Ruminal metabolism and absorption of fatty acids from forages

Forages are low in fatty acids (FA) but these FA are rich in linolenic acid ; it can thus be thought that forages are a valuable source of polyunsaturated FA for ruminants ; however their ruminal metabolism and their digestibility are not well known. Aseries of 4 experiments was carried out on 4 forages (2 cocksfoots, red clover, perennial ryegrass) as fresh grass, silage and hay (or haylage) on cannulated sheep. Forage drying resulted in a decrease in FA content and in the proportion of linolenic acid. Hydrogenation was also reduced by drying, so that the amount of absorbable FA was only slightly modified. Ruminal metabolism produced a large variety of 18-carbon trans monounsaturated isomers, of which transvaccenic acid was the main one, and very limited amounts of conjugated linoleic acids (CLA). This suggests the post-ruminal origin of meat and milk CLA, by desaturation of transvaccenic acid. Fatty acid digestibility was slightly higher for fresh grass and silage than for haylage and hay. An in vivo trial was carried out on cannulated steers receiving perennial ryegrass, white or red clover, and the mixtures of ryegrass and each of the clovers. Hydrogenation of linolenic acid was lower for red clover, alone or mixed with ryegrass. This effect of red clover could be due to its high content in polyphenol oxidase (PPO). This hypothesis has been studied in vitro, with a classical line rich in PPO, a line poor in PPO and the same line in which PPO was inactivated. Plant-induced lipolysis was reduced by high PPO content. These experiments improved the knowledge on FA digestion for forages. However, the specificities of red clover remain to be better known.