AS-445
animal sciences beef
calving Difficulty in Beef cattle
Calving difficulty (dystocia) can increase calf losses, cow mortality, and veterinary and labor costs, as well as delay return to estrus, and lower conception rates. In two studies at the U.S. Meat Animal Research Center (MARC), Clay Center, Nebraska, calf losses within 24 hours of birth averaged 4 percent for those born with little or no assistance compared to 16 percent for those requiring assistance. Percent calf mortality increased by a 0.35 percent per pound increase in birth weight. In a Hereford herd at the Miles City, Montana, Experiment Station, 57 percent of all calf losses were reported to be due to dystocia.
Researchers at MARC noted that the percentage of cows detected in estrus during a 45-day A.I. period was 14 percent lower in those requiring assistance than in those calving with no difficulty. Conception to A.I. was 6 percent lower in cows experiencing dystocia than in those with no dystocia. Pregnancy rate after the entire breeding season (70 days) was 16 percent lower in cows that had been assisted (85 percent vs. 69 percent). At Miles City, pregnancy rate among cows that had caesarean deliveries was 26.6 percent lower (52.4 percent vs. 79.0 percent) than the herd average.
Factors Affecting Dystocia
Calving difficulty is influenced by many factors, including:
1.	Age of dam;
2.	Calf’s birth weight;
3.	Sex of calf;
4.	Dam’s pelvic area;
5.	Dam’s body size;
6.	Gestation length;
7.	Breed of sire;
8.	Breed of dam;
9.	Sire’s genotype;
10.	Dam’s genotype;
11.	Nutrition of dam;
12.	Condition of dam;
13.	Shape of calf;
14.	Position or presentation of fetus;
15.	Geographic regions; and
16.	Other unknown factors.
Several of these factors are interrelated in a complex manner. For example, larger cows of larger breeds have larger pelvic areas, which would be an aid to calving. However, larger cows of larger breeds have proportionately bigger calves, which tends to offset the advantage of a larger pelvic area.
Age of Dam
Table 1 is a summary of calving data from MARC and Colorado State University (CSU), relating age of dam to calving difficulty. These data illustrate that age of dam has a profound effect on the incidence of dystocia. First-calf, two-year-old heifers represent the greatest source of trouble to the beef herd owner. Difficulty in two-year-olds is three to four times as high as in three-year-olds, and three-year-olds have about twice as much difficulty as four-year-olds. By the time a cow reaches four to five years of age, dystocia problems are minimal. Calving difficulty in MARC Hereford and Angus cows was higher than in CSU Hereford cows, presumably because the former tended to be mated to larger exotic sires, whereas the latter were mated only to Hereford sires.
Table 1. Effect of dam’s age on calving difficulty.
	Research station
Dams	
age (years)	MARC	CSU
Percent calving difficulty