SELECTIONS - Summer 2004
What’s Your Voluntary
Waiting Period and Why?
By Mel DeJarnette, reproduction specialist
After calving, cows require time to complete normal
uterine involution before they can be expected to re-breed.
This period of time after calving during which we refrain
from breeding cows even if they show heat is referred to
as the voluntary waiting period (VWP).
Over the years, there has been a trend towards shorter
and shorter VWPs and many herds routinely breed cows as early
as 40 days after calving. What has caused this to occur and
is this really more profitable dairy management?
The VWP is important because it is a sizeable portion
of the overall calving interval and is the "easiest" portion
for the producer to control. Influencing conception rates or
heat detection efficiency is inherently problematic, but
breeding cows earlier is an "easy" procedure to implement.
Thus, short VWPs appear to be largely a "side effect" of
poor reproductive performance. Producers often find it
difficult to pass up a heat detected before the end of the
VWP because they are afraid the cow may not be detected the
next time she comes into heat. This is supported by DairyMetrics
benchmarking analysis, which indicates that in the average
Holstein herd in the United States, only about 50 percent of
cows receive their first breeding between the VWP and 100 days
in milk. Efforts to compensate for the 30 percent of the cows
that do not receive their first breeding until more than 100 days
in milk is likely the primary reason approximately 20 percent
are bred before the VWP.
Another factor that may have contributed to increased
use of short VWPs is the follicular memory theory. This theory
explains how negative energy balance can impact reproductive
capacity for weeks to months after positive energy balance has
been restored. However, there are several ways to interpret how
this information should be "applied" in our efforts to improve
dairy herd reproductive performance. The first is to pay greater
attention to nutrition and body condition at all stages of
production in order to minimize the effects of negative energy
balance on reproduction. The second is to shorten the VWP and
breed cows to early heats produced by follicles that initiated
growth while cows were in a more positive energy state. The latter
is obviously the easiest to implement and appears to have
received the greater emphasis even-though the former route would
likely have the greatest impact.
One of the most important factors in selecting the VWP is
the fertility potential of the herd. Conception rates are basically
zero for the first week after calving and rapidly rises over the
next several weeks as the uterus returns to its normal size and the
cow resumes normal estrus activity. However, the amount of time
required for cows to return to their optimum or maximum level of
fertility can vary greatly from herd to herd depending on a
multitude of environmental and management factors (Figure 1).
In the average herd (Herd B), cow fertility is still on an upward
trend until 65 to 70 days postpartum and plateaus at a high level
thereafter. In less-than-optimal conditions, the interval required
to return to optimum levels of fertility may be extended and the
maximal fertility potential may be greatly diminished (Herd C).
Well-managed herds (Herd A) may achieve optimal levels of
reproductive performance very early postpartum.
Among the example herds in Figure 1, only Herd B would notice
an appreciable difference in conception rates between 45-day and
60-day VWPs. Conception rates in Herds A and C would be nearly
identical at short and long VWPs, but for very different reasons.
Herd A, which has an outstanding management program, achieves high
levels of fertility very early postpartum. The most important question
for Herd A is not "can I breed cows early," but rather, "do I really
want to?" What might be the economic implications of getting too many
cows pregnant too early? In contrast, cows conceiving too early are
the least of concerns for Herd C. Conception rates at 45 and 60 days
are nearly identical because management is limiting. Cows are equally
"infertile" at both VWPs. The question for Herd C is not "can I breed
cows early," but rather, "what can be done to achieve a fertility
potential more like that of Herds A and B?"
Short VWPs will allow some cows to conceive earlier after
calving, which may help to tweak some measures of reproductive performance,
but early breeding in itself is not going to “fix” anyone's reproductive
problems. In fact, it’s usually mathematically impossible for a herd
with poor reproductive performance to achieve their desired calving
interval simply by shortening the VWP, because it usually means they
would have to start breeding cows before they calved.
Another consideration in selecting the VWP is the level of milk
production in the herd and/or individual cow. Many producers have found out
the hard way that they really don't want cows to conceive at 40 to 45
days in milk if it means drying them off while they are still milking
80 to 90 pounds a day. A recent study (Tenhagen et al., 2003) evaluated
the effects of milk production and VWP on conception rates at Ovsynch
timed A.I. At five weeks postpartum, cows were categorized as below
average (less than 55 pounds), average (55 to 77 pounds), or above
average (more than 77 pounds) daily milk production. All average
production cows were bred to Ovsynch between 73 and 81 days in
milk. Among the below average production cows, half were scheduled for
Ovsynch at 73 to 81 days while the other half were scheduled for Ovsynch
three weeks earlier (53 to 59 DIM), with the logic that it may be more
profitable to get the below average producers pregnant earlier. Likewise
among the above-average-production cows, half received Ovsynch A.I.
at 73 to 81 days in milk while the others were not scheduled for
Ovsynch A.I. until three weeks later (94 to 102 DIM), with the
logic that it may be more profitable to extend the VWP and calving
interval in these higher-producing cows. Conception rates for first
services were lower at the shorter VWP in both the above and below
average production groups (Table 1)
and there was actually a numeric
tendency for longer days open in response to the shorter VWP within
the below average production group. This effect is better visualized
by plotting cumulative pregnancy rates for each VWP within each
production group (Figures 2 and 3). Some cows did conceive earlier
in response to the shorter VWP; however, among those cows that did
not conceive, early timed A.I. closed the door on alternative
reproductive therapies until the cows either returned to estrus or where
diagnosed open. Therefore, the later VWP resulted in a crossover
effect wherein greater conception rates at the first A.I. resulted
in greater cumulative pregnancy rates from day 75 to approximately
150 days in milk and it could be argued that the shorter VWP
actually reduced reproductive performance.
With these concepts in mind, it may be time for many producers
to reevaluate their VWPs. Why is it, where it is and is it really working?
It's very important to keep in mind that during the transition to
short VWPs, a new and highly effective VWP management technology has
become available: Ovsynch. Producers no longer need to be afraid of passing
up a heat as both the first and all subsequent breedings can be precisely
scheduled on the day of choice after calving and/or open diagnosis. Producers
should no longer fear that some cows will be greater than 100 days in milk
at their first breeding. Although that still happens in far too many herds,
it does not and should not have to be. One of the most important questions
a producer should now consider is: "If I am going to the trouble and expense
of synchronizing cows for their first breeding with Ovsynch, what should my
VWP be to ensure I am optimizing my return on investment?" Some studies seem
to imply most herds should target cows for timed breeding in the 65 to 75 day
range. That’s not to say a producer should disregard natural heats occurring
45 to 65 days after calving; however, he or she must just be careful not to induce
a high percentage of the herd to be time bred before they are ready. Otherwise,
time breeding with a VWP that is too short could actually backfire and
reduce reproductive performance while wasting hormones, semen, money, and most
importantly, time. Producers opting for short VWPs should carefully reconsider
the reasons they have chosen to do so, as well as the actual level of success
being achieved. It may be more profitable to wait.
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