Managing Reproductive Disorders in Dairy Cows
Early Embryonic Loss
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, 1997; Fricke et al., 1998), whereas conception
rates in dairy heifers are nearly 75% (Pursley et al., 1997). 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.
Timing of Early Embryonic Loss
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 from 28 days post AI to calving
in lactating dairy cows (Vasconcelos et al., 1997). Pregnancy diagnosis was
conducted at 28, 42, 56, 70, and 98 days post AI for 1,600 dairy cows in three
herds with a rolling herd average >23,000 pounds. The conception rate of
cows at 28 days post AI was 32%, and overall pregnancy loss from day 28 to
calving was nearly 25%, with most losses occurring during the first 60 days of
gestation (Figure 3).
Factors Affecting Early Embryonic Loss
Because fertility assessed at any point
during pregnancy is a function of both conception rate and pregnancy loss,
factors associated with pregnancy loss may be similar to those responsible for
low fertility. Nutrition can have a major impact on dairy cow fertility. A
recent review (Ferguson, 1996) indicated that nutritional causes of low
fertility are more like first due to energy management, second to excessive
protein feeding, and third to trace element and vitamin deficiencies. In
addition, greater body condition score losses from calving to breeding result
in reduced fertility (Ferguson, 1996).
Specific physiologic mechanisms
responsible for pregnancy loss in lactating dairy cows are unknown, but may
include lactational stress associated with increased milk production (Oltenacu
et al., 1980; Nebel and McGilliard, 1993), negative energy balance (Butler and
Smith, 1989), toxic effects of urea and nitrogen (Butler et al., 1995) or
reduced ability to respond to increased environmental temperature (Stevenson et
al., 1984; Hansen et al., 1992). Beef cows losing weight have a higher
incidence of early embryonic loss than those gaining weight (Dunn and Moss,
1992). This suggests that negative energy balance may be involved when a high
incidence of early embryonic loss is observed in dairy cows. Recommendations
for minimizing the severity of negative energy balance in high-producing dairy
cows include maximizing dry matter intake in early lactation and feeding diets
containing .78 Mcal NEl per pound and 5%-7% total fat (DM basis).
High circulating urea and ammonia from
the feeding of diets high in degraded intake protein (DIP) may adversely affect
early embryonic development (Butler, 1998). Further, the feeding of excess DIP
may exacerbate negative energy balance and related reproductive problems. Elrod
and Butler (1993) reported increased early embryonic loss for heifers fed an
energy-restricted diet containing high levels of DIP. Ferguson (1996) indicated
that cows fed high amounts of DIP showed more irregular intervals between first
and second service. Dietary DIP should be restricted to 10%-12% (NRC, 1989; DM
basis). Also, nitrate concentrations in water and forages should be evaluated
when herds are experiencing a high incidence of abortions or early embryonic
loss (Davison et al., 1965).
Reproductive 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).
There are several reasons that
transrectal ultrasound is not widely used among bovine practitioners for
pregnancy diagnosis at present. First, ultrasound machines are relatively
expensive, costing between $15,000 to $20,000. Second, most ultrasound machines
are large and require an external power source thereby making them cumbersome
to use under field conditions. Because of these factors, use of ultrasound has
been restricted research or specialized procedures such as fetal sexing, transvaginal
oocyte recovery, or embryo transfer. Fortunately, several companies are
currently marketing newer generations of ultrasound machines that are cheaper,
smaller, and battery operated. Continuation of this trend will foster future
use of this technology by bovine practitioners.
There are two main caveats to using
ultrasound for routine early pregnancy diagnosis in a dairy herd. First, when
using ultrasound for early pregnancy diagnosis, emphasis must be given to
identifying 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). Second, 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, or a combination of
both methods. Unfortunately, the service rate was only 58% when using a system
combining PGF2a and Kamar heat mount detectors (Britt
and Gaska, 1998), probably due to the inherent inefficiencies of estrus
expression and detection in lactating dairy cows.
An attractive strategy for managing
reproduction in a dairy herd would combine use of synchronization of ovulation
and timed AI (Ovsynch), an estrus detection aid, and early pregnancy diagnosis
using ultrasound. Every two weeks, groups of cows past the voluntary waiting
period would receive their first postpartum insemination after synchronization
of ovulation using Ovsynch. This would dramatically reduce median days to
first AI by eliminating estrus detection for the first postpartum breeding.
At the time of AI, an estrus detection aid such as a Kamar device or estrus
detection tail paint would be applied to the cow. This would aid in detection
of cows that return to estrus between 18 to 28 days post AI due to failure
of conception or early embryonic loss. Cows detected in estrus during this
period could then be inseminated based on the detected estrus. At 28 days
post AI, a veterinarian using ultrasound would identify any nonpregnant cows,
which would be scheduled for resynchronization using Ovsynch along with the
next group of cows. This would eliminate reliance on estrus detection for
the next breeding, thereby reducing the interval from pregnancy diagnosis
to rebreeding. All cows diagnosed pregnant at 28 days post AI would be scheduled
for a second ultrasound examination at 56 days post AI to determine if pregnancy
loss had occurred. This is an aggressive reproductive management system that
would improve reproductive efficiency by maximizing AI service rate in the
herd. This is accomplished through use of early pregnancy diagnosis using
ultrasound. Although estrus detection would not be completely eliminated using
this system, it would be minimized through the use of Ovsynch and timed AI.