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Effects of essential oils on rumen fermentation, eating behavior and milk production in lactating dairy cattle Lisa Rachel Tager Manipulation of rumen fermentation to improve feed efficiency is often achieved by feeding sub-therapeutic levels of antibiotics to ruminant animals. However, there has been increased concern over the use of antibiotics in the livestock industry due to the possible rise of antibiotic resistant bacteria. In the last decade, plant essential oils (EO) have emerged as a suggestion for replacement of antibiotic feed additives due to their known antimicrobial properties. Examples include: cinnamaldehyde (CIN), eugenol (EUG), and capsicum oil (CAP), which may act as rumen modifiers. The objectives of the current studies were to: (1) investigate the effects of CIN, EUG, and CAP on rumen fermentation and pH characteristics in continuous culture using a corn-based dairy ration; (2) to assess the in vivo effects of CAP at the recommended dosage, a blend of CIN and EUG at the recommended dosage, and a blend of CIN and EUG at a high dosage closer to levels used in vitro, on rumen fermentation, rumen pH, milk production, and feeding behavior; and (3) to assess the effects of CAP in a larger scale study on eating behavior and milk production in lactating dairy cattle. To study in vitro effects of EO on rumen fermentation and pH, a 12-unit continuous culture system was used in a complete randomized design with treatments including: no oil (CON), CIN, EUG, and CAP [500 mg L –1 d –1 ]. Dry matter (DM) digestibility did not differ among treatments (P = 0.97). Organic matter (OM) digestibility tended to decrease with CIN (P = 0.06). Digestibility of neutral detergent and acid detergent fiber (NDF and ADF respectively) tended to be highest with CAP (P = 0.04 and 0.08). Crude protein digestibility and bacterial nitrogen flow was depressed with CIN and EUG (P = 0.01 and < 0.01 respectively). Cinnamaldehyde tended to decrease microbial protein synthesis (P = 0.06) and increase effluent ammonia nitrogen (P = 0.06). Total volatile fatty acid (VFA) production did not differ among treatments (P = 0.16). Cinnamaldehyde and EUG had higher mean pH, spent fewer hr/d and had smaller area under the curve at pH < 5.6 and 5.8 (P < 0.01). Capsicum had the smallest area under the curve at pH < 5.6 (P < 0.01). A second experiment was designed to examine the effects of these oils in vivo. Seven ruminally cannulated lactating Holstein dairy cows were used in an incomplete Latin rectangle design to assess the effects of 2 commercial EO products on rumen fermentation, milk production, and feeding behavior. Treatments included addition of: 0.5 g/d XT 6965 (CE Lo; 85 mg CIN and 140 mg EUG; Pancosma S.A., Bellegarde-sur-Valserine, France), 10 g/d XT 6965 (CE Hi; 1700 mg CIN and 2800 mg EUG), 0.25 g/d XT 6933 (50 mg CAP; Pancosma S.A., Bellegarde-surValserine, France), or CON. Dry matter intake, number of meals/d, h eating/d, mean meal length, rumination events/d, h ruminating/d and mean rumination length were not affected by EO (P > 0.20). However, length of the first meal after feeding decreased with addition of CE Hi (47.2 min) and CAP (49.4 min) compared to CON (65.4 min; P = 0.01). Milk yield and composition did not change with EO (P > 0.20). Volatile fatty acid profile, acetate:propionate ratio, and ammonia concentration were not affected by EO (P > 0.15). Rumen pH was unaffected by EO (P > 0.30). Total tract digestibility of OM, DM, NDF, ADF, crude protein, and starch as well as in situ DM disappearance of ground soybean hulls was not affected by EO (P > 0.50). However, in situ OM disappearance tended to decrease compared to CON with CE Hi (P = 0.08). Compared to CON, NDF disappearance (41.5% vs. 37.6%) and ADF disappearance (44.5% vs. 38.8%) decreased with addition of CE Hi (P = 0.05 and 0.04 respectively). To further investigate the effects of CAP on eating behavior, 40 lactating dairy cows were used in a replicated crossover design to study the effect of CAP on feeding behavior and milk production using the GrowSafe system (GrowSafe Systems Ltd., Airdrie, AB, Canada). Experimental treatments included: 1 g/cow/d Xtract 6933 (200 mg CAP) or CON. There was no difference in DMI (23.4 kg/d vs. 23.1 kg/d), number of meals/d (12.5 vs. 12.2), total h spent eating/d (3.1 h vs. 3.1 h), mean meal length (16.3 min vs. 16.4), length of first meal after feeding (44.6 min vs. 45.7 min), or eating rate (7.8 kg/h vs. 8.0 kg/h) between CAP and CON (P > 0.30). Milk yield did not differ between CAP and CON (31.8 kg/d vs. 32.3 kg/d; P = 0.81). In summary, the results from these 3 experiments suggest that EO have limited effects, in vitro and in vivo, on rumen fermentation, rumen pH, eating behavior, and milk production in lactating dairy cattle except at very high dosages, which would be both biologically and economically inappropriate in a commercial dairy operation.