Breeding Parity 1 sows at their second postweaning estrus has been shown to increase the number of live embryos detected at Day 30 of gestation. This increase in embryo viability may negate the effects of the second-parity dip in reproductive performance.

Recent work at the University of Alberta has shown excellent reproductive performance of contemporary dam-line, first-parity sows. Even when these sows lose large amounts of body tissues through imposed feed restriction during peak lactation, there is a relative lack of effect on many measures of postweaning fertility. Still, a second-parity “dip” - or lack of an increase - in the size of the second litter is often observed.

In earlier studies, first- and second- parity sows subjected to “skip-a-heat” breeding (bred at the second estrus after weaning) averaged two pigs/litter more than sows bred at first post-weaning estrus. However, this breeding procedure also accumulated 21 non-productive days (NPD), and resulted in a 9% chance that sows would not stand to be bred on the second estrus.

The data on the changing reproductive characteristics of contemporary commercial sows suggests that the response to skip-a-heat breeding in first-parity sows needs to be reevaluated.

A trial was initiated to reassess the effect of breeding commercial sows at first vs. second postweaning estrus on follicular development, size of the largest ovulatory follicle, ovulation rate and embryonic survival to Day 30 of gestation.

Fifty-five, Parity 1 sows were allocated to the study at farrowing. Within 24 hours after farrowing, all litters were standardized to 10-12 pigs.

Sow body weight, backfat depth, loin muscle depth and area, and individual piglet weights were measured on Days 1, 7, 14 and 21 of lactation, at breeding and at Day 30 of gestation.

All sows were fed a standard lactation diet “to appetite” for the 21-day lactation.

Sows were relocated to the breeding area and, from the day after weaning, were allowed twice-daily fenceline contact with mature boars for detection of estrus. All sows were bred using established artificial insemination (AI) protocols to insure insemination within 24 hour of ovulation. Semen from boars of known fertility was pooled and used within three days of collection.

Sows were divided into two treatments:

1. Bred at their first postweaning estrus (PE1), or
2. First postweaning estrus skipped and bred on second postweaning estrus (PE2).

Sows were paired on the basis of similar wean-to-first postweaning estrus intervals and weight, and randomly allocated within pairs to treatment.

PE1 sows averaged 111.7 hours to first postweaning estrus at an average weight of 392 lb. PE2 sows averaged 113.6 hours to first postweaning estrus and averaged 396 lb.

The study showed PE2 sows were heavier at breeding and had a greater positive weight change between weaning and breeding compared to the PE1 sows (Table 1).

Real-time ultrasonography (RTU) was used to estimate the size of the largest pre-ovulatory follicle at PE1 and PE2. At PE1, the size of the largest detected pre-ovulatory follicle in PE1 and PE2 sows was essentially the same (Table 2). However, at breeding, the size of the largest pre-ovulatory follicle detected in PE2 sows (8.2 mm) was greater than in PE1 sows (7.2 mm).