The interval from calving to first oestrus (post-partum interval) is an important determinant of reproductive efficiency in suckler cows. While this interval is influenced by a range of factors including cow age, breed, presence of bull, cow-calf interactions and disease, the most important factors are suckling (frequency / number of calves) and nutrition (pre- and post-partum nutrient intake, body condition at calving and during early lactation). The mechanisms by which these factors, either independently or interactively, affect the length of the anoestrous period are unclear. Similarly, the time post-partum at which suckling exerts its effect on resumption of cyclicity is unknown. The effects of body condition score at calving and of changing the frequency of suckling on the length of the post-partum interval were determined. Preliminary results are presented.