The effects of concentration and type of forage in the diet on lipid content and fatty acid (FA) composition of rumen bacteria were studied in 14 goats fitted with duodenal cannulas. The goats were fed a complete maintenance diet containing 40, 70, or 100% chopped forage (dry matter basis) in two equal meals. Forage was either corn stover or alfalfa hay. Microbial cell matter (MCM) was isolated by differential centrifugation of duodenal contents. The FA content of the MCM varied from 5 to 11% of DM and decreased with forage level in the diet. Main FA in MCM were C18:0 and C16:0: together they accounted for 70% of total FA in MCM. The mono-unsaturated FA and branched-chain FA (iso-FA and anteiso-FA) each represented about 10% of FA by weight. The proportion of even-chain saturated FA decreased and those of odd- and branched-chain FA increased with increasing forage. With the corn stover-based diet even-chain saturated FA were lower than with the alfalfa hay-based diet, whereas the unsaturated FA, odd-chain FA, and branched-chain FA were higher. The neutral detergent fiber content of the diet seemed to explain most of the variation associated with even-chain saturated FA, and odd- and branched-chain FA. Our results suggest that, for diets not supplemented with fat, mixed rumen bacteria accumulated energy reserves, by increasing synthesis of either even-chain saturated FA, or saturated odd-chain FA and saturated branched-chain FA.

Trans-octadecenoic acid composition of goat milk fat was studied by using silver-ion thin layer chromatography combined with gas-liquid chromatography. This analytical procedure also was used to investigate the effect of diet on trans-C18:1 fatty acid content in goat milk. Thirty-two goats were used in a 2 x 2 factorial arrangement and treatments. Two groups of goats received alfalfa hay at either a high or a low level of forage and two other groups received Rumiluz (dehydrated alfalfa from France Luzerne, Châlon en Champagne, France) at either a high or a low level of forage. Trans-C18:1 isomer proportions (relative to total fatty acids) were, respectively, 2.02% for the Rumiluz low-level group and 1.75% for the Rumiluz high-level group versus 1.71% for the alfalfa low-level group and 1.21% for the alfalfa high-level group. Goats fed on Rumiluz thus produced relatively higher levels of trans-C18:1 fatty acids than animals fed alfalfa hay. The results also showed that production of trans-C18:1 fatty acids increased when the level of forage in the diet decreased. Moreover, goat milk trans-C18:1 composition appeared similar to the cow milk profile. Vaccenic acid, trans-11-C18:1, was the major component and represented about 36.2% of total trans-C18:1 isomers.

The extent of disappearance of 10 mono-and sesquiterpenes in presence of mixed rumen microorganisms has been measured in 24-h batch cultures. The molecules that were tested are frequently found in the plants consumed by ruminants in highland pastures (limonene, β-myrcene, β-ocimene, α-pinene, sabinene, γ -terpinene and thymol) or are potential markers of the diet (camphene, β-caryophyllene and α-copaene). All terpenes were tested at 2 µL mL −1 , except α-copaene and thymol, which were tested at 0.9 µL mL −1 and 2 mg mL −1 , respectively. Camphene and thymol were not degraded to a significant extent. The disappearance of part of β-caryophyllene, (−)-limonene and γ -terpinene could not be attributed to the direct action of rumen micro-organisms but rather than to an effect of the extracellular medium. In contrast, a third group of one sesquiterpene (α-copaene) and four monoterpenes (myrcene, β-ocimene, α-pinene and sabinene) was extensively degraded by rumen micro-organisms, at rates of 1.6, 4.5, 3.5, 4.8 and 5.4 µmol mL −1 inoculum day −1 , respectively. The preliminary exposure of rumen micro-organisms to a specific blend of essential oils compounds, containing mainly thymol, guajacol and limonene, increased the extent of disappearance of γ -terpinene only.