Original Research/Independent Study Undergraduate Paper Presentations

S ADSA Student Affiliate Division * Author Presenting Paper Original Research/Independent Study Undergraduate Paper Presentations 957 The luteolytic potential of reduced doses of prostaglandin. J. Brinkerhoff*, R. Silcox, J. Donley, and C. Kubo, Brigham Young University, Provo, UT. Prostaglandin (PGF) and its analogs have greatly contributed to managing the reproductive performance of dairy herds. The most effective route of administration as well as the minimal dose requirement for induction of luteolysis has been investigated previously. This study was designed to compare the luteolytic response of lactating cows treated with a reduced dose of Prostamate administered in the ischiorectal fossa versus the recommended dose given intramuscularly. Non-pregnant lactating Holstein cows with a functional corpus luteum (CL> 20 mm as determined by transrectal ultrasonography) were sorted by lactation into three groups; 1st (n=40), 2nd (n=40), 3rd and greater (3+) lactation (n=40). Cows within lactation group were paired. One cow of each pair was randomly assigned to be treated with 25 mg Prostamate intramuscularly (IM group) or with 15 mg Prostamate given in the ischiorectal fossa (IRF group). Ovaries of cows were examined by ultrasound 72 hours post-injection to determine luteal regression. The functionality and regression of the CL were confirmed through analysis of serum progesterone concentrations in blood samples collected by tail venipuncture at the time of PGF administration, then 24 and 72 hr later. Tail heads of all cows were chalked at the time of PGF treatment. Estrus was detected daily for seven days post-PGF by assessing chalk removal. Luteal regression was induced in 103/120 cows. Luteolytic response was not affected by lactation number (P>.05). Route of administration and dose of prostaglandin did not affect response rate (P>.05; IRF:50/60, 83%; IM:53/60, 86%). Overall estrus detection rate for all cows given PGF was 43% (IM:29/60, 48%; IRF:23/60, 38%; P>.05). Estrus detection rate among cows that responded to PGF did not differ due to treatment (IM:29/53, 55%; IRF:23/50, 46%; P>.05). The IRF is a viable alternate route of administering a reduced (60% of normal) dose of PGF to lactating dairy cows since both luteolysis and estrus are induced at rates comparable to that attained with a normal dose administered IM.