Short luteal phases and ovulations without oestrus in primiparous Borana ( Bos indicus) cows in the central highlands of Ethiopia

Abstract

The incidence of short luteal phases and ovulations without oestrus was investigated in 42 primiparous Borana cows. Based on calving body weight, cows were allocated to either a control group with grass hay only or to a supplement group with a concentrate ration. Within nutritional groups, cows were further assigned to either milked or partially suckled groups. Cows were under continuous observation and vasectomised teaser bulls used in oestrus detection. Blood samples were collected twice weekly commencing on Day 30 postpartum until pregnancy for determination of plasma progesterone levels. A progesterone concentration of above 1 ng ml −1 was taken as a cut-off value between luteal and non-luteal phases and oestrous cycles of 17 days or less were considered as short. By 150 days postpartum, ovarian cyclic activity had commenced in 64.3% of the cows while 35.7% remained anoestrous. The mean (±SEM) postpartum anoestrous interval for the cyclic cows was 104 ± 19 days. Progesterone profiles revealed that up to the first detected oestrus, 97 ovulations occurred and 72% were not accompanied by behavioural signs of oestrus, indicating that an average of 2.6 oestrous periods per cow were silent. Differences in intervals between calving to onset of first oestrus based on visual detection and progesterone concentrations ranged from 5 to 21 days in 22.2%, 24–43 days in 44.4%, 44–63 days in 11.2% and 83–166 days in 22.2% of the cows. Short luteal phases with a mean duration of 10 ± 2 days and a range of 7–14 days were detected in 55.6% of the cows. There were no nutritional and suckling influences on the incidence of ovulation without oestrus and short luteal phases. Progesterone concentrations, however, were significantly ( P < 0.01) lower in short than in normal oestrous cycles. It was concluded that although short luteal phases occurred in a relatively larger proportion of cows, it was not an exclusive prerequisite for the establishment of normal oestrous cycles. The commencement of ovarian cyclic activity in 78% of the cows before Day 63 postpartum also suggests that ovulations without oestrus rather than postpartum anoestrus could be more responsible for extended calving intervals in Zebu cattle. The physiological basis and significance of ovulations without oestrus and short luteal phases in the fertility of Zebu cattle thus need further investigation.