Sarah Fields and George Perry from South Dakota State University published a paper presenting the effects of stress on embryonic mortality in cattle. Shipping cows between days 5 and 42 causes around a 10 percent decrease in pregnancy rates. Research has also demonstrated that shipping cattle 45 to 60 days after insemination can result in 6 percent of embryos being lost. The following table demonstrates how transporting cows at different days after insemination affected pregnancy rates.
Blastocyst formation, hatching, maternal recognition of pregnancy and adhesion to the uterus take place during this early time of pregnancy. If any of these time points are disturbed, then the result leads to increased embryonic mortality and decreased pregnancy rates.
So . . . . . . . . . . . . . . .
When to haul pregnant cows: Days 1 to 4 or days 45 to 60
When to NOT haul pregnant cows: Days 6 to 42
Here is a link to the Factsheet: https://www.sdstate.edu/vs/extension/beef/upload/ExEx2063_Shipping-and-HeatStress-on-EM.pdf