Genetic Improvement of Farrowing Rate in Pigs

By John Mabry, Ken Stalder, and Linda Engblom

In our current times of high feed prices and increasing expense of inputs, the cost of maintaining the sow herd has risen greatly over the past few years.

If a sow is not gestating or nursing a litter she is not generating income for the pork producer. ‘Non-productive sow days’ is the term used for these instances. The major traits impacting non-productive sow days (NPD) are farrowing rate, and the intervals of entry to first service, weaning to service, and weaning to removal. The trait with the most potential to reduce NPD is farrowing rate. When a sow is bred, but does not have a litter, the number of NPD generated is 21 at a minimum, and can be much greater depending on the management of the sow herd. The problem is it is virtually impossible to directly select for farrowing rate. When a sow is bred, but does not farrow, the cause is not clear due to confounding between several factors. Was the sow infertile? Was the semen bad? Did the inseminator do a poor job? Was the weather too hot? When a situation such as this exists, with no clear cause of the litter
being lost, direct selection will not work. However, indirect selection may be possible if there exists another trait that is closely associated with farrowing rate, and is routinely measured at the farm.

Genetic Improvement of Farrowing Rate in Pigs

One trait that is routinely measured on the sow farm, that should be closely associated with farrowing rate, is litters per sow per year (LSY). Litters per sow per year is measured as the annualized proportion of breeding female days attributed to successful gestation days. However, genetic improvement in reproductive efficiency by directly selecting for LSY has not yet been studied in depth. It may be possible to make a permanent genetic improvement in LSY (and indirectly improve farrowing rate) if LSY is found to have appropriate characteristics for making genetic improvement. To accomplish genetic improvement through selection, the trait must be accurately measureable. This is no problem with the computerized sow management software systems in use today. Secondly, the distribution of the trait should be normal. Figure 1 shows the distribution of LSY in a 2,400-sow herd.

As you can see, the distribution is skewed to the right. Preliminary research has shown that this skewness is caused by the parity of the sow. First litter sows have only one litter to spread the NPD due to entry to first service interval, thereby mathematically reducing their LSY. After adjustment of LSY for parity, the distribution of LSY is more normal, as shown in Figure 2, which is LSY adjusted for parity.

Thirdly, the traits must have an adequate genetic control, or heritability to respond to selection. To examine this, we used the data from the same 2,400-sow herd, and estimated the heritability for LSY using variance component estimation software. The result was heritability for LSY of 0.10. While this is in the low range it is similar to that of litter size, which has responded to selection when using a BLUP-based selection approach. This then suggests that LSY has the potential to respond to selection using a BLUP approach. If we can improve LSY via BLUP-based selection, then farrowing rate, as the primary component of LSY, should be improved also.

But just how much economic impact will be made if we genetically improve LSY? Consider the economic value of LSY in relation to other traits, such as litter size and NPD. The economic value of one pig born alive (NSIF guidelines) is approximately $13 per pig. The economic value of one NPD (according to ISU research) is somewhere between $1.50 and $2.50 per day. What do we stand to gain if we can increase LSY? For example, let’s use an improvement of LSY by 0.10. Increasing LSY by 0.10 will reduce NPD in the herd by approximately 11 NPD/sow/ year. If the average litter size is 11 pigs born alive and we get an additional 0.10 litters per sow per year, this would be an extra 1.1 pigs born alive. The economic impact of 11 NPD is approximately (11 x $2) $22 and the economic impact of 1.1 pigs born alive (1.1 x $13) is approximately $14.30. Improving LSY by 0.10 would then offer a payback of approximately $36.30 per sow per year in the herd. If this is genetic improvement, this would be a permanent improvement. For a 2,400-sow herd, this would be more than $87,000 per year.

Genetic Improvement of Farrowing Rate in Pigs

In summary, there is a great potential to improve the profit of our swine herds if we can make genetic improvement in farrowing rate. Direct selection for farrowing rate does not appear feasible, but indirect improvement via selection for litters per sow per year might be possible. To examine feasibility, more research needs to be conducted.

Editor’s Note: JDr. John Mabry, PhD is Director of the Iowa Pork Industry Center. He, Dr. Stalder, PhD, and Dr. Engblom, PhD are in the Department of Animal Science at Iowa State University, Ames, IA.