Inclusion of a daisy plant, Chrysanthemum coronarium, in a dairy sheep diet has been reported to result in increased concentrations of health-promoting rumenic acid (RA; cis-9,trans-11 CLA) and vaccenic acid (VA; trans-11-18:1) in milk. The aims of the present study were to determine if the reported change in milk fatty acid composition was the result of the effects of C. coronarium on the biohydrogenation of linoleic acid (LA; cis-9,cis-12-18:2) by ruminal microorganisms, and to investigate which constituents of C. coronarium may be responsible for the observed effects. Ruminal digesta from four sheep receiving a mixed hay-concentrate diet were incubated in vitro with LA in the presence or absence of dried whole-plant C. coronarium var. Primrose Gem. Rates of LA disappearance and stearic acid (SA: 18:0) production decreased as a result of C. coronarium addition, and VA accumulation doubled. Chrysanthemum parthenium and Chrysanthemum vulgare had much smaller effects on biohydrogenation. C. coronarium added to cultures of the only known ruminal SA-forming bacterium, Butyrivibrio proteoclasticus, also inhibited LA metabolism by, but not growth of, this species. Lipid analysis indicated that C coronarium var. Primrose Gem had a high content of a-linolenic acid (LNA; cis-9,cis-12,cis-15-18:3; 8.79 mg/g DM) compared to the other samples (<0.50 mg/g DM). In fractions derived from differential (Soxhlet) solvent extraction, only extracts containing LNA affected LA metabolism by B. proteoclasticus. LNA and coronaric acid ((+)-cis-9,10-epoxy,cis-12-18:1) were investigated as the main components present in C. coronarium that could have altered the biohydrogenation of LA in vitro. LNA inhibited biohydrogenation of LA causing a slowdown of RA and VA formation and a subsequent increase of the accumulation of RA and VA over time. Coronaric acid showed an inhibitory effect on the metabolism of LA, although it did not correspond to a higher accumulation of intermediates. It was concluded that the combined effect of LNA and coronaric acid in C. coronarium could be responsible for changes in the biohydrogenating activity of ruminal bacteria causing an increase of VA and a decrease in SA in vitro. This effect would lead to an increased flow of VA from the rumen which in turn would lead to an increase in RA and VA in milk from ruminants receiving C. coronarium. (C) 2010 Elsevier B.V. All rights reserved.