There is a growing trend within the ranks of commercial pork producers to switch from purchasing all replacement gilts to producing their own.

This shift to internal multiplication programs is largely driven by health challenges associated with routinely introducing replacement gilts into commercial operations. Generally, purchasing replacement females gained wide acceptance because it maximizes terminal market hog production, allows the genetic supplier to focus on genetic improvement, plus management of the system is relatively easy.

While internal gilt multiplication may present fewer health concerns, it also introduces a new set of challenges and pitfalls that must be taken into account.

Consider the Options

There are several ways that replacements can be produced in an internal gilt multiplication system. The three most popular are a grandparent program, a great-grandparent program and a rota-terminal program.

The challenges in each system are similar, therefore, we will focus on the grandparent program for this review. Figure 1 outlines a typical grandparent program commonly used in commercial pork production.

The first challenge is identifying the number of grandparent females required to ensure a good supply of replacement gilts. Naturally, this number is affected by sow herd cull rates and conception rates, as well as the percentage of gilts successfully bred and placed in the breeding herd.

Using a 600-sow herd as an example, if 40% of the sow herd drops out from culling or mortalities, 240 gilts will be needed annually.

On average, we know that most litters are roughly half boars and half gilts. If we assume that two gilts per litter meet soundness and underline criteria and are successfully bred, then 120 litters are needed to produce the 240 replacements. The 120 litters are then divided by the litters/sow/year that the grandparent females produce. For this example, if we assume the grandparent females produce 2.2 litters/sow/year, we will need 55 grandparent females (120 litters/year divided by 2.2 litters/sow/year).

Should the breeding herd replacement rate increase to 50%, then 69 grandparent females are needed (600 × 50% = 300 divided by 2 gilts/litter = 150 divided by 2.2 litters/sow/year = 69).

If you cull replacement gilts closer and only save 1.5 gilts/litter for the breeding herd, and the herd replacement rate remains at 40%, then 73 grandparent females are needed.

Similarly, if the replacement rate climbs to 50% and only 1.5 gilts are retained from each grandparent litter, then 91 grandparent females are needed.

Generally, the number of grandparent females required to produce sufficient replacement gilts for a commercial operation requires 10-15% of the total sow herd.

Once you've established the number of grandparent females needed, you must decide whether to produce grandparent litters in large lots or small lots spread evenly across each farrowing group. This choice is dependent on whether the operation is breed-to-wean, farrow-to-finish, a sow cooperative or some other type of production system.

Also, since 10-15% of the matings will be devoted to maternal line production, producers must either purchase maternal line boars or maternal line semen to make these matings.

If semen is purchased, it is critical that the grandparent females are mated using the correct semen. This may seem obvious, however, it can be a challenge in some production systems where hired labor is used extensively. Ideally, the owner/operator should take on that breeding responsibility or delegate it to a reliable, detail-oriented employee.

Production Sacrifices

Producers must also remember that half of the offspring produced from grandparent females mated to maternal line boars will be males. As barrows, these are of less value than the offspring obtained from terminal matings. Typically, these animals have poorer growth rate, higher backfat, less muscle (lower % lean), and poorer feed efficiency when compared to the terminal offspring. This can impact the way finishing facilities close out and the profitability of their finishing group.

The situation becomes even more significant in breed-to-wean operations because the barrows from the maternal line matings will be sold along with the terminal weaner pigs. It is critical to tell buyers so they can adjust for the differences in performance. Explain to the buyer that, in your attempt to improve the health of the animals being produced, you have employed a within-herd multiplication system, which naturally produces some maternal line barrows.

Table 1 shows the expected decrease in production values of the maternal line animals relative to terminal offspring, plus a cost estimate. These values can be used to estimate the production costs of running a grandparent internal gilt multiplication program. It must be noted that these costs do not include the increased labor costs of animal identification, tracking and other items.

Table 1. Estimates of the Decrease in Production Performance and Associated Costs of Internal Gilt Multiplication in a Typical 600-Sow Farrow-to-Finish Operation
Item Change in Production Value Per Item, $ Amount of each item1 Dollars Lost
Change in Feed:Gain, lb. 0.3 .064/lb. 80,784 lb.2 $5,170
Increase/Decrease in P/S/Y -1 13.50/pig 55 pigs $743
Change in market value/pig -2.5 2.50/pig 1,224 $3,060
Change in days to market 5 0.17/day 6,120 $1,040
Total $10,013
1 Values based on 600-sow, farrow-to-finish system needing 55 grandparent females producing 2.2 litters/year each for a total of 120 maternal-line litters/year, and 10.2 pigs/litter or 1,224 maternal line animals produced per year.
2 Calculates to 66 lb. more feed required per maternal line animal produced.
P/S/Y = Pigs/Sow/Year

In this example, the production losses incurred by the 600-sow herd producing its own replacement gilts are estimated at over $10,000.

Cannot Cut Corners

Once the correct matings have been made and litters are farrowed, the next challenge is identifying the gilts from the grandparent litters using a tagging, ear notching or tattooing system. It is easy to lose track of these gilts in a typical production system, so it is essential to be able to identify them throughout the growing and finishing phases.

The replacement gilt candidates need to be grown and developed until they reach breeding age and weight. Gilt development costs include facility, vaccination, feed, etc., which can easily add up to $30/gilt or more, depending on the specific situation.

An additional consideration is many operations may not have adequate gilt development facilities, so new or rented facilities may be required.

Using the previous example in which 240 gilts are needed, we know that it will take at least 300 breeding age gilts to fulfill our needs because about 20% will be disqualified for unsoundness, poor quality or failure to breed. Therefore, the development costs in our example would approach $9,000 per year (300 gilts × $30 per gilt). This raises the cost of internal multiplication, including production losses noted in Table 1, to over $19,000 for the 240 replacement gilts needed in our 600-sow herd example (labor not included).

Genetic Selection Required

When all replacement animals were purchased, your genetic supplier was responsible for the genetic improvement. Now that responsibility lands on the producer. This is one of the more hidden challenges of an internal multiplication program and this ongoing genetic selection pressure must not be ignored.

Producers should not rely on the maternal line boars or semen they are purchasing for all of their genetic improvement. When gilts from grandparent litters are born and raised, producers must adopt evaluation procedures that will allow them to accurately select for economically important traits and ensure their maternal line females continue to improve.

Perhaps the simplest example is with feet and leg soundness. Assuming a producer purchases grandparent stock that has sound feet and legs, mates them to maternal line boars, he must then evaluate all gilt replacement candidates for feet and leg structure and soundness before bringing them into the breeding herd. If they are not sound, it won't be long before the producer has a barn full of sows that want to sit on their rear quarters, have difficulty getting up, and a whole host of other soundness problems.

Failure to adopt an effective genetic selection program will result in deterioration of other economically important traits such as number born alive, litter weaning weight, growth rate and backfat. Detailed selection information is available from the National Swine Improvement Federation at

A final note of caution — it is imperative that a commercial operation does not mistakenly bring back females that were sired by boars producing terminal market hogs. Not only are these animals likely to have poorer maternal performance, if they are saved as replacement gilts and bred to the terminal sire line intended for market hogs, on average, 50% of the heterosis will be lost.

Herd Size is an Issue

If your herd is less than 300 to 400 sows, then the feasibility of an internal replacement gilt multiplication program may be questionable. Regardless of size, periodically it will be necessary to introduce females in most internal gilt multiplication systems. The exception is the rota-terminal system where the purchase of females may only be required once. After the initial purchase, all genetic introductions can be accomplished through purchased semen.

With the grandparent and great-grandparent programs, there is a risk of disease introduction anytime new animals enter the herd. Still, in an internal gilt multiplication system, the frequency of introductions and the number of animals brought in is greatly reduced compared to purchasing all of your replacement females.

If the breeding herd staff is adept at using artificial insemination, semen can be used to produce both the maternal and terminal offspring. Vasectomized boars, used for heat detection, can be produced within the herd, thereby limiting disease risk and exposure further.

There are advantages and disadvantages to any gilt replacement system. Certainly, the easiest way to obtain replacement females is to purchase them. The advantages of raising your own replacement females are numerous, including lower herd health risk, reduced acclimation periods and possible total cost savings.

However, the disadvantages include extra demands on management, maternal line females displace females that would otherwise produce terminal market hogs, and evaluation and selection are required to maintain genetic progress.

If producers are not dedicated to the internal multiplication system, it can be extremely difficult to manage. If difficulty occurs, much of the health benefits attained could be lost by reduced productivity.