Dystocia strategies can help ease delivery problems for cows

Editor's Note:This month Dr. Randall Ott, DVM, MS, diplomate, American College of Theriogenologists addresses dystocia and strategies to help prevent problem deliveries in cattle. Ott is a professor at the University Of Illinois College Of Veterinary Medicine in Urbana. He made presentations on the subject at the American Veterinary Medical Association meeting in Nashville.

Q: What impact can dystocia have on breeding programs?

A: Dystocia is a major concern when heifers deliver their calves.In a recent survey by the National Animal Health Monitoring System (NAHMS)16.7 percent of heifers required assistance at calving. Only 2.7 percentof cows required assistance. Dystocia decreases production. Common resultsare delayed conception in the next breeding season or a failure to rebreed.In some cases, injury or death of the calf and/or cow is a result.

In earlier studies dystocia rates were highest with heifers calving as2-year-olds (29.7 percent). The dystocia rate continued to decline untilcows become mature at 5 years of age. For example, in 3-year-olds the ratedropped to 10.5 percent, and 4-year-olds, 7.2 percent.

The negative impact on subsequent pregnancy rate of cows can be substantialif caesarian section is required. In a study of more than 1,300 calvings,the pregnancy rate of cows that required caesarian section was only 52 percent.The subsequent pregnancy rate of cows not affected by dystocia was 79 percent.

Q: What causes dystocia?

A: The rule is larger calf size at birth means more dystocia.The major cause of dystocia is relative fetal oversize, a disproportionatelylarge calf birth weight relative to the pelvic area of the cow.

Above a threshold weight, there can be a 1 percent increase in dystociafor every 1 pound increase in birth weight.

Large mature weight bulls sire heavier calves at birth. One of the mostcommon ways that a calf becomes too large for the cow's pelvic area is throughthe use of sires that have a large mature weight. Weaning weight, yearlingweight and mature weight are heritable and are positively correlated withbirth weight. Over the past several decades, producers have selected bullswith large mature weights to increase weaning and yearling weights, whichare important production traits. As a result, birth weights have increased,and necessarily, so have dystocia rates.

Q: If the trend is to grow bigger calves, how do you prevent dystocia?

A: The best tool currently available is Expected Progeny Difference(EPD). EPD is a prediction of the transmitting ability of a parent animal.The statistical technique used to calculate EPDs are known collectivelyas best linear unbiased prediction.

EPDs include performance information on a bull's ancestors and relatives.The more information included in the calculation of the EPD, the higherthe accuracy of the EPD. EPDs estimate progeny performance as comparedwith the average of that particular breed. EPD is six to 10 times more accuratethan an individual's performance.

EPDs provide an estimate of a particular bull's ability to pass on certaintraits. Traits such as weaning and yearling weight are very important forbeef production. The development of EPDs for these and other traits werean important advance for the cattle industry.

EPDs can help your clients reduce or control dystocia rates in a herd.An EPD for birth weight would indicate the difference in performance thatis expected from the progeny of a sire compared with that expected fromthe progeny of the "average" bull of the particular breed to whichthe bull belongs.

EPDs for birth weight are expressed in pounds (positive or negative).Positive EPDs are desirable for traits like weaning and yearling weights,but negative EPDs might be needed for birth weight.

As an example, consider an Angus bull with a birth weight EPD of +2.4.We could expect offspring produced by the bull to weigh 2.4 pounds moreat birth than offspring of the "average" Angus bull. This wouldnot be a bull to breed to heifers. A bull with an EPD of 0 or a negativenumber would be preferred.