Using Pelvic Areas and Linear Body Measurements in the Selection for Reduced Dystocia Rates in Sussex Heifers

Dystocia (birth difficulty) in cattle is a problem which causes many health issues and economical losses. Dystocia is defined as prolonged or difficult parturition and it is a condition in which the first or, especially the second stage of parturition was markedly prolonged for more than six hours and the cow requires assistance (Abdela and Ahmed, 2016). Dystocia occurs when there is a failure in one or more of the three main components of birth: expulsive force, birth canal dilation, fetal size and disposition (Mee, 2008). Selection of animals based on pelvimetry can be used as a tool to prevent or decrease the cases of dystocia (Van Nieuwenhuizen et al., 2017). Fetomaternal disproportion is the main cause of dystocia in heifers and attempts to prevent it have focused mainly on decreasing the birth weight (BW) of the calf (Holm et al., 2014) and ensuring adequate breeding BW. Calf BW and dam pelvic area (PA) contributes 33 and 12%, respectively, towards dystocia in heifers (Holm et al., 2014). Breed effect on the incidence of dystocia is attributed to differences in the relative BW, pelvic structure and large variation in pelvis dimensions in some breeds (Nogalski and Mordas, 2012; Holm et al., 2014).
       
Measuring the pelvic area is becoming a very vital part of the herd management for most breeders in South Africa (SA) and it should become the basis for selecting female breeders especially in the beef cattle industry (Van Der Merwe, 2017); however, PA measurements are not done in most herds because measuring PA is an operation that requires skill and suitable equipment which is not always available to farmers (Van Rooyen et al., 2012). Heritability of calf BW and PA are reported to be 0.44 and 0.46, respectively, but heritable traits predict calving ease poorly in individuals (Holm et al., 2014).
       
Pelvic area has been seen as a reliable measurement influencing calving difficulty, as larger PA is associated with reduced calving difficulty (Murray et al., 2002) and it is used to identify potential problem heifers with small pelvic sizes (Micke et al., 2010) that may be at risk for dystocia at calving. Moreover, BW and age have both joint and independent associations with PA. In other words, if a number of heifers of the same BW are compared, then the older of those will have larger PA and similarly if a number of heifers of the same age are compared, then the heavier of those will have larger PA (Holm et al., 2014). Van Donkersgoed, (1997) and Holm et al., (2014), reported that calf BW is the most important determinant of dystocia, whereas PA tended to be associated with dystocia only when adjusted to calf BW. Pelvic area is associated with calving rate, it may also be associated with age at calving, which may result in a comfortable effect of PA on dystocia (Zaborski et al., 2009; Holm et al., 2014).
       
Pelvic area is commonly calculated by multiplying the pelvic height (PH) with the pelvic width (PW) which results in a rectangular area (Kolkman et al., 2009). However, to the greatest of our knowledge, use of pelvic areas and certain body measurements parameters in South African Sussex heifers has not yet been reported on literature. Hence, the aim of this study was to use PA measurements and external body measurement in the selection of replacement Sussex heifers to reduce dystocia amongst heifers at parturition, while improving their ease of calving. 

A total number of one hundred eighty-six (186) first calf Sussex heifers approximately 24 months old, weighing approximately 437 kg were used for this study. All heifers used for the study had a relatively good body condition score (BCS) with an average of three and weighed more than 65% of the mature female body weights of the Sussex breed. A number of six, two-year-old bulls, weighing approximately 800 kg were used for mating the 135 heifers during the first trial with a bull ratio of (1:35; 1:35; 1:35 and 1:30). The second trial consisted of 51 heifers with a bull ratio of 1:30 and 1:21. The fertility of bulls was assessed by a private veterinarian before the breeding season. Animals were managed extensively on the veld during the time of the trial, receiving production lick supplements to maintain their body weight. In order to exclude the camp effect, the heifers were rotated every two weeks among the eight camps during the study. The pelvises of all the heifers were measured once before breeding, using a method adapted from (Van Rooyen et al., 2012). The following formula was used to calculate PA:
 
 
 
The general procedure in taking pelvic measurements was to restrain the heifer in a chute using a light squeeze. A comfortable, normal standing position is best for this procedure. Faeces was removed from the rectum and the instrument caliper type pelvimeter (Rice pelvimeter; Manufactured, Studbook Bloemfontein) was carefully placed into the rectum according to the procedure of Van Zyl (2008). After inserting the pelvimeter into the pelvic area of the heifer, pelvimeter was gradually opened by applying light pressure on the handle.  The pelvimeter was then twisted from left to right to feel the ossified joint on the pubic symphysis, as a reference point to measure the height between the dorsa pubic tubercle on the floor of the pelvis and the sacrum (spinal column) at the top.
       
The pelvimeter was then turned 90° sideways to measure the width of the pelvis at the widest points between the right and left shafts of the ilium bones. This is the horizontal diameter of the pelvis (Van Zyl, 2008; Van Rooyen et al., 2012).  After that, pelvimeter was carefully pulled out in the same twisted position to measure the width between the left tuber ischii and the right tuber ischii. The pelvimeter was removed from the heifer. The pelvimeter after used on each heifer was thoroughly cleaned with water, disinfected with a mixture of gel (biosol) and disinfectant (Van Zyl, 2008). All measurements were taken in centimeters.
       
The following body parameters were measured according to methods described by Fourie et al., (2002): live weight (LW); hip height (HH); chest depth (CD); shoulder width (SW); hindquarter width (HW), birth weight (BW), body length (BL), sex of the calf and rump length (RL). These parameters were correlated with the PH, PW and PA. In addition, the heifers were assessed visually for body conformation (BC) and selection type (S), as described by the Sussex breed Standards of Excellence on a scale of 1-5. Conformation scores ranged from one (being very poor) to 5 (being very good). Rump slope (RS) scores ranged from 1 (being very flat) to 5 (being very droopy).
       
Live weight (kg) was measured following a 12-hour fasting period, shoulder height (cm) was measured vertically from the thoracic vertebrae to the ground (Fourie et al., 2002), chest depth was measured from the spianus to the oxyfoid process of the sternum (Fourie et al., 2002), the hindquarter width (cm) was measured between the left  thigh to the right  thigh, rump length (cm) was measured as the distance from the tuber coxae to the pin bone, hip height (cm) was measured as the distance from the ground just in front of the hind hoofs over the hook (hip), birth weight of the calve (kg), sex of the calf (female coded two and male one), body conformation and rump slope.
       
During the parturition process the calving ease score codes were used to score each heifer that calved. The scores ranged from one, (1=no assistance during parturition (normal), 2=heifer assistance as gently pull, 3=heifer assistance as hard pull, 4 =heifer cannot calf, 5=heifer calved a dead calf and 6=heifer calf with abnormal position). Analysis of variance were conducted to determine the statistical significance of the variables using SPSS. A stepwise regression analysis was carried out to determine the individual influence of body measurements on PA. Statistical significance was set at (P<0.05).

Table 1 depicts the means and standard deviations of the external body measurements and pelvic areas, body length 149.24±7.72 cm, chest depth 67.43±4.25 cm, hip height 128±5.57 cm, hindquarters width 52.92±3.72 cm, rump length 47.47±2.43 cm, shoulder height 124.55±4.46 cm; calves birth weight 35.38±5.53 cm and internal pelvic areas (PH 18.0±0.74 cm and PW 15.88±0.75 cm) that were measured during the trial in two-year-old Sussex heifers. The difference in pelvic size is usually attributed to the difference in PH (Anderson and Bullock, 1994; Van Rooyen et al., 2012), but in agreement with Holm et al., (2014) who displayed that breed effect on the incidence of dystocia is attributed to differences in the relative birth weight, pelvic structure and large variation in pelvis dimensions in certain breeds.
 

       
One of the aims of this study was to determine if a correlation between calving ease score (CES) and pelvic dimensions exists. Since CES is an ordinal variable and not a continuous variable the non-parametric Spearman’s rho was conducted to determine the correlations between calving ease score and pelvic measurements. The results of the Spearman’s correlation test are shown in Table 2 indicating that there is a negative correlation between CES and PA, r = - 0.266, P<0.05. The strength of this association is therefore, weak. These results show that as the pelvic area increases, the lower the chances of heifers to experience dystocia. This finding is in agreement with the study of Briendenhann (2010), who revealed that a disproportionally large calf size at birth in relation to the dams PA is one of the biggest causes of dystocia.
 

       
A moderate negative correlation between CES and PH, r=-0.407, P<0.05 was recorded. These results reveal that as the PH increases there is a lower risk for Sussex heifers to experience dystocia (R²=0.17). The results of a one-tailed Spearman correlation test indicate a non-significant (P>0.05) correlation between CES and PW (r=-0.069). This is in contrast to Briedenhann (2010) study, who reported that PW is more important in Bos taurus cattle while PH is more important in Bos indicus cattle to predict dystocia.
       
The study furthermore explored to determine if there is a significant relationship between CES and the following variables: live weight 18-months (LW18 m), live weight at calving (LWC), calf gender and calf birth weight (BW), (Table 3). Pelvic size, calf birth weight and their ratio are the most important for predicting dystocia in Sussex heifers (Van Nieuwenhuizen et al., 2017). Van Nieuwenhuizen et al., (2017) reported that calf birth weight is influenced by genetic, breed of the sire and dam, as well as the nutritional factors and gestation length of primiparous dam.
 

       
Mellor and Diesch (2006) reported that larger heifers have a larger pelvic opening and have higher birth weights. In this study the correlation between CES and LW18 m was (r=0.124, P>0.05). Furthermore, no correlation (P>0.05) between CES and LWC was found. A negative correlation between CES and calf gender was recorded (r=-0.355, P<0.05). Moreover, the chances of a heifer to experience dystocia are more when a bull calf is born compared to heifer calves. These findings are in agreement to Johanson and Berger (2003), who stated that the odds of male calf needing assistance was 25% greater than when the calf was female.
 
A positive correlation between CES and BW, (r=0.312, P<0.05) was found. Higher the birth weights of the calf, higher the probability of a heifer to be prone for dystocia. These finding are in contrast to the report of Johanson and Berger (2003), where they revealed that the significance of calf birth weight diminish when gender of the calf is included in the analysis, while in agreement with Holm et al., (2014), who reported that calf birth weight and pelvis area contribute 33 and 12%, respectively, towards dystocia in heifers. Deutscher (1991) indicated that the major cause of dystocia is disproportion between the offspring birth weight and dam’s pelvic area.
       
Due to the fact that there was a positive correlation between PH, PA and CES a regression analysis was conducted to determine which body measurements explain the largest amount of variation in the PA and PH variables (Table 4). Pearson correlation was conducted to determine if a straight-line correlation exist between PH and body length (BL), chest depth (CD), hip height (HH), hindquarters width (HW), rump length (RL) and shoulder height (SH). A stepwise multiple regression was conducted to evaluate whether all these body measurement variables were necessary to predict the PA variable. Only the chest depth (CD) variable made a statistical contribution to the model and were entered into the regression model. This resulted in a significant model R²=0.344 adjusted. The adjusted R² value of 0.344 indicates that approximately 34% of the variability in the PA could be predicted by the CD variable.
 

       
Van Nieuwenhuizen et al., (2017) reported that the increase in body measurements is related to an increase in pelvic dimension, this applies to body length, heart girth, shoulder height and age of the heifer in their study. The relationship between pelvic dimension and body measurements are still unclear for the Brahman, Nguni and Bonsmara cattle breeds. These finding are in agreement with the current study in Sussex heifers, as it is only chest depth which moderately contributed in the prediction of PA.

The results of this study indicate that PA measurements, measured prior to mating have a moderate significance when sex and BW of the calves are included in the analysis. Overall it can be concluded that there is a high relationship (P<0.05) between PA dimensions in Sussex heifers as they all have a direct influence on the pelvic size. It can also be concluded that PH plays a bigger role compared to PW in predicting dystocia specifically in Sussex heifers. The pelvic area measurements, BW and gender of the calf are the most important parameters in predicting dystocia in Sussex heifers. The results revealed that the relationship between external body measurements and pelvic dimensions seems to be unclear as it is only chest depth that can be used in predicting PA. Further studies on the relationship between certain body measurements and pelvic dimensions are recommended.