Stability of fatty acids in grass and maize silages after exposure to air during the feed out period

Lipids in forages are extensively hydrolysed in the silo with a concomitant increase in the level of free fatty acids (FFA). After opening of the silo, exposure of the FFA to air and light with, a concomitant increase in pH and microbial growth, could induce oxidization. The present study investigated the stability of FA in grass and maize silages exposed to air for 0, 12 and 24h. Eight maize silages were selected with varying dry matter (DM) contents, being very wet, wet, normal and dry. In addition, eight grass silages were chosen on the basis of ammonia (NH₃) concentration and pH level. Grass and maize silages were sampled 8-10 weeks after ensiling and anaerobically transported to the lab in cooled plastic bags. After mixing, each sample was divided into three subsamples and exposed to air for 0, 12 or 24h. Concentrations of individual FA were quantified by gas chromatography. Among the investigated silages, concentrations of total FA varied greatly and ranged from 16.4 to 23.9 and 9.5 to 21.6g/kg DM in grass and maize silages, respectively. Exposure to air up to 24h lowered (P<0.01) the contents of linolenic acid (C18:3), linoleic acid (C18:2), oleic acid (C18:1) and total FA in maize silages. In grass silages, 24h exposure to air decreased (P<0.05) concentrations of C18:3, C18:2 and total FA (P<0.01). In both grass and maize silages a decline in concentrations of major unsaturated FA (UFA) was associated with a concomitant increase (P<0.01) in the proportion (g/g total FA) of palmitic acid (C16:0). The relative decrease in total FA after 24h exposure to air was higher in maize silages with a high moisture content, and progressively decreased with increasing DM content. In contrast, pH and NH₃ levels of grass silages had no effect on the stability of FA during feed out. The present study demonstrates that extended exposure of silages to air during feeding increased the proportion (g/g total FA) of C16:0 and lowered the concentration of polyunsaturated FA.