At first look, it may seem that Heifer Pregnancy and Stayability are measurements of the same character: fertility. However, they are two distinctly different aspects of fertility that need to be considered separately. Their EPDs are not highly correlated with each other which means sires that excel in one trait don't necessarily excel in the other. Another difference is their affect on profit. One trait deals with the number of replacements needed each year and the other is concerned with the number of heifers needed to fill those replacements.
Heifers that don't conceive waste money in the form of used resources (e.g. resources required to develop heifers to breeding season). The RAAA Heifer Pregnancy EPD allows cattle producers to select those bulls with superior genetics for producing daughters that are more likely to conceive to calve as two-year-olds than daughters of inferior bulls. This affects profit by enabling producers to reduce costs. It predicts differences between sires in the number of open daughters which impacts the number of heifers needed for heifer development (e.g. differences in feed costs). Alternatively, heifers with higher genetic merit for conceiving to calve as two-year-olds are likely to require less feed resources during development
Conception for young females depends on various factors including body weight and age at puberty that don't affect older cows in the same way. In contrast, conception in older cows depends on body condition, days postpartum, dystocia of most recent calf, etc. Because of this, the Heifer Pregnancy EPD is only a predictor of pregnancy rate differences for sires' daughters that are bred to calve at two years of age. It does not predict lifetime fertility. (You can read more about heifer pregnancy EPDs in the February 2002 issue of the ARA or at http://www.redangus.org/newredsite/themagazine/feb02/magazine.html).
Once a young
female enters the breeding herd (by becoming pregnant and having a calf), she
starts her productive life. The measure of productive life is distinctly different
than the
initial ability to become pregnant. In order to predict lifetime production
in terms of economics (profit), we needed to define a trait that described the
age at which a cow pays for herself. That trait is Stayability, which was originally
defined by Hudson and Van Vleck (1981) as the probability of surviving to a
specific age given the opportunity to reach that age. The most appropriate age
for RAAA breeders and customers is, on average, six years. This definition implies
that cows culled before the age of six have not lived long enough or produced
enough calves to be profitable (assuming one calf per breeding season). Stayability
differences help predict profit differences by estimating replacement rates.
The percent probability difference between two sires' Stayability EPDs is the
percent probability that one sire's daughters are more likely to be replaced
before they are profitable than the other sire's daughters. That percent difference
can be used in conjunction with individual cow herd sizes and age structures
to determine the cost differences in raising replacements for the sires in question.
Although Heifer Pregnancy and Stayability appear to be similar traits, they have distinctly different definitions. Breeders and producers must realize that Heifer Pregnancy is not contained within Stayability. Stayability begins where Heifer Pregnancy leaves off. The Red Angus philosophy of EPD development is to define traits in terms of economic relevance. That way, Red Angus breeders and their customers can relate genetics to profit by the most efficient and understandable means possible. By separating fertility into two traits, we minimize the number of replacements required and decrease heifer development costs.
The Red Angus Association of America has led the beef industry in describing fertility and reproduction, in general. This year, the RAAA published the industry's first reproduction sire summary (as a supplement to the 2002 RAAA Sire Evaluation). It lists 527 sires along with their EPDs for Calving Ease Direct, Heifer Pregnancy, Calving Ease Total Maternal, and Stayability. A copy of this supplement is included in the 2002 May/June issue of the ARA. Alternatively, the Reproduction EPD Sire Summary can be downloaded from the internet at:
http://www.redangus.org/genpred/f02.repro.sum.xls .
Hudson, G.F.S., and L.D. Van Vleck. 1981. Relationship between production and Stayability in Holstein cattle. J. Dairy Sci. 64:2246.
Q. What data is used to calculate the Calving Ease EPDs?
A. Calving Ease EPDs are calculated using both calving ease scores from first calf heifers and birth weights producing one all encompassing EPD. Although the calving ease scores are the most useful data to predict calving ease, because of the known relationship between birth weight and calving ease, we can use birth weights alone, if necessary, to still predict the genetic merit for calving ease.
Q. What are the units of measurement for the Calving Ease EPDs?
A. Because we are trying to predict the probability of an unassisted calving, Calving Ease EPDs are reported in percent probability. This means that the difference between two sires' EPDs predicts the probability difference between the unassisted calving rates of the sires (or their daughters in the case of Calving Ease Total Maternal).
Q. How do I Interpret the Calving Ease Total Maternal EPD?
A. The actual calving ease score for a calf is a combination of the calf's genetics for calving ease and the dam's ability to calve unassisted. The calf's genetics ("direct" genetics) influence the shape and size of the calf. The dam's genetics ("maternal" genetics) influence the uterine environment, pelvic area, etc. When we refer to Total Maternal Calving Ease, we are describing the combination of direct and maternal aspects that will result in the probability of a daughter having a calf unassisted. The Calving Ease Total Maternal (CETM) EPD combines the two effects and predicts the probability that daughters will calve unassisted as two-year-olds.
Q. Aren't We Confusing the Issue by Adding Another EPD if We Already Have Birth Weight?
A. On the contrary, we are clarifying the selection of calving ease sires. Birth weight is only an indicator of calving ease. When available, Calving Ease EPDs should be used alone (without birth weight) to select calving ease sires. At the very worst, a Calving Ease EPD contains all the information of birth weight with the added benefit of being expressed as percent probability differences. The ultimate goal of any EPD is to maximize the accuracy of selection. That is, we want to choose sires that are going to do what we want them to. Calving Ease EPDs maximize the accuracy of selection for unassisted calvings. Birth Weight EPDs maximize the accuracy of selection for birth weight. Anyone who wants to select for calving ease should ignore Birth Weight EPDs whenever Calving Ease EPDs are available.