All pig producers would like to raise their herd's genetic status without risking its health. The problem in practice may be that they receive conflicting advice from different sources about the right direction to take towards achieving this objective.

Occasionally, the opinions of the advisers clash, perhaps because they come from different parts of the supply sector such as veterinary practices, breeding companies or feedmills. We have been aware of an example recently in which, to protect herd health, some veterinarians recommended to their clients a plan of action that involved closing the herd to the entry of new breeding pigs.

Closed herds do make sense from the viewpoint of biosecurity. But they also present the producer with a dilemma. Put simply, how does the business prevent the erosion of performance that will inevitably accompany the exclusion of fresh genetics?

For bigger enterprises, the answer is often to purchase some purebred dam-line animals and run a miniature breeding pyramid within their herd. It is not easy. The establishment and operation of an effective pyramid takes a great deal of organisation, excellent stockmanship in managing three different types of sow on the farm (pure lines bred pure, purebreds producing F1 parent-stock and also the F1 parents) and detailed record-keeping. However, farms that apply the principles correctly are rewarded with good results.

Criss-cross breeding programme

However, the breeding pyramid is not an option for smaller and medium-sized pig enterprises. It is not feasible to have, say, only three purebred animals that are bred pure. What do you do if one returns or dies?

An increasingly common solution on mid-range units is to try a rotational criss-cross breeding programme. This is achieved by the purchase of F1 parent gilts, which are inseminated using dam-line semen from boars of one of the two breeds forming the F1. Their progeny are mated to the other breed present in this parent generation. The oscillating between the breeds continues by using the dam-line semen, with herd replacements selected from the progeny.

Although a simplistic system, it suffers from the fact that it produces a far inferior product. It also brings a number of challenges for the producer to try to resolve. For example, the F1 parent gilts need to be clearly identified from birth, ensuring that they are retained in the herd rather than selected as slaughter pigs (or the other way round, meaning slaughter pigs being selected wrongly for retention). In addition, selected F1 parents from approximately 60kg need to be raised on a gilt-rearing ration and not a standard finisher diet.

Due entirely to the selection of future breeding stock from its output, the herd will produce 2.5% fewer slaughter pigs. Then there is the impact on the slaughter progeny produced from dam-line matings. These will make up 7.7% of slaughter pigs from the enterprise, so it is economically significant that they can be up to 14 days slower to reach market weight and they will grade at least 0.5 millimetres fatter at 100kg or more at higher slaughter weights.

Loss of heterosis

The performance of the breeding herd will also be affected. Most breeding companies offer the producer an F1 parent. This maximises heterosis, which has a large effect on traits such as litter size. Using a criss-cross programme decreases the heterosis, initially by 50% in the parent sow because she is no longer produced from two unrelated purebred lines. This loss of heterosis can reduce numbers born alive by up to 10%.

The selection intensity of gilts chosen for the herd is typically poor, with producers selecting primarily on type rather than any quantitative traits. The way in which stock for sale is selected by the reputable breeding companies offers a total contrast. Just to show the difference, one single evaluation run at JSR involves using our computer software to solve more than 1.6 million equations.

The cost of implementing a criss-cross scheme and absorbing the inferior performance into the system will vary from unit to unit. Those hit hardest will be the enterprises with 'all-in/all-out' finishing facilities (due to the slower growing dam-line genotypes) and any whose contract with the processor does not tolerate much variation in the slaughter product.

With practical and clear protocols in place, units should be confident about accessing high-quality genetics from their breeding stock supplier in a controlled health environment. Knowledgeable producers choose a source of genetics that operates a robust health-monitoring programme. They will then work on the basis of good veterinary advice regarding unit biosecurity and new stock integration, to minimise any risks associated with the introduction of the replacement breeding gilts and boars purchased to produce slaughter stock with good growth rates and uniformity.

The possible risks and challenges to a herd's health status fall into two categories — those that cannot be controlled by management and those that can. In the first category are the risks related to location. They include the position of the unit in relation to neighbouring pig farms, major roads, temporary road works, slaughterhouses, markets, the prevailing wind and topography (hills and valleys).

Pig pathogens spread best in conditions of moderate temperatures, high humidity and low air turbulence. Therefore, even something as apparently innocuous as the removal of an area of woodland can sometimes cause a unit to break down with disease after years of high health, because trees cause air turbulence that breaks up clouds of airborne pathogens. Often, an outbreak of coughing occurs on a day when the weather is mild and foggy, with little wind.

The distance that airborne pathogens can travel varies with the specific causal organism. The virus of foot-and-mouth disease has been shown capable in ideal conditions of crossing a wide stretch of sea, whereas the pleuropneumonia organism Actinobacillus pleuropneumoniae (APP) will only cross the central passageway of a pig building.

However, a distance of 3 kilometres can usually be considered a reasonable upper limit for airborne transmission of the common airborne pathogens Mycoplasma hyopneumoniae and possibly PRRS virus. Some authorities claim PRRS is airborne only over much shorter distances. An organism such as Streptococcus suis may spread from unit to unit carried by flies. So, if pigs are kept in close proximity to others or have others travelling close by, the risks of a herd health breakdown are high. These risks can best be reduced by providing structures to break up plumes of pathogens carried on the wind and by vaccinating animals to mitigate against serious health issues in the event of a breakdown.

Movements into the unit

The challenges that can be controlled by management relate to the movements of everything that comes into the unit. This covers the full range from people (stock attendants, veterinarians, electricians and so on) to equipment and feed and bedding, in addition to the live pigs themselves. To maintain the highest possible health status, everyone concerned with the unit must have a clear understanding of their responsibilities.

A pig-free period before entry must be agreed and adhered to. Before entry there must be a mandatory change into boots and overalls belonging to the unit. In some cases, a shower and change into clothing provided by the unit is recommended.

Goods should be obtained only from known clean sources, of course. Plant and equipment must not be moved from farm to farm. Make sure to dispose of dead animals safely, preferably by incineration on site. Vehicles delivering feed should not enter the unit.

Back to the question of genetic replenishment, we strongly recommend to all producers that they arrange for their veterinary adviser to discuss with the breeding company the details of all supplying herds, including the latest monitoring results and the length of time the source farm has maintained that health status. Sometimes pre-delivery vaccinations can be arranged to protect higher-health animals from a breakdown on arrival.

Follow good veterinary advice on integration matters such as a period of isolation and acclimatisation to minimise the risk of destabilising the resident herd. The age of gilts delivered might also be adjusted.

Where a very high health status is being protected, it is recommended to operate off-site isolation that includes the introduction and subsequent testing of sentinels and a check-back to the herd of origin after a period of 6-8 weeks.

Transport makes a difference, too. The breeding company should require that the vehicles to be used for deliveries to customers are fully cleaned and disinfected before use and adhere to set protocols. But the producer's own biosecurity provides his best protection against a health risk from the transporter that carries new breeding pigs. The ideal on the farm is to have two loading docks, both of them well-designed, easy to disinfect and well-positioned. One dock is exclusively for incoming animals. The other is only for the departure of slaughter animals or weaner pigs.

This way, there is no danger of incoming gilts walking in with an infection that was left behind by a truck that had earlier collected some departing animals. But the loading docks must be immaculate at all times.

Another essential is that transport drivers must never, under any circumstance, enter a pig unit or walk beyond the driver's side of the ramp. Similarly, unit staff must never step onto the driver's side of the ramp or onto the vehicle itself.

So by taking the right advice, producers do not have to choose between biosecurity or productivity. With attention to detail and robust systems in place, producers can ensure the herd continues to develop its genetic potential without compromising herd health.