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Dose-Response Relationship of Tetronasin Administered to Grazing Steers

Louisiana State University Agricultural Center^,5 Baton Rouge 70893 and Coopers Animal Health Inc., Kansas City, MO 64141

Abstract

Two grazing experiments were conducted to evaluate the dose-response relationship of steers to the ionophore tetronasin. Bermudagrass-based pastures were grazed 126 d in Exp. 1, and annual ryegrass or an annual ryegrass-berseem clover mixture was grazed 112 d in Exp. 2. Tetronasin was administered in ground corn (.91 kg/hd) fed daily to provide dosages of 0, 7.5, 15, 30, 60 or 90 mg. One hundred forty-four steers (220 kg, Exp. 1; 196 kg, Exp. 2) were allocated to treatment groups of six steers within four initial weight blocks in Exp. 1 and within two initial weight blocks, assigned to two forage types in Exp. 2. Initial weight blocks were confounded with four pasture blocks, divided in six 1.35-ha paddocks. Treatment groups were rotated among paddocks within pasture block every 2 wk. Stepwise tetronasin addition resulted in a linear increase (P < .05) in the proportion of propionic acid and a linear decrease (P < .05) in the proportion of acetic acid and the acetic to propionic acid ratio in both experiments. Total VFA concentrations were not altered in Exp. 1 but they decreased linearly in Exp. 2 with tetronasin addition. Maximal observed improvement in daily gain (.1 kg) occurred at a dosage of 30 mg·hd^–1·d^–1 in Exp. 1 and 90 mg·hd^–1·d^–1 in Exp. 2. Daily gain increased linearly (P < .06) with tetronasin level in both experiments. A segmented, linear plateau model (R² = .98) was more descriptive of the gain response to dosage than was a quadratic (P = .19; R² = .14) model in Exp. 1 because of an equivalent response to 30, 60 and 90 mg/d dosages. Tetronasin improved growth rates of grazing steers and was effective at dosages lower than those recommended for ionophores currently available.

¹ Iberia Res. Sta., Jeanerette, LA 70544.

² Current address: USDA-ARS, Coastal Plain Exp. Sta., Tifton,GA 31793.

³ Dept. of Anim. Sci.

⁴ Coopers Animal Health Inc.

⁵ Approved for publication by the Director of the Louisiana Agric. Exp. Sta. as manuscript number 88-70-2468.