RESUMO
Repeat breeding [RB] is a syndrome and several factors have been identified as its causes. Luteal insufficiency is a known cause of embryonic mortality. The aim of this study was to assess the effect of two periods of P4 therapy on the Second service and also their effects on the First plus the second service conception rates [CR] of repeat breeder dairy cows. Cows included in the study were in their first to filth lactation and had 3 to 6 unsuccessful inseminations within the current lactation. They were inseminated according to the AM/PM rule relative to estrus onset, and randomly assigned into 3 groups: [A] CIDR on day 5 after insemination that was removed on day 9 of the cycle [n=40]; [B] CIDR on days 5 after insemination that was removed on day 19 of the cycle [n=36]; and [C] untreated controls [n=40]. Pregnancy diagnosis was conducted by rectal palpation about 45 days after Al in cows no observed in estrus. The second service CR and overall CR in groups A, B and C were 61.7, 42.9.21.4 and 82.5, 66.7, 45 percent, respectively. Group A showed higher level compared to the controls [p<0.05]. In conclusion, repeat breeder cows in groups A and B benefited from progesterone supplementation, but significant effects of treatment for improvement of conception rate was seen in short term treatment [4 days treatment]