Besides production in time and space, there is another efficiency to consider -- that of the animal investment. In PigCHAMP, one measure that approximates the efficiency of the animal investment is pigs weaned/lifetime female (PW/LF).

PW/LF is a measure calculated only for animals that have been removed from the herd (death or culled) during a given period of time. It is defined as the total number of piglets weaned from all removed sows divided by the total number of removed sows.

There are several reasons why PW/LF is difficult to benchmark. Because it is closely correlated with average removal parity, differences in sow herd maturities create considerable farm-to-farm variation for the measure. By their very nature, startup herds will have low PW/LF. Alternatively, older herds that are undergoing depopulation are likely to have higher PW/LF. Herds with restricted replacement rates, whether for performance or financial reasons, would also tend to have higher PW/LF.

The accompanying plots show various relationships with average parity of culled sows and PW/LF for a subset of U.S. herds in 2005. Specifically, herds were included if their inventory was stable and the average parity was greater than or equal to 2.

Using these charts, we can make a few observations. As previously noted, average removal parity is highly correlated with PW/LF. PW/LF does not differ with herd size. There is an extremely weak, positive association between PW/LF and PW/MF/Y. However, there is no association between the average parity of culled sows and PW/MF/Y.

There is no universal target for PW/LF. Ideally, it would be the optimum allowed in an economic balance with production at the level of the sow's time and space. The balance between lifetime productivity and herd productivity merits further consideration because the costs of animal replacements are more complicated than simply their meat values and genetic premiums. Many herds utilize extensive acclimation programs to prepare gilts for entry. Parity 1 offspring leave the sow unit with different pathogen loads and levels of immunity. And, Parity 1 offspring, overall, weigh less at weaning and are at higher risk for mortality.

Ultimately, operation-specific financial conditions and herd performance will dictate where the complicated balance of herd and individual production lies. At present, however, awareness of the individual sow's contribution to the herd is a good place to start.