The objective of regular introductions of new females into the breeding herd is to maximize the amount of lean meat produced/sow/year, and to do it profitably.

The genetic composition, selection criteria and development of the gilt will impact its reproductive ability and longevity. The system through which replacements enter into the herd will influence profit and biosecurity.

Genetic Selection Producers are confronted with the difficult task of selecting replacement gilts from the myriad of available breeds, lines and crosses offered by independent breeders and hybrid swine companies.

Before committing to a genetics program for replacement gilts, producers should analyze their situation. They should first pinpoint the traits needed in their operation that meet their goals before looking for reputable suppliers to select from. Information on suppliers can be collected from other producers and independent sources.

Prior to purchasing breeding animals, evaluate the supplier's genetic improvement program. A sound genetic improvement program should include four points:

1. Accurate, complete performance records;

2. Assessment of genetic merit;

3. Indexes, and

4. Selection of the highest-ranking boars and gilts based on the selection indexes.

The genetic trends for the supplier will show the annual improvements in genetic progress that have occurred.

Because of the sow's integral role in reproduction, representing half the genes in market pigs, focus should usually be placed on several traits by using a selection index.

Maternal line indexes that combine litter size, litter weight, backfat depth and days to market are commonly used.

Also, when making genetic decisions, economically important traits should be emphasized. But it's important to understand the effect that selection has on different traits. Table 1 provides heritability estimates that tell us the strength of inheritance for each trait. Heritability is the percent of the variation in performance that is due to genetic effects. Selection will be less effective for lowly heritable traits, like pigs born alive, because they are affected by environment to a greater extent.

In addition, an estimate of the standard deviation and economic value for each trait is also provided. The standard deviation tells how much variation is present in the population and can be used to estimate where an individual animal ranks in the population.

Economic values indicate how much importance to place on each measure. If two different sources of replacement gilts are expected to differ by .5 pigs born alive, this equates to an expected difference in marginal profit of $6.75. If the females will also produce pigs that are .05 in. leaner when mated to the same terminal sires, there would be an additional economic advantage of $.75 per pig produced. This system can be used to help price gilts based upon their genetic value.

Economic values may vary from farm to farm due to differences in management factors and markets.

Genetic selection of replacement gilts should be balanced with the practical reality of low-selection intensity being placed on commercial replacement females.

If a producer is willing to accept gilts in the upper 75% of a group, then the selection of the leanest, fastest growing, reproductively sound, and structurally correct gilts from maternally selected sows is a valid program.

Visual Evaluation Females that are purchased or retained as replacements should be genetically superior, reproductively sound and structurally correct. Replacement females need to have a thorough visual examination to determine their structural and reproductive fitness. Purchased gilts should also be inspected for mange, lice or other signs of unhealthy conditions before they are allowed to enter the herd.

Feet and legs are important as sows are expected to farrow more than two litters per year, nurse a large litter of pigs for two to three weeks, breed back in seven days or less and live their entire life on solid concrete or wire floors.

An ideal foot on a hog is comparatively large, with both toes the same size. The pastern should be relatively soft (not rigid or erect) and the rear hocks and front knee should be angled so as to not put extreme pressure on the leg joints.

When the animal is walking, the foot should be perfectly flat against the floor and not rotate or turn when the hog takes a step. If the foot rotates or turns on the floor as the animal walks, it sets up a possible sore on the bottom of the foot that can become infected and cause the animal to be in pain and unable to perform. Hogs that have sore inside toes have a tendency to become unsound at an early age. This is due to the uneven balance of weight on the feet and the fact that the foot does not set down on the floor surface evenly.

The overall structure of an animal is the sum total of bone, muscle, fat and skin and how it is assembled to make an animal functional for a specific purpose. The structure of the skeleton is very important because it affects longevity and function. A correct skeleton is one that is shaped in such a way that the hog has ample interior body space for essential organs to function.

In the case of females, a long, wide, deep skeleton allows for more space for reproduction. Correct structure allows a hog to move around on most any surface without difficulty. An incorrect structure will cause a sow to have difficulty getting up and down while she is in the farrowing crate.

The underline of a gilt or sow is extremely important. Replacement gilts must have at least six functional nipples per side and they should be evenly spaced and prominent. The nipples should start far forward on the underline and the underline should be free of pin nipples and inverted nipples.

A pin nipple is any underdeveloped nipple that replaces a functional one. Pin nipples never become functional. An inverted nipple is one where the end of the nipple is held up in the body of the mammary gland and therefore is "inverted." Inverted nipples will sometimes pop out when the sow farrows. But gilts with inverted nipples should generally be eliminated before they are put into the gilt pool because a majority of them will not be functional. Besides underlines, there are other external signs that can be checked for possible reproductive problems.

There are two problems that can be seen in the vulva. The first is a condition where a gilt will show an infantile or extremely small vulva. Infantile vulva is a possible sign of an immature internal reproductive tract.

The second condition is with vulvas that are tipped up on the end. A gilt with a tipped-up vulva may have difficulty getting bred. Gilts with infantile or tipped-up vulvas should not be selected as replacements.

Heterosis Crossbreeding is an important part of commercial swine production systems because of the improvement in efficiency from heterosis and the potential to exploit differences between breeds. A terminal, static cross in which all offspring are market animals takes the greatest advantage of differences in strengths of lines or breeds.

Lines that have superior genetic merit for reproduction should be used to provide the females for a breeding program. Lines that are superior for production traits provide the sires used to produce the terminal market hogs. The pigs marketed then have high genetic potential for production traits and the sow herd has high merit for reproductive traits. Heterosis has the greatest benefit in maternal performance and factors affecting fertility in boars (Table 2).

Ultimately in commercial pork production, selection and crossbreeding combine to achieve the highest level of performance.

An example of heterosis is a cross between lines Y and Z. Let's assume that number of pigs born alive average 10 and 11 for Y and Z, respectively, and that the daughters produced from this cross average 11.5 pigs/litter. The heterosis for number born alive in these YxZ females can be calculated as follows:

Maternal heterosis for number born alive = [(11.5 - ((10 + 11)/2)/ ((10 + 11)/2)] x 100 = 9.5%. Maternal heterosis of 9.5% is equal to the 1 pig/litter advantage the crossbred female has over the average performance of the pure line parents.

In designing and implementing a crossbreeding program, the expected levels of maternal, paternal and individual heterosis are important.

For example, let's compare the F1 female from a YxZ cross to the backcross female Yx(YxZ). The expected heterosis of a cross is determined by the amount of genes the parents have in common. If X and Y are unrelated, that is, they have completely different breed makeup, their offspring will have 100% heterosis.

Therefore, the F1 has 100% heterosis and the backcross has 50% heterosis. In the previous example, the YxZ female had a 1 pig/litter advantage due to heterosis. However, if the YxYZ backcross female were used in the sow herd, this advantage over the purebred lines due to heterosis is reduced to only .5 pigs/litter.

Systems To Produce Replacements Terminal crossbreeding systems offer the best balance of profit per litter and consistencyof production. The terminal crossbreeding system allows the producer to use specialized sire and dam lines and make the most of heterosis. This enables the producer to intensify management, as the genetics of the sows and pigs are consistent.

The problem with terminal crossbreeding programs is obtaining replacement gilts. Gilts can either be purchased or raised on the farm. Purchased replacements can be obtained as weaners or closer to breeding age.

On-farm production can take the form of grandparent (GP), great-grandparent (GGP) or rotaterminal systems and can be structured as a separate herd, contract multiplier or a dedicated portion of the breeding herd.

Producers decide to purchase gilts for many reasons. For small to mid-size production units, purchasing gilts from an outside source is often most practical because within-herd multiplication may not be feasible. This system is the simplest to manage; it maximizes heterosis in market hog production, and allows the genetic decisions and programs to be managed by the supplier. Potential disadvantages can be cost, availability, timing of introductions and increased health risks.

Most producers who raise their own replacement gilts are trying to minimize health risk or reduce total investment in breeding stock.

For within-herd multiplication, a portion of the sow herd (10-15%) is designated to produce replacement gilts for the terminal portion of the herd. These systems lower the potential health risk associated with animal introductions and offer potential cost savings. However, within-herd systems require extra management ability and reduce the number of females devoted to terminal production.

To effectively manage within-herd multiplication, you must be willing to invest extra time, energy and labor into the system. A high level of pig flow management, identification and recordkeeping is required to ensure a consistent supply of quality gilts.

A grandparent multiplier is one of the most commonly used systems to produce replacements. For this system, approximately 15% of the sow herd is made up of purebred or F1 grandparent females and mated to unrelated maternal line boars. Gilts from these matings make up the remainder of the herd (85%) and are mated to unrelated terminal boars with all production going to slaughter. This system can be expanded to a great-grandparent system where 2.5% of the herd are great-grandparents dedicated to producing grandparents, 15% are grandparents producing replacement gilts and 82.5% are used for market hog production.

This system reduces animal introductions and is ideally suited for using AI, but increases management requirements and does not work well in herds less than 600 sows. Grandparent and great-grandparent systems can be formed within herd or established with a multiplier network or user-group. This cooperative approach can be used by smaller producers to maximize genetic improvement and health, while reducing the associated production costs.

Another within-herd system is the rotaterminal. In this system, the sow herd is maintained with a rotational cross of two or more unrelated maternal lines and the top 15% are identified for mating with maternal-line boars to produce replacement gilts. The remaining females and replacements are mated to unrelated terminal boars for market hog production.

In a three-breed rotaterminal, 86% of maternal heterosis can be realized and this is reduced in a two-breed rotaterminal to 67%. This system minimizes animal introductions but requires the greatest level of management.

Tables 3 and 4 compare the market hog production from purchased gilts, grandparent and rotaterminal systems using a four-breed terminal cross.

Maximum heterosis levels are maintained when gilts are purchased, resulting in the most pigs / sow / year. Assuming a profit of $4.00/cwt. and removing differences in breeding animal cost, increased management and lost market hog value, the greatest net return per sow is observed under the grandparent program. These results are only as good as the assumptions in thesimulation (i.e. replacement costs, replacement rates, carcass value program, required management, labor costs.

Another important assumption that was made is that genetic merit is considered to be equal in all three systems. If genetic merit is not equal, the system that offers the greatest genetic potential may have the greatest net return. The program that has the greatest risk of reduced genetic potential is the rotaterminal. Great care must be used to bring in the absolute best maternal boars to produce replacement gilts.

The results of a feasibility analysis conducted by Tom Baas, Iowa State University, are shown in Table 5. Gross margin was compared above all variable fixed costs for F1 grandparent and purchased gilt systems using assumptions for a 600-sow unit. The first column gives the cost to raise or purchase replacement females at 250 lb. The gross profit margin is then shown for different levels of replacement rates and pigs weaned per litter.

These results are somewhat in contrast to Tables 3 and 4, because in the grandparent program we are achieving 100% maternal heterosis through the use of F1s, whereas in the previous examples we were at 85% maternal heterosis by using pure grandparents, so some of the results don't line up exactly.

This table can be used to determine the effect of additional costs that may be incurred in the payment of genetic premiums or royalties, in the evaluation and selection of home-raised replacement females, and to determine the feasibility of various gilt purchase prices compared to the grandparent program. This table also underscores the effect of pigs weaned/litter on profitability by demonstrating the dramatic increase in gross profit margin as pigs weaned/litter improves.

For example, if the replacement rate is 45%, a genetic premium that increases the cost of raising a gilt from $175 to $200/head would decrease gross profit/cwt. from $3.86 to $3.68. A base gross profit margin in a grandparent program of $4.05 corresponds to a cost of raising the gilts of $150/head, along with a market price of $48/cwt., 45% replacement rate and 8.93 pigs weaned/litter. Comparing this base to the purchased gilt program a producer could afford to pay about $220-$225/head for gilts if market price is $48/cwt., replacement rate is 45% and pigs weaned/litter is 9.

The choice of a system for obtaining replacement gilts depends on management ability, herd size, anticipated reproductive performance, cost and availability of breeding stock.

Appropriate age and weight at first mating is very dependent on genotype. Genetically leaner sows will have a larger mature body size than fatter genotypes. Thus, leaner gilts will achieve the same body weight at a younger age than fatter genotypes, but will be physiologically less mature. In many cases, a mating weight of 275 lb.and age of 210 days appears most appropriate for optimum fertility and longevity.

It is also important at first mating that adequate fat stores are available for good lactation and a short weaning-to-estrus interval. This adequate body composition may be represented by a fat depth in excess of .7 in. However, age, fat depth and live weight are not themselves the targets for the right time for first mating but are indicators of age pattern at puberty, the weight pattern of fatty tissue growth, and the relationship between ultimate mature size and the proportion of mature size that is required before reproduction should be initiated.

Maximizing Genetic Potential In order to maximize genetic potential at the commercial level, the nucleus source for breeding herd replacements must be making constant genetic progress. The breeding herd replacements put into commercial production must also be selected according to genetic potential and compatibility (heterosis). When replacement gilts are selected within herd, some objectivity should be used for keep/cull decisions.

To realize genetic potential from an animal, the genotype must be allowed to express itself. This means having access to appropriate feed, water, facilities and absence of disease. Often, the quality of the commercial environment does not allow this full expression.

When making changes or upgrades in the source of breeding animals, an upgrade in management and/or environment may be required for the new, improved genotypes to express their full potential. In order to monitor expression of genetic potential and to make appropriate adjustment to feeding and management programs current, accurate, and useful performance records are required.