Conception rates after AI in Swedish red and white dairy heifers: relationship with progesterone concentrations at AI

Blood samples were collected from 211 dairy heifers at the time of field insemination [artificial insemination (AI)]. Heifers were defined as either first-service heifers (n = 91) or third to eigth-service heifers [presumed (third AI, n = 60)] or well-defined repeat breeders (greater than the fourth AI, n = 60). Plasma progesterone concentrations at AI were evaluated in relation to oestrous behaviour at AI and conception rates post-AI. Conception rates after third AI were good, but conception rates decreased markedly after fourth AI and onwards. Those heifers that did not become pregnant after AI had significantly higher basal progesterone concentrations (>0.5 nmol/l) at AI, so-called suprabasal concentrations, compared with those which conceived after AI (irrespective of the number of AI practised). Relative risk for repeat breeding was 58% after AI performed at suprabasal progesterone concentrations, while it was 42% at basal (0.5) progesterone concentrations. Results from this field study confirm results obtained in earlier controlled studies of repeat-breeder heifers (RBH), indicating that the current definition of repeat breeding should be retained. Analysis of suprabasal progesterone concentrations could be considered as a tool for identification of RBH, provided that heat detection and AI timing are optimal. Besides considering the direct costs involved in repeat breeding, it remains to be determined whether it would be economically beneficial to identify and exclude RBH from the breeding population.