The greatest loss in litter size occurs prior to farrowing. That loss is the embryonic or fetal death loss; this is normally 25-40% of the number of eggs ovulated in swine. Recently a report from Canada (Fengler et al., 1990) demonstrated an improvement in embryo survival of about 14% with the dietary supplementation of oils in early gestation. The Canadian research supplemented the diet at a rate of 4% with either safflower oil or olive oil. The assumption tested and discussed was related to the role of linoleic acid (C 18:2w6) in swine diets. Linoleic acid is thought to be the only essential fatty acid needed by swine and it can be found in large amounts in many fats and oils.

Interestingly, in the analysis of the fatty acid composition of the embryos, the only significant difference in the embryonic fatty acid content was an increase in the omega-3 fatty acid docosahexanoic acid (C 22:6w3) in embryos from those sows fed diets supplemented with oil. Docosahexanoic acid is not found in plant oils; however, through a series of metabolic reactions swine can manufacture this fatty acid from another fatty acid - linolenic acid (C 18:3w3). There are three good sources of linolenic acid from plant origin; those sources are linseed oil (from flax), canola oil (from rapeseed or canola) and soyabean oil. The positive results which they observed then may have been due to trace amounts of linolenic acid in the oils which they evaluated.