Breeding herd reproductive efficiency starts with the gilt pool. The gilt pool is defined as a group of young females selected as potential replacements for the breeding herd.

The term "gilt pool" doesn't refer to a location or facility. Rather, it refers to a group of females available to be bred and added to the breeding herd as needed.

In today's modern farrowing operations, there are a fixed number of farrowing crates per group. An efficient breeding herd manager will strive to keep those farrowing crates full to maximize throughput and maintain pig flow throughout the system.

As breeding herd females are regularly culled for infertility, unsoundness, age, etc., replacement gilts from the gilt pool are needed to complete each farrowing group. Proper use of the gilt pool will ensure that a pregnant female is available to fill each farrowing crate according to a planned schedule. The breeding system and farrowing schedule will determine the number of gilts required in the gilt pool.

Choice of Crossbreeding System The crossbreeding system that maximizes profit for commercial producers in most cases is a terminal crossbreeding system. A terminal cross in which all offspring are marketed offers several advantages over other breeding schemes:

1. Heterosis is maximized;

2. Greater product consistency is possible;

3. It's easier to implement and manage;

4. It allows the best use of genetically selected sire and dam lines. Lines with superior genetic merit for reproductive traits provide females for the crossbreeding system. And specialized lines that excel in growth and carcass traits are used as terminal sires.

Terminal crossbreeding systems have become the industry standard for most operations for several reasons.

First, larger and more specialized pork production units have developed specific mating schemes to make within-herd gilt replacement more feasible.

Second, independent seedstock suppliers and companies have expanded their herds to meet the market for replacement gilts and/or grandparent females.

High herd health practices have made the regular purchase/introduction of these females more practical.

Finally, the use of artificial insemination (AI) and advanced genetic evaluation techniques have made highly selected terminal and maternal sires more readily available.

Optimum Breeding System The purpose of a breeding system is to make the most profit from the operation. The breeding system can be defined as the design and type of crossbreeding program and the breeding stock used in it.

To obtain the most profit, the breeding system has to produce a consistent, high-quality product as efficiently as possible, given a fixed level of inputs (facilities, capital, labor, etc.).

The best breeding system won't always provide the greatest production potential or the least capital cost. It will generate an appropriate number of pigs over time, resulting in "optimum" pig flow, not necessarily maximum pig flow.

Assuming the terminal crossbreeding system is used, a key question is how to obtain replacement females. Whether to purchase all replacement females or establish a within-herd scheme for producing the needed replacements will depend on each producer's situation.

Raising Gilts For Gilt Pool Several requirements should be met before a producer sets up a within-herd gilt multiplication system. First, and foremost, you must be willing to put forth the extra management required. Extra discipline, time and effort are essential. These systems require identification of all nucleus females, an evaluation and selection program to ensure the best sow herd candidates are selected, and, management of the production supply.

Before you decide to raise your own replacements, analyze the potential benefits and the costs associated with the extra effort.

In a within-herd multiplication system, a portion of the sow herd is designated to produce replacement gilts for the terminal portion of the herd. Advantages include lowering the herd health risks involved with introducing new animals into the herd plus a potential cost savings. However, you must be committed to the extra management required and recognize that you are reducing the number of females devoted to terminal market hog production. Several examples of within-herd multiplication of replacement females will be discussed.

Within-Herd Multiplier A within-herd grandparent multiplier is one of the most common systems used. (See example in Figure 1.) Close to 15% of the sow herd is made up of purchased F1 grandparent females (ie.: Hampshire x Landrace) that are mated to unrelated maternal line boars or inseminated using purchased semen. Gilts from these matings are used to maintain the other 85% of the sow herd, mated to unrelated terminal boars with all offspring going to market.

Great Grandparent Plan Another example - an extension of the grandparent system - is the within-herd great-grandparent program. It includes another level where great-grandparent females are used to produce the grandparent females within the herd.

For example, the Hampshire boars and Landrace females would be the great-grandparents in the previous example. Approximately 2.5% of the total sow herd would be devoted to producing these F1 grandparent replacements, while 15% would be used for parent female production, and 82.5% would be used for terminal market hog production. This system reduces the number of outside females that must be purchased and is ideally suited for AI.

However, this program requires even greater management ability and attention to detail, and reduces even further the number of females available for terminal market hog production. It also does not work well in herds under 400-500 sows.

Rotaterminal System A third option to consider is the rotaterminal system in which replacement gilts are produced by approximately 15% of the sow herd which is maintained in a rotational cross of two or more unrelated maternal lines. These gilts are then mated to unrelated terminal boars for terminal production. This system has an advantage in that startup females are purchased only once. It is, however, even more complex to manage since it is critical that rotaterminal females are mated to the correct breed or line of boar in order to maintain maternal heterosis.

In a three-breed rotaterminal, 86% of potential maternal heterosis is realized if the correct rotation of breeds is maintained. If two breeds are used, potential maternal heterosis is reduced to only 67%.

The percentages of the herd needed to raise replacement gilts that are included in these three options are rule-of-thumb estimates. Actual number of females needed will depend on production levels, sow herd culling rate and selection intensity. Breeds shown are examples to maximize both maternal and pig heterosis in each system. Various other breed or line combinations are also available.

Multiplier Options Another popular way to obtain replacement gilts is through a network multiplier or a user-group multiplier. In these systems, a group of producers set up a separate venture to produce replacement breeding stock for the user group. One of the previously described mating schemes is used and the group is generally tied directly to a seedstock supplier. Each member purchases shares (stock or sows) in the user group that fits with their production schemes.

These network multipliers are designed to maximize genetic improvement, health (biosecurity) and reduce costs and management to maintain grandparent or great-grandparent females. Startup costs will probably be greater, but this system has the potential to reduce genetic costs and provide long-term genetic gain.

Regardless of the system used, development of potential replacements starts at birth. A record of specific matings to produce replacement females must be maintained; gilts in those litters must be identified at birth by earnotching and/or eartags.

Some producers choose to raise these potential replacements with their contemporaries, while others may pen them separately for identification and selection. This also allows them to be fed separately and developed on a different plane of nutrition.

Buying Gilts Purchasing all gilts from an outside seedstock source has become popular for numerous reasons. Given a terminal crossbreeding system, it is the simplest system to manage since no breeding decisions have to be made. All terminal boars (or semen) can be mated to any female in the herd. Buying all gilts makes the most of terminal production in the herd. These should be the most profitable pigs to produce since market hogs produced in these terminal mating systems are generally leaner and more efficient than the barrows and cull gilts produced in the maternal-line litters.

If all replacements are purchased, a producer will depend on his seedstock supplier to do his "genetic work" for him. As a result, the genetic merit of the commercial producer's herd will directly relate to the genetic progress made in the seedstock supplier's herd.

Potential disadvantages of buying all replacement females are the initial cost, limited availability, timing of introductions into the herd and the herd health risks involved.

Cost and availability questions are primary considerations. A consistent, reliable supply of gilts is essential when all replacements are bought.

Frequent introductions of breeding age females aren't feasible for herds that have initiated segregated early weaning (SEW), multiple-site systems to improve herd health. Some seedstock suppliers have introduced SEW gilt (<21 days of age) and junior gilt (40-50 lb.) programs to meet the demands of these herds.

Under these programs, gilts are placed in an isolation facility separate from the base herd. Introduction at an early age allows more time for females to be isolated, monitored for disease status, and acclimated to the health status of the base herd.

An acclimation period following isolation allows new gilts to build immunities comparable to the base herd. Normally this is accomplished by introducing a few sentinel animals into the isolation facility after the initial monitoring period. These sentinel animals may not have clinical symptoms of diseases present in the base herd, but differences in immune systems may cause the new gilts to exhibit an immune response upon exposure to the disease pathogens. An adequate acclimation period will facilitate close monitoring of any immune response and allow time to establish a common health status between the new gilts and the base herd.

It should be noted that introducing females at or near market weight (5-6 months of age) increases the temptation to reduce the time spent in isolation and acclimation. The isolation and acclimation periods should be a minimum of 30 days each to allow adequate time for disease monitoring and adjustment to the breeding herd environment. Proper time spent here will boost lifetime reproductive performance and longevity.

Genetic Merit It is important to consider the relative genetic merit of purchased vs. home-raised gilts. Remember that the genetic merit of home-raised gilts in a grandparent system is a function of the genetic merit of the grandparent females and the maternal sires to which they were mated. Gilt selection efforts will contribute very little to the genetic improvement of the herd and are relatively unimportant when compared to the importance of the genetic merit of the grandparents.

Because of the small number of grandparent females needed and the premium generally paid for them, it is very important that they have performance records and are selected from the top end of a herd making consistent genetic improvement.

In addition, AI has become a powerful tool in making the best maternal sires in the industry available to commercial pork producers for replacement gilt production.

Perhaps of even greater importance is the genetic merit of purchased replacement gilts. If these gilts come directly from a multiplier herd that is supplied by a nucleus herd making significant genetic improvement, this improvement will be channeled directly to the commercial herd and genetic lag will be minimized.

But if the multiplier herd is using average or below average females from a nucleus herd, genetic progress in the commercial herd will be limited. It is equally important to select both boars and gilts from herds that have a sound testing and selection program.

Number In Gilt Pool A planned breeding and farrowing schedule, along with previous production records, will help determine how many gilts are needed in the gilt pool. Season of the year, age, environment and genetics are factors that may affect the number of females exhibiting estrus and their conception rates.

Naturally, the number of replacement gilts needed to keep each farrowing group full must be determined in advance. Culling rates and previous conception rates for gilts and the existing sow herd will determine how many gilts should be bred. In general, at least three gilts should be selected for the gilt pool for each farrowing crate to be filled. Even more gilts are needed during problem breeding periods or if pen breeding is used.

It is generally recommended that females are not bred before they reach 7-711/42 months of age. This will ensure that gilts are on at least their third heat period before being bred. This usually results in an increase in ovulation rate and fewer rebreeding and lactation failure problems than if gilts are bred at their first or second estrus. Temporary situations may make it advantageous to breed females earlier than their third heat cycle, but lifetime production will likely suffer.

Cost Comparisons Accurate cost comparisons between the various systems should be made to assess the most cost-effective for each herd. Each system may have a different genetic cost based on structure of the breeding herd, initial purchase price, replacement rate, expected production levels and economic values specific for the herd.

Remember that the value of different alternatives varies from farm to farm and the lowest genetic cost may not be the best. The genetic merit of the pigs produced must also be weighed in evaluating the benefits of the various systems.

Gilt Nutrition Energy intake of developing gilts can be restricted after 180-200 lb. without delaying puberty. This can be accomplished by hand feeding 5-6 lb. per day of a 14-15% protein, well-balanced diet. This program will allow moderate lean tissue growth and save on feed costs over ad libitum feeding. It also allows accumulation of adequate fat reserves necessary to support reproduction without increasing unneeded body weight that may decrease longevity and contribute to unsoundness during development or later in the sow herd.

Summary Effective management of the gilt pool can greatly affect the reproductive performance of the breeding herd. Choice of a system for obtaining gilt pool replacements depends on management ability, herd size, expected reproductive performance and availability and cost of breeding stock replacements. All must be considered when weighing the merits of each system.

The purchase of replacement females for terminal crossbreeding systems is an alternative that should be given careful consideration. If a consistent supply of genetically superior females is available from a reliable supplier, a high level of productivity in an easy-to-manage system can result. Properly designed terminal crossbreeding programs will achieve a maximum level of both maternal and pig heterosis.

Purchasing all replacement females must also be regarded relative to the cost and the potential disease risk involved.

Home-raised females in a grandparent system can be cost effective and greatly reduce the disease risk involved in the continuous introduction of replacement gilts.

AI is well suited to within-herd multiplication systems and is especially helpful in small herds to overcome inefficient boar use with natural matings in grandparent systems. It also allows the use of superior maternal boars that might not be available through natural service.