In this study, two experiments were conducted to evaluate the effect of cumin (Cuminumcyminum) seed extract (CSE) on PUFA biohydrogenation. The first experiment was arrangedin a completely randomized design in which eighteen crossbred goats were assigned tothree dietary treatments: one group received basal diet (control; C0) consisting of forageand concentrate at equal ratio while the other two groups received the same diet withsupplemented CSE (at 1.27% and 2.53% of dry matter intake as C1 and C2, respectively).The rumen content was sampled on day 27 and 29 of the experiment and then subjected tovolatile fatty acids (VFA) and long chain fatty acids analyses. The CSE had no effect on molarproportion of VFA (P > 0.05). However, the addition of the CSE to the diet of goats increasedthe ruminal concentration of rumenic acid (RA; C18:2(c9,t11)) by 34.8% (P = 0.041) andvaccenic acid (VA; C18:1(11)) by 11.4% (P = 0.01) in lower supplemented group in com-parison to the CSE-free diet. In the CSE-fed goats, the concentration of linoleic acid (LA;C18:2(9,12)) and linolenic acid (LNA; 18:3(9,12,15)) was higher (P < 0.05) and stearic acid(SA; C18:0) was lower (P = 0.036) than that of the control group, suggesting the inhibition ofbiohydrogenation. There was no dose dependent effect of the CSE on PUFA content. In thesecond experiment, linoleate isomerase activity and the growth of five strains of Butyriv-ibrio sp. and ten predominant species of rumen bacteria was tested at presence of the CSE.The results revealed that CSE did not inhibit the growth of Butyrivibrio sp. but it decreasedthe growth of some main species of the rumen bacteria such as Prevotella brevis, Strep-tococcus bovis, Peptostreptococcus anaerobius and Lachnospira multiparus (P < 0.05), whichare involved in ruminal biohydrogenation. Moreover, the linoleate isomerase activity ofthe only stearate forming bacterium, Butyrivibrio proteoclasticus, decreased by 53% in thepresence of the CSE (4.25 g CSE/ml; P = 0.002). Initially, the inhibition of LA metabolismby this species decreases the rate of RA and VA formation, but it gradually results in theaccumulation of RA and VA. It was found that CSE altered the bacterial biohydrogenationactivity, thus increasing RA and VA concentration in the rumen.

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