Embryonic growth between d 33 and 45 of pregnancy in lactating dairy cows differing in hormone and metabolite concentrations

Abstract Text: Embryonic loss in dairy cows continues to occur into the fifth and sixth week of pregnancy and there is an association between slower embryonic growth and pregnancy loss during this period. Individual differences in hormone and metabolite concentrations may affect the growth of the embryo via endocrine mechanisms or by affecting nutrient flux across the placenta. The objective was to examine the relationship between postpartum hormones/metabolites and embryonic growth from d 33 to 45 of pregnancy. Holstein cows (n=56; 86±17 d postpartum at AI) were examined by transrectal ultrasonography on d 33, 35, 38, 40, 42, and 45 of pregnancy.  Length (l) and width (w) of the embryo and amnionic vesicle were measured and the volumes for the embryo (e_vol) and amnionic vesicle (a_vol) were calculated [volume = 4/3*π*(0.5*l)*(0.5*w)*(0.5*w)]. There was an effect of day of pregnancy (P<0.001) because e_vol and a_vol increased from d 33 to d 45 (0.14±0.01 cm³ vs. 0.60±0.06 cm³; and 1.52±0.05 cm³ vs. 10.57±0.59 cm³, respectively). Across all days, the a_vol of male embryos was larger than female (4.25±0.20 vs. 3.70±0.20 cm³) but e_vol was similar for male vs. female. Blood was collected on the d of ultrasound and plasma analyzed for glucose, progesterone (P4), growth hormone (GH), IGF1, and insulin (INS).  Plasma hormone and metabolite concentrations were not affected by day of pregnancy (P>0.1) but differed for individual cows (P<0.001; range = 51 to 82 mg/dL for glucose, 4.7 to 13.5 ng/mL for P4, 2.3 to 13.4 ng/mL for GH,  50 to 131 ng/mL for IGF1, and 0.2 to 0.7 ng/mL for INS). Individual cows were categorized as being above or below the median for each blood hormone/metabolite concentration.  Cows that were above or below the median for glucose, P4, GH, or INS were similar for e_vol and a_vol (P>0.1).  There was a category by day interaction (P<0.05) for IGF1, however, because cows with IGF1 above the median (mean = 102.4±16.4 ng/ml) had greater e_vol on d 42 compared with low IGF1 cows (mean = 69.9±13.0 ng/mL) (1.11±0.04 vs. 0.98±0.04 cm³; above vs. below). A_vol was not affected by IGF1 category. Conclusions were that male embryos have greater amnion vesicle volume from d 33 to 45 of pregnancy. Plasma concentrations of IGF1 were positively associated with a larger embryo on d 42. Chronically low IGF1 concentrations in lactating cows may lead to embryonic loss via slower embryo growth.

Keywords: Bovine, embryo, metabolites