Breeding dairy cattle is an inexact science, so many gene-linked traits must be considered. Some of the major ones are quantity of milk produced, its fat and protein content, mothers' pregnancy rates, calving ease and, most recently, stillbirth rate. Such evaluating of genetic traits has allowed dairy farmers to increase milk production to all-time highs.

Scientists recently added calf survival to a series of calculations that lead to what's called a Lifetime Net Merit score. This is an economic evaluation of a bull and--by extension--what he will transmit to his daughters and granddaughters. Sadly, about eight percent of calves born do not survive beyond 48 hours and are considered stillborn.

According to Duane Norman, the research leader of the Animal Improvement Programs Laboratory (AIPL) in Beltsville, Md., when the scientists select traits for Lifetime Net Merit, the animal's economic value, level of hereditary influence and amount of variation are all taken into account. Norman oversees a database that includes not only important yield traits, but also fitness traits that affect animals' health, vigor and profitability, such as mastitis resistance, fertility and longevity.

The AIPL is operated by the Agricultural Research Service, the U.S. Department of Agriculture's principal research agency.

According to AIPL geneticist Paul VanRaden, each genetic trait is given a percentage of emphasis for calculating Lifetime Net Merit. For example, milk production accounts for 46 percent of the score, while calving ease and stillbirth are combined into a calving ability index valued at 6 percent. Stillbirth data are collected on-farm by dairy workers and provided to the National Association of Animal Breeders based in Columbia, Mo.

Having information like this about genetic evaluation traits allows breeders to make selections that best achieve their goals. This may involve doing "corrective breeding" to strike a productive balance. For instance, a dairy farmer may mate a cow with a history of calving difficulty to a sire that's demonstrated his offspring are born relatively easily.

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