Estrous activity and pregnancy outcomes in Holstein heifers subjected to a progesterone based 5-d CO-Synch protocol with or without administration of initial GnRH

Timed-AI (TAI) protocols have been developed to facilitate the adoption of AI in cattle as estrus detection is not needed. However, recent studies have shown that regardless of the TAI protocol used, expression of estrus before TAI resulted in increased pregnancy per AI (P/AI) [[1], [2], [3]], especially when using sex-sorted semen [4]. Protocols based on gonadotrophin releasing hormone (GnRH) and prostaglandin F2α (PGF) are commonly used for TAI in cattle in Canada [5]. Yet, the necessity of the initial GnRH to achieve acceptable fertility in heifers synchronized with a shortened GnRH-based protocol (i.e. 5 d CO-Synch plus progesterone device) has been controversial [[6], [7], [8], [9]].

As approximately 20% of heifers ovulated within 72 h after PGF injection and progesterone device removal, the timing of insemination has been the focus of research in both beef and dairy heifers subjected to 5 d CO-Synch protocols. Some differences may exist whether initial GnRH is given or not. In this regard, Kasimanickam et al. [10] reported that beef heifers inseminated at 56 h had, on average, a 10% higher P/AI than heifers inseminated at 72 h. Both groups of heifers received the initial GnRH and the second GnRH was given concurrently with TAI. Conversely, Lima et al. [11] reported increased P/AI in dairy heifers that did not receive an initial GnRH but received the final GnRH concurrent with TAI at 72 h after PGF compared with those that received the final GnRH 16 h before TAI. Recently, Macmillan et al. [12] reported that the attempts to optimize P/AI in Holstein heifers subjected to the 5 d CO-Synch protocol without the initial GnRH by either altering the timing of second GnRH or TAI were unsuccessful. We concluded that additional studies were needed to optimize the timing of AI relative to the timing of ovulation to improve fertility in heifers subjected to the 5 d CO-Synch protocol.

There is little published information concerning timing of estrus or ovulation in heifers subjected to shortened GnRH-based TAI protocols. Only one study with a small number of animals (n = 64) determined the interval from progesterone device removal to ovulation [6]. However, no-one has reported in detail estrous activity in heifers subjected to a 5 d CO-Synch protocol with or without the initial GnRH administration. Although determining the timing of ovulation via serial transrectal ultrasonographic examinations would be the ideal procedure to optimize the timing of insemination, the use of an automated activity monitor system to estimate the time of ovulation based on estrous activity would be a more practical approach and more acceptable from an animal welfare perspective.

We hypothesized that the administration of GnRH at the initiation of a 5 d CO-Synch protocol would differentially affect the time of onset of estrous activity in Holstein heifers when compared to a protocol in which the initial GnRH was not administered. Therefore, the overall objective of this study was to investigate whether estrous activity and its impact on pregnancy outcomes would differ between heifers subjected to a 5 d CO-Synch plus progesterone releasing intravaginal device (PRID) protocol with (GnRH) or without (NGnRH) an initial GnRH treatment. The primary objectives were to compare the (i) interval from PRID removal to the onset of estrus, (ii) rate of estrus expression following PRID removal and (iii) the impact of these two measures on pregnancy outcomes between GnRH and NGnRH heifers. A secondary objective was to determine the interval from TAI to subsequent estrus between GnRH and NGnRH heifers that were non-pregnant following TAI.

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