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 cost-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 years was established as the target age for stayability. Without records of which females were retained for breeding and every calf they raised, cows with at least one calf before age six years were considered to have opportunity, and successful cows had a calf at age six years 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 years 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 12 years.
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.
There is no adjustment to the binary (0,1) stayability observations.
Contemporary groups for stayability should include all females entering the breeding herd with the same opportunity to stay. Basic information used to form stayability contemporary groups is heifers' weaning contemporary group, and contemporary group of their first calves, when they calved as two-year-olds.
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 random regression approach accommodates observations from all ages of cows, not just six-year-old and older cows, and uses observations from contemporary groups with no variation. Contemporary groups with no variation were removed from the initial MAP evaluations to avoid computational problems.
Stayability EPD predict differences in the percentage of bulls' daughters that will remain in production through age six. Stayability is economically relevant in herds that generate their own replacement females. Herds purchasing replacement females may consider stayability EPD of the sires of those females. Stayability is irrelevant to terminal sires, whose daughters will not be bred.
- W. M. Snelling, B. L. Golden, R. M. Bourdon, Within-herd genetic analyses of stayability of beef females, Journal of Animal Science, Volume 73, Issue 4, April 1995, Pages 993–1001, https://doi.org/10.2527/1995.734993x
- J. Jamrozik, S. McGrath, R. A. Kemp, S. P. Miller, Estimates of genetic parameters for stayability to consecutive calvings of Canadian Simmentals by random regression models, Journal of Animal Science, Volume 91, Issue 8, August 2013, Pages 3634–3643, https://doi.org/10.2527/jas.2012-6126