Mating 15-month heifers by artificial insemination (AI) and keeping their heifer calves is one method to increase genetic gain in the dairy industry. This is also a viable option to reduce the number of bobby calves produced each year. There is a wide belief, and some studies have reported that heifers born to first-calving cows were lighter at birth and grew slower up to first calving compared with heifers that were born to older cows.
If farmers are being encouraged to mate their 15-month-old heifers to AI instead of natural mating with bulls, it is important to know if there are live weight (LWT) OR milk production disadvantages of keeping progeny from these younger dams.
This study being summarised was presented by Rhiannon at the NZSAP conference using a large dataset provided by Livestock Improvement Corportation (LIC) for her PhD studies.
Key Points
Heifers born to 2-year-old heifers were lighter from 3 to 21 months of age than heifers born to older cows (3-year-old and 4 to 8-year-old).
The progeny of 2-year-old and 3-year-old cows produced more milk in first lactation, than the progeny of 4 to 8-year-old and 9-year-old and older cows.
Replacements from cows aged 9 and over was not recommended, as these heifers had lower genetic merit and produced less milk than heifers with younger dams.
The heifers were grouped into 4 groups based on their dam's age at the time of calving.
These were:
2-year-old dams (13,717 heifers; 7.2%),
3-year-old dams (39,258 heifers; 20.7%),
4 to 8-year-old dams (120,859 heifers; 63.6%) and,
dams aged 9-year-old or greater (16,102 heifers; 8.5%).
As would be expected, the majority of heifers (63.6%) were born to mixed-age dams (4 to 8-year-old) and the least amount of heifers (7.2%) were born to first-calving heifers (2-year-old).
Breed composition
On average, heifers with dams aged 2 years old (blue in graph), had lower percentages of Holstein-Friesian (F) and higher percentages of Jersey (J) in their breed composition compared with heifers with older dams. It is likely that most 15-month-old heifers were mated with the semen from Jersey and smaller Kiwicross bulls to help with calving ease. This would also help to explain why there was a greater heterosis percentage (also known as hybrid vigour) in the heifers with dams aged 2 years old (blue in graph).
Live weight
Because of these breed differences among the four age of dam groups, we decided it was best to compare LWT and milk production differences based on all heifers being of the same breed make-up.
For example, we all know that Jersey's are lighter than Holstein-Friesians (prime example in the photo above!), and we just saw that the group of heifers born to 2-year-old dams had more Jersey in them than the groups of heifers from older dams. Based on those two pieces of information, we would expect that the heifers in this study that were born to 2-year-old cows would be lighter than those from older dams, because they were more Jersey!
If the research question is: "Are there LWT disadvantages to being born to a 2-year-old dam versus an older dam?" This doesn't really answer the question does it?
In an ideal world, all four groups would have had the same breed composition, be born on the same day, be fed the same food etc. The only difference would have been the age of the dam. In a nutshell, this is how statistics works, it takes all the information and adjusts the groups to be as equal as possible for all things apart from age of dam, to better answer the research question.
When corrected for breed, heifers born to 2-year-old cows were still lighter from 3 to 21 months of age than heifers born to 3-year-old and 4 to 8-year-old cows. Heifers born to 2-year-old cows were also lighter than those from the oldest dams (9 and over) for all ages studied apart from 12 months of age.
The table below is a modified version of what is in the full paper. If the letters beside the numbers are the same (e.g. both 2-year-old and >9yo have the letter 'a' for 12 mo LWT) then there was no difference. If the letters are different (e.g. 2-year-old has an 'a' and 3-year-old has a 'b' for 12 mo LWT) then this means there was a difference.
However, the differences among the groups was very very small! In a practical sense, the differences between heifers are not likely to have a substantial effect on farm. For example, the difference between heifers from 2-year-old vs 4 to 8-year-old dams at 21 months of age was 411.6 kg vs 415.9 kg.. a 4.3 kg difference!
Based on this information, if farmers were to use similar bulls for their heifers as they have in the milking herd, then we would expect to see a very small difference in LWT for the heifers born to first-calving heifers.
If the main herd are large Holsteins and they are bred to large Holstein sires but the heifers are bred to Jersey sires, then we would expect to see a larger difference in LWT for the heifers born to first-calving heifers, because of their breed composition, not because of their dam's age.
Milk Production
Same as for LWT, the milk yields were adjusted to a similar breed composition for all 4 groups.
Interestingly, even though they were lighter, the heifers that were the progeny of 2-year-old cows produced more milk in their first lactation and their second lactation compared with the heifers that were the progeny of mixed-age dams (blue vs purple in the graph below).
When comparing the heifers born to 2-year-old and 3-year-old cows (blue vs grey), the ones with 2-year-old dams were superior for 2nd lactation milk production, but for 1st and 3rd lactation, they produced similar quantities of milk.
Over all 3 lactations studied, the heifers that had the oldest dams (9 and over) produced the least amount of milk (black in the graph).
Genetic Merit - Breeding Worth and BVs
As we were comparing heifers that were the progeny of younger vs older dams, we expected that there would be some differences in genetic merit among the groups, which may have explained the differences in milk production.
The data used for this study were from cows born between 2006/07 season and the 2013/14 season. One year group was selected (2011/12) and their genetic merit was compared for each of the age of dam categories. Instead of selecting their most recent estimates of genetic merit (breeding worth; BW and breeding values; BVs), we selected the estimates from before their first calving (May 2013). This means that their own milk production had no influence on their BW, it was based solely on the information from their ancestors.
As expected, the heifers born to younger dams (2 and 3-year-old) had higher BW than those from older dams (see Figure 3 below).
Now, BW is a "selection index" which is made up of many different traits; in 2013 these were milkfat, protein, volume, LWT, somatic cell, fertility and residual survival.
For more on BW and BVs - Dairy NZ have some good explanations and for more info on BW back in 2013 - check out the 2012/13 Dairy Statistics (page 45).
In this study, the differences in BW appear to be driven by differences in milk volume and LWT BVs and not by fat or protein BVs. As can be seen in Figure 4 below, the heifers with 2-year-old dams had much lower milk volume and LWT BVs than those from older dams (blue vs grey, purple and black).
Both milk volume and LWT BVs have a negative economic weighting for calculating BW, therefore, lower values are desirable for these two traits. Some of the differences in BW between the progeny of 2-year-old dams and the progeny of older dams are likely to be due to breed, as the progeny of 2-year-old dams had greater proportions of Jersey compared with older dams.
What's next?
Based on this study keeping replacement heifers from first-calving heifers appears to be a good option. They were only slightly smaller than those from older dams and their milk production was equal to that of 3-year-old dams, and greater than that of older dams!
Further from this, keeping replacement heifers from cows that were 9 and over did not appear to be a very good choice; this group of heifers had the lowest estimates of genetic merit and produced the least amount of milk.
Currently I am writing up the results from this study along with the analysis of stayability (remaining in the herd, similar to survival) and calving pattern. The hope is that we can get some answers on how long heifers are surviving in the herd and what their reproductive performance is like if they had older or younger dams.
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