Genetic and environmental factors influencing first service conception rate and late embryonic/foetal mortality in low fertility dairy herds

The objective of this study was to identify factors affecting variation in conception rate to first artificial inseminations (AI) (CR: number of pregnant cows on D80–100/inseminated cows) and the incidence of embryonic/foetal loss (LEM) between 21 and 80 days of pregnancy (number of cows non-pregnant on D80–100/pregnant on D21) in 44 low fertility dairy herds of the west-central region of France. Reproductive status was assessed using progesterone milk concentration on D0 = Day of AI and D21–24, plasma PSPB concentration on D30–35, rectal palpation on D80–100 and observed return to oestrous. The final data set contained 1285 Prim’Holstein cows, 5.0% (64/1285) were inseminated in the luteal phase (progesterone ≥3 ng/ml on D0), 61.3% (787/1285) were pregnant on D21–24 (progesterone <3 ng/ml on D0 and ≥5 ng/ml on D21–24), 15.4% lost their embryo/foetus between D21–24 and D80–100 (198/1285) and 45.8% (589/1285) were pregnant on D80–100. The incidence of late embryonic/foetal loss (LEM) was 25.2% (198/787). Multivariate logistic regression models including the random herd effect were used to analyse the relationship between AI centre, AI sire, cow's sire, parity, interval between calving and AI, milk production, milk protein content, body condition score (BCS) on D0, season of calving, season of AI, estimated genetic index on CR and LEM incidence. CR was significantly related to parity (p < 0.05), milk production after calving (p < 0.05) and estimated genetic value (p < 0.01). A significant difference in CR was observed for calving to AI interval ≤70 days versus ≥90 days, but the overall effect of the interval was not significant (p = 0.11). LEM incidence was affected by period of AI (p < 0.05), milk production (p < 0.05) and BCS (p < 0.05), but was not related to estimated genetic index. In conclusion, in these low fertility herds, the incidence of LEM was high and 25% of the cows lost their embryo after 21 days of pregnancy. LEM was affected by specific factors (season, BCS), which were not related to CR. The absence of a relationship between estimated genetic index and LEM in spite of its effect on CR indicates that estimated genetic merit has a greater effect on early embryonic loss or fertilisation failure than on later stages of embryo development.