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Effects of virginiamycin on milk yield and ruminal pH of lactating dairy Holsteins cows.

M. Gorocica



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Effects of virginiamycin on milk yield and ruminal pH of lactating dairy Holsteins cows.
M. Gorocica*1, M. A. Iglesias2, R. R. Gonzalez2, L. O. Tedeschi3. 1Phibro Animal Health Corporation Teaneck, NJ, 2PB Animal Health de Mexico Guadalajara, Mexico, 3Texas A&M University College Station, TX.

An on-farm study investigated the effects of feeding 500 mg/d of virginiamycin (VM) for 74 d on rumen pH, feed intake, and milk yield (MY) of lactating Holstein cows in Mexico. Cows of similar DIM were randomly split into 2 treatments in 2 pens: control (C) (n = 90) and VM (n = 93), then 15 cows from each treatment were randomly selected to receive indwelling sensors to measure ruminal pH and temperature wirelessly. Measurements included cow ruminal pH every 10 min for 40 d, daily pen ration DMI, daily cow MY, and weekly pen milk fat and protein contents. Daily energy-correct MY (ECMY) was computed by interpolating weekly milk fat and protein contents of each treatment to daily contents and adjusted for 3.5% fat and 3.2% protein. Statistical analyses were conducted as repeated measurement design (RMD) for ECMY and ruminal pH, and one-way ANOVA for DMI. The area under the curve (AUC) and time under the curve (TUC) for ruminal pH below 5.4, 5.8, and 6 were computed daily for each cow, using spline interpolation with the Forsythe, Malcolm, and Moler method. Weekly ruminal pH was analyzed using DMI as a covariate. VM-treated cows produced 5.5 kg/d (40.6 vs. 35.1; P < 0.01) and consumed 2.6 kg DM/d (24.0 vs. 21.4; P < 0.01) more than C cows, increasing the milk-to-feed ratio by 3% (1.69 for VM and 1.64 for C; respectively). The RMD analysis indicated that VM cows had lower minimum ruminal pH than C cows (5.37 vs. 5.49; P < 0.01), but average and maximum ruminal pH did not differ (P > 0.09). The AUC analysis confirmed the RMD findings but also indicated that the AUC and TUC were not different (P > 0.12) between VM and C cows for ruminal pH below 6, 5.8, 5.6, or 5.4. The ruminal pH may have been affected by the 12% greater DMI of the VM compared with the C cows. When ruminal pH was adjusted for weekly DMI and compared at the C DMI (20.4 kg/d), the minimum ruminal pH was no longer statistically different (P > 0.07), but the average ruminal pH tended to be greater for VM-treated cows. The increased milk-to-feed ratio for VM-treated cows suggests VM might have improved rumen health or other metabolic parameters that need to be studied

Keywords: virginiamycin, acidosis, production.