Heifers with high fertility BVs reach puberty earlier than low fertility BV heifers
This week we are taking a look at a large DairyNZ study on dairy fertility breeding values (BV). Heifer calves were bred from low fertility BV cows or high fertility BV cows and raised together from calves as one herd for at least 2 lactations. These high and low fertility BV calves were monitored extensively for many years and some of these results have been recently published.
The exciting thing about this study is that no information on early fertility measures (puberty, pregnant as an R2 etc.) is used for fertility BV estimation. Currently the fertility BV is estimated from information on cows once they have calved. For example, did she calve in the first 6 weeks in her second lactation?
The findings from this study show that there may be potential in collecting earlier fertility measures to increase the accuracy of the fertility BV.
High fertility BV heifers were pubertal earlier than low fertility BV heifers.
High fertility BV heifers were lighter at puberty than low fertility BV heifers.
More high fertility heifers were pregnant earlier than low fertility BV heifers.
The process of selecting heifers to be included in the study was a lengthy one! It started by identifying Holstein-Friesian cows in the national herd as being a High fertility BV cow or a Low fertility BV cow.
In the 2014 breeding season the High fertility BV cows were mated to High fertility BV Holstein-Friesian bulls and the Low fertility BV cows were mated to Low fertility BV Holstein-Friesian bulls.
If these cows had heifer calves born in the 2015 season they were purchased and collected by DairyNZ and formed the basis of this herd.
Only Holstein-Friesians were used for this study as this breed group has the most genetic variation in fertility. What this means is that the Holstein-Friesian breed has a larger range in fertility BVs than other breeds so it was easier to identify the highest of the high and the lowest of the low to be included in this study where the aim was to investigate the most extremes that exist.
565 heifer calves were collected and passed parentage DNA verification from 379 herds throughout the North Island and brought to a rearing facility in Te Awamutu at around 9 days of age.
The aim was to generate heifers that were different in their genetic fertility but similar for all other traits (milk yield BVs, fat yield BVs, liveweight (LWT) BVs, BCS BVs etc.). This meant that their Breeding Worth (BW) was different as we weight fertility BV highly in the calculation of BW.
Growth and development
Heifers were weighed and measured for stature (height, length and girth) regularly. As the heifers were selected to be similar in their LWT BV, it was expected that they would be similar in their LWT measures from collection up until first calving.
As we can see in the graph below, this was generally true. The average LWT of the High fertility BV heifers is in purple and in blue is the Low fertility BV heifers.
In the first few months, heifers of both groups were very similar in their LWT. However as they got older, a slight difference is seen in that the Low fertility BV animals were slightly (8kg) heavier at 21 months of age than the High fertility BV heifers.
In this study, heifers were also monitored for length, height, girth and BCS. There were no differences between the two fertility BV groups for any of those traits, at any age.
The main (and exciting!) results from this study is all around puberty attainment.
The High fertility BV heifers reached puberty earlier and at a lighter weight than the Low fertility BV heifers.
As we can see in Figure 2 below, the High fertility BV heifers were on average 358 days old (nearly 11 months) whereas the Low fertility heifers were 27 days older (385 days old or nearly 13 months) when they became pubertal.
Previous studies have suggested that if we can have heifers pubertal earlier, then they have more opportunities to cycle before they are mated. This should increase their chances of conceiving at their first insemination/mating. Therefore being pubertal earlier is advantageous for heifers.
Interestingly, this study also found that the High fertility BV heifers were 20 kg lighter when they reached puberty compared with the Low fertility BV heifers.
Usually we would expect that heifers of similar breed and age would reach puberty at around the same LWT. In this study, this appears to not be the case! Perhaps the High fertility BV heifers are different in their development and they don't need to be as heavy to reach puberty compared with the Low fertility BV heifers.
At the start of breeding, 94% of the High fertility BV heifers were pubertal and only 82% of the Low fertility BV heifers.
Pregnancy as R2 heifers
The last section I want to cover is on pregnancy rates of the heifers as rising 2-year-olds. The working hypothesis for puberty and pregnancy is that heifers that are pubertal earlier, have more opportunities to cycle before mating commences. As they have had more cycles, they are more likely to get pregnant early on in the breeding period compared with heifers that reached puberty later.
In this case, we would expect that more of the High fertility BV heifers would get pregnant earlier than the Low fertility BV heifers, as they were pubertal earlier.
Funnily enough, this is exactly what happened!
We can see in Figure 3 below that the 3 week pregnancy or in-calf rate was 75% for the High fertility BV heifers and only 62% for the Low fertility BV heifers.
This extended all the way through with High fertility BV heifers having a 90% 6-week in-calf rate compared with 82% for the Low fertility BV heifers, and a higher 9-week in-calf rate for the High fertility BV heifers compared with the Low.
As DairyNZ needed to get as many heifers pregnant in order to continue the study in the first and second lactations, the natural mating period was for 98 days (14 weeks). The final pregnancy rates were 98% for the High fertility BV heifers and 94% for the Low fertility BV heifers, significantly different for the two groups.
What does this mean?
The fertility BV is estimated based on reproductive traits of cows during lactations 1, 2, 3 and 4. The results from this study where animals that were the extremes for fertility BV were measured extensively indicate that puberty and heifer pregnancy rates were affected by the fertility BV.
This means that the opposite may be true, if we can measure puberty and heifer pregnancy rates then, maybe we can get an earlier estimate of an animal's genetic merit for fertility. This would help to increase the accuracy and strength of the fertility BV.
I was actually involved in this study way back in 2015 when I worked at LIC. I assisted in identifying which calves needed to be collected for the trial. It is awesome to see the hard work of so many people and organisations come together with such an interesting result.
Unfortunately the results from pregnancies during the first and second lactations are not yet published in a peer-reviewed journal. They are available on DairyNZ's website if you wanted to read further - https://www.dairynz.co.nz/about-us/research/pillars-of-a-new-dairy-system/fertility-bv-animal-model/
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Meier, S., Fisher, B., Eketone, K., McNaughton, L. R., Amer, P. R., Beatson, P., . . . Burke, C. R. (2017). Calf and heifer development and the onset of puberty in dairy cows with divergent genetic merit for fertility. In S. L. Peterson (Ed.), Proceedings of the New Zealand Society of Animal Production (Vol. 77, pp. 205-210). Rotorua, New Zealand.
Meier, S., McNaughton, L. R., Handcock, R., Amer, P. R., Beatson, P. R., Bryant, J. R., . . . Burke, C. R. (2021). Heifers with positive genetic merit for fertility traits reach puberty earlier and have a greater pregnancy rate than heifers with negative genetic merit for fertility traits. J Dairy Sci, 104(3), 3707-3721. doi:10.3168/jds.2020-19155