الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
This study aimed to : Reproductive efficiency in lactating dairy cattle isn’t optimal, encouraging the development of more different programs of reproductive management focused on developing the use of artificial insemination and conception rates in dairy herds (Thatcher et al., 2006). A long standing objective of reproductive physiologists was to improve a program of hormonal synchronization that could avoid the problems and limitations related to detection of heat in dairy cows. This objective was started in 1995 with the publication of a hormonal synchronization program that combined PGF2α and GnRH to control ovarian physiology and is now usually referred to as the Ovsynch program (Pursleyet al., 1995.The relationship between follicular dynamics and the reproductive efficiency is well started with the advent of ultrasound (Griffin and Ginther, 1992). Nevertheless, it is difficult to precisely know the ovarian follicular development status by a single random examination with ultrasound and it is often not practical to serially examine individual cows at field-level. To avoid these problems, an alternative way is to exogenously control the initiation of follicular wave (Bo et al., 1995). The synchronization programs of follicular wave including GnRH analogue have been designed with the principle of controlled induction of new follicular wave emergence by blocking the suppressive effect of persistent dominant follicle (Kohram et al., 1998). Using Pre-synchronization to onset the Ovsynch program in lactating dairy cattle within early diestrus (Vasconcelos et al., 1999). In addition, cattle which ovulate after GnRH have shorter dominancy intervals, which make ovulation of an earlier oocyte with good quality (Cerri et al., 2009). However, a mature corpus luteum (CL) induced with a pre-synchronization program assures higher releasing progesterone (P4) within follicular development, which has a positive effect in oocyte quality, thus developing fertility (Bisinotto et al., 2010 ; Santos et al., 2010). The Double-Ovsynch programraized fertility to the Ovsynch timed artificial insemination program compared with a Presynch-12-Ovsynch program. Double Ovsynch maximized the percentage of cattle with elevated progesterone at the time of 1st GnRH administration. In addition, one of the studies noted a development in progesterone concentrations at the time of PGF2α injection. Thereby, the use of Double-Ovsynch might effectively treat anovular cattle and might develop synchronization during the final Ovsynch timed artificial insemination program (Souza et al., 2008 ; Herlihy et al., 2012).Several factors affect fertility, containing the characteristics of the animal, environment and climate (Chebel et al., 2004). Cow parity number one of these characteristics. Results of some studies have revealed high fertility in multiparous (Stevenson and Call, 1988), and primiparous cows (Tenhagen et al., 2004). During early lactation, differences in fertility between the 2experimental groups of cattle and between different synchronization programs might be associated with the abnormal metabolic occurrence (Gröhn and Rajala-Schultz, 2000) and cow’s essential trend to experience reproductive disorders (Huszenicza et al., 1987) or anovulation (Silva et al., 2009; Bamber et al., 2009). Other researchers have associated it to a trend by the multiparous cattle to have fewer regression rates as compared with primiparous cattle (Martins et al., 2011), with a suggested program to improve fertility in multiparous cattle after synchronization being additional injections of PGF2α during synchronization (De-Jarnette and Marshall, 2003). Low fertility in dairy cattle is associated closely with heat stress during the hot season. During seasonal periods of heat stress, pregnancy rate declines as a result of decreased conception rates (Thatcher and Collier, 1986) and lowered rates of estrous detection (Her et al., 1988). Therefore, the aim of this study was to determine the effect of pre-synchronization and double-Ovsynch protocols on ovarian follicular dynamics, P4 concentration and fertility at first postpartum artificial insemination in Friesian cows under the effect of parity and season.