Evaluation of a new commercial source of monensin in lactating dairy cows and continuous culture fermenters

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The objective of this experiment was to evaluate a new commercial source of monensin (MON) on performance of mid-lactation dairy cows and fermentation parameters of dual-flow continuous culture fermenters. In Experiment 1, 43 Holstein cows (25 multiparous and 18 primiparous; 175 ± 97 DIM; 45.3 ± 10 kg/d milk yield; 692 ± 68 kg BW; mean ± SD) were used in a randomized block design experiment with a 15-d covariate and a 9-wk treatment period. The first 3 wk of the treatment period were considered adaptation and the last 6 wk were used for data collection and analysis. Treatments were: Control (CTR; no MON added), Rumensin® 90 (RUM; 350 mg/d MON from Elanco Animal Health Inc.), and Monovet® 90 (MVet; 350 mg/d MON from Huvepharma® Inc. US). All cows were fed the same base diet throughout the experiment and treatments were top-dressed during the treatment period. Orthogonal contrasts were used to evaluate CTR vs. MON (RUM + MVet) and RUM vs. MVet. Compared with CTR, MON tended to increase milk yield (43.8 vs. 42.3 kg/d) but did not affect DMI or feed efficiency. The MVet treatment improved feed efficiency compared with RUM (1.70 vs. 1.58). Treatments did not affect milk fat content and yield, milk protein yield, MUN concentration, or energy-corrected milk yield. In Experiment 2, nine continuous culture fermenters were used in a randomized block design experiment with two 10-d periods. Fermenters were fed once a day approximately 100 g DM of the same diet fed in Experiment 1. Treatments were: CTR, RUM (2 mg/L MON from Rumensin® 90), and MVet (2 mg/L MON from Monovet® 90). Monensin did not affect DM, OM, NDF, ADF, and CP degradability or total VFA production (mmol/d), but decreased acetate, butyrate, and isobutyrate molar proportions and increased propionate molar proportion compared with CTR. Both sources of MON decreased acetate to propionate ratio compared with CTR (1.61 vs. 2.40). Adding MON increased daily production of propionate (74.1 vs. 108 mmol/d) and decreased that of butyrate and isobutyrate compared with CTR. Monensin supplementation decreased ammonia-N production (433 vs. 545 mg/d) compared with CTR. Overall, results indicate MON tended to increase milk yield and altered VFA profile in the rumen and in vitro, and that MVet is a suitable source of MON for dairy cows.