Evaluación De La Respuesta Ovárica

EVALUATION OF OVARIAN RESPONSE AND PERCENTAGE OF GESTATION IN MEXICAN TROPIC’S HEIFERS TREATED WITH MELENGESTROL ACETATE
aSalas-Razo G., aVargas, J. P. S., aPerea P.M., aGutiérrez E. V., aJuárez A. C., bGarcidueñas R.P., cEspinoza J.L.V. y.aFlores P.J.P.
aInstituto de Investigaciones Agropecuarias y Forestales de la Universidad Michoacana de San Nicolás de Hidalgo. bFacultad de MedicinaVeterinaria y Zootecnia de la Universidad Michoacana de San Nicolás de Hidalgo. cUniversidad Autónoma de Baja California Sur

Abstract
The aim of this study was to assess the ovarian response and the percentage of gestation in Mexican tropic’s heifers treated with melengestrol acetate (MGA). The work took place in the municipality of Tzitzio, Michoacán, Mexico. Twenty-five, 43±4.6 months old heifers,weighting 312.72±25.78 kg, and recording 3.0±0.25 of body condition were feded with 0.5 mg of MGA pre-blended with 250 g of supplement through 21 days. At the end of the treatment the heifers ovaries were observed over ultrasonography daily until the ovulation time, and then four more times every 5 days to observe the corpus luteum development. The heifers were direct mated, and 88% of themrevealed dominant follicle post-treatment, and had their ovulation on the fifth to tenth day, focused on the eighth day (72.7%). Most of them (95.5%) were pregnant after the mating; realizing that ovaric response of the heifers treated with MGA at a dose of 0.5 mg per day through 21 days standarize the estrous advent. Moreover, it shows that the ovum have the conditions to be fertilized. The MGA may beused as an important constituent to perform a strategy to achieve homogeneous calves lots in suitable times for production, thus contributing to the Mexican tropic’s livestock reproductive parameters improvement.

Introduction.
Bovine production in the Mexican tropic units aims at obtaining calves and they have as goal to obtain one calf per female per year. One of the main constraints toachieve this however, is the nutritional restriction to which are subjected for long periods (time of drought), as well as caloric stress that must be aovercome, altering negatively indicators open days (447±57.8 days), and a late sexual maturity of the heifers, which will result in a low rate in the production of calves, that babies are dispersed in the course of the year, producing lots of calveslittle uniforms (Quezada et al., 2004).
Age at first birth of the calves in the Mexican tropic is 34.7±4.4 months. That is taken as a low reproductive efficiency expressed through late sexual maturity. That’s why is necessary to raise the use of strategies to reduce the age at first birth in these operating systems. One is throughout the use of melengestrol acetate (MGA), which can induce ovarianactivity after oral -treatment in a sincronized way. Based on these considerations arose the present work with the aim of evaluating the ovarian response and the percentage of gestation in the Mexican tropic’s calves, treated with melengestrol acetate.

2. Material and Methods.
This work was carried out in the Limon del Torbellino ranch; municipality of Tzitzio, Michoacán, Mexico, from Aprilthrough May of 2011. The ranch is located geographically at 19° 20´ 57" North latitude and 100 ° 53´ 18" West longitude, at 811 masl. Its climate is tropical tempered with rains in summer. The annual rainfall is 1,397.3 mm3 and its temperatures ranging from 13.1 to 27.2 ° C (INAFED, 2011).
Twenty-five 43 ± 4.6 months old heifers Cebu x Swiss with 312.72 ± 25.78 kg of body weigth, and 3.0 ± 0.25 ofbody condition, according to the scale proposed by Wildman et al. (1982), were supplemented with 0.5 mg of MGA pre-blended with 250 g of supplement in the morning during 21 days. Heifers were feded with 1.5 kg of supplement in the morning and in the afternoon, sorghum forage and water ad libitum.
Once finished the treatment the heifers ovaries were observed via transrectal with a portable...