What's Your Interpretation? (Non-Refereed)

Decrease in litter size with seasonal decrease in ovulation rate

Figure 1 indicates that pigs born alive per litter decreased as wean-to-first-service interval increased. This can be observed in most swine operations. It has been suggested that the decrease in litter size is associated with a decrease in total born, which is due to a decrease in ovulation rate.1

Other factors that can decrease total born include:

  • Poor semen quality,
  • Poor timing of insemination,
  • Poor semen handling techniques,
  • Improper nutrition of the sow or gilt,
  • Moving or mixing animals multiple times through out gestation,
  • Seasonal effects, and
  • Parity.

Other research has indicated that litter size is optimal when sows are bred 4 days post weaning, decreases progressively when sows are bred on days 5 to 7 days post weaning, and remains low when sows are bred on days 8 through 10.2

In my experience, the wean-to-first-service interval is highly influenced by season and parity. Even though the seasonal affects are sporadic in their magnitude and duration, there is a consistent 0.2- to 0.5-pig decrease in total born in most operations some time during the months of October to December. It has also been my experience that parity 1 and parity 2 animals are more susceptible to increased weaned-to-first-service interval.

Hormonal intervention with gonadotropins (eg, PG-600(R), Intervet, Millsboro, Delaware) given at weaning can increase the number of pigs produced per sow weaned by maximizing the number of sows that return 4 to 5 days postweaning.3 This treatment is especially beneficial in parity 1 and 2 animals.


The author would like to thank Dr. Sandy Amass for her assistance in this report.

--Matt Ackerman, DVM
Swine Veterinary Services, PC
Greensburg, Indiana 47240

References -- refereed

1. Deckert AE, Dewey CE, Ford JJ, Straw BE. The influence of the weaning-to-breeding interval on ovulation rate in parity-two sows. Swine Health Prod. 1997; 5:89-93.

2. Dewey CE, Martin SW, Friendship RM, Wilson MR. The effects on litter size of previous lactation length and previous weaning-to-conception interval in Ontario swine. Prev Vet Med. 1994; 18:213-223.

3. Kirkwood RN, Aherne FX, Foxcroft GR. Effect of gonadotropin at weaning on reproductive performance of primiparous sows. Swine Health Prod. 1998;6:51-51.