For herd health reasons, and to a lesser degree costs, many commercial pork producers are implementing internal gilt multiplication systems.

Many swine genetic companies and purebred seedstock producers have programs available where grandparent or great-grandparent stock can be purchased to produce parent females for commercial market hog production.

Elite maternal line boar semen is commonly purchased from the breeding company, purebred producer or AI stud to mate to these grandparent or great-grandparent females. The focus of this article is on the maternal line grandparent program.

Internal Multiplication

The first step toward an internal multiplication program is identifying the source of the grandparent stock. Once this decision is made, the producer and the supplier must agree on the genetics that will best serve maternal line goals. The level of management and the facilities in which both the maternal lines and the terminal offspring produced from the parent stock will be raised must be considered.

The maternal lines of choice should excel for number of pigs born alive, milking ability and longevity. These lines should also have acceptable growth rate and carcass cutability (backfat and loin muscle area), but the most important trait is number born alive.

It is unwise to sacrifice maternal trait performance to make relatively small gains in terminal market hog performance. For example, number born alive is more important, economically, than backfat.

Commonly, gilts produced in an internal gilt multiplication program are not evaluated for growth, backfat and loin muscle area. Commercial producers typically rely on the genetic supplier to make any necessary improvement in these traits. However, minimum standards for those traits should be discussed with the supplier.

Having selected the genetic lines, females from one maternal line are purchased for delivery to the commercial producer. Select females are mated to two types of boars. Some are mated to boars from the same breed or line. Typically, 10 to 20% of the maternal line females are selected for this mating to maintain the “pure” line genetics needed to maintain the grandparent line. Sows with the most outstanding maternal records should be used to make this next generation of pure maternal line females.

The remaining 80-90% of the pure-line females are mated to a distinctly different maternal sire line to produce a two-breed (or two-line) parent female. These two-breed/line cross females, often called F1 females, will have 100% heterosis, which is important to the performance of the reproductive traits in which this parent must excel.

When this two-breed/line or F1 parent female is mated to terminal line boars, the resulting offspring also have the advantage of maximized heterosis. An example of this type of system is shown in Figure 1.

The Rotaterminal Option

Another option many producers utilize for producing their own replacement gilts is the internal rotaterminal program. This program does not require the purchase of pure-line maternal females from a genetic supplier.

Instead, the commercial producer identifies 10-20% of the best sows in the herd, then mates them to a top-rated maternal line boar using semen supplied by their genetic supplier. These top sows should be identified and selected on the basis of their reproductive performance — including number of pigs born alive, litter weaning weight and sow longevity.

Once these females are identified and mated to a maternal line boar, their offspring are mated to a boar of another (unrelated) maternal line to produce the second generation of replacement gilts.

In the third generation, the females from the last crop of females (genetic makeup includes the original female line, the first maternal line and the second maternal line) are mated to a boar from the first maternal line or breed used. In other words, two maternal lines are rotated every generation.

There is a small loss of maternal heterosis in the parent female with this type of system. A maximum heterosis of 67% is eventually attained. However, terminal offspring heterosis will always be maximized when the parent females are mated to a terminal sire from a third, completely unrelated breed or line. An example of a rotaterminal internal gilt multiplication system is shown in Figure 2.

Beyond the Genetics

After the sows have been mated to produce either grandparent or parent replacement females, and they have farrowed, producers must remember to do several things.

First, the females from these matings must be identified so they can be tracked throughout the nursery and grow-finish phases of production. Good options include earnotching, a hole punch in the ear, tagging or any identification method that is easy to see from a distance.

Often these replacement females are raised with pigs intended solely for market, so it is critical that the replacement female candidates can be identified and retained.

As the candidate gilts approach market weights, producers should follow the guidelines of their genetic supplier for proper gilt development. This is a critical step toward ensuring a replacement gilt has a long and productive lifetime within the breeding herd.

Furthermore, even though these gilts are raised within the producer's system, replacement gilts still require proper isolation and acclimation. This program should be developed in consultation with the herd's veterinarian.

Too often, producers find themselves short of replacement gilts, wanting to keep the farrowing crates full, and are tempted to skip proper gilt development, isolation and acclimation.

Don't do it. It will likely contribute to poorer than expected productivity, poor sow longevity and unsatisfactory results.

Another critical step in selecting replacement gilts is evaluating feet and leg soundness, as well as underline quality and genitalia soundness.

The poster found between pages 4 and 5 in this issue of National Hog Farmer offers guidelines to selecting for conformation and structural soundness. Subsequent posters will present guidelines for feet and leg soundness (February) and reproductive trait soundness (March).

Table 1. Mating System Costs, Genetic Cost/Pig, Annual Genetic Savings, and Relative Management Required When the Number and Percentage of Gilts Retained/Sow/Year Differs Using Two Distinct Internal Gilt Multiplication Systems Compared to the Purchase of Replacement Females in a Commercial Pork Operation.1
Mating System Gilts Retained/Sow/Year Percent of Gilts Retained/Sow/Year Genetic Cost/Pig Annual Genetic Cost (Savings) to the Herd Relative Management Required for Producing Replacement Females
Purchase Gilts n/a2 n/a $ 4.55 $ 248,742 03
In-herd purebred — 10% 6 60% $ 2.72 $(106,392) ++3
In-herd purebred — 15% 4 40% $ 3.11 $( 87,132) ++
In-herd purebred — 20% 3 30% $ 3.51 $( 67,132) ++
Internal rotaterminal — 10% 6 53% $ 3.55 $( 71,166) +3
Internal rotaterminal — 15% 4 36% $ 3.84 $( 56,611) +
Internal rotaterminal — 20% 3 27% $ 4.12 $( 42,055) +
1Assumes an operation containing a total of 2,400 sows. Barrows from an internal multiplication system are assumed to have a lower value than the terminal market barrows and all maternal line females have slightly poorer maternal, growth, and carcass performance and are accounted for in the table.
2n/a = not applicable.
3Relative management required assumes that purchasing replacement gilts requires little management and an internal gilt multiplication system that uses purebred or pure lines requires much more management (++), and a rotaterminal internal gilt multiplication system requires an intermediate amount of management (+).

The posters provide illustrations of various defects that should be discriminated against as producers evaluate replacement females. The defects illustrated can adversely impact productivity and longevity of the replacement females. Gilts with one or more of the defects presented in the posters should be considered for culling.

Minimum Growth, Carcass Traits

Commercial producers can also evaluate the replacement females for growth rate by determining age at 250 lb., backfat and loin muscle area. If gilts do not meet some minimum criteria, they should be culled. Like culling for feet and leg soundness or reproductive defects, culling for the growth and carcass traits increases the total number of replacement animals needed.

Some producers choose to make all the genetic progress for growth, backfat and muscle through the sires supplied by their genetic supplier. This approach avoids the costs associated with evaluating these traits (i.e. weighing, ultrasound scanning). However, producers should still cull replacement females that are obviously slower growing, lighter muscled or fatter than their contemporaries.

Calculate Costs, Gilts Needed

As briefly mentioned earlier, the number of sows needed to produce replacement females is dependent upon the size of operation. However, the recommended percentage of maternal matings required to produce both the replacement grandparent and parent females ranges from 8 to 25%. Naturally, the requirement is a direct function of the number or percentage of acceptable quality females that can be produced from those matings.

Table 1 illustrates the number of females that are retained when maternal line matings make up 10, 15 or 20% of total matings within a herd utilizing the pure breed/line and rotaterminal systems.

As you can see, producing your own replacement females requires much more management compared to simply purchasing them. Accurate record-keeping and the ability to track candidate females through the system are essential. Evaluation of production traits and screening for conformation and soundness traits become your responsibility. All of these things require more time, effort and expense. Only you can determine whether such an investment will pay dividends in your pork production system.