Calving difficulty is a subjective measure of the degree of dystocia. Dystocia can be caused by a large pre-natal calf, small pelvic area of the dam, lack of sufficient uterine contractions, insufficient dilation of the cervix, or mispositioned calf prior to parturition. Calving ease EPDs are produced from calving difficulty scores and birth weight observations.
Producers are asked to score the degree of calving difficulty using the following subjective scores:
|1||No difficulty, no assistance required|
|2||Minor difficulty, some assistance|
|3||Major difficulty, usually mechanical assistance|
|4||Caesarian section or other surgery|
Cows that calve without assistance between daily checks can be assumed to have calved without assistance even if the calving was not observed. A score of 1 should be reported for these. All calvings should receive a score even if the calf is born dead.
No adjustments are made to calving difficulty scores.
The same contemporary group definition for birth weight should be used for calving difficulty. This includes year/season of birth, management group. Some organizations that use threshold analysis for determining calving ease EPDs will not divide calving difficulty contemporary group by sex of the calf in order to increase the number of contemporary groups with variation. If other methods for analysis are used such as random regression or a linear model, then the contemporary group should be divided by sex of calf.
Calving ease EPDs are produced from calving difficulty scores. They are published on a probability scale and are then the opposite of calving difficulty, such that a higher value means more ease of calving. Because the dependent variable in the genetic evaluation model is calving difficulty the translation to a probability must be performed, usually using the normal cumulative density function.
Scores of 1 through 4 are used and score 5 observations are not used because this has been shown to not be heritable and is a result of different effects than the other scores.
Models vary between organizations producing calving ease EPDs. Some groups use a threshold model method. This has the disadvantage of requiring variation in observations within the contemporary group. Thus, all data from contemporary groups with no variation are eliminated. This can cause bias in the prediction.
An alternative is to fit a random regression model, or a simple linear model, which has been shown to be sufficient and will then use the data from contemporary groups with no variation in calving difficulty.
Some analyses use calving ease observations from all ages of a female. However, in most breeds, very little variation is observed after dams reach three years or older.
Birth weight observations on all ages of dams should be fit in a multiple trait analysis with calving difficulty scores.
A maternal effect for calving difficulty should be included, and thus a maternal calving ease EPD is calculated. In most presentations of EPDs for making selection decisions, a total maternal calving ease EPD is reported along with calving ease direct: Total Maternal Calving Ease EPD = Maternal Calving Ease EPD + 1/2 Direct Calving Ease EPD. In a few evaluations of indicus-influenced breeds, the maternal is reported but not the total maternal.
Calving ease is an economically relevant trait in all production systems. Producers often place more emphasis on calving ease then an optimal selection index would indicate is appropriate. This is because producers often trade risk for profit maximization and consider the risk of calving difficulty.