August 2010 Articles

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Managing to Avoid Calving Difficulty

Managing to Avoid Calving Difficulty Many people find the time leading up to calving season filled with excitement to see what the next calf crop will look like. I am sure that some also dread some parts of calving season as well, and I bet that part involves using a calf puller! There is not much joy to be found in losing a calf or heifer due to dystocia. The good news is that through good management, several causes of dystocia can be avoided. There are two main causes of dystocia, excessive calf birth weight in relation to the dam’s pelvic area and abnormal calf presentation. There are three major factors affecting the birth weight of a calf. The first is the sex of the calf; bull calves weigh more than heifers. Obviously, we cannot control this efficiently. The second is nutrition. This can be controlled by monitoring the feed intake of heifers and their body condition. The third is the genetic influence of the bull on birth weight. This can be controlled through proper sire selection. One common problem to avoid is improper selection and feeding of your replacement heifers. Small, underdeveloped heifers have more problems with dystocia than larger, properly developed heifers. The reason for this is their pelvic area. The larger heifers usually have a larger pelvic area. Heifers should be fed to weigh approximately 75 percent of their mature weight before breeding (usually around 14 months). They should be fed to a body condition score of 5 or 6 but should never be overfed to cause excessive fat buildup. This can also contribute to dystocia as well as decreased milk production, due to excessive fat in the udder. Structural traits in cattle tend to be highly heritable, and pelvic area is no exception. This means there is a large genetic influence on pelvic area, which results in rapid response to selection. However, pelvic area is genetically correlated with many other traits, so selection for increased pelvic area alone can result in other traits changing for the worse. For example, selecting for increased pelvic area can result in increased birth weight and mature weight. Pelvic measurements should be taken prior to the first breeding season and combined with a reproductive tract examination. The best use of pelvic measurements is to identify abnormally small or abnormally shaped pelvises and cull those animals. These situations left unidentified are often associated with extreme dystocia, resulting in cesarean delivery and even death of the calf or cow. Proper sire selection can solve many problems associated with excessive birth weights. Beef producers have at their disposal excellent tools in the form of Calving Ease and Birth Weight EPDs to assist in the management of calving difficulty. While these two EPDs are highly related and are under the influence of many of the same genes, they describe different traits associated with management of calving difficulty. Consider the following example for two bulls: Calving Ease EPDs predict the ease with which a sire’s calves will be born when he is bred to first-calf heifers. Calving Ease EPDs are expressed as a percentage of unassisted births. Using the example, if we mated both bulls to 100 first calf heifers, we would expect nine more heifers to calve unassisted due to the difference in CE EPD of 9 percent between the two bulls. Higher CE EPDs reflect more calving ease. The Birth Weight EPDs for the two bulls suggest that Bull A’s calves would weigh 2.5 pounds less on average at birth compared to calves sired by Bull B, but we really don’t know if this extra 2.5 pounds will likely lead to any calving problems. There is a strong genetic correlation between calving ease and birth weight, suggesting that sires with favorable CE EPDs also tend to have lower BW EPDs. However, this relationship is not perfect. Calving Ease EPDs reflect multiple factors that contribute to calving ease genetics, including birth weight. Birth weight EPDs, on the other hand, only predict differences in genetics for birth weight. While both EPDs provide opportunities for managing calving difficulty in heifers, Calving Ease EPDs directly estimate the trait of economic consequence – calving ease, as reflected in the number of heifers that are expected to calve without assistance. Therefore, Calving Ease EPDs should be utilized as a primary tool in selecting heifer bulls, in conjunction with other EPDs of economic importance.