The effect of suckling upon the endocrine changes associated with anoestrus in identical twin dairy cows.

Identical twin pairs of dairy cows (one twin being placed with 4 calves for multiple suckling while the other was machine milked) were used to study the hormonal changes associated with anoestrus. Experiments were conducted to examine the effect of multiple suckling on the interval to first oestrus post partum, concentrations of LH and progesterone post partum, and the LH response to LH-RH or oestradiol-17 beta. The effects of hormone therapy on anoestrus were also examined. Multiple suckled cows had a 5-9-week longer interval to first oestrus post partum than did their non-suckling twins in all trials. There were no significant differences between the suckling and non-suckling cows in basal plasma levels of LH. The suckling cows showed a delay of 2 and 6 weeks post partum in their response to LH-RH and oestradiol-17 beta respectively when compared with their non-suckling twins. Treatment with a progesterone-releasing intravaginal device and bromocriptine to lower prolactin levels did not reduce the interval to the first post-partum oestrus while treatment with the progesterone device and PMSG reduced the interval to both first oestrus and conception. These findings suggest that suckling affects the response of the hypothalamic-pituitary axis to both LH-RH and oestradiol in the post-partum cow.