Lactation persistency – a trendy management tool

Flat lactation curves are believed to be associated with fewer health and reproduction problems. Here you can read more about this trend among dairy farmers from a scientific point of view.

Since 2006, the Nordic Cattle Genetic Evaluation (NAV) has calculated breeding values for persistency, which provides information about the shape of the lactation curve.

For example, bulls with EBV (Estimated Breeding Values) above 100 (high persistency) will produce daughters that have a flatter lactation curve with lower yield than expected in the first part of the lactation, and a higher yield than expected in the last part of the lactation, compared to bulls with a lower EBV for persistency.

Recently, dairy farmers have shown an increased interest in lactation persistency as a management tool because flat lactation curves are believed to be associated with fewer health and reproduction problems.

But how is persistency calculated?

Lactation persistency and how to understand it.

According to the literature, persistency can be calculated in many ways using the same general definition of persistency. The method used depends on the purpose.

For example, if the purpose is to feed the cow as cheaply as possible by replacing concentrates with roughage around peak yield, a measure such as Y-index (Production index) at peak yield may be advantageous. A low index value is desired. However, such an index is of course unfavorably correlated to lactation yield.

According to NAV, breeding values for persistency are expressed as the amount of milk lost or gained due to the shape of the animal’s lactation curve from 100-300 days in milk (DIM).

The loss (or gain) is then calculated by multiplying the animal’s EBV at day 100 by 200. This gives the theoretical yield if a constant lactation curve is assumed. This is then deducted from the yield measured by the true genetic lactation curve between 100 and 299 DIM.

The larger the derived value is, the more persistent the lactation curve of the animal is. Breeding values for persistency are calculated for each lactation and weighted into one breeding value using weights of 0.3, 0.25 and 0.45 for 1st, 2nd and 3rd+ lactation, respectively.

In the figure there are examples of 3 Holstein bulls, all with breeding values for milk yield equal to 100, but different breeding values for persistency.

• V Evertop has EBV = 80 for persistency
• VH Evade has EBV = 100 for persistency
• VH Leyton has EBV = 120 for persistency.

For each bull, a simple lactation average for milk yield is shown as unstandardized EBV expressed as the deviation in kg from an unspecified mean (~6 kg). Later peak yield (VH Leyton) results in higher persistency compared to earlier peak yield (V Evertop).

Persistency – in practical use

Persistency is not included in NTM (Nordic Total Merit) mainly because it has no positive effect on economic gain. However, the genetic gain for persistency is positive given the current weighting of traits in NTM.
Higher persistency is favorably associated with yield, longevity, and udder health, whereas the association with milking speed and general health is slightly unfavorable.
For management purposes, a flat lactation curve should always be considered as it puts less stress on the cow, i.e., less negative energy
In special circumstances such as spring calving combined with heavy pasture grazing cow performance in early lactation is important for utilizing the high feed availability, and so lower persistency cows may be better suited leading to better fertility performance.

Text by: Lars Peter Sørensen, Genetic Development Manager, VikingGenetics.

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