Relationships between maternal hormone secretion and embryo development on day 5 of pregnancy in dairy cows
Introduction
Studies of the extent and timing of early embryo mortality in the dairy cow indicate that at least 25% of embryos are lost during the first three weeks of pregnancy (Sreenan and Diskin, 1986, Peters, 1996). Despite extensive research, the reasons for this phenomenon are still poorly understood. A number of factors have been implicated including poor oocyte quality, defects within the embryo, an inadequate maternal environment, asynchrony between embryo and mother and failure of the mother to respond appropriately to embryonic signals. Studies suggest the time at which the embryo enters the uterus (around days 4–5; Guillomot, 1995) to be critical, since embryo mortality has been found to occur by day 5 post-oestrus in the dairy cow (Ayalon, 1978, Wiebold, 1988).
During early pregnancy, progesterone influences the endometrial secretion of nutrients, growth factors, immunosuppressive agents, enzymes, ions and steroids that are essential for embryo development (Graham and Clarke, 1997) and a number of studies have demonstrated lower maternal progesterone concentrations during the first three weeks of pregnancy in those cows in which mating is unsuccessful (see Mann et al., 1999). Further studies have also established a link between lower maternal progesterone concentrations and reduced embryo development with poor development accompanied by both a delayed post-ovulatory plasma progesterone rise and reduced progesterone concentrations (Mann and Lamming, 2001). Other studies in beef cows have shown that administration of progesterone can advance embryo development (Garrett et al., 1988) further emphasising the importance of progesterone in the control of early embryo development. Consistent increases in pregnancy rate are only achieved when administration of progesterone to mated cows is initiated during the first week of pregnancy (Mann and Lamming, 1999). Furthermore, in a large field trial, Starbuck et al. (2001) identified a close relationship between the concentration of milk progesterone on day 5 after mating and the outcome of early pregnancy indicating that the post-ovulatory progesterone rise may be the critical factor of maternal progesterone secretion controlling early embryo development.
In dairy cows, a relationship has been established between maternal progesterone concentration on day 5 after mating and the degree of embryo development on day 16 of pregnancy (Mann and Lamming, 2001). The objective of the present study was to determine if a relationship between maternal progesterone and embryo development could be identified as early as day 5 of pregnancy.
Section snippets
Experimental protocol
The study was undertaken in 20 Holstein–Friesian cows of 1st to 9th parity (mean 3.7 ± 0.5) obtained from the University of Nottingham commercial herd between September and February in accordance with the Animals (Scientific Procedures) Act, 1986. All the cows were at the end of lactation, on average 374 ± 32 days post-partum.
Oestrous cycles were synchronised by two intramuscular injections of 50 μg of the prostaglandin F2α analogue, Cloprostenol (Estrumate; Schering-Plough Animal Health Welwyn
Statistical analyses
Pregnant animals were defined as those animals in which an intact embryo was recovered at the expected stage of development while non-pregnant animals were those in which empty zona pellucidae, unfertilised oocytes or no embryonic structure was found. For all analyses, data from non-pregnant animals were always analysed both including and excluding those animals in which no embryonic structures were found. Unless stated, no difference between the two data sets was identified with respect to the
Ovulation and pregnancy rate
Twenty animals were initially synchronised, although three animals were excluded from all subsequent analysis as they failed to develop a functional CL and had undetectable plasma progesterone concentrations by day 5 post-oestrus. Of the remaining 17 cows, viable embryos (n = 15) were recovered from 11 cows (65%). These animals had embryos at the morula (n = 3 cows), 8–16 cell (n = 3 cows) and 8-cell (n = 5 cows) stages of development. Embryos at the morula stage of development were recovered from the
Discussion
In the present study, we have demonstrated a relationship between maternal progesterone concentration and the stage of early embryo development on day 5 of pregnancy in dairy cows. This extends the findings of an earlier study in which a relationship was established between maternal progesterone and the degree of embryo development on day 16 (Mann and Lamming, 2001).
The role of progesterone as the key hormone in the maintenance of pregnancy is well established. The time at which divergence in
Acknowledgements
This work was supported by a research grant from the Ministry of Agriculture, Fisheries and Food. MPG was supported by a studentship from MAFF.
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Present address: Department of Animal Sciences, University of Missouri, 920 East Campus Drive, Columbia, MO 65211, USA.