Closed herd strategies have received renewed attention as producers focus on disease, seen as the largest constraint to profitability, and as technologies to conserve and improve genetics mature.
In essence, the closed herd concept attempts to augment stringent biosecurity by eliminating live animal inputs as opportunities for disease entry into the herd.
Closed herd systems are also perceived to improve gilt acclimation and stabilize herd immunity. This holds true for many typical swine diseases.
The widespread distribution of porcine reproductive and respiratory syndrome (PRRS) virus, due to animal movements, has led producers to consider closed herd systems for risk management and health improvement.
Poor application of closed herd strategies, however, can lead to a loss of genetic basis and destabilization of health. This is especially true in farms endemically infected with PRRS.
Closed herd systems also present challenges in pig flow and require additional management intensity for success. Figure 1 shows typical pig flow for various production schemes using gilt replacements. In Figure 2, pig flow is diagrammed for a closed herd system with internal gilt multiplication.
For their part, external gilt replacement schemes also carry inherent health risks. Persistent or “carrier” disease status confounds diagnostic testing or results in cost-prohibitive testing regimes to ensure gilt health. Gilt introduction without proper herd acclimation can create endemic disease conditions on farms.
Perceptions aside, closed herd systems don't ensure gilt acclimation to sow farm organisms. In fact, poor gilt acclimation schemes are not improved by herd closure alone.
Closed herds do eliminate communication failures between buyers and suppliers about health status of replacement gilts.
Closed herds also remove the risk of disease breaks from newly purchased gilts created by the lag time of diagnostic tests between disease exposure and test results.
Few epidemiological studies have evaluated the difference in disease risk between closed and open herds. Of note, closed herd status was linked to a statistically significant lowered risk for F18 E. coli in a study of Belgium herds.
Short- and Long-Term Closure Strategies
Historically, closed herd strategies have been applied as temporary interventions to control a particular health problem. Closing the herd for a limited time to allow PRRS immunity to stabilize after a break has effectively shortened the duration of disease in farrow-to-wean farms, for example.
These short-term approaches range in complexity, but they do not provide for maintenance of breeding herd inventory and genetic improvement. The duration of these temporary herd closures is limited by the need to replace involuntary culls and sow mortalities, and by the eventual need to return gilt inventory to normal levels. This influx of gilts increases risk of new disease introduction.
Long-term, sustainable herd closure is more complex. It requires a portion of productive females in the breeding herd be replaced with maternal grandparent sows. These sows are mated to maternal line boars (via semen) to produce replacement gilts. These gilts are then reared on site and mated to terminal sires to produce market animals.
Sustaining genetic input is typically accomplished via semen from monitored boar studs supplying artificial insemination (AI) services. Other methods for genetic input that don't require use of live animals, such as embryo transfer, are being developed. However, these new technologies are not widely used because of cost and low efficiencies.
Frozen boar semen may dramatically increase the biosecurity of semen inputs. Frozen semen would allow continued monitoring of the stud while sufficient time passes to ensure that a boar was clear of disease or infection during the collection period.
Pros and Cons
The primary advantages of closed herd systems are tighter control of gilt supply and potential reduction in the risk of disease introduction.
Depending on the structure and pig flow of the system, additional advantages in gilt acclimation can be gained.
But advantages of closed systems carry a cost. Specifically, increased management intensity is required, and group weight and growth rate variation may increase due to management of multiple genetic lines together in nurseries and finishers. Gaps in biosecurity at other phases of production may jeopardize the sow farm when gilts return.
The economic determination of whether to close a herd and internalize multiplication must include these less obvious factors:
Reduction in growth rates and marketability of maternal line barrows;
Any reduction in market throughput due to devoting space to maternal lines; and
Added labor and training costs of handling multiple genetic lines on the sow farm.
Biosecurity is a key prerequisite to starting a closed herd system. The system inherently limits the opportunity for introduction of new genetic material in return for a greater level of protection from new disease introductions.
However, if current biosecurity practices are subpar, the main advantage of the closed herd will be lost. Infections of closed herds (not due to semen) have been reported for PRRS, porcine respiratory coronavirus and swine influenza virus (SIV). Prior to implementing a closed herd, thoroughly review these biosecurity risks:
Transportation — Clean and disinfect all transports off site; segregate marketing transports from internal pig movements; eliminate visitor vehicles.
Pig flow — All-in, all-out (AIAO) flow maximizes health benefits; segregate/eliminate light pigs that remain on site post-marketing; eliminate rendering visits to sites.
People movements — Control order of site visits/work flow; ban visitors; change boots and coveralls, and wash hands between different age/ phase/ health status groups.
Equipment use and cleaning — Segregate equipment to barns/rooms and wash and disinfect between groups.
Regional risks — Decide if local pig density has contributed to past disease breaks such as SIV and decide if it remains a threat.
While this list is not intended to be all-inclusive, it must be extended to all sites/barns that house gilts returning to the sow farm. Continuous-flow phases tend to serve as reservoirs of disease that impact growth performance, negating the gain afforded by closing the system to outside threats.
Consideration should also be given to the system's disease history. High-health farms using excellent biosecurity and appropriate acclimation on externally supplied gilts are sustainable, given correct relationships and conditions. These production systems may not gain added revenue from converting to an internal gilt multiplication system.
Pig Flow Considerations
Assuming a 50% annual replacement rate, about 3.5% of production will be devoted to producing replacement gilts in closed herd systems.
Another 3.5% of production will be composed of barrows from maternal line litters. Given that they are maternal vs. terminal line progeny, these barrows will increase size variation in growing groups. A plan to manage these barrows is a necessary part of the closed herd system. This can be especially troublesome in the early stages of conversion to a closed herd, when health benefits have not yet offset this growth variation.
In farms under 400 sows, it becomes difficult to introduce similar-aged gilts on a weekly basis, given that gilt demand per week is less than one litter. Batching the maternal litters is an option, but it also introduces variation in gilt age at breeding if a batch of replacement gilts is bred into several different farrowing groups. This requires attention to gilt size and feeding to avoid creating a reproductive problem.
Acclimation is Still Important
In closed systems, acclimation and vaccination of gilts produced internally is as critical, as it is for externally supplied replacements. Ensuring gilt exposure and immunity to sow herd pressures are still necessary.
The immunity desired of a dam is more a reflection of the health challenges for her pregnancy and litter. Vaccination or boostering for reproductive diseases such as porcine parvovirus is frequently still necessary despite previous gilt exposure.
In internal multiplication, gilts shouldn't be expected to provide better lactogenic immunity to preweaned pigs than outside replacements, unless vaccination and exposure are provided during acclimation.
Often overlooked, it is critical that producers understand that a system that poorly acclimates externally supplied gilts will be equally poor in a closed system. Introduction of gilts directly from a continuous-flow finisher barn will make stabilization for PRRS virus elusive.
In PRRS endemic herds, a “cool-down period” is required prior to gilt introductions to the sow herd. This is usually accomplished with a dedicated AIAO gilt acclimation barn.
Experiences with closed systems and recent research would suggest that a closed system where gilts are raised in continuous-flow facilities, then diverted to the sow farm at marketing without an acceptable cool-down period, may actually worsen the clinical PRRS picture on farms.
Though research literature doesn't pinpoint an ideal cool-down period, data on persistently infected pigs suggests that longer is better, assuming this can be accomplished without exposing gilts to other lateral infections.
Moving gilts off the sow farm to three-site or two-site production systems may actually increase risk to the farm when these flows are in pig-dense areas. In these situations, it may be necessary to build facilities to raise gilts on the sow farm site.
Typically, off-site grow-out facilities have lower biosecurity standards, especially if managed AIAO, because of the mistaken belief that any disease will be shipped out with the pigs and not maintained on the site. Incorporating replacement gilts in this flow dramatically changes this perspective, and creates a biosecurity standard for the site equivalent to the sow farm.
And as animals are moved to grow-out facilities off site, transportation biosecurity becomes more important.
Recently published research suggests the virus can exist in pigs as a “quasi-species” with different genetic strains present simultaneously.
Also, continuous passage of the virus, as occurs in continuous-flow nurseries, leads to genetic variation in the virus. Putting gilts through a continuous-flow, PRRS-infected nursery should not be considered safe.
Managing gilts in smaller, AIAO groups with controlled exposure of the entire group at a single time is preferred. This exposure would be followed by a period of isolation from potential exposure, without new animal introductions, so that immunity of the group is uniform when they enter the sow farm.
Maternal lines must be identified and managed as a subpopulation in the sow herd. Likewise, the offspring of these maternal females must be permanently identified to distinguish them from market animals. Generally, reproductive performance of maternal sow lines differ from the sow lines used for commercial production.
If the management strategy is to rely on production records to identify changes and challenges to production, maternal performance should be tracked separately from commercial sow performance. Otherwise, this performance variation between the lines can conceal problems. This is especially true in systems using Six-Sigma or statistical process control (SPC) run charts to monitor performance. The increased variation in sow performance can make the methods less sensitive to change.
Closed systems are generally dependent on boar semen to maintain an acceptable rate of genetic improvement. However, in doing so, this foregoes improvement that could be obtained by critical selection of females.
A selection or indexing strategy for maternal line gilts is necessary to ensure improvement. These strategies, however, imply that an excess of maternal gilts will be available for evaluation. The residual (non-selected) gilts are then marketed as commercial animals. As with maternal barrows, these may add variation to growth rate and performance in grow-out phases and/or weight variation at marketing.
In marketing situations that involve a narrow target weight or percent lean, increased variation in finisher pigs is usually dealt with by marketing over an extended period of time. Depending on flow pressures, this can create a continuous-flow scenario on the finisher site, and it might expose maternal gilts, destined for the sow farm, to disease from market trucks and biosecurity lapses.
In this context, growth variation in closed systems can be self-perpetuating if it leads to disease introduction affecting growth.
Endemic Disease in Closed Herds
Herd closure strategies help maintain and improve herd health by eliminating live animal introductions.
Health improvement stems from the ability of herd closure to stabilize immunity and eliminate active diseases. This process is based on the belief that exclusion of naïve animals will eventually result in the whole population having sufficient immunity to prevent disease.
The perception that simply closing a herd will allow immunity to stabilize and eliminate most diseases may be too optimistic. Research shows that Actinobaccillus pleuropneumonia, rotavirus, Streptococcus suis, salmonella and ileitis can persist in closed herds that are actively farrowing.
There are some big assumptions, outlined below, that must be understood to improve understanding and success:
Assumption #1: “No new naïve inputs are added to the population.” The immune status of an animal is not a static state and, in fact, almost always changes over time. An example that is important to closed systems is the piglet with declining maternal antibodies. Eventually, these piglets become naïve animals in the population. Strategies to manage these pigs before they become a risk include segregated early weaning and other early wean strategies that move them off site before they are susceptible to infection.
Another important example is compromised adult animals. A sow with previous exposure and well-developed immunity may not be able to protect herself from infection if she is immunocompromised.
Factors compromising immunity include poor nutritional status, negative energy balance, and other immunocompromising diseases and environmental stressors such as extreme temperatures.
Assumption #2: “Animals develop sufficient immunity to disease to prevent survival and replication of the disease-causing organism.” Disease organisms differ in the degree of response they illicit from the immune system.
Field studies and controlled research have shown that some organisms can persist in the animal in a carrier state. These organisms, such as salmonella and PRRS virus, survive in the animal despite the immune response mounted.
In contrast, “sterilizing immunity,” which is an immune response that completely eliminates the organism from the pig, clears disease organisms such as Transmissible gastroenteritis from pigs.
Assumption #3: “Eventually, all animals in the population have the same immunity.” Even with diseases that stimulate strong, sterilizing immunity, subpopulations of naïve animals may remain if the disease organism is slow to move through the population.
In this regard, PRRS virus presents an additional challenge since original paradigms of PRRS as a fast-moving, blitzing infection of sow farms don't always hold true in field cases. This is especially true when new strains infect an endemic herd where the rate of transmission may be impacted by animals' partial immunity to a previously related strain of the virus. When diseases with slow rates of transmission or infection are encountered, the population must remain closed until well after all animals have been exposed.
Alternately, vaccination or planned exposure techniques can increase the rate of transmission in the herd.
Assumption #4: “As immunity stabilizes, there will be no host available in which the disease-causing organism can reside.” Rodents, feces, insects, farm pets and dirty equipment all represent potential reservoirs for disease organisms to safely hide for the next round of naïve pigs.
Survivability of swine pathogens in these areas is highly variable. It is critical that these potential reservoirs of disease are identified and eliminated.
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