Repeated oestrus synchrony and fixed time AI in beef cows at pasture

Abstract

The feasibility of breeding spring calving, single-suckled beef cows without the use of natural service was explored over two breeding seasons using repeated oestrus synchrony and fixed-time AI. Initially, cows were oestrus synchronised using subcutaneous norgestomet implants inserted for ten days with prostaglandin injection prior to implant removal. Cows were inseminated once, 56 hours after implant removal and 12 days later all cows were re-treated with norgestomet implants to allow a second synchronised service. Twenty-one days after the first synchronised AI, milk samples were drawn for progesterone assay and the norgestomet implants removed. Cows received a second service 56 hours later if the 21 day milk progesterone assay suggested they were not pregnant. All cows receiving a second service were re-treated with norgestomet implants as before to allow a third synchronised service as necessary. Pregnancy was later confirmed by manual rectal palpation. In year one, 48 cows entered the program and the pregnancy rates to the first, second and third synchronised services were 56%, 69% and 40% respectively with 17% of cows barren at the end of the breeding period. In year two, 69 cows entered the program and the pregnancy rates were 58%, 48% and 33% to each service with 20% of cows barren at the end of the breeding period. The accuracy of milk progesterone assay for pregnancy diagnosis was 84% and 87% in years 1 and 2 respectively. The study showed it is possible to achieve a calving rate of at least 80% following a 46 day breeding period using a combination of oestrus synchrony and fixed-time AI in beef cows at pasture.