Understanding the Key to Successful Reproduction
Early Embryonic Loss in Lactating Cows
Pregnancy
loss contributes to reproductive inefficiency in lactating dairy cows because
fertility assessed at any point during pregnancy is a function of both
conception rate and pregnancy loss. Conception rates at 28 to 32 days post-AI
in lactating dairy cows range from 40 to 47% (Pursley et al, 1997b; Fricke et
al., 1998), whereas conception rates in dairy heifers are nearly 75% (Pursley
et al., 1997b). Similarly, pregnancy loss in lactating dairy cows is greater
than that in dairy heifers (20% vs. 5%; Smith and Stevenson, 1995). Although
the specific factors responsible for early embryonic loss in dairy cows are not
known, they may be similar to those factors responsible for reduced conception
rates.
Early
embryonic loss in cattle is difficult to study because no sensitive test
similar to that used for women and mares exists. The fertilization rate after
AI in beef cows is 90%, whereas embryonic survival rate is 93% by Day 8 and
only 56% by Day 12 post AI (Diskin and Sreenan, 1980). In dairy cattle, only
48% of embryos were classified as normal on Day 7 after AI (Weibold, 1988).
Thus, substantial pregnancy loss probably occurs within two weeks post AI.
Rectal
palpation from 40 to 60 days post AI is the most common method of pregnancy
diagnosis in dairy cattle. Several studies have used pregnancy diagnosis based
on rectal examination to establish a conception rate from which pregnancy loss
can be determined as gestation ensues. Using this technique, pregnancy loss is
about 10%, with greater losses in lactating cows compared with heifers
(Thurmond et al., 1990; Markusfel-Nir, 1997). Furthermore, the risk of
pregnancy loss was more than four times greater during the first compared with
the second and third trimesters of gestation (Markusfel-Nir, 1997).
Recently,
transrectal ultrasonography was used to determine the timing of pregnancy loss
in lactating dairy cows (Vasconcelos et al., 1997; Fricke et al., 1998). Of
cows diagnosed pregnant at 28 days post AI, 14 to 16% experience early
embryonic loss by 56 days post AI. This rate of loss decreased dramatically
after about 56 days post AI.
Management of Early Embryonic Loss
At
present, there is no practical way to reduce early embryonic loss in lactating
dairy cows. However, recognizing the occurrence and magnitude of early
embryonic loss may actually present management opportunities by taking
advantage of new reproductive technologies that increase AI service rate in a
dairy herd. One such technology is the use of transrectal ultrasonography for
early pregnancy diagnosis. If used routinely, transrectal ultrasonography has
the potential to improve reproductive efficiency within a herd by reducing the
period from AI to pregnancy diagnosis to 26 to 28 days with a high degree of
diagnostic accuracy (Pierson and Ginther, 1984). Furthermore, use of ultrasound
could minimize embryonic loss that may occur after manipulation of the
reproductive tract and conceptus during pregnancy diagnosis using rectal
palpation (Paisley et al., 1978; Vaillancourt et al., 1979).
When
using ultrasound for early pregnancy diagnosis, emphasis must be placed on
identification of nonpregnant rather than pregnant cows. Of cows diagnosed
pregnant at 28 days post AI, 14 to 16% experience early embryonic loss by 56
days post AI (Vasconcelos et al., 1997; Fricke et al., 1998). Therefore, cows
diagnosed pregnant at 28 days post AI using ultrasound should be scheduled for
reexamination around 56 days post AI, when the rate of embryonic loss per day
begins to decline (Vasconcelos et al., 1997; Figure 3). Once nonpregnant cows
have been identified, a management strategy must be developed to return the
nonpregnant cows to service as quickly as possible after pregnancy diagnosis.
Such strategies include administration of PGF2a to cows with a responsive CL, use of
estrus detection aids, synchronization of ovulation and timed AI or a
combination of both methods.