Introduction

Historically, sow gestation feed has been considered as a ‘maintenance feed’ – formulated to supply a constant supply of nutrients to support sows until farrowing.

However, two recent advances mean that a more tailored approach needs to be considered:

1. Modern genetics: Modern sow breeds have more lean muscle deposition which comes with altered nutrient requirements – and yet the amount of feed provided has not changed (typically 1,150 to 1,300 kg/year/sow).
2. Hyperprolificacy: The average litter size of modern breeds has increased (in Denmark from 27 to 34 piglets/sow/year since 2008). Alongside this there has been a reduction in piglet survival rates and an increase in litter heterogeneity, the number of low birthweight piglets (intrauterine growth retarded-(IUGR) piglets) and immaturity at birth. Low birthweight piglets exhibit reduced weight gain¹ and immune functionality² that continues right through to the fattening phase.

Optimizing production outcomes in this new environment requires a more tailored approach to diet formulation that better supports the nutritional needs of sows and their developing piglets.

This article explores recent developments in the complex science of sow nutrition, how this impacts on piglet growth and development in utero, and what nutritional tools can be used to maximize production outcomes from today’s hyperprolific sows.

Nutrient allocation during gestation

It must first be understood that reproduction and the in-utero growth of piglets are not the top priority for nutrient allocation during gestation. The top priorities are maintenance and growth of the sow. This means that if nutrient availability and balance in the feed are sub-optimal, the first impact will be on reproduction and litter quality (uterine capacity, size and homogeneity of the litter, piglet weights).

When is litter quality most vulnerable?

There are two critical periods that account for a major portion of litter heterogeneity and low birthweight issues: Early gestation (~d 2 to ~d 35 post-insemination), and late gestation (~d 90 to ~d 110):

• When nutrients are inadequate to support optimal growth during early gestation, this results in IUGR-piglets that are ‘symmetrical’ (all organs are small)³. Symmetrical IUGR is caused by inadequate supply of micronutrients (amino acids (AA), vitamin B12, vitamin D, C and E).
• When nutrients are inadequate during late gestation, this results in ‘asymmetrical’ IUGR (where the liver is small but not the brain)³. Asymmetrical IUGR is caused by inadequate provision of energy, AA, phosphorus, vitamin C and E.

Asymmetrical IUGR accounts for 75% of all IUGR in piglets on-farm. Therefore, optimizing the nutrient content of the diet during late gestation will achieve the greatest benefits.

Energy and AA requirements

During gestation, the majority (95%) of the energy requirement is needed for growth and maintenance of the sow – only 5% is needed for the foetus. However, the foetus needs a higher proportion of the digestible lysine (dig. Lys) requirement (26%), and this doubles after 70 d of gestation due to increased protein deposition (Figure 1)⁴.

In addition to this, the dig. Lys requirement is greater in sows with larger litter sizes (20 vs. 15 piglets) and in primiparous vs. multiparous sows.

Furthermore, the requirement for threonine (Thr) doubles in the last compared to the first month of gestation (from 5 to 12.3 g/d)⁵, linked to the production of mammary glands and colostrum.

These dynamics in the amount and balance of AA required during gestation mean that, ideally, the Lys content of the feed should be increased during gestation, and the Thr:Lys ratio should also be increased, to support the changing needs of the sow and the developing piglets. This can be done either for the duration of gestation or by altering the feed content during the second half of gestation, via a 2-phase feeding program (as is starting to be adopted by some producers in Europe).

Minerals and nitrogen

Foetal requirements for minerals (calcium, phosphorus, sodium and potassium) also increase during gestation, by up to 3-fold (Figure 2)⁶. The trend is similar for nitrogen.

Hormones

Interventions to influence sow hormone levels during gestation are another potential route to reducing heterogeneity in piglet litter quality and low birthweight issues.

One intervention is an approach called ‘flushing’. An additional energy source (usually carbohydrates in the form of sugar) is added to the feed in the period between the previous weaning and 45–50 d of gestation. This restores body condition, supports ovulation through increased insulin production which stimulates luteinizing hormone (LH), and preserves the intrauterine muscule growth of piglets, as shown in Figure 3 (Figure generated using data from Van den Brand et al.⁷.):

Ensuring a positive energy balance during the previous lactation is also important for ovulation, as a negative energy balance significantly reduces follicle size⁸.

Other factors that influence sow health and reproductive success

Apart from the nutrient composition of the diet, other important factors that contribute towards sow health and reproductive success include:

1. Prevention of exposure to mycotoxins – Mycotoxins (DON and BEA) impair cell development including oocyte development and gilt oocytes are more sensitive to this effect than sows⁹.
2. Avoidance of urinary tract infections (UTI) – 20 to 40% of EU sows affected by subclinical UTI and sows without UTI infection perform better (fewer still births, shorter farrowing time).

Dietary tools for supporting muscle development in pigs in-utero

Muscle myogenesis starts from d 35 of gestation and the number of muscle fibres in a piglet is fixed at birth. Precise quantitative and qualitative nutrition during gestation is therefore critical for maximizing piglet muscle deposition both before and after birth.

Feed allowance

One approach to help achieve this is careful regulation of the sow feed allowance, because this has a direct effect on the number of muscle fibres that are formed. This is particularly important during d 25 to 45 of gestation when primary muscle fibres are developing; if gestation feed is reduced from 3 kg to 2.2 kg/sow/day during this period, the number and size of muscle fibres is reduced¹⁰.

Vitamin D supplementation

Vitamin D supplementation is another dietary tool for supporting muscle development of piglets in utero and after birth.

Supplemental vitamin D3 given to sows in the form of 25(OH)D3 (as Hy-D®) has been shown to interact with both foetal and developing piglet tissues. This form of vitamin D is transferred more efficiently from feed to blood than classical vitamin D3 and results in:

• Improved placental development in gestating sows¹¹
• Increased foetal plasma levels of 25(OH)D3
• Increased number of muscle fibres in the foetus and piglets¹¹
• Increased serum bone turnover and bone quality
• Increased production of beneficial gut bacteria metabolites (butyrate) in piglets

Other vitamins

It is also important to pay heed to the adequacy of other vitamins in sow gestation diets.

Levels of vitamin C (ascorbic acid) as well as those of vitamin E (α-tocopherol) and vitamin A (retinol) drop at the end of gestation¹² as the rapid growth requirements of the developing litter create conditions of oxidative stress and drain sow vitamin storage reserves. Diet composition needs to be carefully analysed and monitored towards the end of gestation to ensure these micronutrients do not become deficient.

And finally…the importance of physical checks on sows

The increased nutrient requirements of the developing litter towards the end of gestation mean that there is a risk of sows arriving at farrowing in a catabolic status (where fat and muscle mass is being lost rather than gained).

Sows in a catabolic status have impaired litter characteristics at birth (lower birth weight and higher litter heterogeneity): A loss of 5 mm of muscle depth at the end of gestation (d 84 to 105) results in a drop in average birth weight of 100 g in small piglets and 1.27 kg in litter birth weight, and an increase in litter heterogeneity (+3%)¹³.

Backfat depth is also important. It has been shown that as long as sows have 17 to 18 mm fat at d 85 of gestation, a subsequent loss in muscle mass can improve piglet weight and litter homogeneity at birth, whereas when sows have less than 17 to 18 mm fat, a subsequent loss in muscle mass is detrimental to piglet birth weight and heterogeneity at birth.

Physical checks on sow muscle thickness and fat depth, using available on-farm technologies, can provide useful insights into the nutritional adequacy of the gestation diet.

Conclusions

Gestation is a complex period involving dynamic requirements for nutrients by sows and their developing piglets. Ensuring the diet is nutritionally adequate so that sows arrive at farrowing with adequate reserves of energy and muscle mass, bearing piglets that are uniform in size and not growth-restricted, requires careful formulation and monitoring. Supplementation of vitamin D3 as 25-OH-D3 can be effective in supporting sow and foetal/piglet growth and development.