Herd parity distribution may hold the key to robust system performance. Seasonal productivity effects are most profound among first and second parity animals. The offspring of parity 1 females have lower survivability from nursery to market.

Although mature sow herds may not produce the top-ranking production numbers, the consistency of their performance at both the herd level and among their offspring makes a stable parity distribution a worthwhile goal.

While conceptually simple, however, achieving a stable population distribution is certainly easier said than done. For one thing, it takes time. Large gilt introductions move through herds in cohorts that age simultaneously. With each parity rotation, a portion of the initial cohort is replaced, and at some point, the aged surviving cohort needs to be replaced as well.

From the perspective of gilt flow management, a dilemma arises. Is it better to optimize the value of the individual sow? Or is it better to apply a consistent replacement rate from the beginning?

For many herds, the latter has been the priority. After all, if within-parity survival remains constant across time, a herd would require several years to approach a stable parity distribution. Furthermore, it is conceptually easier to apply an expected average replacement rate and let herd management cull aggressively and force in replacements.

If, however, a herd assumes a long-range perspective that pursues optimization at the level of the sow space, then gilt flow management becomes a more complicated task. In early parity rotations, substantially fewer replacements are warranted. In fact, for a herd where 75 per cent of gilts achieve two litters and 50% are retained through parity 5, annualized replacement rates for the first two years (five parity rotations) should average less than 32%.

In the long run, an optimisation-based replacement strategy would require a higher replacement rate in the fourth year to replace the survivors of the initial cohort. Once mature, its projected annual replacement rate would approximate 47 per cent.

Alternatively, application of a consistent 47 per cent replacement rate beginning in the fourth parity rotation would require nearly 4 per cent more replacement females over the first five years.

The potential benefits of optimisation-based replacement programs merit further consideration. In addition to reducing replacement female expenses, i.e. meat value, genetic premium, development, etc., avoiding unnecessary replacement introductions reduces the proportion of the population susceptible to first-parity production challenges.