Calving difficulty in heifers
The occurrence of calving difficulty varies between seasons and properties, but under Victorian conditions up to 30% of heifers may require assistance at calving.
Other costs associated with calving difficulty are:
- veterinary fees
- cost of labour to supervise and assist heifers during calving
- reduced herd fertility (due to the longer time required by assisted heifers to conceive again)
- increased calf morbidity and reduced colostrum consumption.
Although it is not possible to completely eliminate calving difficulty, a number of steps can be taken to reduce occurrence to a minimum.
Causes of calving difficulty
The two most important known causes of calving difficulty in heifers are:
- excessive calf size at birth
- inadequate size of the birth canal.
Large, heavy calves are more difficult to expel than calves of average weight for the breed, and therefore are prone to more difficulty at birth and a greater rate of stillbirths. Male calves need to be assisted at birth much more frequently than females as they are generally heavier.
Calves that are heavy at birth require a larger birth canal or pelvic area for normal delivery than lighter calves.
In an experiment with 1000 Hereford heifers calving at two years of age, it was found that an extra 200 square centimetres of pelvic area was needed to accommodate each additional 4.5kg of calf if it weighed between 28kg and 36kg. Calves weighing less than 23kg caused almost no calving difficulty, while calves weighing more than 36 kg caused an extremely high incidence of difficulty.
A number of other causes of calving difficulty that account for a relatively small proportion of difficult calvings include:
- abnormal calf presentation
- obstruction of the birth canal by fat deposits
- constriction of the birth canal at the vulva, vagina or cervix
- weak labour or poor muscle tone in heifers that are either very thin or too fat.
Age at first calving
It is often said that calving difficulty results from joining heifers when they are too young. In cases where heifers are accidentally mated at very light weights or young ages, high levels of calving difficulty attributable to a small, immature birth canal do occur. However, do not be deterred from joining well grown British breed heifers (minimum joining weight 275kg, condition score 3), to calve at two years of age.
Calving difficulty is essentially a problem of first-calvers, irrespective of whether they calve at two or three years of age. Although three-year-olds are heavier and have larger pelvic openings than two-year-olds, they have proportionately heavier calves.
Heifers calving for the first time at two and three years of age normally have a similar incidence of calving trouble. More difficulty can be experienced by heifers calving first at three years because they tend to become too fat.
Controlling calving difficulty
Calving difficulty can be kept to a minimum by applying the following management principles.
Bulls of any larger cattle breeds, including Brahmans and European beef breeds, throw heavy calves and may cause high levels of calving difficulty. They should not be mated with heifers of smaller breeds.
There are also important differences between the British beef breeds. Hereford bulls normally cause more calving difficulty than either Shorthorn or Angus bulls. Some beef producers have adopted the practice of mating Hereford heifers with Angus bulls in an attempt to avoid calving difficulty. In some cases this approach has been marginally successful but in others the Angus bulls have caused slightly more difficulty than the Herefords.
The only bull breed which can be relied on to substantially reduce the level of calving difficulty in heifers is the Jersey. However, the value of a beef-Jersey cross must be questioned in view of the difficulty likely to be encountered in marketing the progeny.
Differences between sires of the same breed
Within pure breeds of cattle there are large differences between sires in their ability to throw light calves and cause a minimum of calving difficulty. In large artificial insemination programs, the level of calving difficulty and stillbirths has been reduced by more than 30% by careful selection of sires. No simple method of identifying 'easy calving' bulls on visually determined characteristics has yet been found.
There appears to be little or no relationship between the conformation, degree of muscling or body measurements of a bull and the amount of calving difficulty he causes. The commonly held view that bulls with large heads or protruding shoulders cause higher than normal levels of calving difficulty has not been substantiated by research on the subject.
BREEDPLAN estimated breeding values
Apart from progeny testing, the only useful aids to selecting 'easy calving' or 'low birth weight' bulls are BREEDPLAN 'Estimated Breeding Values' (EBVs) or the bull's own birth weight. As birth weight is a moderately heritable characteristic, bulls that had below average birth weight themselves will sire lighter calves than bulls that had above average birth weights. Where BREEDPLAN EBVs are available, bulls with negative or low EBVs for birth weight should be selected to mate with heifers.
Using litter mate bulls
A disproportion between sire and dam in frame size and birth weight can lead to increased calving difficulties. To avoid this problem, in some large Australian herds, heifers are joined with 'litter mate' bulls, that is, bulls from the same calf drop as the heifers.
This practice ensures that the bulls used are 'genetically compatible' with heifers for birth weight and frame size. Ideally litter mate bulls selected should be of below or average birth weight. The use of litter mate bulls will lead to a level of in-breeding. Therefore, their progeny should not be used for future breeding.
Heifer breed for calving
Breed differences in the incidence of calving difficulty among heifers follow a similar pattern to breed differences among bulls.
Exercise during pregnancy
Exercise is believed to play some part in the avoidance of calving difficulty, although this has not been tested in experiments.
Heifers that have had to walk up and down hills, for example, should have better muscle tone and greater calving endurance than unfit heifers.
Some producers deliberately feed out hay on hill tops, well away from watering points, to ensure that heifers get plenty of exercise during pregnancy.
Disturbance during calving
Experiments conducted in Victoria have shown that calving problems can be induced by excessive disturbance during the calving period.
When heifers are disturbed at the time of calving, muscles along the birth canal fail to relax and the birth process may be interrupted by constriction at the vulva and vagina. A compromise must therefore be reached between the need to observe heifers frequently during calving and the need to avoid disturbing them.
Selection against calving difficulty
The question of applying genetic selection procedures to the problem of avoiding calving difficulty often arises. These approaches are possible, but unfortunately none are likely to be successful.
Culling heifers that have a difficult calving
The daughters of a heifer that had difficulty at calving are no more prone to difficulty themselves than the daughters of a heifer that calved normally. In other words, the heritability of calving difficulty is low.
Heifers that experience difficulty the first time are not likely to have difficulty later. It may be sound economics to cull heifers that produce a dead calf, but no genetic improvement can be expected by culling heifers that had a difficult calving.
Selection against heavy calf birth weights
The main problem with this approach is that her productivity would be reduced. Calf birth weight is closely related to weaning weight, yearling weight and growth rate.
Selection for low birth weights would therefore result in lower weaning and yearling weights.
Selection against small pelvic size
Pelvic size is closely related to heifer skeletal size which in turn is related to calf birth weight. By selecting for large pelvic size, calf birth weight would be increased and consequently no improvement in calving difficulty could be expected.
Using calving induction
When heifers are mated too early or mismated to a bull which is likely to throw calves too large for the heifer to deliver, calving induction may be necessary.
Abortion can be induced by injecting the heifers with the hormone prostaglandin, in the first third of pregnancy, after a minimum of five days after the last observed mating. Prostaglandin can only be obtained with a prescription and must be administered under veterinary supervision.
Nutrition's impact on dystocia
Heifer nutrition has a key impact on the incidence of dystocia.
The growth rate of heifers is closely aligned with pelvic diameter. This means if heifers are grown at a consistent rate from weaning to calving the pelvic diameter at calving will be optimised.
The desirable rate of growth depends on the age/weight at weaning, breed, and genetics of the heifer, however ~0.7kg/day can be used as a guide. It is known that most of the foetal growth occurs in the final trimester.
Some people have limited heifer feed intake in an attempt to reduce birth weight. However, restrictive feeding in the final trimester actually increases dystocia by reducing pelvic diameter and increasing the occurrence of weak calvers.
As a result, it is important to maintain a steady constant growth rate in heifers, so they are well grown without being over fat. If heifers do become over fat in late gestation heifers must still be fed to meet their maintenance, growth and pregnancy energy requirements.