Fertility BV Animal Model

To test the robustness of the current Fertility Breeding Value (BV) and identify new measures of fertility, we generated a unique herd of heifer calves that had a 10-point difference in their Fertility BV (-5% vs +5%).

We observed remarkable differences in reproductive measures between high and low Fertility BV animals during rearing (2015-17), and their first (2017-18) and second (2018-19) lactations.

Find out more on how we conducted this study and the key results:

How we created the Fertility BV Animal Model herd

To identify new fertility traits, we worked with farmers and partners (including NZAEL, LIC and CRV Ambreed) to generate a unique research herd of 550 heifers with high (+5%) and low (-5%) Fertility BV.  This required the contract mating of 2800 cows across the North Island (see Breeding Plan image below).

Breeding plan to generate heifer calves with divergent Fertility BV.

The heifers that make up the Fertility BV Animal Model are Holstein-Friesian. This breed has the greatest genetic diversity in the fertility trait, as well as the largest breed population in NZ.

The study herd were balanced for other traits such as live weight, milk production, and % North American ancestry.

640 heifer calves were collected from across the North Island within 9 days of being born and following DNA parentage testing, we retained 289 High and 276 Low Fertility BV calves for further study.

All animals were reared together in Taranaki for a standard 13-week period from birth to weaning. They were weaned on age, rather than liveweight, as we wanted to measure their growth rates as part of the study. They were then moved to a grazing unit just north of Auckland for the rest of their rearing period.

Heifers were studied closely to understand biological differences in their fertility during the rearing phase (2015-17), and their first (2017-18) and second (2018-19) lactations.

High Fertility BV heifers reached puberty earlier

A key finding during rearing was the discovery that high Fertility BV heifers reached puberty at:

• an earlier age (358 vs. 379 days),
• a lighter live weight (271 vs. 296 kg), and
• a lower % of estimated mature live weight (51% vs. 55%), compared with Low Fertility BV heifers.

Because the two groups had a similar growth rate (see Figure 2), the earlier onset of puberty in the high Fertility BV heifers indicates that the biological “trigger” to start cycling is reached at a lower live weight in more fertile animals.

High Fertility BV heifers had more cycles before mating

The earlier onset of puberty in the high Fertility BV heifers meant that they had one more oestrus cycle before mating start date than low Fertility BV heifers.

This resulted in a greater percentage of high Fertility BV heifers cycling at mating start date relative to the low Fertility BV heifers (93% vs. 76%).

After three weeks of mating, 99% of the high and 87% of the low Fertility BV heifers had reached puberty and could be bred.

High Fertility BV heifers had better reproduction

Overall, reproductive performance of the high Fertility BV heifers was superior to the low Fertility BV heifers.

Using dated pregnancy diagnosis, we estimated that 8% more high Fertility BV heifers were mated during the first 3 weeks of breeding than low Fertility BV heifers (97% vs 89%).

This resulted in more high Fertility BV heifers conceiving earlier in the mating period, with 9% greater 3-wk and 6-wk in-calf rates (Figure 3).

The final not-in-calf rate after 14 weeks of natural mating* was 2% vs 6% for high and low Fertility BV heifers, respectively.

*The maiden heifers were naturally mated to Jersey bulls for 14 weeks. We chose a relatively long mating period to ensure that enough low Fertility BV heifers conceived and remained in the herd for further study as lactating cows.

High Fertility BV cows had over 30% higher 6-wk In-Calf rates

In both lactation 1 and 2, we used reproductive interventions for non-cycling only after 6 weeks of artificial breeding (AB) to enable animals to express their natural fertility.

High Fertility BV cows had approx. 35% greater 6-week In-Calf rates than low Fertility BV cows in lactations 1 and 2 (Figure 4). They also conceived earlier in the mating period: by 7 days in lactation 1 and 13 days in lactation 2.

This meant that high Fertility BV cows were 4 times more likely to be pregnant after 6 weeks of AB.

Final Not-In-Calf rates were very poor in the low relative to the high Fertility BV cows (approx . 40% vs. 14%; Figure 4a and b).

High Fertility BV cows had greater pre-mating cycling and submission rates

The key drivers for the greater 6 week In-Calf rate in high Fertility BV cows were substantially greater:

• pre-mating cycling rates, and
• submission rates.

The submission rates for low Fertility BV cows were extremely poor during both lactations 1 and 2 (Table 1). This was because a large percentage of low Fertility BV cows were still anoestrous (non-cycling) after 6 weeks of AB.  Few of these cows could be rescued with CIDR treatments, indicating issues with prolonged and non-responsive anoestrous.

The low Fertility BV cows that cycled naturally (without interventions) still had a 9 day longer interval from calving to first ovulation. These results indicate that high Fertility BV cows have a significantly greater ability to resume cycling after calving.

High Fertility BV animals had greater survival

During rearing, fewer high Fertility BV heifers were removed from the herd due to ill-health, poor conformation, and on-farm deaths (Table 2). Combined with their greater pregnancy rate, this meant that high Fertility BV heifers had a better survival rate than low Fertility BV heifers going into their first lactation (Table 2).

During lactations 1 and 2, there were similar pre- and post-calving animal removals due to slips, ill-health and other issues such as temperament and udder conformation (Table 3). However, large numbers of low Fertility BV cows were removed each year due to reproductive failure, which resulted in a poor survival rate between lactations (Table 3). After lactation 2, only a quarter of the low Fertility BV animals remained in the herd, whereas two-thirds of the high Fertility BV cows remained.

High Fertility BV cows had a better calving pattern

The greater 6-wk In-Calf rates in high Fertility BV heifers and cows translated to earlier and more condensed calving patterns during lactations 1 and 2 relative to low Fertility BV animals.

During lactation 1, high Fertility BV heifers calved, on average, 4 days earlier. They had a greater proportion of animals calved within the first 3 weeks after the planned start of calving (PSC; 82% high vs 75% low; Figure 5a). However, by 6 weeks after the PSC, a similar number of animals had calved in both groups (93%).

The re-calving pattern during lactation 2 was significantly better in high Fertility BV cows. On average, they calved 12 days earlier than the low Fertility BV group. A greater percentage of high Fertility BV cows calved within the first 3 weeks (62% high vs 41% low) and 6 weeks (82% vs 57%) after the PSC (Figure 5b). However, low and high Fertility BV cows had similar calving rates by 10 weeks (100%), due to the large number of low Fertility BV cows inseminated to CIDR-induced heats after 6 weeks of mating.

Differences in reproduction were not due to milk production

The poorer reproductive performance in low Fertility BV animals could not be explained by greater milk production relative to high Fertility BV animals. Differences in total milksolids yields during lactation 1 and 2 were less than 2%.

Similarly, the prolonged postpartum anoestrous interval in low Fertility BV animals was not driven by greater negative energy balance as there were no differences in metabolic indicators relative to high Fertility BV animals.

As this experiment was set up to test the effect of the Fertility BV trait, these results were not unexpected. This is because significant efforts were made to balance traits between the two groups, including milk production, live weight and BCS.

Podcast: Improving your herd fertility, part 1

Does selecting bulls that have a high Fertility Breeding Value really improve your herd’s fertility? In this episode, DairyNZ senior scientist Susanne Meier talks about what we learned from studying a unique herd of heifers with high and low Fertility BV, during rearing and their first and second lactations.