The Fertility BV (re-calving within 42 days of the start of calving) is currently calculated by eight predictor traits, but new measures of cow fertility could improve its accuracy.
To identify new traits, we generated a unique herd of 550 heifers with high (+5%) and low (-5%) Fertility BV. We studied their fertility and other traits during rearing (2015-17) and their first (2017-18) and second (2018-19) lactations.
- High Fertility BV heifers reached puberty 21 days earlier and 25 kg lighter than the low Fertility BV heifers, despite growing at the same rate.
- High Fertility BV heifers also had a 9% higher 6-week In-Calf rate during their first mating period, compared with low Fertility BV heifers.
- 6-week in-calf rates were over 30% greater during lactation 1 and 2 in high Fertility BV cows.
- The superior reproductive performance in high Fertility BV cows was driven by their greater ability to resume cycling post-calving. This led to 38% and 25% greater 3-week submission rates during lactations 1 and 2, respectively.
- Without hormonal interventions, almost half of the low Fertility BV cows had still not cycled after 6 weeks of breeding during lactation 1.
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What it means
- Genetic selection for cow fertility results in tangible differences to herd reproductive performance.
- A cow’s genetic merit for fertility affects her ability to start cycling post-calving and therefore calve in the first six weeks next season.
- Using high Fertility BV bulls will improve your herd’s fertility.
- There are biological differences controlling reproduction that can be used to identify new traits to improve the Fertility BV.
- Both 'puberty' and 'heifer reproductive measures' are potential earlier predictors of subsequent cow fertility than current measures based upon mating and calving records during lactation.
We are now testing practical ways to measure new traits at a larger scale. We are following a 2018-born cohort of 5000 cows in 54 herds across 3 regions (Waikato, Taranaki, Otago) to estimate the genetic relationships between novel traits and subsequent cow fertility.
This scale-up study began in Autumn 2019 working with farmers with good quality herd records. We are using large numbers of animals with diverse Fertility BV to obtain robust estimates of genetic relationships across a range of farming operations.
To date, we have estimated an age at puberty trait by blood sampling heifers monthly to detect elevated progesterone. Puberty rate was highly variable, so we surveyed farmers to investigate animal and herd management factors influencing puberty onset. This will help design future routine data collection.
We also determined that age at puberty is moderately heritable (between 20% and 30%) indicating it will respond to selective breeding. We will now analyse first and second lactation data to determine if genetic selection for earlier age at puberty improves reproductive performance.
Other measures under investigation include using wearable activity monitoring devices to estimate age at first heat (as an alternative to age at puberty) and the timing and strength of pre-mating heats in first lactation.
The scale-up study will also provide insights into using pregnancy testing records to measure timing of conception. These are now widely recorded by farmers and could provide earlier, more accurate information on a sire’s ranking for fertility.