Effect of monensin and progesterone priming on ram-induced reproductive performance of Boutsiko mountain breed ewes

The effects of monensin and progesterone priming on reproductive performanc e (estrous response, lambing rate and prolificacy) of grazing Boutsiko moun tain breed adult and 18-mo-old ewes at the end of seasonal anestrus were in vestigated. In Experiment 1 the feed supplement with or without monensin wa s offered for 21 d after introduction of vasectomized rams (Day 0). Progest erone was administered to the ewes in the respective groups as a single inj ection at Day -3. Ewes of both age groups were assigned randomly to 1 of 4 treatments: C, C+P, C+M and C+M+P. In Experiment 2 the supplement C or M wa s offered from Day -26 to Day 21. The treatments consisted of C, C+P and CM+P. Blood samples were taken 50 h after ram introduction for determination of plasma concentrations of P and insulin-like growth factor-I (IGF-I). There was a greater increase in estrous response at Days 17 to 19 and at Da ys 0 to 19 when supplementation was offered before rather than after ram in troduction in both age groups. In the adult group ewes synchronization of e strus at Days 17 to 19 was significantly increased by administration of mon ensin (P<0.05) and progesterone (P<0.01) compared with the control group in the first but not the second experiment. The incidence of estrus at Days 1 7 to 19 or at Days 0 to 19 was highest in the adult groups treated with mon ensin and progesterone in both experiments. in 18-mo-old ewes progesterone was effective in synchronizing estrus only in Experiment 2. Mean plasma IGF -I concentrations were increased by monensin treatment (P<0.05) in adult ew es that were at the periovulatory stage at blood sampling time. Correlation coefficients between IGF-I and progesterone concentrations in monensin plu s progesterone group adults were -0.715 (P<0.02) and -0.516 (P<0.01), respe ctively across all treatments. The results suggest that monensin and proges terone priming improved reproductive performance, and the monensin-induced increase in plasma IGF-I levels at the periovulatory stage may be causally related to the ability of ovulatory follicles to develop into functional co rpora lutea (CL). (C) 1999 by Elsevier Science Inc.