Stayability

Stayability is generally defined as the probability of surviving to a specific age, given the opportunity to reach that age. Adapted to beef cows, the general definition of stayability is the probability of a cow surviving to her breakeven age, given the opportunity to reach that age. Cows usually need five consecutive calves by six years of age to generate enough income to pay their development and maintenance costs, so the age of six is was established as the target age for satiability. Without records of which females were retained for breeding and every calf they raised, cows with at least one calf before age six were considered to have opportunity, and successful cows had a calf at age 6 or older. Annual production records required by whole-herd reporting systems enable more rigorous definitions of stayability. Calving first as a two-year-old can indicate opportunity, and a calf every year through age six may be required for success.

A version of stayability, called sustained cow fertility, is provided by the American Hereford Association. It is calculated as a cow's ability to stay in the herd producing calves through age 12.

Phenotype

Observations of stayability are binary, either success (1) or failure (0). An observation should be assigned to all females who are old enough and are considered to have had an opportunity to succeed. Missing values should be assigned to females who are too young or otherwise did not have an opportunity to succeed. That may include cows culled for reasons other than reproductive failure, breeding females sold before the target age, and females used as donors or recipients in embryo transfer and in vitro fertilization programs.

Adjusted Value

There is no adjustment to the binary (0,1) stayability observations.

Contempory Group

Contemporary groups for stayability should include all females entering the breeding herd with the same opportunity to stay. Basically, heifers

Genetic Evaluation

In its initial release in 1995 stayability was analyzed using a maximum a posteriori threshold model (MAP). More recently, a random regression model (RR) has been implemented. The advantage of the random reregression approach includes the ability to use observations from contemporary groups with no variation. The MAP approach was implemented with contemporary groups fit as fixed effects. Including groups with no variation resulted resulted in the problem being non-singular. This is not a problem in the RR approach. Additionally, RR is superior for accomodating observations from all ages of cows.