Basics of reproductive function

Efficient management of the reproductive process is critical to overall dairy herd profitability and productivity. A thorough understanding of reproductive biology is essential to developing an effective reproductive management system. Maximal reproductive performance depends on high rates of heat detection and high conception rates. Effective heat detection requires a focused, structured program. Such a program can be developed using a variety of management tools.

It is well known that the herd’s breeding efficiency impacts the profitability of a dairy farm. The farmer’s goal of have a calving interval (CI) of 12 months applies everywhere in the world, irrespective of the production system used. Even in the US, where the use of BST is widespread, economic studies confirm that maximum profit is reached when the CI is kept between 12 and 13 months.

The main reason for its high impact on farm profitability is that CI directly affects three key areas:

  • Milk production
  • Dry cow days
  • Replacements

The first is related to the milk production curve, which normally describes a parabola that has its maximum at the beginning of the lactation (peak about 90 days) and then declines slowly according to the animals and the feeding strategy of the herd.

Thus, the more peak production periods in the productive life of a cow, the greater the total cumulative milk production. Furthermore, this production is efficient thanks to the good feed cost/milk production ratio that characterises the first part of the lactation. To achieve as many peak lactations as possible, the CI needs to be shortened to 12 to 13 months.

The second area is related to the fact that cows with longer calving intervals tend to dry off. Despite cows compensating by having more days in milk, these dry periods risk being longer than the standard 60 days, which means an unnecessary cost for the system. The table below shows that the average number of dry days increases with longer calving intervals.

Calving interval (months)

Days in milk

Days dry


297 60


305 60
















The third aspect is that long calving intervals mean fewer calves in a year. This affects the replacement possibilities. There is less chance to select heifers for repopulation and there will be fewer sales from the farm.

The table below shows, as a percentage basis, the numbers of calves obtained by farmers with different calving intervals.

Calving interval (months)

Calves obtained 

per year (%)















Research has shown that sub-optimal calving intervals cause large economic losses to dairy farmers, second in importance only to mastitis. Therefore, successful reproductive management has a significant impact on the herd’s overall performance and the net income.

Average Calving Interval (Days)

Production Level (kg milk/cow)

 - 4499

4500 - 5499

5500 - 6499

6500 - 7499

7500 -

< 365






365 377






378 392






393 408






> 409






Table 3:
Distribution of controlled Swedish herds in relation to calving interval and production level (Source: SHS)*
* SHS: Swedish Association for livestock, breeding and production

Well-managed reproduction also reduces the risk of expensive involuntary culling. Information from UK and USA shows that the cost for a one day extended calving interval ranges from £1.5 to £3 per cow ($2.00 - $4.00 US). The portion of the calving interval that it is possible to influence by management is the open days, which are determined by the voluntary waiting period (VWP) and the breeding window (BW). A very common reason for undesired long calving intervals are missed heats. With improved heat detection rate (HDR) and conception rate (CR), through better management and improved timing of inseminations, it is possible to obtain a significantly shorter calving interval.

Factors affecting the calving interval

The Estrus Cycle

As long as a cow or heifer is not pregnant she will normally have a 21-day estrus cycle. The length of the estrus cycle may vary but it usually ranges from about 17 to 24 days. A heifer’s estrus cycle is normally slightly shorter than a cow’s. The cycle will continue until the cow is pregnant. After calving, cows normally undergo a 20 to 30-day period when estrus cycles do not occur.

The estrus cycle is controlled by a complex system involving different hormones produced in the brain and ovary. The figure below shows a simplified picture of how two of these hormones, estrogen and progesterone, vary depending on where in the cycle the cow is.

The Estrus Cycle

Some cows do not follow the normal estrus cycle. For example, a cow can be anestrus, which means her ovaries do not function with the regular 17 to 24-day cycle and she is therefore not observed in heat. Other cows may suffer from ovarian cysts. These cows will show heat at very short intervals and the period that they are in heat will last three to four days.

(See Figure The oestrus cycle)

Detecting Heat

Heat detection and accurate insemination are very important links in the chain to reach high breeding efficiency.

Several studies from around the world show that pregnancy rates (PR) have decreased during the last 50 years. In the US, the PR per lactating cow decreased from 66% in 1951 to about 40% in 1997, whereas in heifers it has remained at 70% during the same period. This difference is not due to differences in genetic selection or semen quality, but can be associated with physiological changes or stress related to a consistent increase in milk production.

Literature from different parts of the world reflects the problems that farmers have in detecting a high proportion of the heats. Normal figures are around 30 to 70%, and we can say that 50% is a very common situation.

The problem can be explained by looking at some of the changes that have occurred with modern cows.

Heat Duration

Data from cows at the Virginia Tech Dairy Cattle Centre

It was previously proven (Roberts 1986) that a cow’s heat lasts for 15 to 18 hours. However, recent research has shown that this period has been reduced. On average, the heat duration is about 7 to 8 hours (Dransfield et al.1998, Walker et. al 1996). It varies significantly between individual cows, but not between herds.

Heat signs

Data from cows at the Virginia Tech Dairy Cattle Centre

It was thought that cows were mounted on a heat 20 to 55 times per heat. A study in the US shows an average of 8.5 mounts per heat. There were differences between individual cows and breeds Holsteins averaged 7.5 mounts and Jersey 10 mounts, but 50% of Holstein cows had lessthan 5 mounts.

Factors that affect the presentation of the heat signs include:

  • Peak yield: Cows with high milk peaks (above 60 litres) have 30% fewer mounts than cows with lower peaks.
  • Extreme temperatures: Extremes of hot and cold reduce mounting activities.
  • Non-slippery surface: Cows must have a solid footing to mount other cows. Dry ground or pastures are the best alternatives. Grooved concrete can, but does not always, provide secure footing.
  • Interaction with other cows: This is one of the most important factors. Crowded areas like holding pens and feed bunks give few opportunities for interaction. Normally, the detected heats in these situations are false. The ideal area for heat detection is a grassy or hard-packed dirt paddock where cows can move freely.

Because of the reduced frequency of mounting in high producing herds, secondary estrous signs may be used to help identify cows in heat. These signs include:

Increased activity
Walking the fence line
Swelling and reddening of the vulva
Mounting other cows
Lower milk yield
Reduced feed intake

The duration of heat varies from animal to animal, but approximately 10-12 hours after the end of standing heat, the egg is released (ovulation) and the heat ends.

While secondary signs of heat can be useful in confirming estrus, breeding only on these signs should be done with caution. Research suggests that 5-30% of all artificial inseminations in the United States are performed on cows that are not in heat. Part of this problem is due to poor heat detection programs. However, another major factor is poor identification of animals.

Heat Detection Efficiency

As discussed above, much of the reproductive process is driven by the biology of the cow. Since management tools cannot currently override biology, managing for reproductive efficiency depends on taking control of events that can be changed.

For most herds, the most important component of reproductive performance is heat detection efficiency. Heat detection efficiency has both a high impact on breeding success and is “manageable”. Heat detection efficiency is almost completely controlled by the heat detection program. However, programs that lack structure and/or focus are unsuccessful.

Manual Heat Detection

Manual heat detection relies on manual observations in the barn. The cows and heifers should be observed for estrus two to three times per day and all observed heats should be recorded whether the animal is bred or not. These records provide the manager with information to anticipate future heats, which will make it easier to distinguish if a cow is in heat or not. Most mounting occurs between 6 p.m. and 6 a.m., and it is therefore worth trying to check for heat during these hours.

The disadvantage with manual detection is that it is very time demanding and requires people with the ability to observe the right signs. This is particularly important when there are no distinct signs of heat. By using estrus synchronization programs, progesterone testing and other tools on the market, the detection rate can be improved.

Automatic Heat Detection

Mounting activity can also be detected using indicator methods. Pressure activated patches (either mechanical or electronic), tail chalk, and tail paint are all common tools for increasing heat detection efficiency. All of these methods require that cows be observed regularly, but they can decrease the amount of time spent in direct observation.

Another way to identify cows in heat is to monitor their activity. During estrus, a cow’s activity can increase up to 8 times compared to the normal level. Her activity can automatically be recorded by using activity meters attached to the neck or leg. By automating the heat detection process, significant time saving and improved calving interval can be achieved.

Timing of Insemination

With artificial insemination, the timing of the insemination is critical to successful breeding. The optimum time for insemination depends on when ovulation occurs in relation to the heat and for how long the sperm is viable. Sperm requires about 6 hours to travel to the oviduct, which is the site of fertilization. Most sperm remain viable for about 24 hours. As illustrated below, ovulation normally occurs about 30 hours after the start of standing heat. The ovum’s “life” is 8 to 12 hours, but optimum fertility lasts only 6 to 10 hours after ovulation. Therefore, ovum viability becomes the most critical component of the breeding window. Highest conception rates are achieved when the insemination is done 8 to 12 hours after the start of standing heat.

In addition to fertilization rate, insemination timing influences embryo viability. Researchers at Virginia Tech have reported high embryo quality (viability) with insemination occurring at the onset of estrus but low embryo quality when insemination occurs 24 hours after estrus begins.

Traditionally the a.m.-p.m. rule was used to determine insemination timing. This rule dictates that cows and heifers first observed in heat in the morning should be bred late in the afternoon. Likewise, cows and heifers first observed in heat in the afternoon should be bred the following morning. The am-pm guideline is most appropriate with frequent (at 2 to 4 hour intervals) observation of estrus.

With automated heat detection systems (examples: Heat Watch, Mount Count), the first mount of the standing heat period can be identified. As a result, optimal insemination time is narrowed down. For systems that identify estrus onset, insemination should occur 4-14 hours after the first mount.

On many large dairies, estrus observations occur less frequently than 6 times per day (intervals of greater than 4 hours). Under these circumstances, cows should be inseminated about 6 hours after the first observation of standing heat since the onset of estrus is not known.

Many producers have implemented once-a-day insemination programs due to labor constraints. In these systems, cows and heifers first observed in estrus in the afternoon or the following morning, should be bred late that morning. From the discussions above, it is obvious that this system results in a higher risk of missing the optimal time for successful insemination than other insemination timing methods. Producers using once a day insemination would be well advised to examine the true costs (including lower conception rates and higher early embryonic death) of the system rather than focusing only on labor savings.

Related Links:

The Reproductive Status of your dairy herd.
J. F. Smith & D.A. Becker. New Mexico State University.

Benchmarks for Evaluating Reproductive Performance of Herd
R. L. Nebel, Virginia Tech

Reproductive efficiency is still very important!!!
R. L. Nebel, Virginia Tech

Your Herd's Reproductive Status
R. L. Nebel, Virginia Tech