Research reported in this thesis was designed to quantify the impact of dystocia in heifers on beef cattle production in the Waggamba Shire of Southeast Queensland in the early 1990?s. The research was also designed to identify factors influencing the incidence of dytocia in beef heifers. It was found that the average beef cattle breeding property in the Waggamba Shire at the commencement of this study was 2090&plusmn;7.7 hectares and ran a total of 336&plusmn;1.6 breeders, of which 78&plusmn;1.1 were heifers. A large proportion (66%) of the breeders were either Hereford or Shorthorn. Seventy nine percent of properties had a 4&plusmn;0.24 month joining period falling between the months of September to March. All properties incorporated one or more of the subjective appraisal criteria of frame size, temperament, age and "soundness of type" when selecting heifers for joining. Nineteen percent of properties weighed heifers prior to joining to assess if critical mating weight had been attained. Forty percent of properties routinely had pregnancy diagnosis performed. The mean percent of calves weaned from the heifers that were pregnancy tested in calf was 81.9%. In the cow herd the mean percentage of calves weaned from pregnancy tested cows was 95.8%. The mean incidence of dystocia in heifers from surveyed properties was estimated to be 10% with a mean of 1.6% of heifers dying from dystocia each year. A mean of 13 working days was spent observing or assisting heifers each calving season. A benefit-cost analysis, incorporating a gross margin analysis and net present value calculations showed there was a $0.13 increase in the gross margin per hectare for each percentage decrease in the dystocia rate, representing an increase in the annual total gross margin of approximately $272 for each percent decrease in heifer dystocia for a 2090 hectare Waggamba Shire beef cattle property. Net present value calculations revealed that over a ten-year period, a property with five-percent dystocia in the heifers would have $34.81 per hectare extra income than a property with 40 % dystocia. This equated to $72440 extra income for an average sized Waggamba Shire property of 2090 hectares at the end of ten years. A significantly higher mean birth weight for calves requiring veterinary assistance for delivery compared to unassisted calves (p<0.01) was found. Examination of pelvic area data showed that assisted heifers delivering male calves had significantly larger mean pelvic areas and higher pelvic area:calf birth weight ratios than assisted heifers delivering female calves (p<0.01). It was found that male calves required a larger pelvic area for delivery compared to female calves, and also needed, on average, an extra 1.94 cm2 of pelvic area per kilogram of calf compared to the females. Results also showed that heifers requiring assistance had significantly higher body condition scores at calving (p<0.01). Heifers delivering male calves had a significantly higher mean body condition score than those delivering female calves (p<0.05). Results from analysing seasonal effects showed that calvings for which veterinary assistance was sought occurred significantly earlier in the calving season compared to unassisted calves (p<0.05). A retrospective study showed a positive correlation between yearling heifer body weight and yearling heifer pelvic area (p<0.01, r=0.29). This demonstrated a mean increase in yearling heifer pelvic area of 0.174 cm2 for each 1kg increase in body weight. The r2 value indicated that 8.9% of the variation in yearling heifer pelvic area was accounted for by changes in heifer body weight. The mean yearling pelvic area of heifers requiring assistance at calving was 182.2 &plusmn; 3.1cm2 compared to 193.5 &plusmn; 1.2 cm2 for heifers not requiring assistance. The difference of 11.3 cm2 was significantly different from zero (p<0.01). The mean Pelvic Area : Birth Weight ratios for assisted and unassisted calvings were 5.4 &plusmn; 0.01 cm2/kg and 6.6 &plusmn; 0.08 cm2/kg respectively and were significantly different (p<0.01). Calf birth weight was not significantly correlated with the heifers? yearling body weight. However, the type of delivery when calving as a two-year-old was significantly affected by the heifers? weight as a yearling (p<0.01). For the sub-population where yearling heifer body weight was measured, the mean yearling weight for the 63 heifers having calving difficulty as two-year-olds was 254&plusmn;5.3 kg, compared to 284.5&plusmn;2.95 kg for the 227 heifers not requiring assistance. The percentage of assisted calvings for the whole heifer population studied was 16.8%. For the sub-population where birth weight was measured, the mean calf birth weights for the 140 assisted and 655 unassisted calvings were 33.6&plusmn;0.36 kg and 31.0&plusmn;0.17 kg respectively and were significantly different (p<0.01). Analysis of variance revealed calf birth weight was significantly different (p<0.01) between males and females (32.2&plusmn;0.25 kg compared to 29.9&plusmn;0.21 kg respectively). Of the assisted calvings, 65% were male calves. The mean second trimester rainfall for assisted and unassisted calvings was 86.5&plusmn;6.0 mm and 110.4&plusmn;2.8 mm respectively. Chi-squared analysis showed an association between assisted birth numbers and rainfall category in the second trimester (X2 =13.4, p<0.01, df=1). There was also a significant association between assisted birth numbers and rainfall category in the third trimester (X2 = 11.9, p<0.01, df=1). The differences resulted from significantly more heifers requiring assistance when rainfall in the second trimester was less than the mean, and rainfall in the third trimester was greater than the mean. When all three trimesters were considered together, there was a significantly higher than expected number of assisted births for the High/Low/High and Low/Low/High groups of heifers (p<0.01), where High and Low refers to whether rainfall was above or below the mean during the trimester under investigation. Analysis of variance revealed significant differences in mean calf birth weight between the High and Low rainfall groups in the first and third trimesters. Mean birth weights of calves from the Low first trimester rainfall group were 1.6kg heavier than those from the High first trimester rainfall group (p<0.01), and mean birth weights of calves from the High third trimester rainfall group were 0.9kg heavier than those with low rainfall in the third trimester (p<0.02). When the analysis focused on the upper and lower rainfall quartiles, the significant difference between mean calf birth weights when grouped by High or Low rainfall in the first trimester was maintained. Mean calf birth weights from heifers exposed to lower quartile rainfall in the first trimester were 3.2kg heavier than the upper quartile group (p<0.01). This latter analysis failed to find a significant difference in mean calf birth weights between those exposed to different rainfall volume during the third trimester. In relation to month of calving, the mean calving dates for unassisted and assisted births of the 25th August and the 13th September respectively, were found to be significantly different (p<0.01). The mean calving date for the whole group was the 28th August. Two studies comparing heifers fed from approximately three months of gestation to achieve a mean condition score of < 3.5 at calving (Fat group), with those fed to achieve a condition score of < 3.0 at calving (Thin group) showed no significant difference in dystocia incidence between groups. Assessment of blood parameters suggested that heifers from the Fat group had increased levels of stress and tissue trauma associated with parturition compared to heifers from the Thin group. Specifically, heifers in the Fat group had significantly higher postpartum plasma CPK concentrations (p<0.05), significantly higher postpartum blood glucose concentrations (p<0.05), a trend towards higher postpartum plasma fibrinogen concentrations (p=0.09) and significantly lower plasma normalised calcium concentrations (p<0.01). Results revealed serum progesterone concentrations during gestation in the Thin group were significantly higher than in the Fat group at the May sampling time (p<0.05) of the First trial and trended that way in August for the Second trial (p=0.07). There was a trend for serum progesterone concentrations to be higher in the Fat group 24 hours postpartum (p=0.07), with the difference becoming significant at 48 hours postpartum (p<0.01). There were no differences in serum progesterone concentrations detected between the Fat and Thin groups at parturition. The presence of progesterone in subcutaneous fat was established, however there was no indication of it influencing serum progesterone concentrations during gestation or at calving. In the second trial, pelvic width was a mean of 0.6 cm wider in the Fat group compared to the Thin group at approximately two months prior to calving (p<0.05). No significant group or group by measuring time interactions were noted for pelvic height or pelvic area measurements. There was a trend for heifers in the Thin group to produce heavier calves than heifers in the Fat group (p=0.1). Two final studies were undertaken to investigate whether the seasonal and nutritional influences on heifer calving ability identified in the previous studies might be associated with dietary protein concentration of the heifers in the first two trimesters. Heifers were divided into High/High, High/Low, Low/High and Low/Low groups according to the dietary protein concentration they received in the first and second trimesters respectively. In both trials, mean calf birth weights and the numbers of heifers experiencing dystocia were not significantly different between groups. In the second trial, calves from the Low/Low and Low/High groups had significantly larger left and right cannon bone lengths (p<0.05). In the first trial the heifers in the High/High group had a greater mean pelvic height at the end of the first trimester (p<0.05), and in the second trial there was a trend heifers in the High/High group to have a greater mean pelvic height and pelvic area at the end of the second trimester (p=0.1 and 0.08 respectively). A significantly larger mean frame score occurred in the High/High group (p<0.05) of the second trial at the end of the second trimester. There was a trend for placental weights to be heavier for assisted calvings in both trials (p=0.07 and p=0.08 respectively). The correlation coefficient between placental weight and calf birth weight was significant (p<0.01, r=0.71) and demonstrated a mean increase in calf birth weight of 2.6 kilograms for each 1 kg increase in placental weight. The r2 value indicated that approximately 50% of the variation in calf birth weight was accounted for by changes in placental weight. In the second trial, heifers in the High/Low group had a significantly lower placental weight (p<0.05). When the data from the first trial was analysed based on delivery type, mean blood glutathione peroxidase activity was significant different, or had trends towards significance, between assisted and unassisted heifers throughout gestation except for one measurement at three months of gestation. Heifers requiring assisted deliveries had significantly higher GSH-Px activity at all of these times compared to unassisted contemporaries. Although this finding wasn't duplicated in the second trial, it was also not contradicted. In specifically addressing the aims of this study, it was concluded that the mean dystocia incidence in this district was approximately the same as that documented for beef heifers in previous studies. Dystocia caused significant economic loss to beef cattle production in this district. Factors identified by this study that were directly associated with increased dystocia incidence were: (a) Lower mean body weight of heifers at 12 months of age. (b) Lower mean pelvic area of heifers at 12 months of age. (c) Lower ratio of pelvic area of heifers at 12 months of age to calf birth weight. (d) Lower mean rainfall during the second trimester of gestation for heifers bred to calve as two-year-olds. (e) Higher mean body condition score at calving. (f) Higher mean calf birth weights. (g) Male calves. (h) A later mean calving date for heifers calving as two-year-olds. (i) Heavier placental weights at calving.