Why are there differences?
The reason the two graphs can have two different calving patterns is that they calculate their results differently. Both use planned start of calving date (PSC) to calculate calving pattern, but they calculate PSC differently.
Calving rate by age graph:
The calving rate by age graph calculates PSC as:
- Last season’s mating start date (MSD) + 282 days (average dairy cow gestation length)
Example – a herd that had a 24 October MSD for the 2018 spring mating will have a PSC of the 2 August 2019.
Fertility Focus report calving pattern:
The Fertility Focus report calculates PSC as:
- This seasons MSD - 83 days
- 83 days is used because there is usually 12 weeks between the planned start of calving and the start of mating
Example – a herd that has a MSD for the spring 2019 season of the 22 October will have a PSC of the 31 July.
What this means
If MSD is the same each year, the PSC used by both graphs will be the same, and therefore the calving patterns will be the same. However, if MSD is shifted earlier/later this year compared with last year, the PSC dates used by the two graphs will not be the same, and so the calving patterns reported will not be the same.
Why do the graphs calculate differently?
The calving rate graph is showing how cows calved as a result of mating last year - it’s showing the outcome of last year’s mating.
The Fertility Focus report is showing how much time cows had to recover after calving before mating started. This is important to know because the more time a cow has to recover, the more likely a cow is to get in-calf in the early part of the mating period.
As a herd, this means that the higher the percentage of the herd that has calved early (by week 3 or 6 of calving), the higher the chance that a high percentage of the herd will get in-calf in the early part of mating, resulting in a high 6-week in-calf rate. So if a herd has a slow calving pattern/low percentage of the herd calved by week 3 or 6 of calving, it is less likely that the herd will have a good 6-week in-calf rate.
For the query I got, the reason the calving pattern was different between the graphs was because MSD was changed this year.
Late year the MSD was the 24 October 2018:
- This gives the ‘calving rate by age’ graph a PSC of the 2 August 2019
- Based on this PSC date, 60% of the herd had calved by week 3 of calving
This year the MSD was the 15 October:
- This gives the Fertility Focus report a PSC of the 24 July 2019
- Having a PSC that is 9 days earlier means that a lower percentage of the herd had calved by week 3 of calving
- Based on a PSC of the 24 July, only 37% of the herd had calved by week 3 of calving
Moral of the story
In short, if you have changed your MSD, you will have two different calving patterns reported by these two graphs. Both graphs are right, it just depends on what you are trying to measure/analyse as to which calving pattern you use.