The vital role of sow’s milk for piglets

Mammary development critical for sow productivity

Mammary growth is a crucial determinant for the milk production capacity of sows. The composition of mammary tissue shifts from a high lipid content to a high protein content between early and late gestation (Ji et al., 2006). This transition is associated with increased circulating concentrations of mammogenic hormones such as oestrogen, relaxin and prolactin (Ji et al., 2006). Mammary development continues during lactation with the average weight of suckled mammary glands increasing linearly by 57% between days five and 21 of lactation. The period of greatest mammary mass coincides with the estimated peak of lactation, occurring between weeks two and four (Hurley, 2001). Parity can affect mammary development during lactation, with primiparous sows showing greater development than those in their second or third parity (Beyer et al., 1994).Mammary involution: How and why does it occur?
Suckled mammary glands undergo drastic changes within the first seven days post-weaning with significant alterations occurring as early as two days after weaning (Ford et al., 2003). Individual mammary glands that are not regularly suckled during lactation undergo involution and become non-functional with no further reductions in parenchymal tissue during the first seven days post-weaning (Ford et al., 2003).Mammary gland involution is reversible within 24 hours of a teat not being used. However, even though it is reversible a gland not suckled for less than 24 hours will have reduced milk production for the remainder of lactation (Kim et al., 2001). Involution becomes irreversible after three days of a teat not being suckled (Theil et al., 2005). Mammary glands that are suckled during lactation are larger than those not suckled by the end of involution, suggesting that more mammary tissue is available for development in the next lactation (Ford et al., 2003). Farmer et al. (2012) demonstrated that a teat not used during the first parity will produce less milk in the second parity.Milk production in sows
After farrowing, nursing can be divided into five phases: initiation, pre-ejection massage, milk ejection, post-ejection massage and end of nursing. At the start of lactation, the sow usually initiates nursing by vocalizing (grunting). This is followed by the piglets gathering at the udder for the pre-ejection massage, lasting up to three minutes. Milk letdown then occurs with milk ejection lasting no more than 20 seconds. This is followed by post-ejection massage, which lasts several minutes (Spinka & Illmann, 2015).The removal of milk from the alveoli and ductal system of the mammary glands requires a neuroendocrine milk ejection reflex. This reflex is triggered by the piglets' massage of the udder (Fraser, 1980), stimulating the release of oxytocin, which leads to milk ejection (Hartmann & Holmes, 1989). Cyclic nursing begins approximately 10 hours after the onset of farrowing (Lewis & Hurnik, 1985) and gradually develops (Algers & Uvnäs‐Moberg, 2007). The frequency of nursing plays a key role in determining milk production, reaching a maximum around days eight to 10 of lactation and subsequently declining (Puppe & Tuchscherer, 2000).Clearly, one of the main factors impacting both mammary development and mammary involution is the suckling of the mammary glands by the piglets. Suckling a mammary gland in one lactation increases milk production in that gland during the next lactation (Farmer et al., 2012), and suckling is crucial for preventing involution during lactation (Theil et al., 2006).In the first days after farrowing, milk production exceeds the piglets' needs, and most (>85%) nursing sessions are initiated by the sow and terminated by the piglets. When milk supply becomes limited (weeks three to four), most nursing sessions are initiated by the piglets and terminated by the sow (Jensen, 1988; Jensen et al., 1991). Sows end nursing by moving away from the piglets or limiting access to the teats by lying belly-side down (de Passillé & Rushen, 1989b). Both the duration and intensity of teat stimulation by individual piglets influence milk production during the first three days of lactation (Algers & Jensen, 1991), and the frequency of nursing also affects piglet weight gain (Spinka et al., 1997; Auldist et al., 2000; Valros et al., 2002).Consequences of lack of stimulation for future parties
Farmer et al. (2012) demonstrated that the lack of suckling of a mammary gland in the first parity impairs its development and milk production in the second parity. In the study, the same teats or different teats were taped during the first and second lactation, so that in the second lactation, piglets had access to teats that were previously suckled or not. Differences between these teats were observed both in animal performance and in variables measured at the mammary gland level. Regarding piglet growth, those suckling previously used teats weighed 1.12 kg more at 56 days of age than those suckling previously unused teats.A difference in piglet growth rate was observed as early as between days two and four of lactation, suggesting a potential beneficial effect on colostrum production. Fraser et al. (1992) also observed that in the second lactation, differences in body weight gain between piglets suckling teats used or not in the first lactation were more pronounced during the first week of lactation compared to the second or third week.

Effects on piglet behavior
Another interesting and novel aspect by Farmer et al.'s (2012) was the discovery that the use or non-use of teats in the first parity affected piglet behavior during the next parity. On the third day of lactation, piglets suckling previously unused teats showed increased aggressive behavior, longer post-ejection phases, and a higher incidence of missed nursings compared to piglets suckling previously used teats (Farmer et al., 2012). These behaviors indicate that the piglets are more anxious to obtain milk, reflecting likely greater hunger. The effects of previous teat use on piglet behavior were not observed later in lactation (i.e., on day 10).Large litters and milk production: What can be done?
Genetic selection for hyperprolificity has resulted in modern sows producing large litters (Foxcroft, 2012). In these litters, there is often a surplus of piglets relative to the sow's nursing capacity. Additionally, these litters are characterized by higher piglet mortality, greater variation in birth weight, and a relatively high proportion of low birth weight (LBW) piglets within the litter (Foxcroft, 2012; Milligan et al., 2002). Consequently, a large percentage of sows are used as nurse sows (Bruun et al., 2016), and many piglets fail to obtain adequate milk intake to reach their full growth potential.In these situations, the use of nurse sows and supplementation with milk replacers is common. While milk replacers can be useful to ensure adequate nutrition for piglets, especially in large litters, they do not fully replicate the benefits of maternal milk. Additionally, it is well known that sow milk differs in nutritional composition compared to cow milk (Spreeuwenberg et al., 2001). Milk replacers often contain cow milk and plant-derived ingredients, which have lower fat content and higher concentrations of lactose and proteins not found in sow milk. Pieper et al. (2016) found that the expression of pro-inflammatory cytokines was higher in piglets fed milk replacer compared to those that suckled sow milk. These piglets also had deeper crypts and lower lactase activity, factors that can be associated with immunological, physiological, and morphological changes strongly related to health status and inflammation in the gastrointestinal tract (GIT) (Pieper et al., 2016). Gut maturation is crucial for the transition period from nursing to weaning, leading to a smoother adaptation from sow milk to a plant-based diet, ensuring healthier pigs. In the study by Amdi et al. (2022), titled "Suckling Induces Differential Gut Enzyme Activity and Body Composition Compared to Feeding Milk Replacer in Piglets," piglets raised exclusively on milk replacer had a leaner body composition at weaning age compared to piglets that suckled sow milk. In terms of health and growth, artificial rearing had the most negative impact, as illustrated by lower daily gain compared to piglets that suckled sow milk.

Intestinal development and nutritional supplementation in piglets
Supporting the benefits that gut maturation has on piglet performance and health, Wang et al. (2018) reported the critical importance of enterocyte development for gut health and post-weaning growth of piglets in "The relationship of enterocyte proliferation with intestinal morphology and nutrient digestibility in weaning piglets." The results demonstrate a positive association between enterocyte proliferation and key parameters of intestinal function, such as villus height and crypt depth. These findings highlight the intrinsic capability of piglet intestines to absorb and digest nutrients effectively when their natural development is allowed and stimulated.Finally, it is important to consider that the health and growth of piglets can be significantly improved with the use of protein isotonic solutions like Tonisity Px. Intestinal development will reflect in all production phases, including the period when they consume milk replacers.Although milk replacers may be necessary in later stages to supplement feeding when the sow's milk production does not meet the increased demand of older piglets, their use should be carefully planned to avoid compromising mammary gland development in females due to the satiety these products can provide.References available upon request.

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