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Factors Affecting The Ability Of The Dam To Give Birth
Dam's Pelvic Area
Pelvic area has recently received considerable attention as a trait that is related to dystocia. Just like birth weight, pelvic area is a threshold trait: if the pelvis of the female is large enough for the calves that she will have, there would be no advantage in a larger pelvis.
Additionally, although pelvic area is roughly correlated to frame size, the correlation is not perfect and as such pelvic area should be measured directly (i.e. it should not be assumed that all large-framed females will have large pelvises or that all small females will have small pelvises).
Heritability estimates for pelvic dimensions range from 0.40 to 0.53, meaning that pelvic dimensions are inherited at a similar rate to growth traits. Producers that would like to reduce the incidence of dystocia in their herds should therefore consider making use of pelvic measurements.
Age And Parity of Dam
First-calf heifers account for the majority of calving difficulties and associated calf losses. High rates of dystocia among first-calf heifers and young cows are mostly due to their smaller size at first parturition than at subsequent calvings. However, other factors may contribute to this pattern, such as the modification in the shape of the pelvic opening during the first calving and that many of the dams that experience difficult calving as two year olds are culled.
Breed And Genotype Of Dam
Differences in calving difficulty among breeds of dam are most likely due to differences in relative pelvic area, muscling or fatness.
Nutrition And Condition Of Dam
Some cattle producers feel that limiting the feed provided prior to calving will reduce calf birth weight and dystocia. However, research has shown that while birth weight can be reduced by limited cow nutrition, calving difficulty may actually be increased in underfed cows despite having lighter calves. Additionally, since underfeeding cows prior to calving will also delay the return to oestrus, this practice should be avoided. Spring calving cows and heifers should have a condition score of 2 - 2.5 at calving, whilst in autumn calvers the condition score should be around 3.