A major portion of the potential benefit of parity-segregated management of sows is due to enhanced reproductive performance. This article will highlight differences in reproductive physiology and performance between gilts, first-parity (primiparous) sows, and second-parity or greater (multiparous) sows in an attempt to identify areas where parity-specific management might help increase reproductive efficiency.

We will also address other key management issues that may have reproductive implications for parity-segregated systems.

For example, there are periods during the reproductive cycle when it is less risky to move females from the gilt farm to the mature sow farm.

Age vs. Parity

For recordkeeping purposes, sows are categorized based on their parity, which is simply the number of litters they have farrowed. Parity is positively correlated with age, but it is important to recognize the two are not directly linked. Differences in the rate that gilts reach puberty, as well as pregnancy failure in some gilts and sows followed by rebreeding, result in considerable variation in the age of sows within a specific parity category.

Based on sow farm production records, reproductive performance generally increases over the first three to four parities, then begins to decline as sows reach the seventh or eighth parity.

It has often been assumed that this initial boost in performance is related to the maturation of some components of the sows' reproductive system. However, there is little evidence of parity-based differences in female reproductive processes such as estrus, ovulation, fertilization and embryo survival. This may be partially due to the fact that few studies have focused on the underlying causes of this change in reproductive performance.

In addition, experiments rarely separate the effects of age and parity. This makes it impossible to determine if differences observed between parity groups were due to the aging process, repeated reproductive cycles, or some combination of these two factors.

It is possible that physiological differences in the reproductive systems of gilts and primiparous sows could be responsible for their decreased reproductive performance compared to multiparous sows.

However, retrospective comparison of the reproductive performance records of groups of different parity females almost always ignores the fact that these groups are not comprised entirely of the same individuals. It is quite possible that reproductive failure of sub-fertile gilts and first-parity (P1) sows removes them from the breeding pool, and in effect increases reproductive performance of the group in the subsequent parity. This natural culling process may be the primary reason that reproductive performance gradually increases over the first three to four parities.

Wean-to-Estrus Interval

Parity 1 sows, and in some cases second-parity (P2) sows, take longer to return to estrus after weaning than sows of third or greater parity. Primiparous sows often exhibit a 0.5 to 2.0 day longer wean-to-estrus interval than multiparous sows.

The size of this wean-to-estrus interval difference depends on a number of factors. Average daily feed intake during lactation is often 2 to 3 lb. lower in primiparous sows compared to multiparous sows. Primiparous sows also have fewer body reserves to mobilize during lactation than multiparous sows because they are still growing toward their mature size. This increased energy demand for body growth and lactation, combined with decreased feed (energy) intake in primiparous as compared to multiparous sows, may result in a greater negative energy balance.

The resulting catabolic (energy imbalance) state inhibits the secretion of hormones that drive the growth of ovarian follicles and delays the postweaning return to estrus. Suckling of the litter is also a potent inhibitor of hormone secretion and follicle growth, and primiparous sows may be more sensitive to these negative effects than multiparous sows.

Sows that take seven to 10 days to return to estrus after weaning often have decreased farrowing rates and litter sizes compared to sows that return earlier. Primiparous sows are more likely than multiparous sows to exhibit such a late return to estrus. Nutritional programs aimed at development of gilt body reserves, and at maximizing the feed intake of lactating, P1 sows, may help minimize this problem. Parity segregation would definitely make it easier to implement such nutritional programs.

Farrowing Rate

Gilts typically exhibit a 10 to 15% lower farrowing rate than multiparous sows. Primiparous sows also have a 3 to 5% lower farrowing rate than multiparous sows. Farrowing rate remains relatively constant from the second through P5 or more, and then begins to decrease significantly around P7 or P8. The lower farrowing rate of gilts and primiparous sows may be due to the presence of more sub-fertile females in these groups than in higher-parity groups, in which subpar females have already been culled.

However, bred P1 sows that return to estrus and require a repeat breeding do not seem to have a higher rate of recurrence of repeat breeding at the P2 and P3 level than P1 sows that do not require a repeat breeding. Litters that result from a repeat breeding tend to be about 0.5 pigs larger than litters that result from no repeat breeding.

These findings add support to the argument that P1 sows that fail to conceive or remain pregnant should get at least one more chance.

Litter Size

Litter size is generally lowest at P1, increases up to P4 or P5, then tends to level off until it begins to decrease around P7 or P8. A P2 “dip” in litter size has been evident in some sow farms. The body condition of P1 sows at farrowing and their management during lactation likely play major roles in whether litter size dips in P2. Parity segregation would allow the body condition of immature females to be more closely managed.

Estrus and Ovulation Timing

The duration of standing estrus is about five to 10 hours shorter in gilts compared to sows. There is considerable variation among farms and individual females, but duration of estrus typically averages 40 to 45 hours in gilts and 55 to 60 hours in sows.

A sow's duration of estrus and onset of estrus-to-ovulation interval are inversely related to her wean-to-estrus interval. In other words, sows that have short wean-to-estrus intervals (three to five days) tend to have a longer duration of estrus, and a longer onset of estrus-to-ovulation interval than sows that have long wean-to-estrus intervals (six or more days).

Given that primiparous sows tend to have longer wean-to-estrus intervals than multiparous sows, it is to be expected that they would have a shorter duration of estrus and a shorter onset of estrus-to-ovulation interval.

In some cases, this difference in wean-to-estrus interval may be large enough to justify a quicker administration of artificial insemination (AI) services after detection of estrus in primiparous as compared to multiparous sows (Figure 1). Gilts clearly warrant a more rapid administration of AI services after detection of estrus compared to sows due to their shorter duration of estrus.

Parity-segregated farms would have the advantage of being able to customize heat check and AI protocols to better fit the shorter duration of estrus in gilts and primiparous sows, compared to the longer duration of estrus in multiparous sows. Such specialization could improve the reproductive performance of gilts and primiparous sows.

Ovulation and Fertilization Rate

The number of eggs released at ovulation (ovulation rate) increases in gilts with each successive estrous cycle, especially in gilts that reach puberty at a young age. This is one of the reasons for recommending that mating be delayed until the second or third estrous cycle in gilts. Sows do tend to have a greater ovulation rate than gilts, but it is not clear if ovulation rate increases over the first four to five parities as litter size does.

There is some good evidence that age, not parity, is the factor that influences ovulation rate and litter size. Regardless, ovulation rate is so high in the prolific genotypes currently in use that it does not limit litter size.

Once eggs are shed at ovulation, they are viable for only about eight hours. On the other hand, sperm can remain viable in the oviducts for 24 hours. This is the primary reason why gilts and sows need to be inseminated prior to ovulation (Figure 1). When insemination occurs within 24 hours prior to ovulation in sows, the percentage of ovulated eggs fertilized (fertilization rate) tends to be high (85% or greater), regardless of parity.

Gilts, on the other hand, seem to require a narrower insemination-to-ovulation interval than sows to achieve a similarly high fertilization rate. Therefore, while one AI service each day of estrus is usually sufficient for sows, gilts may benefit from two AI services each day they are in estrus. Segregation of gilts and P1 sows would make adoption of such specialized AI protocols more feasible.

Embryo and Fetal Mortality

Given the increase in litter size observed up to P4 or P5, one would expect an increase in prenatal (embryo and fetal) survival to account for this change. However, the percentage of embryo mortality during the first 30 days of gestation, before uterine capacity becomes limiting, and the percentage of fetal mortality after 35 days of gestation, when uterine capacity can become limiting, does not seem to differ significantly between gilts and sows. It is possible that an increase of fetal survival (uterine capacity) is responsible for the increase of litter size up to P4 or P5. At present, studies are lacking that have specifically investigated the effects of parity on embryo or fetal mortality.

For maximum reproductive success, movement of P1 sows from the gilt farm to the mature sow farms should probably not occur within the first 30 days postbreeding. Stress during this period can reduce embryo survival. P1 sows could be moved to the mature sow herd either at weaning or after 30 to 40 days postbreeding.

Nutritional Effects on Reproduction

High levels of prebreeding and postbreeding feed intake can affect ovulation rate and embryo survival, but they seem to have different effects in gilts, primiparous and multiparous sows. High prebreeding feeding levels can increase ovulation rate, but this so-called “flushing” effect has mainly been observed in developing gilts and does not generally improve litter size.

In addition, nutritional regimens that increase ovulation rate may also decrease embryo survival, particularly when a high level of feeding is continued postbreeding. Several studies have demonstrated that a high feeding level during the first 10 to 15 days postbreeding decreases progesterone concentrations, and embryo survival in gilts and primiparous sows, but not in multiparous sows.

In contrast, other studies have failed to find a negative effect of high postbreeding feeding levels on embryo survival in gilts or primiparous sows. While it may be a sort of cheap insurance to restrict feeding of gilts and primiparous sows for 10 to 15 days postbreeding, it has not been clearly established that this is necessary.

Lactation Length

Average lactation length on U.S. sow farms currently stands at about 18 days, with ranges of 14- to 22-day lactation lengths probably common within a group of sows.

Primiparous sows are more susceptible than multiparous sows to the increased wean-to-estrus interval and decreased farrowing rate associated with lactation lengths less than 21 days.

In contrast, P1 and P2 sows were less susceptible to reduced litter size at lactation lengths less than 21 days than were P3 or greater sows (in some studies).

Still, from a sow reproductive performance standpoint, it might be ideal to avoid lactation lengths less than 18 days for the primiparous sows in the gilt sow farm. While segregation from multiparous sows would make this possible, it may not be practical, depending on the flow and requirements of the system.

Table 1. Typical Lactation and Postweaning Reproductive Characteristics of Gilts, Primiparous Sows and Multiparous Sows
Gilts Primiparous Sows Multiparous Sows
Parity 0 1 Ž 2
Farrowing body weight 440 lb. 500 lb.
Farrowing 10th-rib backfat 0.8 in. (21 mm) 0.7 in. (18 mm)
Lactation avg. daily feed intake 9-12 lb. 13-16 lb.
Wean-to-estrus interval 5-6 days 3-4 days
Duration of estrus 40 hours 47 hours 57 hours
Estrus-to-ovulation interval 28 hours 33 hours 40 hours
Ovulation rate (no. of eggs shed) 18-23 20-25 22-27
Fertilization rate 80-85% 85-95% 85-95%
Embryo mortality 30% 30% 30%
Fetal mortality 15% 10% 10%
Farrowing rate 75-80% 80-85% 85-90%
Litter size, total born 10.5-11.5 12.0-13.0
Litter size, born alive 9.5-10.5 11.0-12.0


Seasonal Infertility Problems

A seasonal decrease in sow fertility during the summer and early fall is a common and costly phenomenon in many sow farms. From July to September, sows take longer to return to estrus after weaning, and there is a higher incidence of anestrus than at other times of the year. Sows mated during this period typically exhibit a 10% decrease in farrowing rate and sometimes exhibit a 0.5- to 1.0-pig decrease in litter size compared to sows mated during the spring and winter. Extra gilts are usually mated in an attempt to meet farrowing targets.

There is often an increase in irregular returns to estrus (greater than 24 days postbreeding) in sows mated during the period of seasonal infertility. This seems to be related to the failure of some sows to respond to embryonic signals and complete maternal recognition of pregnancy. The fact that these types of failures result in complete rather than partial embryo mortality may explain why a reduction of farrowing rate is more commonly observed than a reduction of litter size in sows mated during the period of seasonal infertility.

Primiparous sows exhibit a greater increase in wean-to-estrus intervals than multiparous sows during the summer period. However, gilts, primiparous sows, and multiparous sows mated from July to September all exhibit a similar decrease in farrowing rate compared to the other three quarters of the year.

Analysis of some records indicates that P1 and P2 sows are less susceptible to reduced litter size during the summer than third or greater parity sows, though the reason is not known.

Lactation feed intake can be reduced during the summer months, and primiparous sows would be the most adversely affected by such a deficiency. This might explain the greater increase of wean-to-estrus intervals in primiparous sows in summer.

Parity segregation would certainly make special feeding of primiparous sows easier. However, there is no other clear advantage of parity segregation in meeting breeding targets during the period of seasonal infertility based on our limited knowledge of the system flow.

Summary

Despite the fact that gilts and primiparous sows have significantly reduced reproductive performance compared to multiparous sows, there is little evidence of any substantial differences in the physiology of the reproductive process between these parity groups (Table 1). This may be due to a lack of experiments designed to specifically examine the effects of age and parity on reproductive processes.

Conversely, the increase in reproductive performance from gilts through the first few parities may be due in part to the removal of subfertile females from the breeding herd and not to some physiological change. Primiparous sows have lower lactation feed intake, longer wean-to-estrus intervals, an increased incidence of anestrus, and decreased farrowing rate and litter size compared to multiparous sows.

Parity 1, and to some extent, P2 sows, are more susceptible to increased wean-to-estrus intervals and decreased farrowing rates than P3 or higher-parity sows after short lactation lengths.

During the period of seasonal infertility, primiparous sows exhibit a greater increase in wean-to-estrus intervals, but a similar decrease in farrowing rate compared to multiparous sows.

High levels of feed intake postbreeding may exert differential effects on embryo survival in gilts and primiparous sows vs. multiparous sows.

Parity-specific AI schedules may be beneficial given the longer wean-to-estrus interval, shorter duration of estrus, and shorter onset of estrus-to-ovulation intervals of primiparous as compared to multiparous sows (Figure 1). Given the shorter duration of estrus in gilts as compared to sows, it is clear that administration of AI services to gilts should not be delayed after the onset of estrus has been detected.