A seasonal effect was demonstrated on the occurrence of oestrus and on the length of oestrous cycles in Bunaji and Bokoloji cows. There was a gradual lengthening of oestrous cycle which resulted in fewer cycles occurring in the dry and pre-rainy seasons. Oestrous cycle length was the same for Bunaji (22.89 +/-0.70 days) and Bokoloji (23.76 +/- 0.65 days) cows (P less than 0.05). Season had an equally depressing effect on the duration and intensity of oestrus in both breeds of cows. During the dry and pre-rainy seasons the behavioural signs of oestrus were poorly manifested and lasted for only a short period. During the rainy and pre-dry seasons, the duration of oestrus and behavioural signs were much more pronounced.

In a study of 3000 cattle of different breeds and origin (Bos indicus and Bos taurus) in Northern Nigeria, an overall 22.7 per cent incidence of anoestrus associated with organic genital abnormalities was observed. Of this 19.3 per cent was due to ovarian atrophy. This was observed mostly during the pre-dry and dry seasons, periods of poor and inadequate feed. The average incidence of atrophic ovaries in the exotic breeds (Bos taurus) was 17.1 per cent, while an incidence of 20.4 per cent was observed in the indigenous (Bos indicus) cattle; there was no significant difference between the two groups. A relatively higher incidence of functional anoestrus (27.5 per cent) which showed no seasonal pattern and no significant difference between the breeds was observed. Inadequate nutrition was the major cause of the ovarian atrophy and subsequent anoestrus. Other clinical genital abnormalities included hypoplastic ovaries (1.9 per cent), follicular cysts (4.5 per cent), pyometra (4.5 per cent) and freemartinism (0.3 per cent). The influence of anoestrus (50 per cent total incidence) on the economic aspect of livestock production in Northern Nigeria is discussed.