What contributes to calving difficulty and future reproduction? University of Nebraska researchers evaluated 550 spring-calving, two-year-old, first-calf heifers over a three-year period to determine just that.
The heifers were composite MARC II yearlings mated to Angus sires that were either low (-0.9 lb.) or high (+ 6.2 lbs.) in birthweight EPD.
At calving, heifers were given a calving difficulty score (CDS) on a 1 to 5 scale (1=no assistance, 5= C-section). In addition, a gauge attached to the calf puller recorded traction pressure. Following is a summary of the results:
Physical traits of the dam - Heifers that were heavy at birth experienced more calving difficulty because they had heavier birthweight calves, which was probably due to genetics.
Heifer weight, hip height and condition score at 12 and 22 months of age did not affect CDS.
However, dams requiring a C-section had significantly smaller pelvic areas at 12 and 22 months of age. There were no significant differences in pelvic areas among the other CDS groups. Pelvic area was the second most important trait accounting for calving difficulty; thus, yearling pelvic measurements could be useful in detecting heifers requiring C-sections.
Traits of the calf - Traction pressure significantly increased as CDS increased. However, the traction pressure system was able to detect only a slightly larger amount of the variation affecting dystocia than the five-point scoring system Birthweight increased as CDS increased and was the most important factor of those evaluated, accounting for approximately 33% of the total variation in calving difficulty.
There was a tendency for CDS to increase as external measures of calf size increased (head circumference, foot circumference, width of shoulders and depth of chest). However, the results were not consistent across CDS groups and width of hips was not related to CDS. Of the measurements taken, head circumference and width of shoulders seem to be the best indicators of degree of dystocia.
Calf vigor decreased as CDS increased up to CDS 4. Score 5 was not different from CDS 1, indicating that calves born through C-section did not experience any more stress than calves born unassisted.
Sire effects - Calf birthweight was significantly different between low- and high-birthweight sire groups (72.8 vs. 79.6 lbs.). Percent calving difficulty was lower for heifers bred to the low EPD sires (37% vs. 49%).
Calf head and foot circumferences were significantly different between low- and high-EPD sires, even when adjusted for calf birthweight. No significant differences were found for width of shoulders, width of hips or depth of chest among sires.
Reproductive traits - No differences were found among CDS groups in percentage of heifers having estrous cycles before breeding season.
Significant differences were found in conception date between CDS 1 compared with 3 and 4. As CDS increased from 1 to 4, conception date increased from June 13 to June 24. Average conception date for C-section heifers was June 15, similar to that of CDS 1.
There were no significant differences for percentage of heifers pregnant after the 75-day breeding season among CDS groups. This is counter to previous U.S. Meat Animal Research Center (MARC) research reporting that dystocia resulted in lower pregnancy rates.
No significant differences were observed among CDS groups for second-calf birthweight. However, second calf birthweights were heavier than those of the first calf.
Calf growth - Calves delivered by C-section were lighter at weaning than other calves, but had similar slaughter weights. Researchers speculated that the surgery could have negatively affected milk production of the heifers.
The only calves gaining significantly slower in the feedlot were the CDS 1 groups. These calves were the smallest at birth and may have had less genetic growth potential.
Weather effects - Average winter temperatures increased (became warmer) each winter during the three-year study.
Calf birthweights were significantly higher after the cold winter compared to the warmer winter (+10.1 lbs.). Moreover, calving assistance rates declined over the three-year period (58% to 47% to 35%).
These results suggest that greater calf birthweight and increased calving difficulty may be expected in the spring following severe winter temperatures.
From this information, researchers suggest the following management strategies:
* Selecting sires with low-birthweight EPDs helps produce calves with reduced bone size and birthweight, along with less calving difficulty.
* Major calving difficulty may be reduced by culling yearling heifers with heavy-birthweight calves and small pelvic measurements
* Climate can affect calving difficulty. Calving difficulty can affect subsequent conception date.
For more information contact Gene Deutscher, University of Nebraska, at 308/532-3611 ext. 136.