Aim: To evaluate the conception rate to fixed-time artificial insemination (FTAI) of two oestrus synchronisation programmes in dairy heifers on eight farms over 2 years.
Methods: The study was conducted in 2008 and 2010 on eight farms near Palmerston North, New Zealand. Nulliparous Friesian and Friesian×Jersey heifers (13-15 months of age) were randomly allocated to one of two oestrus synchronisation programmes. Group 1 (GPG+P4; n=330), received gonadotrophin-releasing hormone (GnRH) I/M on Day 0, a progesterone (P4)-releasing intravaginal device from Days 0-7, prostaglandin F2α (PGF) I/M on Day 7 and a second dose of GnRH at the time of FTAI on Day 9. The second group (P4+PGF; n=343) received a P4-releasing intravaginal device from Days 0-7, PGF on Day 6 and FTAI on Day 9. Pregnancy was diagnosed from Days 42-52 by transrectal ultrasonography.
Results: The overall conception rate was 52.4% and 54.8% for the GPG+P4 and P4+PGF groups, respectively. The odds of conception for the two treatments were not different (OR=0.90; 95% CI=0.67-1.23), nor was there any difference between groups in different years (p=0.58). Farm affected conception rate (p=0.002), but there was no interaction with treatment (p=0.92) .
Conclusions: This study has shown that an alternative synchronisation programme can produce similar results in terms of conception rate to the GPG+P4 treatment, currently commonly used in heifers. More research is required to establish whether other modifications to the GPG+P4 programme can produce similar results at lower costs, and to identify and quantify farm factors which affect the economic benefit of heifer synchronisation.
Clinical relevance: This study indicated that synchronising heifers with P4 and PGF resulted in conception rates equivalent to those resulting from a GPG+P4 treatment, but with reduced drug costs. However, because heifers in the GPG+P4 group received the second GnRH injection at the time of AI, they needed only three yardings as opposed to the four required for the heifers treated with P4 and PGF. Thus, the choice of programme for an individual farm will depend on that farm's circumstances, in particular the cost of yarding the heifers.
Keywords: GPG; Oestrus synchronisation; conception rate; heifer; progesterone.