The effects of long-chain polyunsaturated fatty acids (LC PUFAs) have been frequently investigated in sows because the profitability of pig production depends mainly on reproduction performance. According to the results of the previous studies, n-3 fatty acids can postpone the time of parturition, decreasing the synthesis of prostaglandins, which are necessary for uterus contraction during labour. These effects could also be useful during the post-weaning period when low prostaglandin levels are indispensable for embryo survival. The n-3 fatty acids fed during the lactation period secreted in milk, may improve piglet performance. Despite the numerous previous field trials, there are no current feeding recommendations available for PUFAs in swine nutrition. In finding the appropriate dose of n-3 PUFAs in terms of sow nutrition, the n-6 PUFAs levels in the given feeds must be taken into account to ensure that there are no significant reductions in the final n-6/n-3 ratio.
long-chain polyunsaturated fatty acids (LC PUFAs) have been a focus of scientific research and practical nutrition for a long time. LC PUFAs are composed of at least 18 carbon atoms and contain at least two unsaturated double bonds in their acyl chain. PUFAs are classified according to the position of the first double bond, relative to the terminal methyl group. Consequently, the short names given to PUFAs show how far the first double bond is from the end of the molecule (e.g., the third carbon atom in the case of n-3 and sixth in the case of n-6 PUFAs) .
Among the various LC PUFAs, linoleic acid (LA; C18:2, n-6) and α-linolenic acid (ALA; C18:3, n-3) are considered essential for vertebrates because they are unable to synthesize them and must be acquired from their diet.
LC PUFAs have diverse effects in the body. They are involved in building the outer membranes of cells (phospholipid bilayer), preserving cell integrity and the fluidity of membranes supporting some intracellular signal transduction mechanisms. They are involved in the biosynthesis of certain hormones (e.g., prostanoids) and immune processes. Moreover, they can affect the expression of specific genes that play a role in the regulation of lipid metabolism and cholesterol synthesis .
According to Wathes et al. , LC PUFAs influence the reproductive performance in such a way that they positively affect follicle development and the expression of genes encoding the enzymes necessary for the formation of prostaglandins and sex steroids. The pig oocyte has high levels of LC PUFA, particularly the n-6 fatty acids, such as LA and ARA, indicating a local role in actively producing PGs.
LC PUFAs can pass through the placenta into the foetus and can be excreted through sow milk . Due to a selective transportation process through the placenta, mainly from the second trimester onwards, a much higher concentration of ARA and DHA can be found in the foetus than in maternal blood in humans and swine .
The n-6 fatty acids play many physiologically essential roles in the body. For example, the rate of embryo growth is directly connected to the amount of ARA. Since the presence of EPA and DHA is considered to depress the synthesis of ARA-derived PGs, they may decelerate the growth of embryos at the end of pregnancy and postpone the term of birth. The n-3 LC PUFAs that are unrelated to eicosanoid synthesis can affect myometrial contractions through direct impacts on ion channels and cell signalling, which can also elongate the duration of gestation. This influence of LC PUFAs on the different ion channels of muscle cells is quite well-documented in human models. Several researchers also found that the administration of n-3 LC PUFAs during the gestation period increases the length of pregnancy in swine.
Studies reporting the effects of n-3 PUFAs on the pre-weaning performance of piglets remain controversial. Some researchers found no significant difference between pre-weaning growth or the mortality of piglets when sows were fed with control or n-3 fatty acid-supplemented diets . A very recent study found that litter birth weight and pre-weaning mortality was impaired when 1% salmon oil was added to gestation and lactation diets beside soya oil as a control supplementation . However, some researchers experienced an improvement in the performance parameters of current and subsequent parturition when sows were fed the n-3 PUFAs supplemented diet during the lactation period. Rooke et al.  and Mateo et al. reported that piglet weight at weaning was higher than the control if the sows were fed a diet supplemented with n-3 PUFAs from fish oil or marine algae during the second half of gestation. Luo et al.  also reported an improved daily weight gain of piglets (0 to 21 days of age) when sows were fed a 70 g/kg fish oil-supplemented diet during lactation. Innis found the same effect in human infants and suggested that n-3 LC PUFA was a pivotal factor for the growth of children. Lavery et al.  fed sows with soya and fish oil from day 105 of gestation to weaning and, although there was no effect of oil type on the number of total born, born alive and piglets stillborn, there was a tendency for a reduced pre-weaning mortality rate for sows offered a diet containing salmon oil. These litters also had an increased litter weight gain in the second part of lactation.
According to human, animal and field studies, it seems that n-3 PUFAs, fed during the gestation period, lead to increased lengths in pregnancy times due to a negative impact on ARA production, which exhibits a linear correlation with embryo size. Without the induction of farrowing (e.g., with PGF2α), n-3 PUFA administration may not negatively influence the birth weights of piglets, as foetuses have more time (some days) to develop before parturition. Supplementation with n-3 PUFAs during the lactation period may improve the quality of ovarian follicles, thereby improving the survival rate of embryos and the subsequent litter size, as was demonstrated by several studies, when there was an ongoing subclinical deficiency in this nutrient. These biological reproductive effects will be reinforced if supplementation is elongated until the end of the first trimester of gestation when ovulation, fertilization, and implantation occur, and the skeletal frame of embryos develop. During this period, the effect of n-3 PUFA on follicle maturation and progesterone synthesis via CL size could help to maintain the pregnancy and reduce the rate of early embryo loss, which is specific to pigs.
Besides reproduction aspects, n-3 LC PUFAs are also transferred into the bodies of suckling piglets through colostrum and sow milk, helping to provide these essential nutrients of piglets in high-performance modern herds.
In case of deficiency, it seems that n-3 PUFAs could have favourable impacts on performance parameters during the whole production cycle of the sow, but especially during the lactation period, when piglets could also benefit from the supplementation directly. In other production periods, the dose, basal sow diet, and n-6/n-3 PUFA ratio may have a significant influence on the outcome.
As there are no current feeding recommendations available for PUFAs in swine nutrition, more research is required with standardized metabolic trials and comparing the different practical feeding situations for the establishment of PUFAs, including n-6/n-3 ratio requirements. More studies should be performed to find a threshold for PUFAs requirements that does not improve production and reproduction results further. The appropriate dose of n-3 PUFA in sow nutrition and the n-6 PUFA levels in the feeds must be taken into account to ensure that there are no significant reductions in the n-6/n-3 ratio. Despite the previous field trials, further examinations will be needed to certify the assumptions and conclusions of this review.