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Effects of Zinc and Sodium Monensin on Ruminal Degradation of Lysine-HCl and Liquid 2-Hydroxy-4-Methylthiobutanoic Acid^*,

¹ Department of Dairy Science, LSU Agricultural Center, Louisiana State University and A & M College, Baton Rouge 70803
² Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University and A & M College, Baton Rouge 70803
³ Southeast Research Station, LSU Agricultural Center, Franklinton, LA 70438
⁴ ADM Animal Health and Nutrition Division, Quincy, IL 62305

Corresponding author: H. G. Bateman, II; e-mail: hbateman{at}agcenter.lsu.edu.

Four nonlactating, mature, Holstein cows were fitted with ruminal cannula and used in a 4 x 4 Latin square-designed experiment to evaluate the impact of supplemental Zn and monensin on ruminal degradation of Lys and liquid 2-hydroxy-4-methylthiobutanoic acid (HMB). Cows were fed 4.54 kg (as fed) of alfalfa hay top-dressed with 4.54 kg (as fed) concentrate once daily. Concentrates were formulated to provide 0 or 500 mg/kg of Zn as ZnSO₄ and 0 or 40 mg/kg of monensin in the total diet. Zinc supplementation provided approximately 22-fold greater dietary Zn than estimated by NRC requirements. On d 14 of each period, cows were dosed via the rumen cannula with 50 g of HMB and100 g of Lys-HCl, and the concentrations of Lys and HMB were monitored every 0.5 h for 8 h. Supplemental Zn tended to decrease the proportion of acetate in ruminal fluid postfeeding and increased the proportion of propionate in ruminal fluid postfeeding. Supplemental Zn increased mean fluid passage rate from the rumen. Monensin decreased the proportion of acetate and increased the mean proportion of propionate in ruminal fluid, resulting in a decrease in the ratio of acetate to propionate. Monensin also increased the mean fluid passage rate from the rumen. Neither Zn nor monensin affected the apparent rate of ruminal disappearance of HMB or Lys. However, Zn and monensin interacted to alter the ruminal degradability of free Lys but not HMB. These data indicate that Zn and monensin may interact to alter ruminal degradability of free amino acids.

Key Words: monensin • zinc • amino acid degradability

Abbreviation key: HMB = liquid 2-hydroxy-4-methylthiobutanoic acid, K_d = rate of disappearance, K_p = fluid turnover rate of the rumen, SBM = soybean meal

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