| Abstrakt: |
In the first of three experiments, 100 bred cows (three age groups) were group fed diets containing four levels of monensin: 0, 50, 200 and 300 mg/head/day, provided in .9 kg of supplement. Cows fed 50, 200 and 300 mg monensin were given 95, 90 and 90%, respectively, the amount of forage (sorghum-sudan-grass hay) fed to the control cows. Supplements were fed for 194 days (112 days before calving started and 82 days after the start of calving). During the precalving period, the cows in the groups fed 0, 50, 200 and 300 mg monensin gained 33, 30, 33 and 25 kg, respectively. These gains were not different (P>.05). From before calving until the end of monensin supplementation (beginning of pasture period), the cows lost 40, 46, 44 and 57 kg, respectively. These losses were not different (P>.05). There were no differences (P>.05) in calf weight gains, onset of estrus after calving or pregnancy rates associated with level of monensin. In the second experiment, 64 bred, 2- and 3-year old cows were assigned to two treatments (0 to 200 mg monensin/head/day in .9 kg of supplement). There were two replications of each age group. The basal diet consisted of grass hay fed at a fixed intake to provide .20 to .3 5 kg gain/ head/day before calving. The supplements were fed for 170 days (from 90 days before calving started until 80 days after the start of calving). The cows given 0 and 200 mg monensin gained 30 and 34 kg, respectively, before calving and lost 18 and 24 kg from the start of calving to the end of monensin supplementation (beginning of pasture period). These weight changes were not different (P>.05). There were no differences (P>.05) due to monensin in calf weight gains, onset of estrus after calving or pregnancy rates. In the third experiment, 32 bred, yearling heifers were used in two replications of two treatments (0 and 200 mg monensin/head/day in .9 kg supplement fed on native winter range pasture). The supplements were fed for 175 days (from 84 days before calving started until 91 days after the start of calving). The heifers fed 0 and 200 mg monensin gained 56 and 46 kg, respectively, before calving and lost 18 and 11 kg, from the beginning of calving until the end of monensin supplementation (beginning of pasture period). These weight changes were not different (P>.05). The calves from heifers fed monensin had higher (P<.05) birth weights than those from control heifers, but the difference in weight was not significant by the time of weaning. The heifers fed monensin held small (P>.05) advantages in onset of estrus after calving and in pregnancy rate. |
