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31Efecto del nivel de suplementación con propilenglicol durante el período de transición a la lactancia sobre actividad ovárica y desempeño reproductivo en vacas HolsteinEffect of supplementation level with propylene glycol during the transition period to lactation on ovarian activity and reproductive performance in Holstein cows AbstractIntroduction. This paper mainly focuses on the study of the inclusion of propylene glycol (PG) as energy compound and its possible effect on minimizing the negative energy balance. Objetive. Evaluate on reproductive parameters, specifically on rates of calving to first service, days open, calving to first observed heat, as well as its effect on insulin secretion and ovarian activity. Materials and Methods. 20 Holstein cows were assigned to one of four supplementation levels: 0, 300, 500 and 700 grams of PG / day mixed in food, from day 10 prepartum and through day 20 postpartum. Blood samples were collected on 10 and 5 prepartum and days 0, 5, 10 15 and 20 postpartum to determine the concentration of insulin were also done to determine transrectal ultrasound follicular growth and ovarian resumption. All data for the evaluation of reproductive parameters were taken from the day of delivery, making daily observations of animals 30 minutes in the morning and 30 minutes in the afternoon to record possible heat presented. Results. There were no significant differences for the variables within the different treatments (p> 0.05), only a trend to decrease the open days when you include a dose of 500 g of propylene glycol with respect to the other groups.Conclusions. The inclusion of propylene glycol had no significant effect on insulin secretion, or the number and size of preovulatory follicles (p> 0.05). It is very likely that the propylene supply mode and time of collection of blood samples affect the identification of the increase in insulin secretion.