BY ALEX ASHWOOD

BRAHMAN NEWS SEPTEMBER 2010 Issue #168

Calving difficulties (dystocia) cause major economic losses to beef enterprises through cow and calf mortalities, increased labour and veterinary costs and reduced cow production (reproduction and milk).

“Approximately 55-60 percent of calf deaths are due to dystocia”

After the failure of cows and heifers to conceive, calf losses due to calving difficulties are the next major cause of reduced calf drop percentages (Table 1).

TABLE 1: FACTORS THAT REDUCE CALF CROP PERCENTAGE

FACTOR	PERCENT
Cows fail to become pregnant	17.4
Calves lost during gestation	2.3
Calves lost at birth	6.4
Calves lost birth to weaning	2.9
Total losses	28.9
Net calf crop percentage	71.1
Source: University of Minnesota (1992)

Note: In general dystocia occurs when:

• The size of the foetus is incompatible with the size of the pelvic opening (ie the foetus is too large or the cow is too small).
• The cow does not have a normal calving due to nutritional weakness or stress (eg mineral imbalances or low plane of nutrition).
• The foetus is abnormally presented (eg breech birth) which frequently due to oversized calves.

The majority (70 percent) of calf deaths due to calving difficulties occur in the first 24 hours and dystocia is the largest single cause of perinatal mortalities.

Postnatal losses in the first week of the life due to calving difficulties are the result of calves suffering damage to joints, bones and organs reducing the ability of the calf to nurse the dam resulting in illthrift, disease and death (eg pneumonia and scours).

Studies show that pregnancy rates for assisted v’s unassisted cows is reduced by 16 percent due to delayed returns to service.

“Dystocia also reduces breeding performance”

Although studies clearly show that birth weight is the most important factor affecting calving difficulty, there are a number of interrelated factors that impact on calving difficulties and the ease of calving.

CALVING DIFFICULTY FACTORS

The majority of calving difficulty studies have been conducted with bos taurus cattle. Whilst it is generally assumed that bos indicus cattle have less problems, the greater emphasis on frame score and growth rates has increased the risk of calving difficulties in bos indicus and bos indicus x cattle.

Factors that affect calving difficulties include:

• Presentation of the calf
• Birth weight of the calf
• Weight ratio
• Gestation length
• Sex of the calf
• Shape of the calf
• Pelvic area of the dam
• Age of the dam
• Dams influence
• Sires influence
• Nutrition and condition of the dam
• Calving ease (EBV’s)

Note: Because many of the causes of dystocia (eg abnormal calf position, uterine torsion) are sporadic and unpredictable, prevention is primarily focused on correcting foetal/maternal disproportion and nutrition.

BIRTH WEIGHT OF THE CALF

As shown in Figure 1, as birth weight increases the risk of calving difficulties increases in a curvilinear mannner.

FIGURE 1: THE EFFECT OF BIRTH WEIGHT (BW) ON PERINATAL MORTALITY (PM) WITH 500-600KG COWS

Studies clearly show a high correlation (+0.44 – +0.54) between increased birth weights and the incidence of dystocia.

There are highly significant correlations between body weights, growth rates and mature weights all of which are highly heritable (Table 3).

The incidence of dystocia, particularly in virgin heifers can be reduced by lowering the calves birth weight relative to the size of the dam. However, when a threshold weight is reached, no further improvements in calvings are evident.

The actual threshold value depends on the genetics, management, the breeding goals of the herd and the nutritional environment.

“Different herd have different threshold values”

Since birth weight is highly correlated to weaning, yearling and mature weights (Table 3), producers may choose bulls that produce calves close to the threshold value for that herd. As long as the birth weights are close to the threshold value, calving difficulties will be minimised.

WEIGHT RATIO

Estimating the calf’s weight relative to the dam’s weight (ie weight ratio) can assist in predicting calving difficulties. For example, if it is calculated that a 600 kg cow will have a 45 kg calf, the weight ratio is 0.075 or 7.5 percent Providing the pelvic area dimensions are acceptable, calves with a birth weight between 7-8 percent of the cow’s weight will maintain calving difficulties at a manageable level and allow suitable growth rates. The weight ratio for heifers is slightly lower (ie 6-7 percent).

GESTATION LENGTH

The influence of gestation length on dystocia is indirect through increased weight of the calf. Bodyweight gains as low as 5-10 percent in late gestation can lead to calving problems. Brahmans apparently have slightly less foetal growth in the last 20 percent of gestation than other breeds. However, the variable gestation length (271-310 average 292 days) for Brahmans can place high birth weight calves (particularly bulls) at risk if the pregnancy is prolonged. As gestation length increases, birth weight increases by 0.15–0.36 kg/day of gestation. As birth weight increases, the percentage of assisted births increases by 3.0-4.0 percent per kg of birth weight (eg an extra 4 kg increases the risk of dystocia by 10-16 percent). Recent studies suggest that gestation length is a trait that could receive fuller consideration in the future. In the meantime, selecting for moderate birth and suitable growth rates is a more effective means of reducing the risk of dystocia.

SEX OF THE CALF

As with gestation length, the sex of the calf has an indirect effect on calving difficulties through increased birth weight. Compared to heifer calves, bull calves have larger gestation lengths and weigh 2-5 kg more at birth which can cause 10-20 percent higher assistance rates at calving. A further factor that impacts on calving difficulties is the shape of bull calves compared to female calves.

SHAPE OF THE CALF

Structural traits in cattle are heritable and calf shape is no exception. It is generally accepted that bulls which have a smooth appearance without thickness of the chest and shoulders and course bone tend to sire calves that have less calving difficulties.

“Avoid using bulls with extreme structural traits”

Producers suggest that some high frame score brahman bulls despite producing angular shaped calves are responsible for calving problems due to the excessive height, length and large bone structure of the calves. 

“Shape, high frame score and mature weight impact on calving difficulties”

TABLE 2: EFFECT OF BIRTH WEIGHT ON LEVEL OF CALVING ASSISTANCE WITH MEDIUM-LARGE FRAMED COWS

	LEVEL OF ASSISTANCE
	NORMAL	HAND PULL	MECHANICAL PULL	CAESAREAN
BIRTH WEIGHT (KG)	37	40	45	55
(%)	57	28	13	2

Note:

1. This range of BW’s is similar to the BW’s of US Brahman bulls
2. Whilst bos indicus cattle generally have less calving problems than bos Taurus cattle, 2) producers need to consider the mature weight of bulls used on cows and heifers
3. Brahman cattle generally have 30 percent less calving difficulties than British breeds and 50 percent of the problems of European cattle. The reduced birth weight of calves 3) born in prolonged exposure to high temperatures also favours Brahman cattle

PELVIC AREA OF THE DAM

A major cause of dystocia is the disproportion between the size of the calf and the dam’s birth canal.

Studies in Dakota (USA) showed that the incidence of calving difficulties is twice as high in heifers with below average pelvic areas compared to females with above average pelvic areas. In Oklahoma studies, 85 percent of heifers with small pelvic areas experienced calving difficulties.

Like birth weight, there is a threshold value for pelvic area, ie if the pelvic area is large enough for calving, there is no further advantage in having larger pelvises.

Some interesting points are as follows:-

• Heifers of increased skeletal size usually have larger pelvises.
• Large framed females tend to have larger calves, hence selection for pelvic area needs to be considered in conjunction with the mature size of the bull used at matings.
• Selection based on pelvic area must be related to the size of the cow.
• Heifer age and development is highly correlated to pelvic area.
• External dimensions (eg width of the pins) are not good indicators of pelvic area.
• Although pelvic area is positively correlated to frame score (ie large females have large pelvises) the correlation is not perfect and there can be within breed differences.
• Females with small pelvises and large calves will always experience calving difficulties.
• It is generally accepted that the ratios of birth weight to pelvic area and birth weight to dam size are important guidelines to determining calving difficulties.
• Pelvic area is moderately heritable but it is also genetically related to other structural traits thus pelvic area should not be selected in isolation of structural and growth traits.
• Measurement of the pelvic area requires special equipment, experience and interpretation of measurements and conversion factors.
• Pelvic area, whilst extremely important is only one part of the calving difficulty complex.

AGE OF THE DAM

The dams age has a major influence on the incidence of dystocia. Some general points are as follows:-

Calving difficulty is higher with 2 year old heifers compared to 3 year olds, and 3 year olds have more problems than 4 year olds. By maturity 4-5 years, the problems are minimised due to structural growth and many of the problem females have been culled.

Despite the fact that heifers are generally observed more closely and assisted more readily at calving the incidence of dystocia can be 10 times more than mature cows.

Second calf heifers (3 year old) or 3 year old virgin heifers require close supervision particularly if mated to high frame score bulls.

Some heifers continue to have calving problems despite skeletal development and changes to the pelvic canal after calving.

DAM’S INFLUENCE

Larger cows have larger pelvises but can have calving problems if mated to sires that produce large calves. The size of the calf is a combination of both the sires and the dams growth genes, ie 50% of the genetic material comes from each parent.

“Small cows mated to high frame score bulls is an accident waiting to happen”

SIRES INFLUENCE

The sires mature weight and shape has a major influence on birth weight and calving difficulties. Birth weights and growth traits are highly heritable and closely correlated to calving difficulties (Table 2).

Selection for high birth weights and growth traits is very effective in increasing growth rates in beef cattle. Adversely, light birth weights and reduced growth rates may negatively impact on productivity and profitability of the business. 

Continued selection for lower birth weights without consideration to satisfactory growth rates may eventuate in smaller females which have poorer calving ability.

NUTRITION AND CONDITION OF DAM

The foetus grows relatively rapidly in the last 90 days of gestation but reducing feed intake does not reduce calving difficulties.

Results of trials indicate that whilst foetal size can be restricted by underfeeding energy, calving difficulties and days to first estrus after calving are increased.

Feeding excess protein (18 percent CP) in the late stages of gestation can lead to increased birth weights. Underfeeding protein (<10 percent CP) can result in poor calves, illthrift and reduced cololstrum and milk production.

Heifers are the most nutritionally challenged females at calving. Achieving breeding and calving targets weights are the most important ways of reducing the risk of dystocia.

“Rations should be adequate and balanced”

Studies show that heifers that are bred at 65 percent of the mature weight and calve at 85 percent of the mature weight have less calving problems.

Developing heifers on a low plane of nutrition increases the incidence of calving difficulties due to reduced skeletal growth and smaller pelvises.

Overfeeding heifers (and cows) causes internal fat deposition which obstructs the pelvic canal. Overweight females can have calving difficulties as severe as underfed stock.

CALVING EASE (EBV’S)

Calving ease is affected by environmental, management and genetic factors. Calving ease EBV’s look at the genetic factors (eg gestation length, birth weight and sex of calf) together with pedigree correlations and field information.

Calving ease EBV’s subsequently involve considerable amounts of information and their accuracy is low without substantial amounts of records.

The Brahman Society have made provision for the calculation of birth weight and calving ease EBV’s. Members are encouraged to forward the relevant data to allow the calculation of these increasingly important EBV’s.

SELECTION OUTCOMES

Birth weight is the primary factor affecting dystocia but there are several interrelated factors that impact on the incidence of calving difficulties.

Producers are therefore advised to consider genetic, management and environmental factors to minimise the risk of dystocia.

Managing the herd with the goal of reducing calving difficulties should result in vigorous calves that achieve the desired growth rates along with improving the dams reproductive performance.

The following points may help minimise the risk of calving difficulties.

• Consider birth weight in conjunction with growth rates.
• Mate heifers and small cows to bulls that sire smaller calves and avoid bulls with excessive mature size.
• Develop heifers to breeding and calving weight targets.
• Ensure cows and heifers are in good not thin or fat condition (ie condition score 6).
• Group heifers separately to cows to allow suitable feed intakes and close observation of growth rates.
• Cull females with repeated calving problems and cull daughters of cows that have a record of calving problems.
• Select female replacements from well grown weaners since these stock will generally have larger pelvises.
• Monitor and record “easy” calving females and select female replacement from these cows.
• Do not use unproven bulls on heifers and small cows.
• Select bulls that match the size of the females and the nutritional management of the herd.
• Check the pedigree for extreme bulls, particularly on the maternal side (ie examine genetic contribution of the dams sire).
• Check that the genotype so called “smaller” bulls matches the phenotype.
• Because heifers are generally smaller than cows, they have an increased risk of dystocia. The size of heifers at breeding should be 65 per cent of their mature weight and 85 percent of their mature weight at calving.
• Know when to assist at calving and seek experienced assistance for difficult calvings.

TABLE 3: HERITABILITIES OF GROWTH TRAITS AND THEIR GENETIC CORRELATION TO TABLE 3 : BIRTH WEIGHT (GCBW)

TRAIT	HERITABILITY (%)	GCBW
Birth weight	44	–
Weaning weight	32	+0.58
Yearling weight	58	+0.61
18 month weight	50	+0.60
Gain birth – weaning	31	+0.38
Mature weight	84	+0.68
Source: Michigan State University