Date of this Version
Embryonic loss may represent the single greatest economic loss for cow/calf producers. With 40,000,000 beef cows and heifers exposed to breeding each year in the U.S., annual losses exceed $1.2 billion. The mechanisms involved in pregnancy establishment and maintenance are complex, and based on the literature, we have made little progress reducing embryo wastage in the past 90 years. This paper will focus on when and why pregnancy failures occur and discuss some management practices that may alleviate these losses.
In beef cattle, it is generally accepted that fertilization rates to a single service are between 90 and 100% regardless of whether natural service or artificial insemination is utilized (Sreenan and Diskin, 1983). Yet, rarely more than 70% of matings result in a positive pregnancy diagnosed 30 days later and even fewer result in a live birth. Embryonic losses are defined as those losses that occur from fertilization until day 42 of pregnancy when differentiation and implantation has occurred. Losses after day 42 are generally referred to as fetal losses. Embryonic losses are further divided into two categories and classified as Early Embryonic Mortality (EEM; fertilization to day 27) and Late Embryonic Mortality (LEM; day 28 to 42). The majority of embryonic mortality is early embryonic mortality, with rates ranging from 20 to 44% reported in beef cattle (Humbolt, 2001). Late embryonic mortality occurs in 3 to 14% of beef cows and heifers (Humbolt, 2001; Perry et al., 2005; unpublished data). A wealth of information exists on embryonic mortality in dairy cattle, but very little information exists in beef cattle. Maurer and Chenault, (1983) suggest the type of pregnancy failures among beef cows and heifers differ. In that study, the majority of pregnancy failures in heifers were due to fertilization failure rather than embryonic loss. The embryo or unfertilized egg recovery rate was lower among heifers than cows, suggesting that perhaps a portion of heifers failed to ovulate after being in heat. Accurate measurement of embryonic mortality is complicated by the fact that we are unable to assess pregnancies until approximately day 27 (via ultrasound or pregnancy specific blood indicators) without harvesting and collecting reproductive tracts at slaughter.
No single factor has been proven to prevent early embryonic mortality. However, if we could prevent embryo wastage in just 5 out of every 100 cows, we would wean an additional 2,100 pounds per 100 cows.