| dc.description.abstract | Two trials were conducted to compare the effectiveness of fixed-time artificial
insemination (AI) to AI based upon visual detection of estrus following superstimulation
of donor beef cows. In Trial 1, multiparous beef cows (n = 31) were randomly allotted to
one of three treatments following superstimulation and removal of an intravaginal
progesterone insert (CIDR). Cows in the Control group were inseminated at 12 and 24 h
after onset of estrus. Cows in the Estradiol group were injected with estradiol-17β (1 mg,
im) at 12 h and inseminated at 24 and 36 h after CIDR removal. Cows in the pLH36
group were injected with porcine LH (Lutropin, 12.5 mg, im) at 24 h and inseminated at
36 and 48 h after CIDR removal. Mean numbers of viable embryos were 7.8, 3.6 and
9.6 for Control, Estradiol and pLH36 treatment groups, respectively (P > 0.10). In Trial
2, multiparous beef cows (n = 22) were randomly allotted to one of three treatments
following superstimulation and removal of a CIDR. Sixteen of the cows were
superstimulated a second time approximately 50 days later and allotted to one of the two
treatments that differed from the initial treatment group. Cows in the Control group were
inseminated at 12 and 24 h after onset of estrus. Cows in the two pLH groups were injected with porcine LH (Lutropin,12.5 mg, im) at 24 h after CIDR removal and were
inseminated with either one unit of semen at 36 and 48 h (pLH36) or with two units of
semen at 48 h (pLH48) after CIDR removal. Mean numbers of viable embryos were 3.0,
6.4 and 3.8 for Control, pLH36 and pLH48 treatment groups, respectively (P > 0.10).
These data indicate that administration of pLH can facilitate use of fixed-time AI in
superovulated beef cows without sacrificing embryo production.
The second study evaluated the efficacy of resynchronizing beef cow recipients
using CIDR devices for only 7 or 14 d. Recipient cows received CIDRs either on the
day of transfer (n = 88) or 7 d post-transfer (n = 230). All CIDRs were removed on d 21
and cows were observed for estrus between d 22 and 24. Cows that displayed estrus
were ultrasounded on d 30, those cows not pregnant that possessed a CL had an embryo
transferred that day. Cows were later examined for pregnancies approximately 23 to 30
d later. There were no differences in pregnancy rates between cows with 7 or 14 d
CIDRs and therefore data were combined. Pregnancy rates at two different ranches
indicate that beef cow recipients can be successfully resynchronized by insertion of a
CIDR without compromising pregnancy rates of transferred embryos. At Center Ranch
the pregnancy rate for the first transfer was 57% while the resynchronized group that
received the second transfer had a pregnancy rate of 55%. At Mound Creek Ranch the
first transfer of embryos produced 59% pregnancy rates while the second transfer had a
pregnancy rate of 71%. No significant differences (P > 0.05) were observed between the
pregnancy rates of the initial transfer and those of the resynchronized transfer using only CIDRs, indicating that resynchronization using CIDRs can be used without reducing
pregnancy rates. | en |
