Suppression of oestrus in Zebu heifers with melengestrol acetate

Abstract

The value of melengestrol acetate (MGA) as a means of suppressing oestrus and its administration by utilising the licking behaviour of the animal were investigated in 3 groups of 30 Sahiwal cross heifers. For 63 days 0.5 mg MGA was either individually fed daily (fed group) or applied daily with molasses to the midrib of heifers to be licked off (licking group). The third group received no treatment (control group). When treatments ceased, fertile bulls were introduced for 11 weeks. The heifers were then slaughtered.

Control animals exhibited oestrus regularly throughout the treatment period while those in the treated groups failed to exhibit oestrus after the first week. Post treatment oestrus was synchronised in the fed group but only partial synchronisation occurred in the licking group.

Pregnancy rates from the first post-treatment oestrus of 63, 17 and 33% for the control, fed and licking groups respectively were significantly different (P<0.01). Pregnancy rates of 87, 60 and 80% from the first plus second post-treatment oestrus for the control, fed and licking groups respectively also differed significantly (P<0.05). There was no significant difference in the cumulative pregnancy rates after the third post-treatment oestrus.

This study demonstrated that daily oral administration of MGA suppresses oestrus in heifers, but pregnancy rates following cessation of treatment are depressed. Application of MGA with molasses to an area on the coat to exploit the licking behaviour of the animal appeared successful. However, the practicability of the technique needs verification under extensive grazing conditions.