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
|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|
|Net calf crop percentage||71.1|
|Source: University of Minnesota (1992)|
Note: In general dystocia occurs when:
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.
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:
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.
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.
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.
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|
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:-
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.
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”
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.
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 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.
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.
TABLE 3: HERITABILITIES OF GROWTH TRAITS AND THEIR GENETIC CORRELATION TO TABLE 3 : BIRTH WEIGHT (GCBW)
|18 month weight||50||+0.60|
|Gain birth - weaning||31||+0.38|
|Source: Michigan State University|