Absence of response to oestrus induction and synchronization treatment is related to lipid mobilization in suckled beef cows.

Energy status, follicular growth, oestradiol and LH secretion were investigated in 17 suckled Charolais cows synchronised 59.0 +/- 3.6 days after calving with a 10 day ear implant containing 3 mg of Norgestomet. The cows received 3 mg of Norgestomet and 5 mg of oestradiol valerate by IM injection at implant insertion (day 0) and 600 IU PMSG at implant removal (day 10). They were artificially inseminated (AI) 48 and 72 h after implant removal. Energy status was assessed by measuring weekly plasma concentrations of non-esterified fatty acids (NEFA), beta hydroxy-butyrate (BHB), glucose and insulin 7 weeks before AI. Progesterone plasma concentrations were measured during the same period to assess the presence of a functional corpus luteum. Follicular growth was followed daily by ultrasonography from day -3 to day 13. Oestradiol secretion was measured on day -3, day 6 and day 10 from five hourly samples. Oestradiol and LH plasma concentrations were measured hourly from 29 to 48 h after implant removal for seven cows. Cows were checked for pregnancy by ultrasonography 45 days after AI. Pregnant cows (P) were compared with non-pregnant cows (NP) for energy status, follicular growth, and oestradiol secretion by split-plot ANOVA. Two cows (11.8%) were cyclic before treatment, seven ovulated after treatment (41.2%) and five were found pregnant 45 days after AI (29.4%). There was no difference in body condition score and body weight between P and NP cows on day 0 (2.5 +/- 0.2 and 685 +/- 24 kg vs 2.5 +/- 0.1 and 670 +/- 13 kg; P > 0.05). Mean plasma NEFA concentrations before treatment were significantly lower in P than in NP cows (218 +/- 29 mu eq/L vs 279 +/- 18 mu eq/L; P < 0.05). No significant differences between P and NP cows were found for BHB, glucose and insulin concentrations. P cows presented more medium sized follicles (5 mm < or = diameter < 10 mm) than NP females during the period of observation (2.65 +/- 0.19 vs 2.50 +/- 0.12; P = 0.05). Plasma oestradiol concentrations were not different between P and NP cows on day -3 (8.4 +/- 0.7 pg/mL vs 7.7 +/- 0.4 pg/mL, P > 0.05), day 6 (10.4 +/- 0.6 pg/mL vs 9.8 +/- 0.4 pg/mL, P > 0.05) but were higher in P than in NP cows on day 10 (10.9 +/- 0.6 pg/mL vs 7.8 +/- 0.4 pg/mL; P < 0.05). After implant removal, oestradiol secretion only increased in P cows and a LH peak occurred whereas no increases in oestradiol (11.0 +/- 0.4 pg/mL vs 6.3 +/- 0.3 pg/mL, P < 0.05) and LH (6.0 +/- 0.5 ng/mL vs 1.2 +/- 0.5 ng/mL, P < 0.05) secretion were observed in NP cows. The conclusion was that follicular growth, oestradiol secretion, ovulation and pregnancy rate after oestrus synchronisation treatment are related to mobilization of energy stores before treatment in suckled beef cows in the same body condition score.