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Selecting Your Source of Breeding Stock

(52) The selection of the correct health status appropriate to your herd and location is vital before breeding stock are purchased. Your primary reason for purchase is to genetically upgrade your herd. Major requirements will be that they are available when you want them, in the numbers that are needed and at a price you can afford. But an overriding requirement is that they will not cause disease in your herd and lower your overall health status. At the onset therefore, consult with your veterinarian and ask him to determine at a veterinary level the information available about the proposed donor herd.

The investigations should include the disease history since its inception and those of any daughter herds that have been established from it. Also the health status and disease history of other herds it supplies. All veterinary reports should be requested and examined together with the results of tests for specific diseases and the frequency of such tests. The breeding history on the farm, should be checked together with any evidence of infectious reproductive disease. A detailed study of records of production parameters, growth and food conversion rates may be helpful. The biosecurity of the breeding pyramid should be checked along with details of the health programme. The biosecurity of the donor herd itself must be assessed including the methods by which pigs or genetic material are brought into the herd. Finally a written veterinary statement should be obtained indicating that on both clinical and pathological grounds those selected diseases that you wish to keep out of your herd have not been diagnosed in the donor herd.

Buying breeding pigs - The ground rules

Step 1 - Select the source based on:

  • Availability.
  • Genetics (including fecundity).
  • Health.
  • Market acceptability.
  • Quality control.
Step 2 - Determine with your veterinary advisor the health status of your own herd.
Step 3 - Request veterinary liaison with the suppliers' veterinarian and get clarification of the health status of the donor herd.
Step 4 - Assess the compatibility of health status. Step 5 - Determine the isolation requirements for incoming stock. Step 6 - Decide on vaccination and acclimatisation procedures.

The donor herd
The suppliers may want to know the health status you require and offer you a choice of sources. Always purchase from a DHHS herd or equivalent if available.

What are the methods and risks of pig movement
Since incoming pigs are probably the greatest potential source of infection to your herd, the methods by which they are introduced or other methods by which you improve the genetic potential of your herd are vitally important.

Five methods are available:

  1. By introducing live pigs.
  2. By segregated early weaning SEW.
  3. By hysterectomy.
  4. By embryo transplants.
  5. By artificial insemination (AI)

Live pigs

a) Mature gilts and boars
Live pigs can be brought into your herd from a source herd of matching health status, or through SEW or hysterectomy and fostering if the source herd is of known but lower health status (depending on the disease to be eliminated).

If live pigs are brought into your herd with or without SEW it is advisable to hold them in isolation for a period before integrating them into your herd to check whether they develop disease and whether disease breaks out in the source herd. If the isolation premises are in a different site to your herd and not of the same biosecurity standards as your recipient herd, there could be a greater risk in holding them there rather than integrating them directly into your herd. The dangers of integrating them directly into your herd are obvious, namely, that if they are incubating an infectious disease sub-clinically then ultimately your herd will become infected. Perfect separate quarantine facilities are rarely available to commercial herds, particularly smaller enterprises but isolation that falls short of complete quarantine (e.g. on the same site) can be surprisingly effective. The incoming stock could be moved into a separate building on the same site preferably over 50 metres distant and this should be reasonably effective, provided separate boots and coveralls are used to tend the animals and provided the drainage from the building does not flow into your other pig buildings. If a separate building is not possible then a separate room sealed off from the main body of the herd is better than direct integration into the herd.

How long should the incubation period be? Here the importance of veterinarian liaison to match respective health status has already been highlighted.

If your herd is believed to be enzootic pneumonia (EP) free then it is advisable to place the incoming animals in isolation for a period of eight weeks. At the same time sentinel pigs (i.e. pigs from your herd due for slaughter) should be moved in and blood tested and / or slaughtered prior to the entry of the new pigs and their lungs examined for EP freedom. If your herd is not free of EP, the length of isolation is debatable. Some veterinarians would advise six weeks but four is more practicable.

Should enteric or respiratory disease appear during the four week period either in the pigs in isolation or in the source herd the chances of preventing further damage by immediate slaughter would be reasonable.

b) Breeder weaners
Instead of buying in mature replacement gilts and boars you could buy in so-called breeder- weaners, say, 30kg live weight. This has the advantage of allowing them a long period of acclimatisation to your herd before you breed them. It also enables you to rear them yourself in the way you think best for future breeding gilts and allows you to carry out your own selection at slaughter weight. A disadvantage is that boars cannot be performance tested and therefore it is not feasible. Also, if you sell your pigs at 25-30kg or at weaning, you probably do not have the facilities to rear such pigs.

The advantages of buying in breeding stock at a commercial level, compared to the selection of the home produced gilt are its low cost, the availability of gilts when they are required, the genetic potential is constantly improved and if done carefully presents few problems. Some farms however prefer to breed their own breeding females and thereby only introduce into the herd, a small proportion of grand-parent females and boars. This policy often fails because of the difficulty of rearing the future female replacements within a commercial operation, the poorer reproductive performance and the fact that the gilts reared on the farm are often not available when required. This system is also a high cost one and often results in lower numbers of pigs reared. Extensive experiences have shown that provided there is good health liaison and sensible practical procedures then the herd health status can be maintained with the purchase of breeding stock.

Segregated early weaning (SEW) - Modified Medicated Early Weaning (MMEW)
The second method of bringing in live pigs from another herd is through a modification of the medicated early weaning (MEW) technique called by many segregated early weaning (SEW) and by one breeding company Isowean. This is based on the principle outlined earlier under "How infectious agents are spread". By the time females reach their first farrowing they have developed a strong immunity to the more serious enzootic pathogens in the herd and have eliminated most of them. Furthermore they pass such a strong maternal immunity to their offspring that the piglets are resistant to infection by most of these pathogens for varying periods depending on the pathogen. Thus if they are weaned immediately from the sow and moved to isolated premises at the appropriate age they will be free of the pathogens you wish to eliminate.

Thus if you wished to obtain future breeding stock from a particular herd but your veterinarian thought that the general health of that herd was below that of your own you could obtain higher health status pigs free from the unwanted pathogen. If the pathogen you wished to avoid was Mycoplasma hyopneumoniae, (enzootic pneumonia), you could vaccinate the dams in the donor herd ahead of time to boost their immunity, put the sows and newborn piglets on an anti-mycoplasma medicine such as tylosin or tiamulin and wean the pigs at ten days to the isolation facility on your farm. Isolation is necessary because if an unknown pathogen enters the donor herd it could go through the SEW system during the incubation period.

The SEW system is discussed in detail in chapter 3.

Hysterectomy and fostering
The fourth method of introduction of live pigs is through hysterectomy and fostering the piglets onto a newly farrowed sow in the recipient herd. This operation is carried out on day 113 of pregnancy when the sow is slaughtered. The womb containing the piglets is either removed 50 meters away to a pig-pathogen-free environment where the piglets are removed or it is passed through disinfectant trap into a sealed room. The litter is then immediately taken into the recipient herd and suckled onto a newly farrowed sow. If done properly the mortality rate is as low similar to that of your naturally farrowed piglets.

The whole operation is synchronised using prostaglandins so that newly farrowed sows are available to act as foster mothers. (See chapter 15). Ideally the sow selected for the operation should be moved into isolation approximately eight weeks prior to the due date and monitored for evidence of disease. At the same time it should be blood sampled and tested for aujeszky's disease (AD) (pseudorabies), swine influenza, PRRS and other relevant diseases that could pass through the placental barrier including leptospirosis and brucellosis. The reason for blood testing for these diseases is that they are capable of passing from a recently infected mother to her piglets in the womb. This is most unlikely to happen with aujeszky's virus and PRRS if they are obviously immune, but it could happen with Leptospira bratislava and possibly L. pomona. If a sow is serologically positive for leptospira the risk can be diminished by treating her with either streptomycin or amoxycillin antibiotics prior to hysterectomy. If a sow is sero-positive for brucellosis it is better to discard her. It would appear also that porcine coronavirus does not cross the placenta and hysterectomy pigs from positive herds can be introduced into negative herds safely.

Hysterectomy is a safe procedure and in many hundreds of operations known to the authors there has been no evidence of transfer of disease.

Embryo transfer
Embryo transfer has been used successfully in several countries for the introduction of new genes but it has not been widely adopted probably because it requires two skilled teams, one to flush the fertilised eggs from the donor sow and one to insert them in the recipient sow. It has not been performed on anything like as big a scale as hysterectomy and fostering and therefore there is not the volume of field evidence to underline its safety, but in theory and on the limited evidence it is safe.

Its drawbacks are (1) that it needs two skilled teams, (2) it requires immaculate synchronisation and timing, (3) the embryos cannot be kept viable for more than a few hours and (4) unless done expertly results in a high failure rate and small litters. For practical purposes, SEW, hysterectomy and AI are much simpler.

Artificial insemination
The sixth method of introducing genes is by artificial insemination (AI). It is known that viruses of swine fever, aujeszky's disease, PRRS, parvovirus, and leptospira bacteria and Brucella suis could be introduced through AI mainly during the early stages of infection of the boar. If the boars first go thorough a true quarantine procedure and are screened for these infections then housed in an isolated AI stud (i.e. one in a secure location), with high standards of biosecurity and hygiene during the production of semen, then field experience indicates that the risks are very small. The advent of frozen semen, which hitherto has been largely unsatisfactory but which is now looking more promising, renders the use of AI much safer since the semen can be stored for a month or two, time enough to be sure that no new infection was incubating in the AI stud. AI does however, have the disadvantage that only half the genes are introduced into the herd.

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