Resumo

VITTONE, J.S et al. Early weaning in beef cows treated with intravaginal progesterone and their reproductive performance. Arch. zootec. [online]. 2011, vol.60, n.232, pp.1065-1076. ISSN 1885-4494.  https://dx.doi.org/10.4321/S0004-05922011000400022.

The objective of the present study was to determine the effect of early weaning time on the ovulation characteristics and the reproductive efficiency of treated beef cows with an intravaginal progesterone device (DIV). Two experiments were performed utilizing Hereford and Polled Hereford cows. Exp. 1: 38 low body condition score cows (BCS; 3.3±0.8, mean±SD, range from 1 to 9) with 57.5±8.8 days postpartum and Exp. 2: 35 high BCS (5.4±0.9) cows with 57.5±15.9 days postpartum. On day 0, all cows received an DIV containing 0.5 g progesterone plus 2 mg of estradiol benzoate i.m. (EB). On day 8, the DIV was removed and 500 ìg cloprostenol was injected i.m., followed by administration of 1 mg of EB i.m. 24 h later. Animals were assigned randomly to each of four treatments:T1) Early weaning at 10 days before the DIV insertion; T2) Early weaning the day of the DIV insertion; T3) Early weaning the day of withdrawal DIV and T4) No early weaning, calves were left with their respective mothers throughout the study period. All cows were fixed-time artificially inseminated 56 h after the DIV removal. In Exp. 1 and 2, follicular dynamics, time and distribution of the ovulation and pregnancy were evaluated by means of transrectal ultrasonography. Plasma progesterone concentrations were determined to evaluate the functionality and lifespan of the corpus luteum generated by the induced ovulation. At the time of the DIV removal, the dominant follicle diameter was larger (p<0.05) in T2 (11.2±2.5 mm) than T3 (7.3±2.0 mm) in cows with low BCS (Exp. 1), but did not differ with T1 and T4 (10.3±2.4 and 8.2±2.8, respectively). The T1 and T2 treatments presented a higher (p<0.05) ovulation rate than T3 and T4 (77.8; 88.9; 40 and 40% respectively). Pregnancy rate was higher in T2 (77.8%) than in the rest of treatments (T1= 55.6%, T4= 30%, T4= 20%). In Exp. 2, no differences were found among treatments for all the evaluated parameters. In non-pregnant cows the corpus luteum had a normal lifespan, independently of treatment or BCS. In summary, early weaning performed 10 days before or the day of treatment start with progesterone/estrogen increased dominant follicle diameter (pre-ovulation) and ovulation rate in low BCS cows. Nevertheless, pregnancy rate was only improved when early weaning was done the day of the DIV insertion in the low BCS cows assigned to Exp. 1.

Palavras-chave : Beef cows; Fixed-time artificial insemination.

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