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Effects of thermal stress on pregnancy characteristics in dairy cows
Nanas, I.; Dadouli, K. Chouzouris...
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Objectives: Due to global warming, the adverse effects of heat stress (HS) on dairy cattle welfare and production are gradually extending in northern latitudes. The combination of high ambient temperatures and relative humidity (THI) in dairy cows decreases appetite, milk yield, estrous expression, estrus length and pregnancy rates of inseminated animals. The extensive use of cooling systems for the abatement of the negative effects of HS, proved to be successful in preserving milk production, but their impact on fertility records are rather negligible. This is related to the high sensitivity of the maturing oocyte and the early embryo to the elevated temperatures as well as to the modifications that increased temperatures bring about at follicular, oviductal and uterine environment.
The trophoblastic cells of the bovine placenta express a series of glycoproteins named Pregnancy Associated Glycoproteins (PAGs) whose concentration is steadily increased with pregnancy progression, culminating at calving. PAG concentration of inseminated cattle is used as a biochemical marker of pregnancy and a reliable indicator for the wellbeing and functionality of the placenta. Progesterone (P4) is of paramount importance for pregnancy establishment and preservation, however, the limits between optimal and suboptimal concentrations is difficult to be accurately defined, as P4 concentrations are related to various environmental and metabolic factors.
Here, we focused to estimate the association between circulating concentrations of PAG, P4 and the early embryo mortality rates during summer and winter.
Materials and Methods: Two trials were conducted in two Holstein dairy farms of central Greece. Both farms are equipped with effective cooling systems comprising electronic assessment of THI and automatic activation of fans and sprinklers. In addition, according to thermal load all lactating cows had intensive cooling (soaking–drying) two to four times daily.
Τhe purpose of the first trial was the quantification of the early embryonic mortality rates under HS or thermoneutral conditions. In 2018, a total of 279 cows were used (144 during the winter, group W, and 135 during the summer, group S). On days 24-25 post AI, all cows were diagnosed as pregnant by transrectal ultrasonography; on days 34-36 they were re-examined, and a blood sample was withdrawn for PAG determination.
The second trial lasted from 2014 to 2019 and aimed at PAG levels evaluation in 6109 blood samples submitted for routine pregnancy diagnosis. All samples were collected 29 to 36 days after breeding and assayed by a commercial ELI- SA kit (DG29, Conception Animal, Canada). According to THI, the samples were allotted into group S THI≥76 n=2388, or into group W THI≤68, n=3721. A sample was characterized as positive, ambiguous, or negative (PAG ≥1000pg/ml, 500 to 999pg/ml or <500, respectively).
Progesterone concentration was also determined in 50 pregnant animals from each group in serum samples collected on days 31 to 33 post AI by a commercial ELISA kit (DRG, Germany).
Results: In trial 1, the pregnancy maintenance rate differed significantly (p=0.03) among groups (136/144 - 94.4% and 81.5% - 110/135, for groups W and S, respectively). According to the PAG concentrations all animals were characterized as pregnant. In trial 2, PAG levels on days 33 to 36 after breading were significantly higher (p=0.043) in group S (2546.8±63.1 ng/ml ) compared to those of group W (2385.51±42.4 ng/ml). Progesterone was lower (p<0.05) in group S (6.76±0.96ng/ ml) compare to W (8.66±1.27). The proportions of ambiguous pregnancies were lower (p<0.001) in group S (7.88%) compared to W (11.10%).
Conclusions: From the data presented here we infer that during summer months the embryos that can survive HS develop a well-functioning placenta. This is in concert with our in vitro studies where we provided evidence that blastocysts produced from oocytes exposed to HS have high expression of genes related to placentation. On the other hand, the combined results of embryo mortality and progesterone might be indicative that during the winter, slow developing or weak embryos have more chances to survive.
Acknowledgements: Work co‐financed by the EU and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH – CREATE – INNOVATE (project code:T- 1EDK-01078, the Summer Cow).
I.N is financially supported by IKY (project MIS-5000432, co-financed by Greece and E.U.-ESF).
Keywords: Heat stress, progesterone, PAGs, early embryonic death, dairy cattle.
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Affiliation of the authors at the time of publication
Univ. of Thessaly, Vet Faculty, Dept of Obstetrics & Reproduction, Karditsa, Greece;
Univ. of Thessaly, Faculty of Medicine, Larissa, Greece.
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