Influence of Deslorelin (GnRH-agonist) implant on plasma progesterone, first wave dominant follicle and pregnancy in dairy cattle

The objectives of this study were to investigate the effect of a synthetic GnRH-agonist (Deslorelin) implant on CL function and follicle dynamics when administered 48 h after PGF(2 alpha), in a timed-insemination protocol and to determine if the incorporation of a Deslorelin implant into a timed-ins emination protocol to synchronize ovulation would be beneficial to the esta blishment of pregnancy. In Experiment I, 15 non lactating cyclic Holstein c ows received Buserelin (8 mu g, im) on Day -9, Lutalyse (25 mg, im) on Day -2, and then on Day 0 received either a Deslorelin implant (700 mu g, sc; n =5), Buserelin (8 mu g, im; n=5), or no treatment (control; n=5). Blood sam ples were collected on Days -9, -2, 0 and thereafter daily until the next o vulation Ovaries were scanned by ultrasound on Days -9, -2, 0, 1 (day of ov ulation) and 3 times a week thereafter until a subsequent ovulation. From D ays 0 to 15, the rate of increase of plasma progesterone (P-4) was greater (P<0.01) for Deslorelin than for control and Buserelin. Establishment of th e first-wave dominant follicle (FWDF) as a Class 3 (>9 mm) follicle was del ayed (P<0.01) with Deslorelin (14.2+/-1.3 d) compared with the control (4.6 +/-1.3 d) and Buserelin (5.0+/-1.5 d) treatments. The FWDF resumed growth a fter Day 13 in all 5 Deslorelin-treated cows, and 2 cows ovulated spontaneo usly. In 1 Deslorelin-treated cow, the FWDF regressed, and a second-wave do minant follicle ovulated, while 2 other Deslorelin cows failed to ovulate u ntil after Day 36. The cumulative numbers of Class 2 and 3 follicles was lo west in the Deslorelin group (P<0.01), while the cumulative number of Class 1 follicles was highest (Deslorelin>Buserelin>Control; P<0.01). The number of days to CL-regression and days to subsequent estrus did not differ (P>0 .05) among treatments. In Experiment II 16 lactating potentially subfertile (body condition score 2.25) cows received Cystorelin (100 mu g, im; Day -9 ), Lutalyse(25 mg, im; Day -2), and either a Cystorelin injection (100 mu g , im; n=8) or Deslorelin implant (700 mu g, sc; n=8) on Day 0 and inseminat ed 16 h later. Deslorelin-treated cows had a higher plasma P-4 concentratio n between Days 0 and 16 (P<0.05) than the 2 other groups, and 5 of the 8 co ws in this group were pregnant (Day 45, palpation) compared with 1 of 8 cow s in the Cystorelin group (P<0.05). Incorporation of a Deslorelin implant i nto a timed-insemination protocol enhanced the pregnancy rate in cows of po or body condition. The results support the hypothesis that enhanced CL func tion and delayed establishment of the first-wave dominant follicle may enha nce embryo survival. (C) 1998 by Elsevier Science Inc.