My high school agriculture teacher always told us the only good thing about gilts was that some become sows. The struggle to turn gilts into productive sows remains today.

Modern pork production systems require groups of high health gilts that will successfully make the transition into a productive sow herd. The success of terminal crossbreeding programs has put greater emphasis on the importance of growing groups of high value gilts.

A successful replacement gilt program includes the purchase of genetically superior animals, proper nutrition and health programs during the growing period, and acclimatization of young gilts to the production herd health status. Producers have a large investment in gilts when they enter into the breeding herd. Long herd life of the gilts increases producer profits.

Herd life is very important if pork production costs are to remain competitive with other meat producers. Benefits of longer herd life include:

* Sows have larger litters with heavier pigs in later parities;

* Sows have acquired immunity to herd diseases and pass protection to their pigs. A sow with porcine reproductive and respiratory syndrome (PRRS) immunity is more valuable than a naive replacement gilt, for example. Fewer new animals introduced into the sow herd helps maintain a 'stable' herd health status;

* Sows grow during life, therefore older, larger sows have more salvage value;

* Specialized replacement gilt purchase and grower programs are costly.

Cohort Analysis Sheds New Light What is a cohort? The new National Pork Producers Council (NPPC) Pork Production and Financial Standards Technical Manual describes a cohort as 'a group of animals that share a common event within a defined period of time.'

This definition allows several ways to form cohorts of sows for analysis. A group of females, regardless of their age, being mated during a given period of time, is one example. Another cohort option would be to group sows by their age and their mating period.

For this analysis, the females were grouped into cohorts by age group, genetic line and sow unit. This grouping procedure yielded 130 cohorts of 25 gilts each. The gilts were assigned to cohorts at about 165 days of age when they were moved to one of two sow units.

On entry into a sow unit, each gilt was given an electronic eartag. Eartag readers allowed us to take inventories each month. This system captured accurate records of each sow's activity, monthly.

These monthly inventories often sent managers back to check for 'missing' sows that were often unrecorded culls or sows that died.

A great advantage of the electronic eartags was that managers knew exactly which sows were missing - not just how many. In a 1,600-sow unit, it is extremely difficult, if not impossible, to find sows if their exact identity is not known. With the electronic eartag and a computerized recordkeeping system, it often takes just a few minutes to check the missing sow's last activity and determine her most likely location. Accurate, long-term sow identification is essential to do a correct cohort analysis.

Value Of Cohorts Replacement gilt cohorts are likely to become a useful analysis tool. The challenge of maintaining high health sow herds leads to a more orderly, less frequent, larger group replacement gilt development program. Purchase of very young gilts probably assures producers they will receive their share of the best growing gilts. A successful replacement gilt program may increase the proportion of useful gilts.

Cohort analysis takes into account a combination of fertility, longevity and prolificacy performance. All of these measures influence profitability directly. Another important feature of cohort analysis is that 'sums' of sow performance are evaluated rather than individual sow performance.

With cohort analysis, every sow day, from the time a gilt enters the sow unit until she dies or is sold, is included. It is too easy, and probably too common, to delete records for a gilt that is sold before mating, fails to conceive or dies. It is important to remember to account for the facility space, feed, labor and veterinary expenses these gilts took. Cohort analysis is unforgiving - it records all the sows, all the time.

The economic value of these measures must be modeled in a defined farm cost structure. Amount of investment in gilt development, breeding and farrowing environments and the level of management of each activity must be defined. See also, "Gilt Growth Rates' Impact On Lifetime Reproduction," beginning on page 14, where Ken Stalder evaluates the value of National Genetic Evaluation Maternal Line Program (MLP) gilts from different weight classes and expresses it as a Net Present Value (NPV). Additionally, see "Economics Of Longevity Using An ROE Model," beginning on page 52, where Will Marsh applies NPPC's Production and Financial Standards Return On Equity (ROE) model, which combines production and financial aspects of a pork production system.

MLP Cohort Results The MLP results show the lifetime performance of six genetic lines, through four parities. To explain the total performance of each line, the gilts were randomly assigned to cohorts by line, entry day and breeding-gestation-farrowing (BGF) unit. These assignments were made when the gilts were moved to one of the two BGF farms.

There are 130 cohorts with 25 gilts in each, about 22 cohorts per genetic line. Performance is shown from 150 days of age until the last sow weaned her fourth-parity litter.

Gilts could be removed only for death, injury, failure to show estrus by 300 days of age or failure to conceive after three mating periods. Sows could be removed for death, injury, failure to show estrus 50 days postweaning or failure to conceive after three mating periods.

Removal rates for each line by age period are shown in Table 1. Getting gilts bred is still a challenge for most lines. Sixteen percent to nearly 21% of the gilts are removed before they reach a year of age, with the exception of the excellent performance of the Dekalb Monsanto MXP200 line.

A slaughter check analysis of culled gilt reproductive tracts reinforces the breeding managers' ability to identify cycling gilts by confirming many had not cycled by the 300-days of age cutoff.

The cohort review showed there is continued sow loss by age period after one year of age, but no differences by line. Notice how large the influence of gilt conception rate is on sow survival rate to fourth parity.

Naturally, producers are concerned about sow mortality. Table 2 shows the distribution of culls and deaths in the MLP program. Some of the cull sows were injured and later destroyed.

There appears to be some difference between lines in mortality. The Dekalb Monsanto MXP200 line has fewer culls for failure to breed and is lowest in mortality rate.

Cohort analysis includes all the nonproductive days and costs of females that did not produce pigs due to death, injury or reproductive failure. All the 'sow days' from 150 days of age (entry to herd) are used to calculate line efficiency measures.

The sows weaning a fourth-parity litter in the program averaged about 810 days of age. Subtract the 150 days of age when they entered the BGF unit, and the average 'herd life' per sow is 660 days. Therefore, the potential herd life of each sow (660 days) divided by the days/year (365) is about 1.8 years/female or 45.2 sow years for a cohort. Table 3 shows results of cohort analysis of herd life, litters born, number pigs born alive and number pigs weaned.

The Dekalb Monsanto MXP200 line dominated all traits that combined fertility, longevity and prolificacy in an unusually productive way.

A 25-sow cohort could produce about 16,500 sow days if every sow had four litters (100 litters). This is not very likely since death and injury will usually eliminate some sows. The cohort analysis shows the average number of litters farrowed during a four-parity term, by genetic line. However, it is also important to recognize the number of litters farrowed by the 130 MLP cohorts ranged from 34 to 94. These results show a great opportunity for improvement exists.

Cohort results also show differences in pigs per litter born and weaned on a per-sow-day basis. With the exception of the Dekalb Monsanto MXP200s, other lines had about the same number of litters and herd life days but vary in pigs born alive and pigs weaned. Therefore, their reproductive performance rates differ. These production differences combine with herd life to describe herd profitability. The Dekalb Monsanto DK44 line had the best rates of production among the five shorter lived lines.

Results of litters/sow year, number pigs born alive/sow year, pigs weaned at 15 days of age/sow year and sow years per cohort are shown in Table 4.

The litters/sow year are the same for all lines except the Dekalb Monsanto MXP200 line, reinforcing similar fertility of sows in the other five lines.

Number of live pigs born/sow year and pigs weaned/sow year, a measure of prolificacy, varied among the lines. The Dekalb Monsanto MXP200 line excelled in both areas, and the Dekalb Monsanto DK44 ranked next best.

Sow years/cohort is a measure of longevity. A 25-sow cohort has a potential of 45.2 sow years through fourth parity. The Dekalb Monsanto MXP200 cohorts were the longest lived with 37.4 sow years in the program. The 130 cohorts ranged from 23.8 to 42.4 sow years, showing occupancy of 53% to 94% of possible sow spaces.

Popular measures of prolificacy, such as number of pigs born live and number of pigs weaned, showed the most variation in the MLP. Although heritability of litter size born is considered low (10-15%), large differences between lines were found.

Some producers calculate and report only those sows farrowing or only sows weaning a litter. Cohort analysis gives the most conservative (lower) prolificacy measure since all sow days are included.

Feed Intake Differences Sow feed intake was measured and adjusted for body condition and size in the test. The program goal was to feed each sow to her needs in each gestation period.

Table 5 shows the daily feed intakes, by genetic line. Even though sows were fed differently in gestation (4-7 lb./day), on average, sows of all lines were fed about the same amount each day.

When sows were challenged to maximize intake during lactation, there were differences between lines. The American Diamond Genetics sows ate the most, while the Dekalb Monsanto MXP200 sows ate the least per day. It is interesting to note that the number of pigs weaned was greater for the Dekalb Monsanto MXP200 sows, but the American Diamond Genetics pigs were heavier at weaning.

The number of weaned pigs/ton of total feed eaten by the sows was much better for the Dekalb Monsanto MXP200 line although their pigs were smaller at weaning. The survival of their weaned pigs did not differ from other lines although a weight difference of 1-1.5 lb./weaned pig existed among the lines. This is an important consideration because some segregated early weaned (SEW) pig contracts penalize smaller pigs. This is justified to reduce losses from the poorest 3-4% of the young pigs. However, a line with healthy, smaller pigs at weaning would be penalized unfairly for good-doing pigs that may not meet the weight standard. Producers must negotiate prices of young weaned pigs based on their vitality as well as size.

Cohort Summary The cohorts vary tremendously in production efficiency. A few cohorts had more than 90 litters born (100 possible) showing the possibilities of greater performance. These cohorts would fill production space for 80-90% of the potential time. On the other hand, some cohorts couldn't fill 60% of potential production occupancy.

This cohort analysis shows that fine-tuning genetic line selection, gilt nutrition program and herd health programs offers producers greater opportunities for increased profitability.