title: Follicular growth and ovulation in postpartum beef cows following calf removal and GnRH treatment.
abstract: This study investigated the effects of calf removal (CR) and gonadotrophin releasing hormone (GnRH) administration on the duration of the postpartum anoestrous period in suckled beef cows. Experiment 1 involved 20 multiparous suckled cows that were assigned to each of two treatments on Day 61 postpartum: (i) unlimited access to their calves (C; n=10) and (ii) calf removal for a period of 96 h (CR96, n=10). Experiment 2 involved 24 multiparous cows that were assigned to each of two treatments on Day 63 postpartum: (i) CR96 (n=12); and (ii) CR96 plus 250 microg of GnRH administered on the day before calf return (CR96+GnRH, n=12). Experiment 3 was a 3x2 factorial experiment, involving 48 multiparous cows assigned to the experiment on Day 58 postpartum. The factors were C, CR96 and calf removal for 144 h (CR144), and 0 or 250 microg GnRH administered on the day prior calf return. In Experiment 1, the number of cows that ovulated within 12 days of calf removal was higher (P<0.05) in CR96 group (3/9) compared to the C group (0/10). In Experiment 2, all 12 cows in the CR96+GnRH group ovulated. In contrast only 4/12 cows in the CR96 group ovulated in response to calf removal. The diameter of the ovulatory follicle tended (P=0.06) to be smaller in CR96+GnRH cows (9.8 +/- 0.3 mm) than in CR96 cows (11.3 +/- 0.9 mm). The maximum diameter attained by the corpus luteum (CL) also tended (P=0.08) to be smaller for cows in the CR96+GnRH than for cows in the CR96 group (12.1 +/- 2.4 mm versus 16.7 +/- 7.5 mm, respectively). Plasma progesterone concentrations 12 days after calf removal tended (P=0.06) to be lower in CR96+GnRH cows than in CR96 cows (0.66 +/- 0.1 ng/ml versus 2.00 +/- 1.1 ng/ml, respectively). Few cows in the CR96+GnRH group regained normal cyclical activity and the interval from onset of calf removal to conception was longer (P<0.05) compared to cows in the CR group (52.2 +/- 5.7 days versus 20.0 +/- 6.6 days). In Experiment 3, 5/8 cows on the CR144 group and all 8 cows in the CR144+GnRH group ovulated. However, the interval from CR to conception was similar for all treatments. Temporary (96-144 h) calf removal, particularly in combination with GnRH treatment, can induce a high proportion of beef cows to ovulate, but the restoration of oestrous cycles may not be achieved.
authors.affiliation: National Institute of Agricultural Research (INIA), Ruta 8 km 281, 33000, Uruguay. quintans@inia.org.uy
authors.forename: G
authors.initials: G
authors.lastname: Quintans
authors.forename: C
authors.initials: C
authors.lastname: Viñoles
authors.forename: K D
authors.initials: KD
authors.lastname: Sinclair
mesh_terms: D000818:Animals; D000833:Animals, Suckling / Q000502:physiology; D001823:Body Composition; D001835:Body Weight; D002417:Cattle / Q000502:physiology*; D003338:Corpus Luteum / Q000033:anatomy & histology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage*; D006080:Ovarian Follicle / Q000033:anatomy & histology / Q000502:physiology*; D010060:Ovulation; D049590:Postpartum Period; D011374:Progesterone / Q000097:blood; D014886:Weaning; D015430:Weight Gain
chemical_list: D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone