Tuesday, June 6, 2017

BIF 2017: Evaluating Sustained Cow Production: Alternative Definitions of Stayability

Guest post by Tamar Crum, University of Missouri


Scott Speidel
Colorado State University

The concept of stayability was developed ~23 years ago. Stayability is simply, generally, the survivability to a certain age given the opportunity to reach that age.

Multi-breed stayability analyses are difficult because of different definitions of stayability.

  • Is the female present in the herd at 6 years old? (basic/generic definition)
  • Did she wean a calf at 6 years old?
  • Did she wean a calf at 6 years old while also calving at 2 years of age?
  • Did she calve 5 consecutive times within the same calving season?

Successful females for stayability varies depending on which of the definitions of stayability the breed association has adopted.

It is shown that stayability to 6 years of age is a heritable trait.

Stayability is a HUGE driver of herd profitability and accounts for 53-77% of the value of of the maternal indices.

Currently, stayability is recorded as a binary trait. This means that a '1' means that the animal is successful at staying in the herd past 6 years of age and a '0' means the animal was not successfully in the herd at 6 years old.


In the traditional model for evaluation of stayability the following un-successful females would have been equivalent under the basic definition of stayability.
0 = Did not calve in that year
1 = Calves in that year
- = No data


Year of Age 2 3 4 5 6
Unsuccessful Female #1
1
0
-
-
0
Unsuccessful Female #2
1
1
1
0
0

Unsuccessful female #1 calved at 2 years of age, did not calve at age 3, and was subsequently removed from the herd. She earns a stayability score of '0' at year 6 for not successfully being present in the herd at 6 years old.

Unsuccessful female #2 calved at 2, 3, and 4 years of age but did not calve in year 5 so was subsequently removed from the herd. She, too, earns a stayability score of '0' at year 6 for not being present in the herd at that point.

Which female is clearly more valuable? The problem with the traditional model of stayability does not take this into account and considers both females equivalent when it comes to stayability.

Phenotypic data records for stayability causes quite a bit of time lag. A bull will be at least 7 years of age before stayability data is able to be recorded. Daughters of the sire have to reach 6 years of age to be considered successful for stayability.

New models for the evaluation of stayability, using a random regression model, allows us to account for these differences in value of females.

While females might not meet the criteria or definition of stayability at year 6, their value and performance in years prior is still considered in the new model.

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