The problem of high mortality in neonatal piglets is one that has existed for a very long time. Approximately 15 to 20% of all piglets born die either during the farrowing process or in early lactation.
Sows do not produce enough milk to sustain maximal growth of their suckling piglets.
Developing strategies to enhance mammary development is an interesting avenue to increase milk yield.
Research emphasis must be put on the study of colostrogenesis, feeding of replacement gilts, transition feeding, and fine-tuning of nutrient requirements in late gestation.
Introduction
Thirty years ago sow productivity was not optimal. Around 20% of newborn piglets died before weaning and the sow could not produce enough milk to sustain maximal growth of the remaining piglets. Today, in 2022, we are still faced with the same dilemma. Even though many scientific advances were made to help elucidate the various causes of neonatal piglet mortality, a preweaning mortality rate of 15% is common. Piglets die either during the farrowing process or in early lactation. The problem of inadequate sow milk yield still persists because as we have improved husbandry and nutritional practices to enhance milk production, the genetic selection pressure to increase litter size has led to the development of new hyperprolific sow lines. Hence, the 10 liveborn piglets per litter generally seen a while back is now easily 16 to 18 piglets. The sow is a great milk producer, on a per kilogram body weight basis, she can produce more milk than a dairy cow. Nevertheless, with the increase in litter size, each piglet is now ingesting less milk. The total amount of colostrum produced by sows is not related to their litter size, so that colostrum intake by newborn piglets is also inadequate in many instances, leading to poor survival and growth.
Decreasing preweaning mortality of piglets
The two most important causes of death are stillborn piglets and crushing by the sow, but there are many predisposing factors leading to these problems (Farmer and Edwards, 2022). Newborn piglets (Figure 1) are most vulnerable for a number of reasons. Namely, they have a very low body weight compared to that of the sow, they have very poor energy reserves (1% to 2% carcass fat), a low metabolic rate, and basically no immune protection. Liver and muscle glycogen are important energy stores for newborn piglets, yet they are depleted soon after birth. Piglets also experience a thermoregulatory shock at birth because of the sudden 15 to 20 °C drop in their thermal environment, and because of their low provision of energy and metabolic immaturity, they must rely on external sources of heat for thermoregulation (Farmer and Edwards, 2022). Colostrum is therefore essential to provide piglets with a source of energy and passive immunity, to activate heat production, and to supply various growth factors and bioactive compounds. A newborn piglet of average body weight (1.4 kg) must ingest a minimum of 250 g of colostrum to survive and grow, and approximately one third of sows do not produce enough colostrum to provide this critical amount to each of their piglet with current litter sizes (Quesnel and Farmer, 2019). The production of colostrum is highly variable between sows and is difficult to increase. Furthermore, there is a lack of studies on the mechanisms controlling colostrogenesis, most likely due to the difficulty in measuring colostrum yield in sows. Useful findings were nevertheless made and the common management procedure of farrowing induction with the use of prostaglandins was shown to reduce colostrum yield if done too early in gestation. On the other hand, the availability of colostrum to piglets can be prolonged with one injection of a very high dose of oxytocin given 12 h after the onset of farrowing (Farmer and Edwards, 2022). Nutrition in late gestation has a greater impact on the composition of colostrum than on its amount. Indeed, the fatty acid content of colostrum greatly depends on the amount of lipid provided in the sow diet, whereas the fatty acid profile of colostrum is largely influenced by the type of lipid being fed. Furthermore, various ingredients (such as fish oils, prebiotics, and probiotics) appear to have immuno-modulating effects that could increase concentrations of colostral immunoglobulins when provided during the last weeks of gestation (Quesnel and Farmer, 2019).
Newborn piglet.
Special attention was recently focused on feeding practices during the period transitioning between gestation and lactation. This transition period is generally defined as lasting from 7 to 10 days prepartum to 3 to 5 d postpartum (Theil et al., 2022) and is highly important because it is when the majority of piglet deaths occur. Nutrition during this critical period can greatly impact fetal growth, mammary development, farrowing, and colostrogenesis. Around parturition, the sow often lacks energy reserves to meet the demands for nest-building, uterine contractions, and colostrum production, and such an inadequate energy status may prolong the farrowing process, thereby increasing the rate of stillbirths. Stillbirths generally account for 30% to 40% of all preweaning mortalities and the main cause is a lack of oxygen during the farrowing process. Such deaths are most prevalent in modern hyperprolific sows because of their longer farrowing process which is a major risk factor (Farmer and Edwards, 2022). An important finding with regard to transition feeding is that sows need to be provided adequate feed supply, at least three daily meals, or supplemental energy around farrowing to help alleviate this problem of stillbirths (Theil et al., 2022). Feeding diets with a greater content of certain fiber types (e.g., sugar beet pulp) is also beneficial to reduce the risk of constipation and to provide a more constant energy supply postprandially, thereby helping to shorten the farrowing process. On the other hand, piglet birth weight appears to be only slightly responsive to maternal nutrition of pregnant gilts. In terms of colostrum production, energy and lysine intakes seem to play important roles, and feeding high-fiber diets may potentially improve both yield and composition of colostrum. However, the responses are variable depending on the fiber source (Theil et al., 2022).
Of importance is the fact that the high incidence of pre-weaning mortality is not only associated with the low birthweights of piglets but also with the presence of intra uterine growth-restricted (IUGR) piglets. Such IUGR piglets are commonly seen in litters from hyperprolific sows as a result of insufficient placental transfer of nutrients (Farmer and Edwards, 2022). The vitality of individual piglets combined with the behavior of their dam and the adequacy of the housing conditions will determine their survival. Hence, regardless of the nutritional strategy used in late gestation, providing assistance to piglets after birth can increase their chance of survival. Various approaches can be used, such as 1) optimizing the farrowing environment, 2) supervising farrowing and assisting newborn piglets, 3) using cross-fostering techniques, 4) providing nurse sows, and 5) providing artificial milk.
Increasing sow milk yield
Sow milk yield is affected by numerous sow-related factors (genetics, breed, parity, litter size, stage of lactation, suckling interval) and also by external factors (photoperiod, temperature, stress, hormones, nutrition). The nutrient requirements of lactating sows have been well established, but the problem resides mostly in getting the sow to consume enough feed to obtain these nutrients. Such was the case 30 years ago and still is the case today, mainly considering that the drastic improvement in reproductive performance of sows achieved over the years has led to greater nutritional needs. Any means of increasing sow feed intake during lactation should therefore be beneficial in terms of milk yield and piglet growth.
An aspect that was overlooked in the past but that now receives more attention is mammary development (Figure 2). Indeed, the number of milk-synthesizing cells present in mammary tissue at the onset of lactation has a great impact on the amount of milk produced. Yet, the optimal conditions required to support maximal mammary development are not known. Hormonal status, nutrition, and management all have an impact on the extent of mammary development (Farmer, 2013), which occurs at three specific stages in the life of a pig. Namely, from 90 d of age to puberty, from day 90 of gestation to farrowing, and during lactation. The key, but not sole, hormone involved in mammogenesis is prolactin. The essential role of prolactin for mammary development in late gestation and during lactation was demonstrated via classic inhibition studies (Farmer, 2022). Providing an exogenous source of prolactin to growing gilts in the critical prepubertal phase increased deposition of mammary parenchymal tissue (where milk is being synthesized) by 116%, whereas injecting prolactin during lactation had no effect on mammary development (Farmer, 2013). However, when a dopamine antagonist was used throughout lactation to increase prolactin synthesis by the sow, milk yield was improved (Farmer, 2022). Increasing circulating concentrations of the growth factor IGF-1 (insulin-like growth factor-1) via somatotropin injections from days 90 to 110 of gestation brought about a 22% increase in mammary parenchymal tissue mass (Theil et al., 2022). An important research avenue would therefore be to develop feeding strategies that could lead to an increase in IGF during late pregnancy.
Sow nursing her litter.
In terms of nutrition, feed restriction of growing gilts from 90 days of age to puberty is not advisable because it will reduce mammary development (Farmer, 2018). Yet, recent results suggest that only a drastic reduction in feed intake (from 2.88 to 2.11 kg/d of a diet containing 13.8 MJ ME) will negatively impact future milk yield. During late gestation, overfeeding energy (44 MJ of ME/day) will negatively affect mammary development, and this is likely related to body condition because obese gilts (36 mm of backfat) have poorer mammary development than gilts with 25 mm of backfat. Furthermore, gilts that are too thin (16 mm of backfat or less) have poorer mammary development than gilts with 17 to 26 mm of backfat (Farmer, 2018). Even though many studies were performed on the amino acid requirements of gestating sows, more work is needed. Indeed, recent findings demonstrated that increasing lysine intake by 40% above current recommendations as of day 90 of gestation, by including more soybean meal in the diet, augmented mammary parenchymal tissue mass by 42%. Such findings indicate that current feeding recommendations for lysine are underestimated for late-pregnant gilts. However, since soybean meal also contains amino acids other than lysine and has a high protein content, it is essential to determine whether this beneficial effect was due specifically to a greater intake of lysine.
An important management practice in first-parity sows that will affect mammary development and milk yield in second parity is ensuring that all teats are suckled. The suckling stimulus by piglets is the key factor necessary to maintain lactation (Farmer, 2019). Primiparous sows that have a poor body condition at farrowing may hinder their long-term reproductive performance if they produce too much milk in the first lactation. Producers may therefore decrease the litter size of such animals. However, it is now known that a teat that is not suckled in the first lactation will be less developed and produce less milk in the second lactation. It is also known that suckling of a teat for the first 48 hours after farrowing in the first lactation is adequate to ensure that milk yield from that teat will not be impeded in second lactation (Farmer, 2019). Therefore, cross-fostering to decrease litter size of very thin primiparous sows can be done but only on the third day postpartum.
Conclusion
Many scientific advances were made in terms of management and nutrition of sows with the goal of maximizing their lactation performance and one may think that no further research is necessary, but that is definitely not the case. Sows are still currently limiting the growth rate of their suckling piglets because of inadequate milk production and this is partly linked with the development of new hyperprolific sow lines. Hence, research emphasis must continue to be put on areas such as developing strategies to enhance mammary development, defining optimal feeding of replacement gilts, fine-tuning nutritional requirements in late pregnancy, and establishing the best transition feeding practice. Special attention also needs to be given to the process of colostrogenesis to increase colostrum intake by newborn piglets. Many new fields of research have emerged in recent years in animal science. More focus is put on important aspects such as environmental impact and sustainability. Yet, one must not forget that productivity is still a major concern in some species, such as lactating sows.
About the Author
Dr. Farmer is a research scientist at the Sherbrooke Research and Development Centre of Agriculture and Agri-Food Canada who has been working in sow lactation biology for the past 35 years. She obtained her BSc from McGill University, her MSc from the University of Saskatchewan, and her PhD from Pennsylvania State University. She was a pioneer in studying mammary development in pigs. She published 159 scientific manuscripts and 9 book chapters and edited 2 books that were translated and published in Chinese. Dr. Farmer’s expertise is recognized internationally. She was guest-speaker in 13 European countries, China, Australia, Canada, and the United States. She received two awards from the Canadian Society of Animal Science and was made a fellow of this Society in 2017. In 2022, Dr. Farmer received the American Society of Animal Science president’s Award for International Achievements in Animal Science. She is currently on the editorial board of the journals Translational Animal Science and Domestic Animal Endocrinology.
