High and low Fertility Breeding Value heifers before puberty.

Key points

• Puberty is blocked at the brain until a threshold liveweight (~50 percent of mature liveweight) is reached.
• Management factors during heifer rearing affect when this threshold liveweight is achieved and, therefore, the timing of puberty.
• Genetics also influence the timing of puberty, as puberty occurs at an earlier age and at a lighter threshold liveweight in heifers with a higher genetic merit for fertility.
• ‘Age at puberty’ may be a useful predictor trait to evaluate cow fertility earlier and more accurately.

Why is ‘age at puberty’ important?

First-calving heifers make up approximately 20 percent of the herd. To calve between 22 to 24 months old, heifers need to get in calf between 13 to 15 months old. This timeframe helps them calve within the first three weeks of the season, giving them a better chance of getting back in calf early and remaining in the herd^{1, 2}. To achieve this timeframe, heifers need to reach puberty (i.e. sexual maturity) early enough to conceive during the first three weeks of the heifer mating period.

Removing the ‘brain-block’ to puberty

Puberty is triggered by signals from the brain to the ovaries. By eight months old, the hypothalamus and pituitary glands in a heifer’s brain are developed sufficiently for the heifer to start cycling.

However, first heat and ovulation are ‘blocked’ because high numbers of receptors in the brain receive the ovary-produced sex hormone, oestradiol, as a strong negative feedback loop that prevents frequent pulses of luteinising hormone secretion required for ovulation (Figure 1). Removal of this oestradiol ‘brain-block’ involves a complex hormone pathway that reduces the brain number of oestradiol receptors, and is influenced primarily by liveweight gain and genetics.

Liveweight gain drives the timing of puberty

It is well-established that heifers reach puberty by about half their expected mature liveweight. For example, a 450 kilogram (kg) mature liveweight cow will reach puberty by about 225kg. Hence, poor growth rates will delay the time to puberty⁴.

To attain puberty before the start of mating at 13 to 15 months old, heifers should be reared to achieve liveweight targets of 30 percent, 60 percent and 90 percent of estimated mature liveweight at six months, 15 months (mating), and 22 months (pre-calving). Check out best practice growth and rearing information at dairynz.co.nz/incalf

A recent study of 10 commercial farms⁴ revealed only 60 percent of heifers had reached puberty by the start of mating (range between farms was nine percent to 93 percent). Predictably, younger animals and those at a lower body condition score (BCS) below 4.5 units were at most risk of being prepubertal at mating start date. A similar trend was reported in an Irish study¹; thus, heifers that are at least 14.5 months of age and 4.5 BCS units at the start of mating are more likely to have reached puberty.

The large percentage of pre-pubertal animals at mating start date on many farms indicates more attention is required to ensure heifers reach liveweight targets to prevent subsequent issues with reproductive performance.

A heifer mob being brought in for weighing.

Genetics also affect the timing of puberty

Genetics modify the effect of liveweight on the timing of puberty. Studies in beef cattle indicate approximately 30 to 40 percent of the variation in puberty onset between animals can be explained by their genetics^{5, 6}. This moderate level of heritability is comparable to milk production traits and demonstrates the puberty trait will respond to selective breeding. Evidence in dairy cattle is limited, but recent work* supports a similar heritability⁷.

Several studies indicate that animals with a greater proportion of North American Holstein-Friesian ancestry and/or a heavier liveweight Breeding Value (BV) take longer to reach puberty^{8, 9, 10}, putting them at a greater risk of not cycling before the start of mating.

DairyNZ-led research* has recently demonstrated that heifers with a high Fertility BV (+5 percent) reached puberty 21 days earlier than those with a low Fertility BV (-5 percent), which meant they were 25kg lighter and at a lower percentage of mature liveweight (51 vs. 55 percent)¹¹ (Figure 2). These groups grew at the same rate and did not differ in other traits.

These results indicate the oestradiol ‘brain-block’ was removed earlier in genetically fertile animals; consequently, 93 percent of high Fertility BV heifers but only 76 percent of low Fertility BV heifers reached puberty by the start of mating.

Puberty as a predictor trait of genetic merit for fertility

The earlier onset of puberty in high Fertility BV heifers indicates it could be a useful predictor trait for cow fertility in genetic evaluation. Earlier information would also be available on a sire’s offspring than calving- and mating-focused cow fertility traits.

Puberty also appears to be a better genetic indicator of subsequent lifetime fertility than heifer in-calf rates, which don’t appear to have such a strong genetic relationship to future fertility².

Further DairyNZ-led research* now underway will determine the genetic relationship between puberty and cow fertility traits using several thousand animals, while ensuring earlier onset of puberty doesn’t compromise genetic gain in other economically important traits (e.g. milk production) that make up Breeding Worth (BW).

Improving the rate of genetic gain in fertility

Heifers reach puberty at about half their expected mature liveweight.

Although fertility is a low-heritability trait, genetics will influence a cow’s lifetime reproductive performance. The overall variation in reproductive performance among animals is very high, meaning the genetic contribution is still well worth capturing through selective breeding.

The Fertility BV is currently estimated using routinely recorded calving and mating traits:

• PM21 (inseminated within 21 days of planned start of mating in lactation 1, 2 and 3).
• CR42 (calving rate in the first 42 days after planned start of calving in lactation 2, 3 and 4).

Although these are robust values, they don’t fully describe the genetic variation in the fertility trait.

It is hoped that the moderately heritable ‘age at puberty’ trait will allow us to evaluate cow lifetime fertility earlier, and with increased precision, so the Fertility BV can better capture the genetic variation in this trait. The more genetic variation we can capture, the better the rate of genetic gain in fertility the sector can achieve.

* This research has been carried out under Pillars of a New Dairy System, which is funded by dairy farmers through DairyNZ and by the Ministry of Business, Innovation and Employment, with aligned core funding for fertility from AgResearch. Additional funding and resources provided by Fonterra, LIC and CRV Ambreed support this key science platform. For more information, see dairynz.co.nz/pillars

References

1. Archbold, H., L. Shalloo, E. Kennedy, K. Pierce, and F. Buckley. 2012. Influence of age, body weight and body condition score before mating start date on the pubertal rate of maiden Holstein-Friesian heifers and implications for subsequent cow performance and profitability. The Animal Consortium 6:1143-1151.
2. Pryce, J. E., B. L. Harris, and L. R. McNaughton. 2007. The genetic relationship between heifer and cow fertility. Pages 388-391 in Proceedings of the New Zealand Society of Animal Production. Wanaka, New Zealand.
3. Day, M. L., K. Imakawa, P. L. Wolfe, R. J. Kittock, and J. E. Kinder. 1987. Endocrine mechanisms of puberty in heifers. Role of hypothalamo-pituitary estradiol receptors in the negative feedback of estradiol on luteinizing hormone secretion. Biology of Reproduction 37:1054-1065.
4. McDougall, S., F. M. Rhodes, and C. W. Compton. 2013. Evaluation of three synchrony programs for pasture-based dairy heifers. Theriogenology 79:882-889.
5. Morris, C. A., and N. C. Amyes. 2010. Correlated responses following genetic selection to change in age at puberty in Angus cattle. Pages 202-205 in Proceedings of the New Zealand Society of Animal Production. Palmerston North, New Zealand.
6. Martin, L. C., J. S. Brinks, R. M. Bourdon, and L. V. Cundiff. 1992. Genetic effects on beef heifer puberty and subsequent reproduction. Journal of Animal Science 70:4006-4017.
7. Price, M. D., M. D. Camara, J. R. Bryant, S. Meier, and C. R. Burke. 2017. Genetic parameters of puberty estimated using two genetically divergent groups of Holstein Friesian dairy heifers. Pages 529-532 in Proceedings of the Association for the Advancement of Animal Breeding and Genetics. Townsville, Queensland, Australia.
8. Garcia-Muniz, J. G. 1998. Studies of Holtein-Friesian Cattle bred for Heavy or Light Mature live weight. PhD. Massey University. Palmerston North, New Zealand.
9. Macdonald, K. A., L. R. McNaughton, G. A. Verkerk, J. W. Penno, L. J. Burton, D. P. Berry, P. J. Gore, J. A. Lancaster, and C. W. Holmes. 2007. A comparison of three strains of Holstein-Friesian cows grazed on pasture: growth, development, and puberty. Journal of Dairy Science 90:3993-4003.
10. McGrath, M. E., J. F. Mee, S. E. M. Snijders, and D. O’Callaghan. 2001. The effect of genotype on the onset of puberty and subsequent fertility in dairy heifers. M. G. Diskin (Ed.) Page 46 in Proceedings of the Agricultural Research Forum. Tullamore, Ireland.
11. Meier, S., B. Fisher, K. Eketone, L. R. McNaughton, P. R. Amer, P. Beatson, J. R. Bryant, K. G. Dodds, R. Spelman, J. R. Roche, and C. R. Burke. 2017. Calf and heifer development and the onset of puberty in dairy cows with divergence in genetic merit for fertility. Pages 205-210 in Proceedings of the New Zealand Society of Animal Production. Rotorua, New Zealand.

This article was originally published in Technical Series June 2019