Searching for nutritional solutions to intrauterine growth restriction
Tuesday, April 2, 2013
There seems no simple way to prevent IUGR based on results of overall dry sow nutrition or dietary protein level studies. However, interesting findings have emerged from recent studies with bioactive compounds such as carnitine, arginine and glutamine
by JANICE MURPHY
Intrauterine growth restriction (IUGR) is a naturally occurring phenomenon that results in under-nourished piglets with low birth-weight at farrowing. In pigs, IUGR develops somewhere from day 30 to 45 until term and tends to be more severe than in other meat-producing animals. It is caused by placental insufficiency or lack of blood flow to the placenta that delivers vital oxygen and nutrients.
Piglets with IUGR are anatomically and physiologically different from piglets with an average or "normal" weight. In addition, their immune systems can be compromised, ultimately hampering resistance to certain pathogens.
In modern production systems, low birth-weight has a negative impact on the bottom line in a variety of ways. Long-term selection programs for larger litters have increased litter size from less than nine to more than 12 in recent years; however, the survival rate of small littermates has decreased.
These smaller piglets have a higher mortality rate, in part due to an inability to compete for the best teats, accompanied by lower consumption of colostrum and milk which leads to a lower nutritional status. In addition, low birth-weight piglets who manage to survive often suffer from reduced performance, including lower weight gain, reduced feed intake, poorer feed conversion and lower lean percentage.
Nutrient restriction during gestation can have permanent effects on physiology and metabolism later in life, a concept known as fetal programming. The long-term consequences of IUGR may result in modifications to the number of muscle fibres and rate of muscle deposition in the growing pig. As a result, IUGR pigs may differ in meat quality traits from average or heavy birth-weight littermates.
Studies have consistently shown that pigs born small have a reduced performance level compared to piglets born large, although the numerical difference varies depending on feeding strategies, breed, litter size and production phases. In a recent study in Denmark, researchers found that average daily gain was reduced by 30 grams a day in low birth-weight pigs in the lactation period and that this deficit persisted from weaning to slaughter (at day 150 of age) by 87 grams a day compared with heavy birth-weight pigs.
Post-weaning feed intake was reduced by 160 grams a day and feed conversion ratio increased by 80 grams a kilogram gain in low compared with heavy birth-weight piglets. In that particular study, there was no difference in lean meat percentage of the carcasses between low and heavy weight pigs.
Research has investigated alternative dry sow feeding strategies to prevent IUGR. Providing dry sows with sufficient nutrients to be channeled to the fetus is critical for fetal growth. In general, pregnant sows are limit fed and energy requirements are targeted at 20 and 35 MJ ME, corresponding to an intake of two to three kilograms a day.
Several studies have reported that increasing overall dry sow nutrition had no beneficial effect on performance and muscle growth traits in piglets. Feeding excess dietary protein was also ineffective at influencing muscle growth traits, whereas moderately decreased dietary protein resulted in decreased muscle fibre number and poorer performance in the offspring.
Recently, Danish researchers have investigated more moderate gestational protein restriction. They reported no significant effects on postnatal growth performance in offspring after feeding pregnant gilts a 30 per cent reduced dietary protein level during gestation and lactation (Table 1).
Dietary protein levels did not show any effect (Table 2) on meat quality traits in offspring, though the researchers did identify some interesting gender-by-protein level interactions in meat quality traits. The results indicated that meat quality traits improved in male pigs when born to gilts fed a low protein diet, with an increased pH45 (a meat quality indicator measured 45 minutes post-mortem) and a corresponding decrease in thawing loss and shear force. The increase in pH45 in muscle may be in response to a different ability to cope with stress prior to slaughter. Citrate synthase activity in muscle was greater in the male pigs, providing a possible explanation for the higher pH45 in male pigs born to gilts fed a low protein diet, which may also improve the water-holding capacity.
There does not appear to be a simple way to prevent IUGR based on results of overall dry sow nutrition or dietary protein level studies. However, interesting findings have emerged from recent studies with bioactive compounds such as carnitine, arginine and glutamine.
There is some evidence that adding carnitine to the dry sow or lactation diet may increase birth- and weaning-weights or the muscle fibre number, respectively, in low birth-weight piglets. Recent studies have reported that feeding 400 milligrams a day of carnitine by syringe during suckling positively increased the number of muscle fibres at weaning. It is unclear whether the increased number of muscle fibres will translate into an increase in daily gain up to market weight and, likewise, whether it is even possible to increase piglet uptake of carnitine by feeding the sow carnitine through gestation and lactation. These questions require further investigation in order to validate the positive effect of carnitine.
Arginine is used in protein synthesis as well as being a precursor for compounds with biological activity, such as nitric oxide and polyamines. Nitric oxide is a powerful vasodilator and stimulates angiogenesis, the formation of new blood vessels, which both reduce resistance to blood flow. The hypothesis is that including arginine in the diet may increase blood flow across the placenta and, in turn, increase the transfer of nutrients from the sow to the fetus.
Polyamines stimulate cell proliferation and differentiation of muscle cells and may, therefore, have a positive impact on myogenesis or muscle growth. Research in Texas has shown that dietary inclusion of one per cent arginine from day 30 to farrowing increased the reproductive performance of sows by increasing the total number of piglets born alive per litter by two, as well as increasing the litter weight and reducing the number of piglets stillborn by 1.2. However, the variation in birth weight was not affected by dietary inclusion of arginine.
Glutamine, like arginine, belongs to the group of functional amino acids and may stimulate protein synthesis. Another research group, also based in Texas, top-dressed a mixture of eight grams of arginine and 12 grams of glutamine onto a dry sow diet of two kilograms a day from day 30 to 114 of gestation, resulting in an increase to the total number of piglets born alive, total litter weight at birth for all piglets born, and litter weight for piglets born alive. In this experiment, the variation in birth weight among all piglets born and among all piglets born alive decreased. Intrigued by these results, the researchers are interested to determine whether arginine and carnitine act additively, antagonistically or synergistically.
Since genetic selection programs will probably attempt to push litter sizes ever higher, it is likely that average birth-weight will continue to decline, increasing the number of low birth-weight piglets born and consequently resulting in a greater mortality rate. Despite the depth of knowledge surrounding the biological basis of IUGR, there remains much to learn about the underlying principles before specific dietary strategies for sows and piglets can be developed to prevent IUGR.
Further research exploring the mode of action of arginine, glutamine and carnitine in order to prevent IUGR would be very valuable. BP
Janice Murphy is a former Ontario agriculture ministry swine nutritionist who now lives and works in Prince Edward Island.
Source:
N. Oksbjerg, P. M. Nissen, M. Therkildsen, H. S. Moler, L. B. Larsen, M. Andersen and J. F. Young. 2013. In utero nutrition related to fetal development, postnatal performance and meat quality of pork. J.Anim.Sci. Published online January 7, 2013 as doi:10.2527/jas.2012-5849.
