Managing herd fertility is one of the most important considerations of dairy producers. Poor fertility can have severe economic consequences and problems can easily go unnoticed if adequate monitoring is not in place. Measuring fertility in dairy herds is the first step towards successful management practices. To do this, many farmers use a variety of fertility metrics.
Fertility has been widely studied, particularly in recent years. The relatively recent move towards higher yielding cows produced a subsequent decline in fertility. Producers, herd managers, vets and researchers have developed a wide range of different metrics and terms to quantify performance. These help farmers make decisions around fertility, calving and herd management.
A Few Different Fertility Metrics
1. Calving-Conception Interval (CCI) and Calving Index
The calving-conception interval of a cow is the number of days between calving and successful insemination. This is also known as ‘days open’ as the cow is not pregnant (open). Calving index is the average CCI for all cows in a herd.
This index is both easy to calculate and easy to understand. However, it can hide extreme variation between individuals of the same herd. Also you can only calculate it on a historical basis, and so it is a less immediate measure than others. Calving index can also exclude any culled cows from the calculation, which can skew the data. In seasonally calving herds, it is of limited use. In these herds, service usually starts on a specific calendar date, rather than at any particular time after calving.
The average gestation length for a cow is around 280 days. To make sure a cow will calve every year, the calving interval must therefore be around 85 days. It is important to be aware that calving index is an average measure so outliers can have a significant impact and variation is minimised. For example, if half of the cows in a herd had a calving interval of 30 days and the other half at 110 days, this would not be ideal. However, the calving index for this herd would still be 70 days, so would appear to be on target.
2. Conception Rate
This is a measure of how successful insemination is. It is the percentage of served cows which subsequently fall pregnant. For example, if a herd of 150 cows receive 300 inseminations, but only 75 become pregnant, then the conception rate would be 25%.
A poor conception rate can be a sign of poor heat detection, or fertility problems. As it is calculated from herd data, it is not an immediate measurement. As such, farmers can use the conception rate to make changes and plan for the future.
3. Calving to First Service Interval
Similarly to calving interval, this measure counts the number of days between calving and first service. This is usually between 40-80 days for most herds. Lower intervals (less than 50 days) can have a detrimental effect on pregnancy rates. Very long intervals can indicate problems with heat detection, or potential cows in anoestrous.
The length of this interval is dependent on the voluntary waiting period. This is the time between calving and when a cow is ready to be served again. As this, in turn, depends on factors such as milk yield and age, the ideal can vary between cows. Most systems aim for an interval of around 65 days, as, combined with reasonably good conception rates, this produces about one calf a year.
4. Three-week Submission Rate
In a three-week period (21 days), the percentage of eligible cows which receive service is the submission rate. An average heat cycle lasts for 21 days, so in this period, all cycling cows should come into heat at some point. If the three-week submission rate is low, this implies that either cows are not coming into heat, or heat detection is poor.
As farmers can calculate this metric at any time and it only takes a few weeks to measure, it is a good immediate measurement of fertility. Unlike other metrics, it allows quick management decisions and any fertility problems to be spotted early.
Other Fertility Metrics
There are many other fertility metrics which can give farmers an indication of their herd’s fertility. While some rely on historical data, others use current data and provide real-time fertility information. Averages are useful to gather an understanding of the whole herd, but can hide variation. On top of that, genetics and body condition play a huge role in fertility, so awareness of nutrition and breeding is a must.
The best method of fertility management is usually a combination of different measurements and practices. For all systems, it is important to keep good records. Without knowledge of each cow’s history, it is almost impossible to get an accurate understanding of her fertility.