The mean interval to the first observed post partum oestrus was 14 days in milked cows. In contrast, none of eleven nursed animals showed behavioural oestrus by Day 30 post partum, although two ovulated before that time. Only one of seven anaemic animals (haemoglobin levels of 9 to 10 g/100 ml) showed behavioural oestrus by Day 22 post partum; two others ovulated, but showed no signs of oestrus. Thyroprotein-fed cows also had prolonged periods of post partum anoestrus.
Pregnancy corpora lutea were quite degenerate by Day 7 post partum. At Day 14 post partum, they were approximately 9·0 mm in diameter and remained at this size, at least to Day 30. There was an increase in follicular size following parturition. At Day 7, the mean diameter of the largest follicle was 9·6 mm; at Day 14, it was 11·3 mm; at Day 30, the largest follicle averaged 13·0 mm.
Neither the presence of suckling calves, nor anaemia delayed uterine involution. Caruncular tissue sloughed and was expelled from the uterus between Day 7 and Day 14 post partum. Regeneration of the surface epithelium over the caruncles occurred by growth from the surrounding tissue, and was complete in a majority of animals slaughtered at Day 30 post partum.