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Every year livestock producers replace cows they cull from their herds with young animals often selected from progeny of the cows they currently own. Since replacement rates for beef cattle in Nebraska tend to average between 16 to 20 percent annually, this is not a trivial undertaking. Much of the talk among producers tends to concentrate on muscling and other physically observable characteristics, and the genetic composition of animals to be selected as replacements. These criteria are important, but what must also be considered is the ability of the selected replacement animals to perform reproductively. Producers expect cost to be reduced by using heifers that become pregnant easily, re-breed, and have fewer problems calving, no dystocia. Cows that remain a productive part of the herd longer result in larger savings, since the value of culled animals is about half that of a productive one.
Data used to create a Maturity Index (MI) and relate it to pregnancy rates and dystocia, were taken from two previous experiments used to identify breeding readiness of grouped beef heifers, conducted at the University of Nebraska-Lincoln Gudmundsen Sandhills Laboratory (GSL). This work has been published in the 2002 and 2005 Beef Cattle Reports, pp. 4-7 and pp. 3-6, respectively. These studies included 500 heifers, but only those heifers that had a calf were included in this analysis (n=448).
It has been found that replacement heifers perform differently according to maturity, as measured by the MI score which is predicted by: weight at breeding, birth weight, age at breeding, dam’s weight and nutritional level between weaning and breeding (see Equation 1).