Most calving problems occur in first-calf heifers. The greatest cause is a disproportion between calf size and dimensions of the dams birth canal. Calving management requires frequent observation (every two hours, maximum) of the heifer herd so obstetrical problems can be detected and corrected early. Time to assist calving is an experienced judgment call and you must be familiar to determine if the process is occurring normally. There are excellent textbooks that describe the calving process and review some common obstetrical problems and visit with your veterinarian for recommendations.
Research at Miles City, Montana by Dr. Bob Bellows and his group sheds light on when to assist. The research is outlined as follows: heifers (280 head) received assistance either early or late in labor (Stage 2). The early-assistance group had the calf delivered with obstetrical assistance as soon as the calf's feet were visible protruding from the dam's vulva, regardless if the calf could have been delivered without assistance. The late-assistance group received assistance on an emergency only basis, i.e., experienced herdsmen judged that without assistance the calf and possibly the dam would be lost. They assisted 88% of the early-group heifers and 18% of the late-group heifers. Results: (early versus late assist): dead calves, none; calf vigor at birth, no difference; dams in heat by beginning of breeding, 91% and 82%; subsequent pregnancy rate (45-day breeding), 92% and 78%; daily calf gains, 1.74 and 1.63 pounds. This research was conducted by experienced herdsmen trained to give correct obstetrical assistance, using good facilities and equipment, and no calf was assisted until the dam's cervix was fully dilated. This research confirmed autopsy findings that showed assisting too early was not the problem, but assisting too late was. A heifer should be restrained, examined vaginally, and an assisted birth proceed if significant progress in delivery has not been made after 1 hour of intense labor.