Forty-five Bos indicus (zebu) cows were blocked by body condition (BC) score at calving and allocated by body weight (BW) to three treatments: untreated controls (CON), and progesterone-releasing intra-vaginal device (PRID)-treated at either 45 (T45) or 60 days (T60) post partum. Cows were given food individually and blood-sampled three times a week for progesterone. Calves sucked twice a day and were weighed weekly. PRIDs resulted in only 21% of cows responding at 60 days and they did not reduce anoestrus (PPI) or calving interval (CI) (P > 0·05). Dry matter (DM) and metabolizable energy (ME) intake were 7·1 (s.e. 0·6) kg and 54·3 (s.e. 4·6) MJ/day, respectively. BC score and BW at calving, and ME intake influenced cow and calf average daily gain ranging from −580 to 625 and 331 to 868 g/day, respectively. Milk yield during the first 4 months was 507 (s.e. 97) l. Cows gained 25 kg per unit increase in condition but lost 50 kg for a unit decline. CI (mean 14·2, range 10 to 24 months) was inversely related to BC score at calving (r = −0·31) and was delayed by a long PPI (r = 0·51), ranging from 38 to 297 days. Most (95%) cows experienced one or more ‘silent ovulations’ before first oestrus, the number extending PPI length (r = 0·49, P < 0·001). Efficiency of ME utilization was higher for smaller cows of 240 (s.e. 28) kg at calving. It is concluded that: (1) PRID treatment before 60 days post partum is unlikely to reduce anoestrus in Boran zebu cattle; (2) given adequate energy intake zebu cattle in thin condition at calving initially replenish body reserves at the cost of lactation and experience more ‘silent’ ovarian activity before returning to oestrus; (3) cows in good condition at calving express maximum lactation potential, mobilizing their own body reserves to support milk production, yet resume oestrous activity earlier despite weight losses to support higher milk yields.