This probably stems back to the dim and distant past when sows were less productive and rather more hardy than today’s high-octane females. These days, a replacement rate of 50 per cent is a lot more realistic.
Replacement rate can be calculated, providing the average number of litters weaned per sow lifetime is known. Most herd-recording systems show this figure, although it is seldom used as a performance indicator. Personally, I find it a valuable measure of how well producers are doing at keeping sows in the herd and achieving a long, productive lifetime.
Unfortunately, over the last 10 to 15 years, litters per sow lifetime has been going down, due to higher culling rates and death loss. The fewer litters a sow produces over her lifetime, the quicker she will have to be replaced. Also, the more productive she is, in terms of litters/sow/year, the shorter the time she will stay in the herd, assuming that litters/sow lifetime is constant. Therefore, knowing these numbers allows the correct replacement rate to be calculated and set as a target for the herd.
Herd-recording schemes show that the average litters/sow lifetime is probably in the region of 4.5, with better farms achieving 5.0 to 5.5. (I have rarely seen a farm where the average is more than six.) Assuming a figure of 4.5 and a farrowing index of 2.4, the whole herd will be replaced in 4.5/2.3 = 1.96 years. That will lead to a replacement rate of 100/1.95 = 51 per cent.
If the farm is able to keep sows in the herd so that they have an average of 5.5 litters per lifetime, that number will drop to 44 per cent. In a 500-sow herd, that would reduce the number of gilts required each year by 35 which, at a cost of $350 per gilt, would result in savings of $12,250. That saving, though, pales into insignificance compared to the value of the performance benefits from keeping more females through the most productive parities, 3 to 6.
If a figure for litters/sow lifetime is not shown in the herd-recording program, it can be calculated from litters/sow/year and replacement rate. If 50 per cent of sows are replaced each year and sows average 2.4 litters weaned per year, then they produce an average of 100/50 x 2.4 = 4.8 litters per sow lifetime.
This is certainly a figure worth knowing and benchmarking against industry standards.
Many producers think that their sows produce more litters in a lifetime than they actually do! The target should be to achieve a minimum number of 5.0 and, ideally, 5.5.
Replacement rate is defined as the sum of culling rate and death rate, assuming the herd size stays constant. Clearly, the fewer sows die, or are removed from the herd, the lower the replacement rate and the higher the litter per sow lifetime. Herd-recording systems show that reproductive problems and lameness, especially in young females, are the most important reasons for enforced culling. An analysis of one’s own herd data will indicate not only the reasons for culling, but also the age profile of culled females. If the dropout rate is too high in gilts and second-litter sows, then a review of feeding and management from gilt introduction through the first two parities should be carried out. Areas such as body weight at first breeding, body condition scores, lactation feed intake, flooring quality and health management routines should be checked. I like to use the weaning-to-breeding interval as an indication of the quality of management of young females. If the interval for gilts weaning their first litter is more than one day higher than the herd average, then action needs to be taken. The best herds achieve a figure of about 0.5 days and this small difference can result in a significant improvement in lifetime performance.
One situation where replacement rate may be increased above the normal level is when parity structure is poor. This may be the result of inadequate numbers of gilts entering the herd, perhaps due to economies being made during a period of poor profitability. The end result is most likely a parity profile with a higher number of older sows — typically defined as parity 8 and over. In order to remove these sows, additional gilts will need to be phased in according to the numbers of sows that will need culling after their next farrowing.
Determining the appropriate replacement rate for the individual herd is an important part of breeding herd management. Getting it wrong can mean herd size drops below the target level if enforced culling is higher than planned. This is one area where being generous is the correct strategy because it helps to ensure that the target output of piglets from the breeding herd is maintained.