Control of calving difficulty in beef heifers
Calving difficulty in beef heifers can be a major source of financial loss, due to a calf death rate of up to 10% in heifer herds and in some cases, loss of the heifer as well. 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, the cost of labour to supervise and assist heifers during calving, and reduced herd fertility due to the longer time required by assisted heifers to conceive again. 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 and 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 sq cm of pelvic area was needed to accommodate each additional 4.5 kg of calf if it weighed between 28 kg and 36 kg. Calves weighing less than 23 kg 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, producers should not be deterred from joining well grown British breed heifers (minimum joining weight 275 kg, 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. Consequently, heifers calving for the first time at two and three years of age normally have a similar incidence of calving trouble. Some producers claim that more difficulty is experienced by heifers calving first at three years because they tend to become too fat.
Control of 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. Unfortunately, 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.
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.
Use of 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.
Breed of heifers
Breed differences in the incidence of calving difficulty among heifers follow a similar pattern to breed differences among bulls. Herefords, the predominant beef breed in southern Australia, have comparatively high levels of calving difficulty.
Feeding during pregnancy
Body condition at calving has an important influence on the amount of calving difficulty experienced by heifers. High levels of calving difficulty can be expected in fat heifers due to high calf birth weights, obstruction of the birth canal by fat deposits, poor uterine muscle tone and weak labour. Similarly, in very thin heifers, high levels of calving difficulty are caused by poor uterine tone and weak labour. Heifers should be in medium body condition at calving.
The amount of feed eaten and the growth rate of heifers at any stage of pregnancy is known to affect the birth weight of the calf. High weight gains at any stage are associated with heavy calves and should therefore be avoided. During spring, for example, it may be necessary to run heifers at a very high stocking rate to slow down their weight gains. The aim should be to keep heifers growing at a moderate rate throughout pregnancy (up to 0.5 kg/day).
Unless heifers are likely to gain weight rapidly, there is no advantage in restricting their feed intake during late pregnancy. Many experiments have shown that although calf birth weight can be reduced by this practice, there is no reduction in the level of calving difficulty. When heifers have been allowed to lose weight during late pregnancy, the incidence of calving difficulty has been increased due to retarded pelvic growth and weak labour.
Season of calving
As the calving season progresses, calf birth weight and the incidence of calving difficulty increase. The reason for this is not clear, but it does not appear to be related to nutritional factors during pregnancy. Experiments with Hereford heifers at Hamilton indicate that an increase in calf birth weight of 2.5 kg can be expected between April and August. This is more than enough to influence ease of calving. Heifers calving in spring invariably experience more calving difficulty than heifers calving in autumn. Although this phenomenon is not fully understood, it is clearly beneficial to have heifers calving as early in the year as possible.
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. Three 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. Furthermore, heifers that experience difficulty the first time are not likely to have difficulty later; that is the repeatability of calving difficulty is also low. 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.
Use of induction as a management tool.
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 withthe 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.
Geoff Kroker, Bendigo and Lisa Clarke, Hamilton