Oestrus synchronisation with a prostaglandin analogue: II. Systems in maiden heifers

Abstract

Groups of maiden heifers (105 dairy and 119 beef) were treated twice with an 11-day-interval between injections of Cloprostenol (0.5 mg/i.m.), and were then inseminated at 72 h and 96 h after the second injection. The pregnancy rate (PR) was 39% in both types of heifers. Significant PR differences between groups of heifers were largely due to differences in the proportion of non-cycling (anoestrus) animals within each group. Excluding these animals from the analyses increased PR's to the set-time inseminations by over 11%.

An alternative treatment regime was used in a second series of trials. Entire bulls fitted with chin-ball mating harnesses were run with groups of heifers. Those heifers served in the first 11-day-period were subsequently injected with 0.5 mg of Cloprostenol 6 days later and inseminated 72 h and 96 h after treatment. The second sub-group of heifers served in the second 11-day-period received the same injection-insemination sequence. Unmated heifers were examined at the same time as the second sub-group was injected. The PR among the 90 treated dairy heifers (from a total of 102 animals) was 69%, and 58% in 163 treated beef heifers. Almost all of the heifers not served by bulls in the pretreatment period were subsequently found to be anoestrus, pregnant to a previous (unrecorded) mating, or abnormal (freemartin).

The use of oestrus synchronisation in maiden heifers can facilitate the use of AB with associated advantages through genetic improvement. The additional advantages in aspects of herd management, particularly in the reduction in the time-span of conventional AB programmes, may be its greatest use. In either case, the pretreatment identification, and the exclusion of heifers which cannot be successfully synchronised, will be important in obtaining satisfactory results.