Introduction

Throughout history pork producers have progressively reduced average weaning age (lactation length) in the US. A few decades ago, 5 to 6 week lactation lengths were common and reduction of lactation length was primarily driven by the potential to increase sow productivity by decreasing the interval between farrowings. More recently, improved weaned pig nutrition and the potential pig health and growth benefits derived from the segregation of early weaned pigs (SEW) to a separate, 'high health' facility have contributed to driving this transition. Recent statistics from a large US records database reflect this change since average lactation length was 18.9 days and approximately 25% of the farms had an average lactation length 16.2 days (PigCHAMP®, 1999). With this gradual change in lactation length has come a change in the definition of just what is consider 'early' weaning. Conventional (traditional) weaning, early weaning and ultra-early weaning may be considered as lactation lengths > 18 days, 14 to 18 days and < 14 days, respectively.

Unfortunately, there appear to be biological limitations to the sow's ability to return to estrus, conceive and farrow a respectable number of pigs following a short lactation length. Evaluation of the specific mechanisms responsible for these limitations is still in its infancy (Cosgrove et al., 1997) and the reproductive response of sows to short lactation lengths can vary widely between individual sows and individual sow farms. Fortunately, both experimental and field data have revealed some important factors that seem to explain some of this variation and may be useful to develop appropriate counteractive management strategies.

Effect of Lactation Length on Sow Reproductive Performance and Productivity

Since gestation length is essentially fixed and the weaning-to-estrus interval is influenced by many hard to control factors, lactation length represents the one phase of the sow's reproductive cycle that can be easily manipulated by management. In theory, reducing lactation length would decrease the interval between farrowings and increase litters/sow/year, pigs weaned/sow/year and the number of pigs weaned/farrowing crate/year if sow weaning-to-estrus interval, conception rate and subsequent litter size remained constant. As one would expect, in practice, this attractive theory does not always hold true. Some studies based on retrospective analysis of production records from hundreds of farms have suggested pigs weaned/sow/year increases as lactation length decreases from 28 to 14 days (Dial et al., 1995; King et al., 1998) and others have suggested there is no change due to the reduced sow reproductive performance and increased non-productive days associated with reducing lactation length (Xue et al., 1993).

Figures 1, 2 and 3 were constructed based on the available experimental and retrospective records analysis data present in the literature on the effect of lactation length on sow reproductive performance (Belstra, 1999). Since the data on which these figures are based was summarized from about 30 different studies, spanning nearly 50 years, that utilized a wide variety of sow genotypes, parities and management techniques, they should be interpreted with some caution. However, the basic relationships that they illustrate are probably similar to those that exist in current commercial production. Polynomial regression lines were inserted for illustration purposes only.

Reduced sow longevity has also been associated with early weaning since herds that practice it sometimes have a lower average parity at removal (Xue and Dial, 1995; Xue et al., 1997). This is probably due to the increased culling of sows for the potential reproductive failures mentioned previously. Some people have even suggested that early weaning is detrimental to sow welfare and the European Economic Community (EEC) has banned weaning at < 21 days. However, there is no evidence to indicate that early weaning is a greater stressor on sows than weaning after a conventional lactation length. In fact, short lactations should minimize loss of sow body reserves (fat and muscle) which should theoretically improve sow welfare and reproductive performance.

Mechanisms Mediating Reduced Reproductive Performance

There appear to be two major obstacles to subsequent reproductive function following short lactation lengths. The first is that suckling of the litter inhibits the release of one of the key hormones that governs follicular growth (luteinzing hormone, LH) and thus limits estrogen production and return to estrus. This suckling-induced inhibition of LH secretion is gradually overcome during lactation but is very potent during early lactation (Quesnel and Prunier, 1995). This results in a longer weaning-to-estrus interval for sows weaned during early lactation or sometimes a failure to return to estrus (anestrus).

About 3 days of suckling are required to establish this suppression of LH secretion and sows that are weaned before this time will develop large preovulatory follicles. However, these follicles usually not ovulate and become cystic, causing sows to exhibit an erratic estrus or remain anestrus and blocking any chance of conception (Elliot et al., 1980). Therefore, when possible sows should not be weaned with less than 3 days of suckling stimulus, unless they are to be culled.

The second obstacle, that is thought to be responsible for the reduced subsequent litter size and possibly the reduced conception rate of early weaned sows, is incomplete uterine involution. Uterine involution is the degenerative and regenerative process that the tissues that make up the uterus and oviducts undergo post-farrowing as they return to a non-pregnant state. Even though much of this process seems to occur during the first week postpartum, the competence of the uterus to support embryonic development may not be fully restored until 3 weeks postpartum (Varley, 1982). Short lactation lengths do not seem to reduce the number of eggs shed (ovulation rate) or the percentage of those eggs that are fertilized (fertilization rate)(Hays et al., 1978; Marsteller et al., 1997) but, the percentage of embryos that survive (embryo survival rate) is substantially reduced by day 25 to 30 postmating (Figure 4). Again, figure 4 is based on data from the available literature and it suggests a 15% reduction in embryo survival by day 30 in sows that lactate < 19 days (dark circles vs. > 19 d open circles; Belstra, 1999). Even though a lack of time to complete uterine involution is generally accepted as the cause of this phenomenon, the specific aspect of the uterine environment that is deficient is unknown.

Despite the seemingly inevitable reproductive consequences associated with lactation lengths < 21 days described above, field data indicate that some farms that practice early weaning experience little difficulty maintaining an acceptable level of reproductive performance. Further, on some farms there is a problem with weaning-to-estrus interval and conception rate but not subsequent litter size when they adopt short lactation lengths and vice versa on others. Several factors that may explain why certain sows and certain sow farms can cope with short lactation lengths while others cannot have recently been suggested.

Parity Primiparous sows are often still building body reserves as they grow towards their mature size when we mate them and the dual energy demands of this growth and conceptus growth during their first gestation in combination with a tendency to eat less than their multiparous counterparts during lactation can result in excessive loss of body reserves and reduced reproductive performance postweaning (i.e. the second parity dip). Primiparous sows also seem to be the most susceptible to the negative effects of short lactation lengths on subsequent reproductive performance (Dial et al., 1995). For example, Mabry et al. (1996) found that parity 3 and older, parity 2 and parity 1 sows could return to estrus in 7 days on average and have a farrowing rate > 70% at lactation lengths as short as 9, 12 and 19 days, respectively. Similarly, Koketsu and Dial (1997a) suggested a lactation length of 11 to 13 days for parity 2 to 6 sows and 17 to 19 days for parity 1 sows. Given this information, it should be beneficial to allow primiparous sows to lactate a few days longer than their multiparous counterparts.

Lactation Feed Intake Retrospective analysis of production records has suggested that sows that consume > 12.3 lb. feed/day on average do not exhibit a large increase in weaning-to-estrus interval or a decrease in conception rate and subsequent litter size, whereas sows that consume < 9.2 lb. feed/day do exhibit a large reduction in reproductive performance (Koketsu et al., 1997; Koketsu and Dial, 1997b). Restriction of nutrient intake during early lactation can reduce LH secretion and increase the weaning-to-estrus interval of early weaned sows (Koketsu et al., 1998; Jones and Stahly, 1999) so management to maintain high feed intake seems warranted. Since primiparous sows represent a large portion of weaned sows (due to their high replacement rate), tend to have low lactation feed intake, and are the most sensitive to the negative effects of early weaning, it would be wise to pay special attention to their needs. Keeping sows cool and avoiding overfeeding during a single feeding can help increase lactation feed intake but, one of the best things you can do, is to measure and record how much feed sows are consuming on feed intake (i.e., disappearance) cards. Without that information it is very hard to determine if and when there is a need to improve lactation feed intake. Reducing the weaning-to-estrus interval though increased lactation feed intake may also improve other aspects of reproductive performance since sows that return to estrus within 3 to 6 days postweaning typically have a higher conception rate and subsequent litter size than those that return in 7 to 10 days (Vesseur et al., 1994; Steverink et al., 1999).

Genotype Short lactation lengths increase the weaning-to-estrus interval and reduce the conception rate and subsequent litter size of all sow genotypes but, the rate of this change as lactation length decreases varies between genotypes (Dial et al., 1995). Some sow genotypes are more sensitive than others to the negative effects of short lactations but, there is not enough data at this point to determine which breeds or genetic lines are the most resistant to reduced reproductive performance.

Stockpeople Farms with the same sow genetics, nutrition program, building design, health status and standard operating procedures (SOPs), which are common in North Carolina, can have very different reproductive responses to short lactations (Almond, 1998). The most logical explanation for this phenomenon is that the people who carry out those SOPs create these differences. Superior stockpeople, who provide superior sow management, are likely a large part of the difference between reproductive success and failure in early weaning systems.

Counteractive Management Strategies

Management strategies to counteract the negative effects of early weaning on subsequent sow reproductive performance typically focus on one of two goals. Stimulate a more rapid return to estrus postweaning to avoid the extended weaning-to-estrus interval and anestrus problem (1, 2, 3) or delay the return to estrus and(or) mating to avoid the reduced conception rate and subsequent litter size problem (4, 5, 6). The most appropriate strategy is partially dependent on which problem(s) is(are) the greatest in the sow herd but, usually only strategies aimed at stimulating a more rapid return to estrus (1, 2, 3) are practical and economically feasible.

1. Lactation Feed Intake: As previously mentioned, sows that have a high feed intake (> 12.3 lb./day?) during lactation seem to be more resistant to the negative effects of short lactations on reproductive performance. Implementing strategies to increase feed intake could reduce sow body reserve loss, increase LH secretion, reduce the weaning-to-estrus interval and improve other aspects of postweaning reproduction.

2. Split Weaning: Another method to increase LH secretion during lactation and postweaning and reduce the weaning-to-estrus interval is split weaning. This usually involves weaning half of the litter (the heavier pigs) about 2 to 4 days before weaning the remainder. Split weaning has reduced the weaning-to-estrus interval of sows that lactated for 3 to 4 weeks (Matte et al., 1992) but, few data sets for shorter lactation lengths exist (Foxcroft et al., 1987). However, some farms that produce early weaned feeder pigs are using it to increase pig uniformity and help meet the minimum pig weight requirements of their customers. Split weaning may or may not reduce the weaning-to-estrus interval of early weaned sows.

3. Gonadotropin Treatment (P.G. 600®): P.G. 600® contains two hormones (400 IU eCG + 200 IU hCG) that mimic the actions of FSH (follicle stimulating hormone) and LH, respectively. P.G. 600® can be injected at weaning or shortly thereafter to stimulate follicular growth, speed the return to estrus and reduce the incidence of anestrus but, it will probably not increase the conception rate or subsequent litter size of treated sows (Bates et al., 1991; Kirkwood et al., 1998). However, this strategy may be practical if extended weaning-to-estrus intervals and anestrus are a major problem in the herd. Cost is somewhere around $3.75 to $4.50/dose so it is wise to treat only those sows that need it. This may be only the primiparous sows or it may be the sows with the shortest lactation in a given wean group that should be treated. Some producers even wait until 7 days or so postweaning and treat only sows that have failed to return by this time. Utilize production records to determine which sows are having problems and might benefit from treatment. Keep in mind though that exogenous hormones can be useful in some situations but, they can never replacement good sow management.

4. Skip-a-Heat Breeding: Instead of stimulating return to estrus, an alternate strategy is to delay mating until the second postweaning estrus. This sometimes results in an increased ovulation rate and often improves conception rate and embryo/fetal survival resulting in a .8 to 1.2 pig born alive increase in subsequent litter size (Moody et al., 1969; Love, 1979; Clowes et al., 1994). However, if farm 'X' is producing 22 pigs per sow per year, divided by 365 days in a year, that is equal to .06 pigs per sow per day. So for skipping 1 estrous cycle (21 days), that is a loss of 1.26 pigs (21 .06) for that time (not counting feed costs) versus a potential gain of .8 to 1.2 pigs born alive. In addition, sows have to be bred to fill the farrowing crates and skip-a-heat breeding could create some serious scheduling problems. Therefore, even though it offers improved reproductive performance, skip-a-heat breeding is probably not economically feasible in most situations.

5. Progestagen Treatment (Regu-Mate®:): Continued feeding of the orally active progestagen Regu-Mate® suppresses follicular growth and estrus until it is withdrawn causing a highly synchronous return to estrus. Regu-Mate® has been fed for different intervals during lactation and(or) postweaning and its use may allow increased ovulation rate, embryo survival and subsequent litter size (Koutsotheodoros et al., 1998). The flexibility of being able to delay the estrus of weaned sows for any interval is attractive and approval by the FDA for Regu-Mate®s use in swine is near but the product's cost and the inconvenience of having to mix the oil-based product with the ration will likely limit its use. The added non-productive days are fewer than with skip-a-heat breeding but, are still an added cost.

6. Inducing a Lactational Ovulation: A somewhat similar strategy is to use a natural source of progesterone, the corpus luteum (CL), to suppress follicular growth and return to estrus. This has been accomplished by treating sows with an LH-like hormone (hCG) soon after parturition to induce ovulation and formation of CL that can then be lysed using a PGF₂ analog like Lutalyse® causing the sow start a new cycle and return to estrus. This strategy is experimental and since a number of sows do not ovulate in response to the hCG and the CL formed on those that do may not lyse in response to PGF₂ treatment, it will remain experimental unless improvements in efficiency are made (Armstrong et al., 1999; Kirkwood et al., 1999).

Take-Home Message and Top 10 Recommendations

The negative effects that early weaning has on subsequent sow reproductive performance can be large enough to offset the potential sow productivity benefits of a short lactation length. However, there are a number of factors that seem to determine how severe of a reduction in reproductive performance individuals sows and sow farms exhibit. Overall, sow management and the stockpeople who provide it seem to be one of the major factors in this response. The following is a list of 10 things you may want to consider to improve the performance of your sow herd.

1. Avoid weaning at lactation lengths that significantly reduce sow reproductive performance unless necessary (i.e., herd health status).
   

2. Monitor and maximize lactation feed intake (> 12.3 lb./d?).
   

3. Avoid weaning primiparous sows as early as their multiparous counterparts.
   

4. Avoid weaning sows you plan to keep and rebreed at < 3 days of lactation. A minimum of 3 days of suckling are necessary to suppress LH secretion and avoid the formation of follicular cysts postweaning.
   

5. Consider the use of P.G. 600® on all sows at weaning or treat only high risk sows primiparous and(or) sows with the shortest lactation length in the group.
   

6. Consider the use of split weaning and wean the heavier half of the litter 2 to 4 days before weaning the remainder.
   

7. Reevaluate your heat detection and insemination program to improve sow conception rate if it is a problem.
   

8. The range or variation in lactation length can be more important that the mean. Small 1 day changes in lactation length when weaning at < 17 days can result in large changes is sow reproductive performance.
   

9. Work with your veterinarian to weigh the potential pig health and growth benefits of early weaning against the expected reduction of sow reproductive performance before implementing short lactation lengths.
   

10. Remember, optimal lactation length is farm specific and sow management seems to play a significant role in determining the reproductive response of sows to short lactation lengths.
    

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