The economic cost of sub-optimal fertility or missed heat in a dairy herd varies between farms, depending on many factors including the herd’s baseline fertility, the mean calving date of the herd and the duration and severity of the effect observed.
The key reproductive targets in seasonal-calving dairy herds include:
- A 90% six-week calving rate;
- A 365-day calving interval;
- A planned empty rate of less than 8% at the end of a 12-week breeding season.
Four factors that underpin achieving high levels of reproductive performance:
- Body Condition Score (BCS) – increasing plane of nutrition throughout the breeding season;
- Disease control – a well-executed and monitored disease control programme;
- Genetics – cows of greater fertility sub index tend to have better reproductive performance;
- Mating management – farmers who use heat detection aids and who carefully herd monitor reproductive performance tend to achieve better results.
Slippage in Calving Interval
In the Irish grass-based milk production system, one of the key ways of maximising profitability is through the synchronisation of feed supply and demand throughout the year.
This is achieved by managing stocking rate, calving date and supplementation strategies. A consequence of a ‘missed heat’ is a slip in the calving date of affected cows in the following and subsequent years.
Therefore, the first major economic cost of infertility is on the cow calving date.
The immediate consequence is a slippage of 11.55 days in the mean calving date of the affected cows in the following year, relative to that achieved following a conception rate of 55%.
Each one day slippage in calving date reduces net profit by €3.81 per cow per day. Based on a reduction in profit associated with 11.55 days in the first calving season and 3.55 days in the second calving season, the total cost amounts to €57.50 per heat missed, associated with calving date change.
Not all cows go in-calf
However, in reality, not all of these cows would have gone in-calf in the first place. Based on a twelve-week breeding season, 91.9% would be in-calf and based on 15-week breeding season 95.7% would be in-calf based on assumptions of 85% submission and 55% conception rates.
Therefore for a 12 and 15-week breeding season, the calving date effect is discounted by 8.1% and 4.3% respectively.
This corresponds to the calving date slippage effect under a 12 and 15-week breeding season amounting to €52.90 and €55.10 per heat missed respectively.
Increases in empty rate
The second consequence of a ‘missed heat’ occurring in the first three weeks of the breeding season relates to a cow’s chance of going in-calf. The likelihood of that cow going in-calf before the end of the breeding season is reduced.
If a heat cycle is missed, then the breeding season is effectively reduced by one cycle (three weeks).
For a herd operating a 12 and 15-week breeding season, the empty rate of losing one cycle increases by 7.1% and 3.8%, respectively.
The costs associated with each 1% increase in empty rates reduces net profit by €13.68 per cow.
Therefore based on a 12-week breeding season effectively being shortened to nine weeks, the impact is an increased replacement rate which is equivalent to €96.60 per heat missed (7.1% X €13.68).
Similarly, based on a 15-week breeding season effectively being shortened to 12 weeks, the impact is an increased replacement rate is equivalent to €51.40 per heat missed (3.8% X €13.68).
Economic cost
Total costs per ‘heat missed’ amount to €52.90 for the slip in calving date and €96.60 for the increased likelihood of cows not in-calf in a 12-week breeding season.
Using a 15-week breeding season, the figures are €55.10 and €51.40.
Therefore the total costs for both scenarios correspond to €149.50 and €106.50 depending on whether the farm was operating a 12 or 15-week breeding season.
Any system change to counteract the effect of poor heat detection (longer breeding season/milking cows over the winter) would have huge consequences for the overall farm system, not least in respect of labour requirements, and has not been included in this analysis.