Two Holstein cows were fed Rumensin for 12 weeks, beginning
approximately 90 days postpartum. Milk production increased 7%, whereas
percentage fat in milk decreased 10.3%. Percentage protein in milk increased 9.6%
in the early stages and decreased 10.2% in the late stages of the study. Daily fat
yield did not change, whereas protein yield increased in the early stages, but did
not change in later stages. Fat corrected milk (FCM) did not change with
treatment, but dry matter intake (DMI) tended to increase when Rumensin was fed.
Acetate concentration did not change whereas propionate concentration increased,
causing a significant decrease in ruminal acetate:propionate ratio. Rumen pH
showed a nonsignificant numerical increase.
