Mating 15-month dairy heifers by artificial insemination (AI) and keeping their heifer calves is one method to increase genetic gain in the dairy industry.
Previously, we have summarised the liveweight and milk production of heifers born from heifers (2-year-olds) or older cows. You can read this first part here: Keeping replacements from first-calving heifers. This second part covers a subsequent analysis looking at stayability (remaining in the herd, similar to survival) and calving pattern. Here we were trying to answer how long heifers are surviving in the herd and what their reproductive performance is like if they had older or younger dams.
This study being summarised was completed by Rhiannon using a large dataset provided by Livestock Improvement Corporation (LIC) for her PhD studies and has recently been published in the Journal of Dairy Science.
Key Points
Heifers born to 2-year-old cows had similar stayabilities to 1st, 2nd, and 3rd calving compared to that of heifers born to older cows (4 to 8-year-old and 9-years-old or greater).
Calving and recalving rates was similar in first-calving heifers of all age-of-dam classes.
Second and third calving rates were similar for the progeny of 2- and 3-year-old dams.
Stayability
A measure of cow survival that does not require super accurate recording of cull data is called stayability. Stayability is defined as the probability (or chance) of a cow surviving to each calving year given that she calved the year before. We have a summary on how heifer liveweight may impact stayability that can be found here: Do heavier heifers survive longer in the herd? for more detail on stayability.
The average stayability for heifers that were born to 2-year-old heifers was 93% for first calving. This means that on average 93% heifers born from first-calving heifers that were reared, ended up calving for the first time as 2-year-olds themselves and 7% (100 - 93) did not calve as 2-year-olds.
In the graph below, we have heifers with dams that were 2 years old (blue), 3 years old (grey), 4-8 years old (purple) and 9 or older (black) when the heifer was born. The left-most set of bars are for stayability to first calving, the middle set of bars are for stayability to second calving (what percentage of heifers that were reared made it to second calving) and the right-most set of bars are for stayability to third calving (what percentage of heifers that were reared made it to third calving).
The stayability of the heifers born from 2-year-olds was similar to the other age-of-dam classes that were studied. The little "a" and "b" letters show us which bars are similar (same letters) and which bars are different (different letters) within each of the first, second or third calving stayabilities. So for stayability to first calving, we can see that those with 2yo dams have both "ab", this shows they are similar to the "b" bars as well as the "a" bar, however, for those heifers with 9yo and older dams, their own stayability was slightly less than that of heifers with 3yo (grey "b") and 4-8yo (purple "b") dams.
When we look at stayability to second and third calving, we see that on average, heifers born to 2-year-old heifers had similar stayabilities to that of heifers born to 4-8yo and 9yo or older.
For the heifers that had dams aged 3yo when they were born, their stayability was slightly better on average than that of heifers with 2yo dams for both second and third calving.
Even though these are statistically significant differences, on a per farm level, these differences between animals with different aged dams are probably not meaningful... For an average New Zealand herd of 430 cows with 86 replacement heifers reared each year, the difference between those that had 2yo dams and 3yo dams would be 1 cow surviving to third calving (52 vs. 53 cows).
On the flip side, these results highlight that there are unlikely to be any negative impacts on future survival by keeping replacement heifers from first-calving heifers compared with older cows.
Calving rate
The definition of calving rate in this study was "the proportion of animals that calved no more than 21 days after the planned start of calving (PSC) provided they calved that year". In other words, of the cows that calved that year, which ones calved in the first 3 weeks after PSC.
For first calving, there were no discernable differences among the age-of-dam categories for 21-day calving rate (as can be seen in the graph below).
Similar to the stayability results, there were small differences in second and third 21-day calving rates between heifers with different aged dams. Namely, the heifers with 2yo dams had slightly lower 21-day calving rates than heifers with dams aged 4-8yo in second calving. By third calving, the heifers with 2yo dams had lower 21-day calving rates than both heifers from 4-8yo and 9yo and older dams.
For first, second and third calving, heifers born from 2yo and 3yo dams had similar 21-day calving rates.
Again, similar to the stayability results, on a per farm level, these differences between animals with different aged dams are probably not very meaningful... and are not enough to warrant not keeping replacement heifers from first-calving heifers.
Re-calving rate
Ideally we would have liked to use in-calf rates rather than calving rates, but at the time the study data was collected (2015) there wasn't enough pregnancy diagnosis data available to us.
The next best thing we could use was "re-calving rate". This is similar to calving rate however it is calculated using the cows that calved the year before rather than the "current" year.
The definition of re-calving rate for this study was "the proportion of animals that calved no more than 21 days after PSC provided they calved the year prior", this makes it more similar to a pregnancy rate, (give or take a bit) and a lower value than the calving rate.
Just as with calving rate, for first calving, there were no detectable differences among the age-of-dam categories for 21-day re-calving rate (as can be seen in the graph below).
There were small differences in second and third 21-day re-calving rates between heifers with different aged dams. Namely, the heifers with 2yo dams had lower 21-day re-calving rates than heifers with dams aged 4-8yo and 9yo and older in second calving and third calving.
What does this all mean?
Including the highest Breeding Worth (BW) cows and mating 15-month-old heifers with artificial insemination of high genetic merit bulls is recommended as a method to generate enough replacement heifers of high genetic merit.
What we have summarised here (and in this previous summary) shows that heifers that are the progeny of first-calving heifers are expected:
to be slightly smaller than the heifers of older dams, provided the bulls used for the first-calving heifers are of a similar breed to those used in the main herd
to have superior genetic merit than that of heifers born from dams aged 9 and over
to produce similar amounts of milk to that of heifers born to 3yo dams, and more milk than that of heifers born to dams older than 4yo
to have very slightly worse reproductive performance (calving and recalving rates) compared with that of heifers born from dams aged 9 and over. However, the difference was very small and on a per farm basis unlikely to be noticeable.
The results from this study indicated that keeping replacements from cows aged 9yo and older was not recommended, as these heifers had lower genetic merit and produced less milk than did those with younger dams.
Furthermore, there were no major disadvantages to keeping replacements from first-calving heifers.
What did you think about this week's post? After reading it is there anything you would consider doing differently on farm? Let us know your thoughts on our facebook page (www.facebook.com/agrisciencer) or in the comments below.
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