Effects of monensin fed to replacement dairy heifers during the growing and gestation period upon growth, reproduction, and subsequent lactation.

Sixty Holstein heifers initially weighing 196 kg were assigned by weight to treatments of 0, 200, and 600 mg of monensin. These treatments were fed daily until calving to determine effects of monensin on growth rate, feed consumption, feed efficiency, reproductive performance, and subsequent lactation. Throughout the feeding period the control (0 mg) heifers received sufficient feed to allow a calculated growth rate of .68 kg/day. Monensin treatment groups were fed the same amount of the identical ration over the 448-day experiment. Daily gain, feed intake, and feed efficiency for the 0, 200, and 600 mg treatments were .60, .69, .69 kg, 7.47, 7.46, 7.43 kg, and 12.41, 10.81, 10.81. Although days on trial to first estrus were not different among treatment groups, heifers fed monensin at 200 and 600 mg/day conceived 38 and 34 days sooner than 0 mg heifers. Percent conception, number of calves born, calf birth weight, and calving difficulty were not affected by treatment. Milk production was measured for 120 days subsequent to the experimental feeding of monensin. During this period heifers were fed a common ration ad libitum. Milk production of 17.1, 16.9, and 17.0 kg/day for 0, 200, and 600 mg was not affected by prior treatment.