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Laidlomycin Butyrate—An Ionophore with Enhanced Intraruminal Activity

Syntex Research^,2, Palo Alto, CA 94304 and and Santa Ynez Research Farm, Santa Ynez, CA 93460

² Correspond to: Syntex Research, 3401 Hillview Avenue, Palo Alto, CA.

Abstract

The polyether ionophore, laidlomycin, and several acyl derivatives were tested for their ability to favorably alter fermentation in two types of in vitro rumen fluid incubations. Dose response data were used to estimate the concentration (µ/ml) of each ionophore required for either 50% maximal enhancement of propionic acid production (EC₅₀) or 50% maximal inhibition of lactic acid production (IC₅₀). Acylation of laidlomycin with straight-chain acyl groups from two to 12 carbon atoms tended to improve the potency of laidlomycin, especially for inhibiting lactic acid production. Comparative incubations using laidlomycin butyrate, laidlomycin and monensin indicated that both laidlomycin butyrate (EC₅₀=.3) and monensin (EC₅₀=.7) were more potent enhancers of propionic acid production than laidlomycin (EC₅₀=2.0; P<.05). Laidlomycin butyrate (IC₅₀=.3) was a more potent inhibitor of lactic acid production than either laidlomycin (IC₅₀=1.8) or monensin (IC₅₀=1.3; P<.05). In a continuous culture experiment, three chemostats each received laidlomycin butyrate or monensin at the rate of .5 µg/ml of effluent/d while two flasks remained as control. Propionic acid production was increased (P<.01) from 22.9 mmol/d in control flasks to 30.5 and 33.7 mmol/d in flasks treated with monensin and laidlomycin butyrate, respectively. Concomitant decreases in production rates of acetic, butyric and isovaleric acids also were observed (P<.01). Thirty-six steers were used in a 56-d trial to evaluate effects of laidlomycin butyrate and monensin, at 33 mg/kg of diet, on feedlot performance. Average daily gain was 8% better and feed efficiency was improved 20% (P<.01) in steers fed laidlomycin butyrate. Monensin also improved feed efficiency by 15% (P<.01). Acylation of laidlomycin to yield laidlomycin butyrate resulted in a potent manipulator of rumen fermentation and an effective feed additive for improving feedlot performance.

¹ The authors thank Vicki Freitas and Bonita Bowers for technical assistance with in vitro studies.