Swine production in the U.S. has experienced rapid growth in efficiencies over the past few decades. Much of this growth has been the result of improved sow productivity through genetic selection for litter size, which has increased the total number born in modern sows (Rutherford et al., 2013; Tokach et al., 2019).

However, selection for total number born has also resulted in higher incidences of piglets with low birth weights, increased weight variability within litters, and increased preweaning mortality (Foxcroft et al., 2006; Quesnel et al., 2008).

There are known negative aspects related to low-birth-weight piglets within the preweaning period. More importantly, these piglets — if they survive to weaning — continue showing poor performance during subsequent growth phases and may even present compromised carcass and meat quality at slaughter (Alvarenga et al., 2012; Alvarenga et al., 2013; López-Vergé et al., 2018).

In this context, the swine industry has started to shift the focus away from the number of pigs produced per sow per year as a principal indicator of productive efficiency, toward kilograms of pork per sow per year as a better indicator of productivity (Kitt, 2020).

Thus, the quality of pigs produced by each sow becomes more important than the quantity in assessing sow productivity. Variables associated with offspring health and productivity (e.g. birth weight variation, piglet survivability and offspring postweaning growth) are of key relevance in assessing sow performance, and therefore sow nutritional and management strategies.

From a nutritional perspective, there are many ways to improve pig health and productivity; however, nutritional strategies in postnatal life may be less efficient when certain traits have been programmed in prenatal life (Jamin et al., 2010; Ji et al., 2017).

This is clearly evident in piglets with intrauterine growth restriction, which present increased rates of preweaning morbidity and mortality, low feed efficiency and growth performance, and even reduced carcass and meat quality (Wu et al., 2006; Alvarenga et al., 2013).

Compared to piglets of normal birth weight at the same age, intrauterine-growth-restriction piglets are unlikely to achieve the same growth rate over the course of their life toward market (Beaulieu et al., 2010).

Therefore, maternal nutrition may represent a viable strategy to benefit postnatal offspring performance, especially of low-birth-weight piglets (Douglas et al., 2013; Krueger et al., 2014).

The objective of this review is to consider maternal nutritional strategies, specifically amino acids, as an opportunity to influence prenatal development (i.e., developmental programming) and subsequent pig productivity.

Maternal nutrition
The concept of developmental programming states that factors associated with early development, especially within the uterine environment, have long-term effects on subsequent health and performance (Barker et al., 1997).

Although the precise mechanisms involved are not fully understood, prenatal programming imparts stable and heritable alterations of gene expression through modifications of DNA — namely, epigenetics (Ji et al., 2017).

In this sense, some organs and tissues (e.g., small intestine, muscle, adipose tissue) appear to be more susceptible to alterations during developmental programming and have been the primary emphasis of investigation due to their importance for survival and subsequent health and performance of the individual (Meyer and Caton, 2016; Zhang et al., 2019).

In the livestock industry, developmental programming has been mainly associated with nutritional factors, where inadequate or improper maternal nutrition can lead to compromised postnatal growth rate, reproductive performance and meat quality (Alvarenga et al., 2012; Smit et al., 2013; Du et al., 2015).

For example, a maternal diet without sufficient protein during gestation negatively impacts offspring's muscle tissue development, including decreased numbers of primary, secondary and total muscle fibers; impaired protein synthesis in skeletal muscle; decreased carcass weight at slaughter; and compromised meat quality traits (Rehfeldt et al., 2012; Oksbjerg et al., 2013; Liu et al., 2015).

Thus, the potential impact of maternal nutrition on developmental programming during the prenatal phase and the effects on postnatal performance of offspring emphasize the importance of sow nutrition — and, within that, consideration of ways to improve overall swine industry efficiency.

Maternal amino acid supply
With respect to maternal AA supply in gestation on piglet quality, the research emphasis has been on late-gestation supply and piglet characteristics at birth. Numerous small- and large-scale studies have consistently reported a minimal impact of increasing AA supply in late gestation on piglet birth weight, litter size and within litter coefficient of variation (Ampaire, 2016; Goncalves et al., 2016; Cloutier et al., 2018; Mallmann et al., 2018, 2019a, b; Che et al., 2019; Thomas, 2019; Bruhn et al., 2020; Stewart et al., 2020).

While there are few assessments of the postnatal growth of offspring, particularly beyond weaning, available evidence supports beneficial developmental programming on piglet quality when considering postnatal growth.

Goncalves et al. (2016) reported a preweaning mortality reduction of 1.2% and 2% in sows and gilts, respectively, with no difference in within-litter coefficient of variation or piglet birth weight when supplied 20 grams lysine per day in late gestation.

Similarly, improved daily gain in the suckling period with no effect on birth weight, colostrum or milk composition when sows received 20.6 grams lysine per day from Day 90 of gestation was reported by Che et al. (2019).

The largest impact, up to weaning, of variable lysine supply (i.e., increasing each day) in gestation in gilts and sows maintained on their respective gestation feeding regimen over two successive parities was the reduction in prewean mortality — 4% versus 7% in adjusted vs. constant feeding regimen (Cloutier et al., 2018).

Given the similarity in piglet weight at birth and milk nutrient supply, improved performance in the suckling period suggests alterations in developmental programming due to a greater AA supply in late gestation, culminating in enhanced offspring quality.

Adjusting daily feed allotment to achieve predicted AA requirements in late gestation results in the concomitant increase in energy intake.

Based on common dietary lysine percentages in swine gestation diets (i.e., 0.55% to 0.65%), daily volume required to meet AA requirements results in excess energy supply.

Stillborns, lightweights
Intakes greater than 7.2 megacalories net energy per day have consistently demonstrated negative consequences on the probability of stillborns and sow lactation feed intake (Goncalves et al., 2016; Mallmann et al., 2018, 2019a), although the latter is more likely related to maternal fat deposition than fetal programming.

Considering increases in total feed intake in late gestation, Mallmann et al. (2019a) reported the lowest proportion of lightweight piglets at birth (i.e., 14% of piglets per litter weighing less than 1,000 grams) occurred at maternal intake of 2.3 kg per day (equivalent to 14.7 grams standardized ileal digestible lysine per day and 7.6 megacalories of net energy per day) with no effect on piglet weaning weights.

However, a tendency for improved daily gain in the suckling phase was reported for offspring from gilts fed 2.2 kg of feed per day in late gestation (Mallmann et al., 2018) and a lower proportion of lightweight piglets in gilt litters fed 3.5 kg feed per day from Day 90 of gestation (Mallmann, et al., 2019b).

Besides evidence of improved piglet quality in the suckling phase with maternal late-gestation AA supply, three independent small-scale studies report enhanced piglet quality postweaning in connection with altered AA supply throughout gestation.

Ampaire (2016) followed gilts provided constant intake (2.21 kg per day) or bump intake (2.61 kg per day) from Day 90 of gestation (diet contained 1.66 gram SID lysine per megacalorie of metabolizable energy), or altered lysine:energy ratio (1.53 and 2.13 grams SID lysine per gram) in early, mid- (breeding to Day 89) and late (days 90-112) gestation, respectively over two successive parities.

The greatest impact on offspring quality was observed in Parity 2 offspring which were 1 kg heavier at weaning and 2 kg heavier at 48 days of age.

In a single gestation cycle, Hansen et al. (2020) and Bruhn et al. (2020) reported offspring from females provided altered lysine:energy ratio during gestation were 2 kg heavier at 66 days of age and 4 kg heavier at 139 days of age, respectively, resulting in fewer days to achieve market weight.

The consistency in response among these independent studies suggests maternal AA supply in late gestation also positively influences developmental programming of postweaning piglet quality. Large-scale studies conducted over multiple parities are necessary to confirm these beneficial impacts on key productivity indicators.

In commercial swine feeding, AA supply in early gestation is typically 20% to 30% above National Research Council (2012) predicted requirements for the majority of the sow herd, because dietary AA levels are set to meet gilt predicted requirements.

In general, detrimental effects of overfeeding AA in early gestation appear limited (Buis, 2016; Thomas, 2019; Bruhn et al., 2020; Hansen et al., 2020; Stewart et al., 2020).

However, in a 2 x 2 factorial with two levels of feed intake (1.8 and 3.5 kg per day) in two stages of gestation (days 22-42 and days 90-110), gilts supplied 1.8 kg in early and 3.5 kg in late had the lowest percentage of stillborns across all other parity and treatment combinations.

The concern with excess AA supply in early gestation relates to greater diet cost and increased nitrogen excretion.

Recent evidence suggests that NRC (2012) overestimates lysine and threonine requirements by up to 20% in early gestation for gilts (Navales et al., 2019; Ramirez-Camba, 2019), representing considerable potential to reduce diet cost and nutrient excretion without any negative impact on offspring quality.

Cloutier et al. (2018) estimated diet cost savings of approximately $3 per sow per year, where excess AAs in early gestation are redistributed to late gestation within a phase feeding program.

Maternal functional AA
Recently, the effects of maternal functional nutrients on sow reproductive performance and the growth and health of offspring have become topics of interest among nutritionists and producers (Ji et al., 2017; Pereira et al., 2020).

Functional AAs during gestation have attracted attention due to their participation in metabolic pathways related to animal reproductive and growth functions (Wu et al., 2017).

Arginine, in addition to participation in muscle tissue protein synthesis, serves as a precursor of biologically active molecules, such as polyamines and nitric oxide, which play a key role in fetal development (Wu et al., 2013; Palencia et al., 2017).

It has been proposed that dietary arginine supplementation during gestation produces long-term morphological or functional changes in the offspring, which potentially improves their health and performance (Mateo et al., 2007; Hsu and Tain, 2019).

In swine, maternal dietary supplementation of 1% of arginine during gestation improved birth weight of piglets born alive by 24.18%, 2.3%, 10.75% and 10.96% in the studies of Mateo et al., (2007); Li et al., (2010); Gao et al. (2012); and Wu et al. (2012), respectively.

Quesnel et al. (2014) reported a 17% reduction in the within-litter variation of piglet birth weight when 1% of arginine was supplemented to sows during the last third of gestation.

Likewise, it was reported that supplementation of 1% of L-arginine during the final third of gestation reduces the incidence of unviable piglets (i.e., under 800 grams) and improved litter uniformity (Moreira et al., 2019).

However, other studies report minimal or indifferent response to arginine supplementation during gestation, and the long-term effects on offspring growth have not been explored (Novak et al., 2012; Li et al., 2014; Garbossa et al., 2015).

Thus, investigations into the impact of supplementation level and duration, influence of parity and effects on offspring performance postweaning are key to understanding the potential of arginine on overall sow productivity.

As members of the arginine family, glutamate and glutamine have been related to similar functions as arginine; their supplementation can enhance fetal growth, reduce within-litter weight variation at birth, and prevent the occurrence of intrauterine growth restriction (Wu et al., 2010; Zhu et al., 2018).

Other AAs with potential functional activities but minimally explored within sow nutrition include: 1) branched-chain amino acids (leucine, isoleucine and valine) related to enhancing placental growth and embryogenesis; 2) citrulline as an arginine precursor; and 3) methionine related to improving antioxidant capacity and intestinal microbiota composition of the offspring (Yuan et al., 2015; Wu et al., 2017; Palencia et al., 2018; Azad et al., 2018).

Final considerations
This review has discussed recent approaches related to maternal AA supply influences on the productivity of the offspring.

Taken together, the information presented highlights the need to better understand the nutritional demands during gestation and stimulates the continued search for strategies to provide adequately balanced diets to sows that can also positively influence lifetime performance of offspring.

Altered lysine:energy ratio and functional AAs were presented as nutritional opportunities to influence prenatal development and postnatal pig performance. Also, postweaning performance was presented as a key variable to better explore the potential of nutritional strategies during gestation.

Perez-Palencia, Samuel and Levesque are all in the Department of Animal Science at South Dakota State University.

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