Measuring fertility in dairy cows
Fertility in dairy cows is the ability of the cow to conceive and maintain pregnancy if served at the appropriate time in relation to ovulation. All farmers know that poor cow fertility costs money. Managing fertility is therefore a critical element of profitable dairy farming. It requires regular and accurate measurement of the fertility status of the herd, at least every six months.
Good records facilitate the assessment of fertility by identification of problem areas and measurement of performance against set targets. The following records are required to properly evaluate the fertility status of a herd.
- Calving records,
- AI / service records
- Culling dates,
- Pregnancy test results
Analysis of these records is essential if fertility is to be improved. The following fertility measurements have their own advantages and disadvantages and for that reason it is recommended to use a range of measurements when benchmarking fertility.
1) Non-return rate
This is the percentage of inseminated cows which are not inseminated again for a set period – usually between 30 – 60 days, often 49 days as this covers two potential heats. It is an estimate of the proportion of cows that get pregnant to each insemination.
- Cheap – only requires insemination records
- Quick – fertility data is available within 30 – 60 days
It is inaccurate because it relies on heat detection. Often more than 30% of the cows which are not inseminated again are actually not pregnant
At least 75% - but in problem herds non-return rates can be very high even though pregnancy rates are very low
2) Calving-conception interval (CCI)
This is the average number of days a cow takes from calving down to being successfully inseminated. It depends on how soon cows are inseminated after calving down and the proportion of cows that get pregnant to each insemination. An increase in the former will increase CCI as will a decrease in the latter.
The number of open days is a similar measurement that includes days from calving to culling or death for those cows which do not conceive.
Accurate – based on a positive diagnosis of pregnancy
Simple figure – easy to understand and fits into yearly planning
Recording – reasonable records required for accuracy and pregnancy diagnosis
Management-related – A breeding policy to delay first service increases CCI
A CCI of 85 days results in one calf per year. In seasonal herds this should be the target. In higher-yielding non-seasonal herds a target of 95 days is more reasonable.
3) Calving-to-first service interval
This is the average number of days from calving to first insemination. It is a major factor in CCI; most herds with a poor CCI will also have a long interval to first service.
Prospective – target set for breeding that can be used to identify problems early.
It depends on management where the breeding policy matters. The voluntary waiting period, i.e. the time between calving down and when a cow is eligible to be served for the first time, may depend on such factors as age and milk yield which are not directly related to cow fertility.
An average interval of 65 days, in combination with reasonably successful insemination, results in one calf per year. An average interval of 75 days may suit some systems better. However, it is important that this is an active decision, not just the result of poor management.
4) Pregnancy (conception) rate
This is the percentage of served cows which become pregnant. This needs to include cows which are culled after service. It is usually expressed as a pregnancy rate per service. So, if 100 cows receive 170 inseminations over a 12 month period and 85 become pregnant then the pregnancy rate is 50% (85/170*100). Pregnancy rates to specific services can be calculated for specific services – commonly the first service pregnancy rate is determined.
The inverse of the pregnancy rate is services per pregnancy (conception). In the example above there were 2 services per conception
Accurate – based on confirmed pregnancies
Good records – all services need to be recorded, not just the most recent
Missing data – No account is taken of presentation for AI. You can have very good pregnancy rates but poor fertility because not enough cows are presented for service
Pregnancy rates greater than 50% are achievable, but many farms struggle to maintain averages greater than 40%. Pregnancy rate is probably the hardest fertility measure to improve, so any strategy to increase it should not expect rapid improvement
5) Heat detection assessment
The area where management can have the most impact on fertility is improved identification of cows in heat. Estimating the quality of estrus detection and identifying areas for improvement is essential. Unfortunately there are no measurements that simply and accurately assess heat detection.
Some cows show less intense signs of estrus and for shorter duration. Often these periods tend to occur at night making it difficult to discover if estrus took place or if the cows are cycling at all.
One useful measurement of heat detection is the duration from calving down to the first observed estrus. A target of 80% seen in heat before 50 days post calving down is often recommended. If the figure is significantly less than this then it gives an early warning that things are not as they should be.
The most commonly used measurement of heat detection is submission rate. This is the proportion of cows which are eligible to be bred during a 21 or 24-day period that actually are bred. On well managed farms this should be well over 70%. However, it is easily manipulated by presenting more cows for AI.
Directly measuring estrus detection efficiency is extremely difficult; various estimates are available but all have some measure of error. They are best used when there is some concern that the accuracy of heat detection is low, such as when submission rates are high but pregnancy rates are very low.
6) Composite measurements
Other measurements of fertility have been developed such as the fertility factor, FERTEX score and fertility index. All these aim to include as much information as is reasonable to produce a single figure which can give an overall picture of fertility. A good example of such an index is the In-calf rate. The usual measure quoted is the 100-day in-calf rate. This is the proportion of cows intended for rebreeding that are back in calf again within 100 days of calving. This is superficially similar to the CCI, but it includes cows that are culled and the influence of breeding policy is not so marked. In seasonal herds, the six week in calf rate, the percentage of cows that are pregnant within six weeks of the start of the breeding season is a better figure as this takes into account calving pattern which is important in seasonal herds.
Simple to understand – single figure comparative across farms
Easy to calculate
Heat detection quality included
Requires positive identification of pregnancy
Need to drill down further if problems are identified
Average 100-day in-calf rates are around 50%, but figures over 65% are achievable. Six-week in calf rates average around 60% but 70% is feasible on well-managed farms.