The regular creation of pregnancies is essential to providing the steady stream of fresh cows necessary to maintain a good milk production average along with the heifer calves that will become the future replacements for a dairy operation.
A common measurement of reproductive performance on dairies is Pregnancy Rate which measures the percentage of pregnancy eligible cows that actually become pregnant during a 21-day heat cycle. The two factors driving Pregnancy Rate are Service Rate (or Heat Detection Rate) and Conception Rate. A simple way to calculate Pregnancy Rate is to multiply the Service Rate x Conception Rate.
Although the national average is estimated to be around 15%, many herds consider a Pregnancy Rate of 20% a desirable and attainable goal. Of course the ultimate success of a repro program is not measured in percentages but in the hard number of pregnancies created. Creating a pregnancy starts with heat detection or service rates.
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Service Rate (SR) – The total number of cows inseminated divided by the total number of Pregnancy Eligible Cows during a 21-day cycle. The term Heat Detection Rate is often used interchangeably with Service Rate. Service Rate is perhaps a more accurate term as it includes all cows inseminated, including synchronized cows that were not actually detected in heat.
Conception Rate (CR) – The total number of cows that become pregnant divided by the total number of cows inseminated during a specific time frame.
Pregnancy Rate (PR) – The total number of cows that become pregnant divided by the total number of pregnancy eligible cows during a 21-day cycle.
Service Rate or Heat Detection Rate is often the greatest limiting factor of the equation. A high Conception Rate is impressive but must be complemented by an adequate Service Rate if the ultimate goal of a high Pregnancy Rate is to be achieved. Before a cow is inseminated we must be reasonably sure she is in heat or trust in a synchronization program to cause her to ovulate at the right time if we time-breed her.
Among the factors that can contribute to a poor Service Rate:
- Inadequate heat detection skills: A technician lacking good heat detection skills may miss some subtle signs of heat and breed fewer cows resulting in a low service rate. Conversely, an inadequately trained technician may achieve a good service rate by breeding many cows that are not actually in heat. This will be reflected in low conception rates but is really a result of poor heat detection technique. Insemination is a process that can be learned but heat detection is more of an art requiring the development of keen observation skills.
- Noncycling cows: Nutritional, metabolic and disease issues along with a premature Voluntary Waiting Period may result in a high number of noncycling cows. According to Fertility Specialist, Dr. Arun Phatak, “In the average herd about 10% of the cows will not be cycling and show any signs of heat until about 75 days in milk. If the number exceeds 10% then nutrition and intakes need to be examined. Also, first lactation cows need their own pen.” The consequence of higher numbers of noncycling cows will be lower heat detection rates along with lower conception rates for cows bred by appointment.
- Inadequate facilities: Overcrowded pens and slippery concrete may limit mounting and other estrus activity. Inadequate lock-up capacity and poor lighting may also hamper a technician’s ability to evaluate cows for signs of estrus.
To demonstrate the importance of Service Rate, commonly observed performance ranges for Service and
Conception Rates are listed below along with possible combinations and resulting Pregnancy Rate outcomes.
Common Range for Conception Rate= 25%-40+% | Common Range for Service Rate= 35%-70% (See Table 1.)
Compared to service rate the performance range for conception rate is relatively lower and narrower, thus it has less potential impact on the final Pregnancy Rate. The wider and higher range for service rate has more potential to affect the final Pregnancy Rate.
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The economic returns are dramatically different under the different scenarios too. At an estimated $10 per point of improvement in PR, we can estimate the payback per cow. (See Table 2.)
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Except for the rare situation of herds that breed their cows entirely by appointment, heat detection skills are critical for achieving a desirable Service Rate. One of the more popular methods of heat detection for large herds involves tail chalking and evaluating cows while walking behind them during a daily lock-up period. Rubbed off cows have a high probability of being in heat but according to Phatak, only 40-50% of cows will be mounted long enough to completely rub off the chalk. Therefore the ability to recognize the more subtle secondary signs of heat is critical in order to find those extra cows to breed and ensure an adequate Service Rate.
Synchronization programs can be an excellent tool for increasing service rate if they are well managed. Strict compliance to protocols is absolutely necessary for positive results. To save on drug expenses and maximize conception rates the majority of herds using a synchronization program will breed any cow that shows an active heat (assuming she is past her VWP) and remove her from the program. Therefore heat detection skills are also critical for this scenario.
A less common but interesting method of heat detection involves the use of an activity monitoring system that predicts estrus by measuring an increased activity level of a cow compared to her norm. Pedometers or neck mounted sensors count the number of steps taken by a cow at regular intervals and downloads the data to a computer program. When cows exhibit an increased level of activity they show up on a ‘hot cow’ list. Activity systems work very well on some dairies but not all. Research and visits to farms using the technology are recommended for those considering a purchase of an activity system.
If one of the goals for your dairy operation is improved reproductive performance, make sure and evaluate the number of cows you are breeding each month. Monitoring only CR, gives only half the picture. After all, the conception rate for a cow not inseminated is always zero!
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Source: AltaGenetics
