The effect of different intravaginal progesterone doses on the conception rate of beefmaster cows and heifers following fixed time artificial insemination

Abstract

The study was carried out to establish whether or not a decrease in the progesterone concentration of an intravaginal device used to synchronize oestrous in Beefmaster heifers and cows would lead to an increase in conception rates. The study evaluated the effect of different intravaginal progesterone device doses on the conception rates of Beefmaster cows and heifers following fixed time artificial insemination (FTAI). A total of 100 stud Beefmaster cows (Cow group) and 100 stud Beefmaster heifers (Heifer group) were used in the study. The animals were year round grazing residents of the farm Oribilaagte near the town of Vrede in the Free State province of South Africa. The Cow and Heifer groups were split into four experimental groups, respectively, and treated with either a 0.5 g, 1.0 g, 1.9 g or 1.9 g re-used intravaginal progesterone device during the spring synchronization program (P4 device insertion and 2 mg i.m. oestradiol benzoate treatment on d 0, 250 μg s.c. PGF2_ treatment on d 7, P4 device removal on d 8, 1 mg i.m. oestradiol benzoate treatment on d 9, and FTAI on d 10). Blood samples were taken on the day of intravaginal insert removal (d 8), and on the day of FTAI (d 10), to monitor the blood serum P4 concentrations (solid phase RIA analysis) between the experimental groups. The experimental groups were examined for pregnancy 5 months post AI by means of rectal palpation. The Cow group recorded a pregnancy rate of 65%, 75%, 67% and 68% for the 0.5 g, 1.0 g, 1.9 g and 1.9 g re-used intravaginal progesterone device groups respectively. The Heifer group recorded a pregnancy rate of 19%, 39%, 41% and 47% for the 0.5 g, 1.0 g, 1.9 g and 1.9 g re-used intravaginal progesterone device groups respectively. The study concluded that a decrease in the progesterone concentration in the intravaginal device did not affect the serum progesterone concentrations in the Heifer and Cow groups on the day of the FTAI, and did not negatively affect the synchronization program. The study further concluded that, although a decrease in the progesterone concentration in the intravaginal device did not statistically influence the conception rates in the Heifer and Cow groups, the Cow group did record a higher numerical conception rate when the progesterone concentrations in the intravaginal devices were reduced.