Currently, one of the most commonly used indicators internationally for evaluating sow productivity and performance is sow productivity per year, meaning the number of piglets weaned per sow per year. It is also sometimes expressed as Piglets Sow Year, abbreviated as PSY. It is calculated as the number of piglets weaned per sow per litter × the number of litters per sow per year. This indicator emphasizes such measures as the number of liveborn piglets, pre-weaning piglet mortality, the sow’s production cycle, and the sow’s non-productive days. Although it is a composite indicator, its components suggest that relying on it over the long term in production may lead producers to place excessive emphasis on farrowing performance, resulting in selection that favors litter size alone.
In fact, for high-producing sows, overemphasis on litter size as a breeding objective has already had some negative effects on measures such as piglet quality, weaning weight, growth rate before market, and feed conversion. According to reports from Hypor, piglet quality traits contribute as much as 33% to the evaluation of sow productivity. In addition, a sow’s age at first farrowing and productive lifespan have a substantial effect on her lifetime production performance. The quality of weaned pigs also has a significant effect on their survival and growth rate, while weaning weight is an important measure of a sow’s lactation performance. Therefore, the industry needs a new, more comprehensive indicator to evaluate sow productivity, in line with current pig breeding conditions and objectives.
1. The concept of sow weaning capacity
Sow weaning power was first reported by Hypor. Weaning power refers to the total weight of piglets weaned by a sow over her lifetime. It is calculated as follows: number of piglets weaned per litter × average weight per piglet at weaning × total number of litters over the sow’s lifetime. It can be seen that sow weaning power is determined by three factors: the number of piglets weaned, the weight of piglets at weaning, and the total number of litters the sow farrows over her lifetime. Compared with annual sow productivity, it highlights the total number of litters during the sow’s productive life and the quality of the weaned piglets, namely weaning weight. Clearly, sow weaning power covers more than annual sow productivity; it is more comprehensive and representative, better indicates a sow’s production capacity and value in use, and better reflects a farm’s management level. The research in Table 1 indicates that the average annual weight of piglets weaned per sow is a more suitable measure of lifetime sow performance than the average annual number of piglets weaned. Increasing sow weaning power can reduce the number of sows kept under the same conditions, thereby saving feed, reducing housing and labor requirements, lowering production costs, and improving profitability. Table 1 shows that the weight of piglets weaned has a greater impact on a farm’s economic returns than the number of piglets weaned.
Table 1 Comparison of the potential benefits of two farms with different sow weaning power
Note: The 5% higher performance and 15.3% lower income figures are based on the British calculation method. This table is sourced from Weizhu Technology.
2. Factors affecting sow weaning power
2.1 Number of piglets weaned per sow per litter
The number of piglets weaned per sow per litter depends to a large extent on the number of live piglets born per litter and the piglet mortality rate during lactation.
Many factors affect the number of live piglets born per litter, mainly including genetics, disease, nutrition, the environment, farrowing management, and breeding techniques. Genetics mainly includes the breeds and lines of boars and sows, hereditary diseases, chromosomal abnormalities, and other genetic factors. Disease mainly refers to reproductive disorders, including non-infectious reproductive disorders such as endometritis, and infectious diseases that can cause mummification, stillbirths, and abortions, such as porcine reproductive and respiratory syndrome, swine fever, and parvovirus disease. Nutrition mainly refers to high-energy diets that make sows overly fat, causing fat to accumulate in the fallopian tubes, uterine horns, and ovaries; degeneration of follicular cells then leads to infertility in sows. In some cases, a severe lack of energy and protein in the diet causes follicular development to stop or ovarian cysts to form. Likewise, deficiencies in certain vitamins or minerals can also cause reproductive disorders in sows. The environment mainly refers to the comfort of the housing during pregnancy, such as temperature and humidity, space for movement, and various stressors. Farrowing management mainly refers to whether attendants have made adequate preparations before the sow is due to farrow, whether farrowing procedures are carried out correctly, and whether non-breathing piglets receive timely assistance. Breeding management mainly includes checking the quality of boar semen, timing of mating or insemination, and breeding techniques.
Factors that mainly affect piglet mortality during lactation include piglet birth weight, care during the neonatal period, and feeding and management during lactation.
2.2 Weaning weight of piglets
Piglet weaning weight has a significant impact on the economic performance of a pig farm. Weaning weight is determined by birth weight, the sow’s lactation performance and the feeding and management of suckling piglets, as well as the length of the suckling period and the age at weaning. As the saying goes, “A difference of one liang in piglet birth weight means a difference of one jin in weaning weight; a difference of one jin in weaning weight means a difference of ten jin in market weight.” This shows how greatly piglet birth weight affects weaning weight and growth and development after weaning.
2.3 Lifetime number of litters per sow
The lifetime number of litters per sow is affected not only by the sow’s age at first mating and age at culling, but also by factors such as the sow’s non-productive days and the length of the lactation period. The sow’s non-productive days are negatively correlated with the number of litters produced per year. The length of the lactation period is also negatively correlated with the number of litters produced per year. Clearly, sows that reach sexual maturity early, produce more litters per year, and have a longer productive lifespan will have more litters over their lifetime.
3. Measures to improve sow weaning performance
3.1 Increase the number of healthy piglets born per litter
Increasing the number of healthy piglets born per litter is the foundation for increasing the number of piglets weaned. This indicator is influenced by both boars and sows.
3.1.1 Select high-performing, high-quality sows
Select sow lines with high litter size, strong milk production, and good breeding ability. Choose sows with good body conformation, such as a broad, full hindquarter; normally developed external genitalia; a full vulva of moderate size; firm, well-structured udders with more than 7 functional teats, evenly spaced; moderate backfat; an early age at first estrus; relatively high birth weight and litter weaning weight; and origin from a large litter. Therefore, when purchasing replacement breeding pigs, be sure to buy from a pig farm that conducts timely performance testing and keeps complete records. Commercial pig farms can make full use of the maternal heterosis.
Generally, F1 sows can achieve a farrowing rate 10%–20% higher than purebred sows, producing 1–2 more piglets per litter. If F1 crossbred sows are mated with a boar of a third breed, litter size can be further increased by 5%–10% over F1. Crossbred piglets have strong adaptability, grow and develop quickly, and have high feed efficiency. Therefore, commercial pig farms can use crossbreeding to increase the annual number of weaned piglets produced per sow.
3.1.2 Select semen of high quality
The quality of boar semen is affected by both genetics and the environment. First, use boars with high reproductive output. When selecting or purchasing a new boar, in addition to considering its growth and meat production performance, also consider its reproductive traits, such as testicle size and position, the arrangement, number, and shape of the teats, and preputial development. More importantly, select based on its production performance, including the performance of its parents and its own performance. Therefore, breeding boars should be purchased from a pig farm that conducts comprehensive performance testing and keeps complete records. Complete pedigrees and performance testing data help improve selection accuracy.
Many environmental factors affect the semen quality of boars. The main factors include the frequency of boar use or semen collection, the ambient temperature in the boar house, boar nutrition, and techniques for semen collection, processing, transport, and storage. In actual production, boars are often used or collected from either too frequently or too infrequently. In fact, overly frequent use can seriously affect the quantity of sperm and the libido of boars. Infrequent use is also undesirable: as the interval increases, more aged sperm accumulate in the epididymis, reducing the proportion of viable sperm in each semen collection. Therefore, a reasonable frequency of use should be determined based on the boar’s age, nutrition, and exercise.
3.1.3 Provide sows with appropriate sexual stimulation during artificial insemination
Many pig farms do not provide sows with appropriate stimulation during artificial insemination. Although the sows may still conceive, their final litter-size results are not as good as those at farms that provide stimulation. Providing appropriate sexual stimulation before artificial insemination can promote oxytocin secretion in sows, accelerate uterine contractions and semen uptake, and reduce semen backflow. There are two types of sexual stimulation: boar stimulation and handler stimulation. Boar stimulation can involve allowing the sow to come into contact with an adult boar before insemination, or placing an adult boar directly in front of the sow during insemination. Handler stimulation mainly involves appropriately stimulating the sow’s sensitive areas. For example, the inseminator can sit on the sow’s back; press both sides of the sow’s abdomen with a fist or knee; lift the sow’s inguinal folds upward; press below the sow’s vulva with a fist; or press and massage the sow’s hindquarters with a knee. It has been reported that, during artificial insemination, if the inseminator places a hand on the sow’s back or applies some form of pushing stimulation, the number of piglets born alive can increase by 6%. According to a report from Denmark (translated by Qu Xiangyang), when carrying out insemination, if the inseminator actively reinforces the stimulation of the sow by taking the actions described above while a boar is used for stimulation, litter size will increase by 0.5～1 piglets.
3.1.4 Improve the feeding and management of pregnant sows
Extensive research has confirmed that nutritional management measures can increase sow litter size and improve the health of newborn piglets. First, gestating sows should be maintained in ideal body condition. The body condition of gestating sows is closely related to piglet birth weight, litter uniformity, and litter size. Highly productive sows often face considerable metabolic pressure. When energy intake is insufficient and the ability to mobilize maternal tissues is limited, it is difficult for sows to provide fetuses with enough nutrients to support their growth and promote the development of their own mammary parenchymal tissue. Therefore, although the traditional feeding principle of “preventing excessive weight gain during gestation” has some merit, it is essential to ensure that gestating sows receive a certain amount of nutrients to support fetal growth. It is also necessary to increase dietary antioxidants to address DNA damage in late gestation. Gestating sows must reach ideal body condition and maintain consistent backfat thickness to achieve high reproductive performance. Reports indicate that adding L-arginine, unsaturated fatty acids, vitamins, and organic trace elements to gestating sow diets can improve reproductive performance.
Stress in gestating sows should be avoided through good management. During the first 3 to 4 weeks of gestation, embryos are highly susceptible to death and are then absorbed by the uterine wall. If no more than 5 embryos remain, gestation will terminate and the sow will return to estrus. Management should therefore aim to avoid all forms of stress during gestation. At present, gestating sows on many farms are housed in individual stalls, preventing embryo deaths caused by sows fighting over feed troughs. However, the risk of abortion or stillbirth caused by handlers treating gestating sows roughly must not be overlooked.
3.2 Reducing piglet mortality during lactation
Deaths among suckling piglets are generally caused by either disease or management, but ultimately, poor management is to blame. Statistics from one farm showed that more than 80% of deaths among piglets during lactation were caused by management oversights and errors on the part of farm staff.
The most important 8 hours in a pig’s life are immediately after birth. Therefore, farrowing care must not be neglected. Providing piglets with some care after birth can save some apparently stillborn piglets and can also save healthy piglets the time and effort of competing to get colostrum. While promptly providing adequate warmth, pay attention to how long piglets stay in the warming box before nursing; ideally, this should not exceed 10 minutes. Once the piglets are dry, they can go out to nurse on colostrum. Give extra care to weak piglets and provide them with sufficient colostrum to improve their resistance and chances of survival. For sows that have just finished farrowing, an appropriate amount of oxytocin can be injected. At the same time, place the piglets in the warming box to prevent them from nudging the sow’s teats. After 1–2 minutes, collect colostrum by manually expressing the teats and feed it to weak piglets. If a large amount of colostrum is collected, the excess can be refrigerated for later use, ensuring that weak piglets receive additional colostrum within 24 hours after birth.
3.3 Increase the daily weight gain of suckling piglets
As mentioned above, when nutrition, environment, and other conditions remain unchanged, birth weight largely determines the daily weight gain of piglets during lactation. To achieve higher daily weight gain, the first step is to increase piglet birth weight. Differences in piglet birth weight are closely related to piglet nutrition during the first month of pregnancy; the quality of the fetal membranes and umbilical cord, as well as the nutritional supply from the uterus, all play important roles. During the final three weeks of pregnancy, piglets nearly double in weight, which requires them to obtain more nutrition from the sow. Therefore, it is very important to ensure that pregnant sows continue to eat and drink efficiently.
The farrowing room should strictly implement an all-in/all-out system to reduce the incidence of disease, provide extra care for newborn and weak piglets, ensure piglets receive colostrum promptly, and manage cross-fostering properly. Begin providing piglets with supplementary feed and creep-feeding training from 5 days of age, and administer the three health-care injections. Provide additional warmth to meet the different temperature needs of sows and piglets. Only by creating a comfortable environment for piglets can their full growth potential be realized.
3.4 Reduce the number of non-productive days in sows
3.4.1 Promote normal estrus in sows
It is important to take effective measures to promote normal estrus in sows. Only when sows are in estrus can they ovulate, be bred, and conceive. This minimizes the non-pregnant period, shortens the reproductive cycle, and increases the number of litters per year.
Ensure appropriate feeding, increase feed intake during lactation, and provide a sufficient, nutritionally complete and balanced diet for sows to meet their production needs. Prevent excessive body weight loss and excessive loss of backfat thickness during lactation. At weaning, a sow’s backfat thickness of 18～23mm is ideal and should be no less than 17mm; otherwise, estrus after weaning will be delayed. Based on an average daily feed intake of approximately 5 kg/day for lactating sows in large-scale pig farms, the digestible energy level of the diet for lactating sows is recommended to reach 13.5～14.5 MJ/g, with a crude protein content of around 18% and a lysine content of 0.99～1%.
For multiparous sows that have not come into estrus for a long time, and replacement gilts that have reached the appropriate stage, estrus stimulation should be carried out. For example, the sow can be exposed to a boar with strong sexual desire; the urine or semen of a boar, or the urine of a sow in estrus, can also be used to stimulate a sow that is not in estrus; a sow that is not in estrus can also be allowed to watch a boar mate with a sow in estrus; alternatively, sows that are not in estrus can be group-housed or moved to another pen to induce estrus. In special circumstances, medication can be used to induce estrus, such as injecting PG600; if a sow is not coming into estrus because of a persistent corpus luteum, cloprostenol can be used to induce estrus. If a sow still has not come into estrus 25 to 30 days after weaning, she must be culled. An excessively long estrous interval is an important reference factor for culling sows.
3.4.2 Breed at the right time to improve the conception rate
Many farms choose to breed sows within 20 to 36 hours after the standing reflex appears. However, this assumes that breeding personnel know the exact time when the standing reflex appeared. In practice, many breeding personnel do not check for estrus frequently enough and therefore do not know exactly when the standing reflex appeared. By the time they notice that a sow is showing the standing reflex, it may already have been present for some time. If they then wait 20 hours before carrying out the first breeding or insemination, the timing will be too late. Therefore, to breed at the right time, estrus checks must first be carried out properly: at least twice a day, in the morning and evening, to determine the exact time when the sow's standing reflex appears. On farms where timely estrus checks are not possible, breeding or insemination should be carried out as soon as a sow is found to be showing the standing reflex. It is advisable to breed 2 to 3 times per estrous cycle.
3.4.3 Carry out pregnancy diagnosis properly
So far, no reports have shown a sow conception rate of 100%. This means that a certain proportion of sows do not conceive after mating, and even those that do become pregnant may miscarry for various reasons. Therefore, it is important to perform early pregnancy diagnosis and monitor for estrus during the middle and later stages after mating.
The first pregnancy diagnosis after mating should be performed at 3～4 weeks and can be based on the estrous cycle. The second pregnancy diagnosis should be performed at 7～8 weeks using an ultrasound diagnostic device.
Diagnosis during the middle and later stages is relatively simple: check the sow’s behavior and appearance. For sows with no obvious external signs, watch for signs of estrus. On farms where pregnant sows are generally kept in individual stalls, an adult boar can be used to check for estrus; this is simple, accurate, and saves labor and time. If a sow is found not to be pregnant or to be in estrus, take steps to induce estrus and remate her.
3.4.4 Carefully raise pregnant sows
After breeding, sows should preferably be housed individually to prevent competition for feed and fighting. The floor of the pen must not be slippery, to prevent mechanical abortions caused by falls and similar incidents. Nutritional provision should be appropriate to avoid deficiencies in vitamins and minerals. In particular, the feed must provide adequate vitamin A, D, and E, folic acid, and B vitamins; iron (Fe), zinc (Zn), manganese (Mn), and others should be added at normal levels. Moldy feed must never be used. Mycotoxins in feed (such as aflatoxins, zearalenone, vomitoxin, fumonisins, and others) can cause abortions and stillbirths in sows, and can also cause anestrus or false estrus.
3.5 Increase the total number of litters before sow culling
Factors affecting the total number of litters produced before sow culling comprise the time from first farrowing to culling, that is, productive lifespan, and the reproductive cycle. Productive lifespan refers to the period from first mating to culling. The reproductive cycle is affected by two factors: non-productive days (as mentioned above) and the length of the lactation period.
3.5.1 Extend sows’ productive lifespan
First, promote timely estrus in replacement gilts. For a given breed, nutrition, season, lighting, housing conditions, and whether boar stimulation is provided all affect the onset of puberty in gilts. In general, when replacement gilts (of foreign breeds) reach a body weight of 70 kg, they should be fed a grower diet ad libitum to maintain ideal body condition. This not only helps induce estrus but can also increase litter size. Research indicates that the optimal breeding body condition for first-parity Landrace × Large White and Large White × Landrace gilts is a P2 score (that is, a backfat thickness of 18–23 mm, measured about 6 cm from the dorsal midline at the last rib). To promote estrus, replacement gilts should be housed in small groups and given as much sexual stimulation from boars as possible. The ideal procedure is to place the gilts in an adult boar’s pen for 20 minutes every day until they come into estrus.
Second, reduce the sow culling rate. In fact, the rate of involuntary culling of sows at many pig farms reaches 20%. The main reasons are an extended interval from weaning to estrus, repeated unsuccessful breeding or very small litters, and disease. A major reason for these outcomes is that sow management is not sufficiently scientific: appropriate nutrition is not provided, and the sows are not given a better environment. In the past, sow nutrition was generally based on a litter size of 9–12 piglets per farrowing. Now, however, as a result of genetic improvement and advances in breeding techniques, many sows produce 13 piglets or more per litter. Accordingly, nutritional provision for sows at each stage must be adjusted. For sows with 12 piglets per litter, feed intake should reach 8–8.5 kg on day 10 after farrowing; if a sow has 14 piglets per litter, feed intake should exceed 9 kg. Insufficient feed intake leads to substantial weight loss in lactating sows, affecting timely estrus and ovulation after weaning. The same principle applies to nutritional provision for pregnant and non-pregnant sows.
Some culling is also caused by diseases resulting from postpartum infection, a characteristic disease of sows. During pregnancy, adjust the sow’s body condition and prevent her from becoming too fat or too thin. Restrict feed before farrowing and prevent constipation to reduce the risk of dystocia. Farrowing is a high-risk process because it involves substantial tissue injury and sows have reduced immunity; antibacterial and anti-inflammatory treatment must be provided. The longer it takes a sow to expel lochia after farrowing, the greater the risk of uterine infection. This is especially important for sows that have experienced dystocia and older sows, for whom postpartum uterine cleansing should be carried out. In addition, keep the farrowing room and the sow’s body clean to prevent infection.
3.5.2 Early weaning
The lactation period is positively correlated with the sow’s reproductive cycle; shortening the lactation period can shorten the sow’s reproductive cycle. Therefore, many pig farms practice early weaning, but earlier is not always better. The weaning time must be determined based on the actual conditions of the farm. It is generally considered reasonable to wean at 21 to 28 days. If ultra-early weaning is practiced, higher demands are placed on piglets’ feed intake and digestive capacity, the environmental conditions in the nursery, and the quality of nursery pig feed.
4. Conclusion
In summary, the sow weaning capacity indicator covers not only the elements included in the sow’s annual productivity—such as the number of piglets born per litter, the number of piglets weaned per litter, and the number of litters farrowed per sow per year—but also reflects the value of piglet growth performance and the sow’s productive lifespan to the overall herd’s production performance. It can both assess the sow’s actual production performance and reflect the management level of the pig farm. To improve sow weaning capacity, efforts should address every stage, including breed selection for boars and sows, management of replacement gilts, breeding techniques, gestation management, sow and piglet management during lactation, weaning techniques, and disease control.