Friday, March 29, 2024

Looking under the bonnet

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Cows are like cars. You rank your potential new vehicle on how it looks, the size of its engine, fuel consumption, safety rating and power. You rank cows in the same way, based on their looks, Breeding Worth (BW), Production Worth (PW) and Lactation Worth (LW).
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These indices are all expressed relative to a group of typical cows of all breeds born in 2000, the average of each index for this group of cows is zero, ie the genetic base cow. The indices are reported with an associated reliability, which indicates how likely the index is to change as extra information is gathered.

The reliability of BW indicates the level of confidence in the index value and can change depending on the source and volume of information available for that animal. Low reliability is strongly based on ancestry; high reliability is mainly based on actual performance. For example, a bull with no daughters has about 27% reliability because there is only ancestry information to work off. A bull with 10,000 daughters will have an index of about 99%. Once a bull has a 75% reliability he is eligible for inclusion in the Ranking of Active Sire (RAS) list.
But what do these figures mean? What goes into them and how does that affect your bank balance?
The old adage that production is 5% breeding and 95% feeding has gone out the window with modern dairy farming. Research shows on average 30% of the difference in milk production between animals of the same age, stage in lactation and breed, is because of differences in genetic merit.

Breeding Worth
BW is the cow’s ability to breed profitable replacements. It is an economic index used to aid farmers in selecting bulls for artificial insemination and cows to breed from and choosing replacements. The index also allows animals across breeds, herds and ages to be compared.
The seven traits of a highly efficient cow – protein, milk fat, milk volume, liveweight, fertility, somatic cell count and residual survival, are included in the BW index as they are deemed to have an economic value to a farm. In 2016 Body Condition Score (BCS) will be included in BW traits. Breeding values for these traits are estimated using a Test Day Model (TDM) from herd-test data. The TDM allows the environment specific to each test day to be taken into account, resulting in an improved accuracy of evaluation. Breeding values and BW calculations compare a specific animal to a genetic base cow.
These breeding values are multiplied with economic values and added together to produce the BW. By doing this, traits are measured by economic attractiveness of that trait. For example protein has a high economic value but somatic cell count doesn’t so has a negative value. Economic values are recalculated every February using farm economic models that take into account a number of factors from milk prices to farm expenses.
Putting it into perspective: A BW of 69/56 indicates that the cow is expected to pass onto her progeny an extra $34 (she passes half her genes on to her daughters) net farm income per year (per five tonnes of feed on a drymatter basis) with a reliability of 56%.

 Production Worth
PW is the cow’s own productive ability. The index measures the ability of a cow to convert feed into profit over her lifetime. This index is commonly used to make culling and purchasing decisions because the index ranks cows that are most valuable over their lifetime. Like BW, PW has an associated reliability figure. A well-recorded heifer starts her life with a PW reliability of 20%. After five recorded lactations, and liveweight recording, the reliability will increase to about 85%.
Putting it into perspective: A PW of 78/56 indicates the cow is expected to generate an extra $78 net farm income per year (per five tonnes of feed on a drymatter basis) with a reliability of 56%.

Lactation worth
LW is the cow’s ability to convert feed into profit in the current season. Early in the season the LW has to be estimated from very little information, so is likely to change very noticeably over the course of the season. LW has limited use for decision-making because of the number of factors that can affect a cow’s current performance but might not apply in future seasons, such as temporary lameness.
Putting it into perspective: a LW of 98/56 indicates the cow is expected to generate an extra $98 net farm income per year in the current season with a reliability of 56%.
As with everything else on the farm, you can manage better what you measure.

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