morphology also show other differences which indicate reduced maturity at birth. Brains from low birth weight foetuses are less myelinated and have less dendritic development than the brains from normally grown littermates. That might contribute to the ten- dency towards a lower vitality score in s-IUGR piglets. Fur- thermore, low birth weight pigs show compromised muscle, cardiac and renal physiology. Digestive organs are also affected by IUGR. In low birth weight piglets, the relative weight of the pancreas is reduced, and IUGR is associated with a reduction in the wall thickness of the stomach, small intestine and colon. The intestinal sur- face area for absorption is highly reduced, with the average number of villi per unit area and the height of villi being 15% to 20% lower. Hormonal status, at birth and subsequently, is also affect- ed by IUGR. However, the mechanisms by which these permanent effects occur and the interrelation between hormones remain unclear.


Consequences of IUGR The most obvious consequence of IUGR is the increased mortality of afflicted piglets during the neonatal period. The relationship between birth weight and survival is well estab- lished. Pre-weaning survival rates decreased progressively from 95% to 15% as piglet birth weight decreased from 1.80kg to 0.61kg. In addition, low birth weight piglets that survive show consistently lower postnatal growth rates. Long-term effects on piglet welfare have received insufficient study to date.


Genetic selection against IUGR One approach to reducing the problem of IUGR piglets is to employ genetic selection against this characteristic.


Selection for birth weight Piglet birth weight may be considered as more than a single trait depending on the level that is under investigation: indi- vidual piglet birth weight at the offspring level, or as litter traits at the dam level – average birth weight, within-litter birth weight variability, total litter weight. Figure 3 shows the relationship between litter size and aver- age piglet birth weight obtained in a recent UK study. As litter size increased, the average birth weight decreased and the proportion of IUGR piglets (as determined by head shape) increased.


Selection against IUGR morphology Selection for an increase in average birth weight or litter weight can be applied, but other traits relating to the IUGR phenotype should also be considered. IUGR-linked traits such as PI and BMI or cranial circumference, which are based on weighing or measuring individual piglets, are time


consuming to record and not suitable for large-scale data col- lection on commercial breeding units. However, the propor- tion of piglets in a litter showing IUGR head morphology is a simple measure and does show a level of heritability which would allow it to be selected against in sows.


Aiding survival of IUGR piglets A number of management interventions are required to pro- mote the survival and welfare of low birth weight and IUGR piglets, and the extent to which these strategies are success- ful partly depends on the IUGR status of the piglet. Prompt humane euthanasia should also be recognised as an


Figure 1 - The relationship between litter size and early piglet mortality (stillborn or dead before first processing at 12-24 hours) from a sample of 1,575 farrowings in a UK herd. Ave Born Dead


Linear (Ave Born Dead) 2.5


Ave Dead by Processing Linear (Ave Dead by Processing)


4.5 2


3.5 4


1.5


2.5 3


1


1.5 2


0.5


0.5 1


0 <6 7 8 9 10 11 12 13


Litter size 14 15 16 17 18 19 20 >21


Source: S.M. Matheson, G. Walling, S.A. Edwards, unpublished. 0


Figure 2 - The birth weight distribution of pigs classified by head morphology as normal, moderate (m-IUGR) or severe (s-IUGR) in a UK herd (20,991 piglets from 1,575 litters).


3000 2500 2000 1500


1000 500 0


Normal


Slightly IUGR Head shape


Source: S.M. Matheson, G.A. Walling, S.A. Edwards, unpublished. IUGR


▶ ALL ABOUT FEED | Volume 28, No. 5, 2020


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Weight at processing (g)


Average number of piglets dead by processing


Average number of piglets born ded


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