Detection of estrus (heat) is often cited as the most costly
component and undoubtedly, the major limiting factor to the success of A.I. programs on many dairy farms. Incorrect detection
of estrus is related to loss of income due to extended calving
intervals, milk loss, increased veterinary cost, increased heifer
rearing cost, and slowed genetic progress. To achieve excellent
heat detection, many factors have to be taken into account. First,
the cow must express behavior and physiological changes, and
secondly, these changes must be detected to determine if and
when insemination should occur. It is clear that an excellent
rate of heat detection is vitally important. Some herds have
exceptional fertility while others struggle with conception rates,
calving intervals, pregnancy rates and other parameters, which
might be caused by inefficient heat detection. Numerous factors,
environmental, managerial, and cow-related, play a role in
estrus expression and detection. Many devices are commercially
available to assist with heat detection and each producer must
decide which works the best in their dairy. The time of ovulation
and age of the egg at sperm penetration is critical for conception,
so the goal of a heat detection program should not merely be to
attain a high detection rate but to achieve a high detection rate
with a corresponding high conception rate.
BEHAVIORAL CHANGES
The occurrence of estrus is due to specific influences of ovarian
steroid hormones on behavioral centers in the brain. As a growing
follicle matures under the stimulation of follicle-stimulating and
luteinizing hormones (FSH and LH) during the last three to four
days of the estrous cycle, it synthesizes and secretes increasing
quantities of estradiol. A threshold level of estradiol is reached
which triggers two closely linked events – the behavioral
response known as estrus and a surge of pituitary hormones,
primarily LH.
It is useful to point out that the maturity of the Graafian
follicle that regulates the amount of estradiol synthesized
regulates its own time of ovulation and concurrent maturation
of the oocyte.
Traditionally, the cow that stands still and allows others to
mount her is in “standing heat.” Standing is the primary sign
of estrus. Ovulation usually occurs approximately 24 to 32
hours after the onset of standing estrus in dairy cows. After
ovulation, there is only a short period when ova can be fertilized
(Figure 1). Optimal fertility of ova is projected to be between six
and 12 hours after ovulation. The viable life span of sperm in the
reproductive tract is estimated at 24 to 34 hours.
The average period of “standing heat” is usually less than
10 hours and consists of about one standing event per hour.
Secondary signs of heat may be an indication that the cow may
soon display standing estrus, is currently in estrus, or has already
gone out of standing heat. Since the degree of these advanced
signs vary in length and intensity, a combination or having
multiple secondary signs increases the reliability of the decision to
breed. It can also help to increase the amount of cows submitted to
A.I. by allowing the insemination of cows that cannot be detected
otherwise.
Secondary signs are caused by elevated levels of estrogen on
the day the animal is in heat. They are also most likely caused by estrus-related activities and events. The primary sign of estrus
is a cow standing to be mounted by another cow(s) even though
she could have resisted the mounting activity. The reason why
secondary signs of estrus get this name is they can also be
caused by events other than estrus and are secondary to standing
to be mounted as the definitive sign. Therefore, a single secondary
sign of estrus should not be enough to make the decision to
inseminate; if a cow is not seen standing to be mounted and is
suspected in estrus, it will be necessary to have a combination of
secondary signs to confirm that the cow is really in estrus. Below
are some of the most commonly recognized secondary signs of estrus.
• Rubbed marks – When a cow dismounts another, she slides
down her tail head and rump. Therefore she puts
considerable pressure on the pin bones and backbone, and
this repeated abrasive action pulls out hair and may produce
red, bloody or swollen sores. Size, appearance, and freshness
of these marks along with the fact that few other events
can cause similar signs; these rubbed marks are one of the
most reliable secondary indicators of estrus. Additionally,
you may find that the flanks of the cow in question have dirt
or manure marks from the hooves of other cows riding them
and are another indication that riding events have recently
happened.
• Mucus – Many technicians would state that mucus is the
most liked secondary sign of estrus. Experienced
inseminators put a profound credence on this sign when
deciding to breed or not to breed a cow, and sometimes they
even massage the cervix and anterior vagina to express a
mucus discharge. Discharged mucus dries quickly so finding
dried mucus on the tail, flanks, or legs is just as good a
secondary sign as seeing a string of clear viscous mucus
coming from the vulva.
• Sweaty appearance – Some cows will develop a sweaty
appearance when in estrus. This “wet” appearance, even
though it is easily detected, is frequently overlooked and may
be where the slang term “hot” cow originated.
• Swollen vulva – Rapidly growing follicles produce high
circulating levels of the hormone estrogen that increase
blood flow to the reproductive tract. The vulva increases in
size and takes on a pinkish swollen appearance. Upon
opening the labia an intense dark pink to red and highly
moistened vagina is present if the cow is in estrus. In
contrast, the vagina will appear dry and pale to white in color
when the cow is not in estrus.
• Chin resting – Chin resting is thought to be testing by herd mates to determine if an individual is receptive to being
mounted. This testing is performed by first resting a chin on
the back of the cow. Considerable salivating and licking
usually takes place during this testing process so you should
inspect the loin and tail head area for saliva.
• Bellowing and urination – There is a tendency for increased
urination. The cow will begin holding her ears erect, become
restless and nervous. Cows coming into heat will become
more active and will spend more time walking around
rather than lying down chewing their cud. Be aware that
during movement, like to and from the milking parlor, is an
ideal time for a cow to mount another and therefore a great
time to detect mounting activity. Some cows may appear to
be standing when in fact they just couldn’t get away because
of crowding. Extra care should be taken to avoid false
positives like this one.
• Bloody discharge – A streak of blood in the mucus usually
means that that cow had a high peak of estrogen one to three
days ago. It is therefore recommended to record that heat
and date it two days ago. This only indicates that she has
been in heat. It has no relationship with timing of ovulation
or whether or not she conceived.
• Grouping – Cows in heat tend to look for willing partners to
get involved in estrus-related activities. These sexually active
groups are a clear indication that at least one cow inside one
of these groups is in estrus.
FACTORS THAT INFLUENCE DETECTION
Cow factors
• Heritability – The heritability of estrus expression is very
low and varies between cows, even for the same cow, from
one estrus period to another. Just because a cow is very
active today does not mean she will be at her next estrus
period during this lactation or in subsequent lactations.
However, there are differences between breeds. In general,
Jersey cows and heifers have more intense and longer
periods of estrus expression than Holstein cattle.
• Days in milk – Silent heat (more correctly, silent ovulation),
is common at the first ovulation after calving. Progesterone
released from the corpus luteum (CL), formed after the silent
ovulation, appears to favor estrus expression during the next
cycle.
• Lactation number – A report from Spain in 2006 revealed
a 21 percent decrease in walking activity with each
additional lactation. A study from the United Kingdom in
2009 reported a significant increase in walking activity
during estrus for heifers versus first lactation cows, and a
significant decrease between first and later lactations, but
no difference in walking activity during estrus for cows
between second and later lactations.
• Milk production – There is no correlation between estrus
expression and milk yield; however, the metabolic clearance
of steroid hormones related to high milk production probably
reduces behavioral expression of estrus. In a study with 267
lactating dairy cows, cows with daily milk yield greater than
57 lbs. (39.5 kg) per day had lower blood estrogen levels and
shorter duration of estrus than herdmates producing less
than 87 lbs. (39.5 kg) of milk daily.
• Lameness – Lameness is classically associated with a
reduction of estrus intensity. Lame cows spent more time
lying and less time standing and walking during estrus. One
study reported an overall reduction of approximately 37
percent in estrus intensity for lame cows.
• Hormonal treatments – Progesterone increases the
sensitivity to estrogen and usually results in increased
estrus expression, specifically mounts, chin resting, and
sniffing (which may explain why after removing a CIDR®
insert, increased estrus expression is common). In contrast,
Gonadotropin-releasing hormone (GnRH) reduces or
suppresses the expression of estrus by causing the early
ovulation of the developing follicle prior to peak estrogen
levels that prompt the expression of estrus.
Environmental factors
• Season – Most studies have reported a depression in estrus
expression during extreme temperatures, either hot or cold.
The inability to have a period of recovery from high
temperatures during the day has also been reported to
negatively affect estrus behavior. Heavy rain, strong wind,
and high humidity also reduce or suppress estrus behavior.
• Nutrition – Loss of body reserves (negative energy balance)
can negatively affect estrus expression. The presence of
mycotoxins, especially vomitoxin and zearalenone, reduce or
suppress estrus expression.
• Housing – The type of floor surface affects estrus behavior.
Duration of estrus and number of mounts were longer (13.8
versus 9.4 hours) and greater (7 versus 3.2 times) on dirt
than on concrete surfaces. Covering a concrete grooved
floor with perforated rubber mats improved the ability of
cows to express normal behavior activity.
• Herd size and over-crowding – The number of social
interactions between cows is greater when the herd size is
larger. However, over-crowding will reduce expression
of estrus by limiting the space available for socially active
groups to form and interact. The degree of estrus expression
and therefore, the possibility of detection, can be
dramatically favored by the number of cows in estrus
simultaneously. This is the reason PGF injections work to
increase detection, especially prior to first insemination.
Each additional cow in heat at the same time has been
associated with a six percent increase in walking activity.
Records
A good record keeping system is one of the most valuable
tools in any detection program, mostly because it will increase
the accuracy of your decisions. All heats must be recorded even
if the cow is not bred at that heat. The pivotal question is when
the last insemination occurred. Having an interval of 18 to 24
days makes the decision to inseminate easier. Breedings that
occur with an estrus interval of four to 16 days usually result in
less than desired conception rates and should be avoided unless
the secondary signs strongly sway the decision to inseminate.
Also knowing if the previous breeding was a result of timed A.I.
or from a standing event may alter the decision. These off-cycle cows could be palpated for the presence of a clear stringy
mucous as the final and definitive secondary sign to confirm the
decision to inseminate.
TIMING OF INSEMINATION RELATIVE TO ESTRUS
For the past 65 years, researchers have investigated the
optimal time at which to inseminate cows relative to the stage
of estrus. 
Trimberger (1948) found that conception rates were
highest when cows were inseminated between six and 24 hours
before ovulation. This early work led to the establishment of the
a.m./p.m. recommendation. This guideline suggests that cows
in estrus during a.m. hours should be inseminated during the
pm hours, and cows in estrus in the p.m. should be bred the
following a.m. However, research with large numbers of cows
indicates that maximum conception rates may not be achieved
using the a.m./p.m. recommendation. A large field trial (44,707 cows) found no difference in the
percentage of non-return rates at 150 and 180 days (which
would indicate pregnancy) between cows bred either the same morning as observed estrus, between noon and 6 p.m. on the
day of observed estrus, or cows bred the following morning after
observed estrus the previous evening. This indicates that a
single mid-morning insemination for all cows observed in estrus
the night before or the same morning should yield acceptable
conception. Also, cows bred once daily (between 8 a.m. and
11 a.m.) had similar non-return rates as cows bred according
to the am/pm guideline. When knowing the onset of standing
behavior, research suggests that cows be bred earlier than the
a.m./p.m. guidelines. Highest conception rates for A.I. occurred
between four and 12 hours after the onset of estrus (Table 1). Cows inseminated 16 hours after the onset of estrus had lower
conception rates than cows bred between four and 12 hours after
the onset of estrus.
The effects of interval to A.I. on conception rates of dairy cows
(4,126 breedings) having high activity using the SelectDetect™
activity system were consistent with similar studies based on
observed mounting activity (Figure 2). he mean duration of
high activity was 10.5 hours with a median of 10 hours. Among
first lactation cows (Figure 2), optimum conception occurred
at A.I. intervals of eight to 16 hours after the onset of high
activity and trended lower for both earlier and later A.I. intervals.
Among second lactation and older cows, conception rates were
similar until 16 hours after the onset of high activity. Optimum
conception rates were obtained at A.I. intervals proximal to 12
hours after detected estrus with shorter intervals appearing
to be less compromising to conception rates that are of longer
intervals.
WHEN SHOULD DAIRY COWS BE INSEMINATED?
The traditional a.m./p.m. recommendation works best with
twice daily observations but may not provide the best conception
rates because several cows will be bred too long after the
onset of estrus, so the chance for successful fertilization may
be missed. The exact onset of estrus is usually unknown. For
example, according to the a.m./p.m. guideline, a cow beginning
estrus at 1 a.m. and observed in estrus at 6 a.m. would be bred
approximately 18 hours after the onset of estrus. Breeding cows
at this time would reduce the number of cows that become
pregnant (Table 1). Cows should be inseminated within four to
16 hours of observed estrus when the precise onset of estrus is
known (Figures 1 and 2). If estrous detection is conducted twice
daily, most cows should be within this time period. However,
a single mid-morning insemination of cows that have been
observed in estrus the same morning or the previous evening
should provide acceptable conception rates.
SUMMARY
Traditionally, the cow that stands still and allows others to
mount is in “standing heat.” Standing is the primary sign of
estrus and determines the time of insemination since ovulation
occurs 25 to 30 hours after the onset of standing activity.
Secondary signs of estrus may be an indication that the cow
may soon display standing heat, is standing now, or has already
gone out of standing heat. Since the degree of these advanced
signs vary in length and intensity, a combination or having
multiple secondary signs increases the reliability of the decision
to breed. Making the decision to inseminate often will require the
use of secondary signs of estrus. There is a fine line that will only
come with experience when using secondary signs to make the
decision to breed or not to breed. Because of biological variation
in the time of ovulation in respect to the onset of estrus, sperm
transport time in the female reproductive tract to the site of
fertilization, and life span of both gametes (sperm and ova) there
is a broad window for the optimum time of A.I., of approximately
12 hours. The key component to timing of A.I. is frequent and
accurate observation periods to determine the onset of estrus.
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