government regulatory bodies would assist with efforts to develop a culture of continuous improvement within industry. The control of feedlot-related salmonellosis is based on efforts to increase the level of host resistance and reduce the size of the Salmonella challenge. Because the current standard of management in feedlots is high, further progress in controlling salmonellosis is likely to be incremental.
a. The strategic use of sheds in paddock-based feedlots
The strategic use of sheds in paddock-based systems is likely to be the most effective means to control feedlot-related salmonellosis. Sheds offer a range of advantages during feedlotting, including increased flexibility in sheep selection and management, reduction (and potential elimination) of outbreaks of salmonellosis, reduced labour costs, income from the sale of manure and reduced environmental concerns. It is important to note, however, that there are also a number of concerns relating to the introduction of sheds, including the high capital cost, concerns about the most appropriate floor design and potential concerns with sheep management in a partially-shedded feedlot.
b. Additional strategies to increase the level of host resistance
Vaccination is currently not a viable option.
Consistent feed intake, which affects host resistance, appears to be critical to the prevention of feedlot-related salmonellosis. Immediate opportunities available to industry to increase host resistance and reduce the risk of feedlot salmonellosis include:
Imposing curfews where producers withhold water but provide dry feed to sheep
Using feeding systems that enable feed to remain dry, and with minimal faecal contamination, throughout the feedlot period
Holding sheep in paddock-based feedlots for the shortest time possible before loading on-ship
c. Additional opportunities to reduce the size of the Salmonella challenge
Efforts to reduce the size of the Salmonella challenge will play an important role, albeit secondary to efforts to increase host resistance, in controlling feedlot-related salmonellosis.
