Somatotropin Application and Sampling. Heifer
BW was recorded at the beginning and end of the experiment
(d 28 and 55). On d 28, heifers were randomly
assigned to receive, in a crossover design containing 2
periods of 14 d each (d 28 to 42 and d 42 to 56), 1)
subcutaneous (s.c.) injection containing 250 mg sometribove
zinc (BST; Posilac; Elanco, Greenfi eld, IN) or 2)
s.c. saline injection (0.9%; SAL). Treatments were applied
once, at 0800 h, during the fi rst day of each period
(d 28 and 42). Heifers also received a new CIDR at the
beginning of the second period concurrently with treatment
administration (d 42). The BST treatment was developed
on the basis of previous research demonstrating
that application of 250 mg of sometribove zinc every 14
d increases plasma IGF-I concentrations in beef heifers
(Buskirk et al., 1996).