Anoestrous dairy cows: GnRH-based vs progesterone-based synchronisation programmes for timed AI — POSSIBLE

POSSIBLE: In anoestrous dairy cows, programmes that include a progesterone device give somewhat better pregnancy rates to timed AI than GnRH-based programmes without progesterone, but the advantage is modest in the largest trial and large only in one older trial's anoestrous subgroup. In Shephard 2013 (PMID: 23829605; 1,906 anoestrous cows from 12 New Zealand herds, randomly assigned), conception to fixed-time AI was 0.34 (95% CI 0.29-0.37) with the GnRH-based GPG programme alone (gonadorelin day -10, cloprostenol day -3, gonadorelin day -1) vs 0.38 (95% CI 0.34-0.43) when a 1.38 g progesterone CIDR device was added for days -10 to -3 and 0.38 (95% CI 0.33-0.42) with a 1.0 g DIB progesterone device — absolute differences of about 4 percentage points (relative risk about 1.12) — and pregnancy by day 28 was 0.55 (95% CI 0.51-0.60) for GPG vs 0.57 (95% CI 0.52-0.61) for CIDR and 0.56 (95% CI 0.52-0.60) for DIB, with no difference between the two progesterone devices; adding equine chorionic gonadotrophin to the DIB programme (0.41 conception, 0.63 pregnant by day 28) helped only in cows more than 60 days calved, where it beat GPG (p < 0.001). In Cavestany 2003 (PMID: 12695051; 742 lactating Holstein cows, 27.4% anoestrous by milk progesterone), overall pregnancy rates were similar (20%) between Ovsynch and Ovsynch plus a medroxyprogesterone acetate sponge, but in anoestrous cows specifically pregnancy rate was 34.9% with the progestin added vs 11.1% with Ovsynch alone (treatment-by-cyclicity interaction P < 0.001; absolute difference 23.8 percentage points). Design class (b): randomisation is established by Shephard 2013, which randomly assigned anoestrous cows to the GnRH-based programme with or without a progesterone device and so answers this shelf's own comparison in this shelf's own population; blinding is not established in either carded record (device insertion cannot readily be masked and neither record states masked outcome assessment — pregnancy was diagnosed objectively). The stamp is POSSIBLE, not LIKELY, because the two carded trials point the same way but with very different effect sizes, the larger and more recent trial's device effect is small and its confidence intervals overlap the GnRH-only arm, and the large anoestrous-subgroup effect comes from a mixed cycling/anoestrous trial using a medroxyprogesterone sponge rather than a progesterone device. Tier C is not engaged: the primary claim is a pregnancy-rate benefit, which is not a mortality, fetal-loss or survival claim. One-home cross-points: Ovsynch vs insemination at detected oestrus homes on shelf #359 and the CIDR programme vs no treatment in anoestrous cows homes on shelf #360 (whose McDougall records are not re-carded here); progesterone supplementation after AI in repeat breeders is topic #875 and ewe sponge vs CIDR is topic #913 — different comparisons and populations. A further near-miss, not carded: Ozturk 2010 (PMID: 20022363) randomised 51 anoestrous primiparous cows between two GnRH-based protocols (Doublesynch vs Ovsynch, overall pregnancy 72.8% vs 29.8%) — a comparison within GnRH-based programmes with no progesterone arm, so it cannot carry this shelf's claim. Drafted Nov 10, 2026 (VetMed build, topic #874); figures from the abstracts, fetched and verified at drafting.