Can We Improve Fertility And Mastitis Resistance Through Selection? (1/31/08)
Written by Kent Weigel, University of Wisconsin
In the past decade, a great deal of emphasis has been put on developing and refining methods to improve ‘fitness’ traits or ‘health’ traits through genetic selection. This effort has involved scientists, breeding companies, and producers; both domestically and overseas. For a dairy cow, it’s hard to imagine any traits that are more critical to fitness and health than fertility and udder health. In fact, mastitis and infertility are the leading reasons for culling on U.S. dairy farms, and collectively these two problems account for more than half of the annual turnover rate in the national dairy herd.
Can we improve these traits through selection, or do we just need to do a better job managing our cattle? When I was in graduate school 17 years ago, the popular line of thinking was that ‘low heritability’ traits, like fertility or mastitis resistance, should be addressed through changes in management, not selection. If a trait has low heritability, that means that most of the differences we see between animals are due to management or environmental causes, rather than genetics. But in recent years we’ve learned that large differences do exist between sire families in these traits.
It’s true that producers with reproduction or milk quality problems should review their management practices and protocols ?rst, but once these issues are corrected they can make additional gains by considering such traits during sire selection as well. Why would anyone want to use a bull whose daughters are known to be prone to infertility or mastitis based on progeny testing in dozens or even hundreds of other herds?
Direct selection for mastitis resistance isn’t yet possible in the U.S., because we don’t have a national system for recording clinical episodes of this disease. However, in Somatic Cell Score (SCS) we have a very good indicator trait that can be used for the purpose of ranking dairy sires according to their daughters’ propensity to mastitis infections. The genetic correlation between mastitis and SCS is very high – around 70%, and somatic cell score is recorded on the vast majority of U.S. dairy farms as part of the DHI milk recording program. Genetic evaluations for SCS have been provided by the USDA-ARS Animal Improvement Programs Laboratory since 1994..
Daughters of the best and worst bulls for SCS differ by nearly a full point. Average bulls, like Wrangler, will have daughters with around 3.00 SCS in first lactation, which corresponds to a somatic cell count of about 100,000 cells/ ml. First lactation daughters of the best bulls, like Billion, will have somatic cell count of approximately 75,000 cells/ml, whereas first lactation daughters of the worst bulls for SCS, like Mission, will have somatic cell count of about 134,000 cells/ ml. Research shows us that a 1-unit change in SCS corresponds to a 13% change in the lactation incidence rate of clinical mastitis, so the infection rate of first lactation daughters of Mission should be about 11% higher than that of first lactation daughters of Billion.
The ARS Animal Improvement Programs Laboratory introduced sire rankings for Daughter Pregnancy Rate (DPR) in 2003. Data consist of insemination records, pregnancy examination outcomes, and calving dates reported within the DHI milk recording program (often electronically through transfer of data that were recorded previously in on-farm herd management software programs). Genetic evaluations are expressed in terms of 21-day pregnancy rate, which refers to the percentage of eligible cows (i.e., non-pregnant cows that have passed the voluntary waiting period) that become pregnant during a given 21- day period of time.
Daughters of the best and worst bulls differ by more than 6% in daughter pregnancy rate. Producers who are more comfortable using days open as a measure of reproductive effciency can take advantage of the fact that a 1% increase in daughter pregnancy rate corresponds to a 4-day decrease in days open. Average bulls, like Bob, will tend to have daughter pregnancy rates around -0.40. Production and reproduction are negatively correlated, therefore, the average active A.I. sire is slightly less than zero. The 21-day pregnancy rate for daughters of the best bulls for DPR, like Alfie, will be about 3% above average, which means that they will have about 12 fewer days open per lactation than the average cow. Daughters of the worst bulls for DPR, like Harry, will have 21-day pregnancy rates that are about 3% below average, so they will have about 12 additional days open per lactation than the average cow.
In Summary, Consider These Key Points.
• Genetic differences in udder health between sire families are large – under similar management conditions daughters of the best and worst bulls will differ by up to 1 unit of somatic cell score and 13% mastitis incidence rate per lactation.
• Genetic differences in female fertility between sire families are also large – under similar management conditions daughters of the best and worst bulls will differ by up to 7% in 21-day pregnancy rate and nearly 30 days open per lactation.
• There is a slight negative correlation between milk yield and these health traits – sires that transmit high milk yield will tend to have daughters with more mastitis and poorer fertility, but there are many sire families that excel for production, fertility, and udder health.
Written by Kent Weigel, University of Wisconsin
In the past decade, a great deal of emphasis has been put on developing and refining methods to improve ‘fitness’ traits or ‘health’ traits through genetic selection. This effort has involved scientists, breeding companies, and producers; both domestically and overseas. For a dairy cow, it’s hard to imagine any traits that are more critical to fitness and health than fertility and udder health. In fact, mastitis and infertility are the leading reasons for culling on U.S. dairy farms, and collectively these two problems account for more than half of the annual turnover rate in the national dairy herd.
Can we improve these traits through selection, or do we just need to do a better job managing our cattle? When I was in graduate school 17 years ago, the popular line of thinking was that ‘low heritability’ traits, like fertility or mastitis resistance, should be addressed through changes in management, not selection. If a trait has low heritability, that means that most of the differences we see between animals are due to management or environmental causes, rather than genetics. But in recent years we’ve learned that large differences do exist between sire families in these traits.
It’s true that producers with reproduction or milk quality problems should review their management practices and protocols ?rst, but once these issues are corrected they can make additional gains by considering such traits during sire selection as well. Why would anyone want to use a bull whose daughters are known to be prone to infertility or mastitis based on progeny testing in dozens or even hundreds of other herds?
Direct selection for mastitis resistance isn’t yet possible in the U.S., because we don’t have a national system for recording clinical episodes of this disease. However, in Somatic Cell Score (SCS) we have a very good indicator trait that can be used for the purpose of ranking dairy sires according to their daughters’ propensity to mastitis infections. The genetic correlation between mastitis and SCS is very high – around 70%, and somatic cell score is recorded on the vast majority of U.S. dairy farms as part of the DHI milk recording program. Genetic evaluations for SCS have been provided by the USDA-ARS Animal Improvement Programs Laboratory since 1994..
Daughters of the best and worst bulls for SCS differ by nearly a full point. Average bulls, like Wrangler, will have daughters with around 3.00 SCS in first lactation, which corresponds to a somatic cell count of about 100,000 cells/ ml. First lactation daughters of the best bulls, like Billion, will have somatic cell count of approximately 75,000 cells/ml, whereas first lactation daughters of the worst bulls for SCS, like Mission, will have somatic cell count of about 134,000 cells/ ml. Research shows us that a 1-unit change in SCS corresponds to a 13% change in the lactation incidence rate of clinical mastitis, so the infection rate of first lactation daughters of Mission should be about 11% higher than that of first lactation daughters of Billion.
The ARS Animal Improvement Programs Laboratory introduced sire rankings for Daughter Pregnancy Rate (DPR) in 2003. Data consist of insemination records, pregnancy examination outcomes, and calving dates reported within the DHI milk recording program (often electronically through transfer of data that were recorded previously in on-farm herd management software programs). Genetic evaluations are expressed in terms of 21-day pregnancy rate, which refers to the percentage of eligible cows (i.e., non-pregnant cows that have passed the voluntary waiting period) that become pregnant during a given 21- day period of time.
Daughters of the best and worst bulls differ by more than 6% in daughter pregnancy rate. Producers who are more comfortable using days open as a measure of reproductive effciency can take advantage of the fact that a 1% increase in daughter pregnancy rate corresponds to a 4-day decrease in days open. Average bulls, like Bob, will tend to have daughter pregnancy rates around -0.40. Production and reproduction are negatively correlated, therefore, the average active A.I. sire is slightly less than zero. The 21-day pregnancy rate for daughters of the best bulls for DPR, like Alfie, will be about 3% above average, which means that they will have about 12 fewer days open per lactation than the average cow. Daughters of the worst bulls for DPR, like Harry, will have 21-day pregnancy rates that are about 3% below average, so they will have about 12 additional days open per lactation than the average cow.
In Summary, Consider These Key Points.
• Genetic differences in udder health between sire families are large – under similar management conditions daughters of the best and worst bulls will differ by up to 1 unit of somatic cell score and 13% mastitis incidence rate per lactation.
• Genetic differences in female fertility between sire families are also large – under similar management conditions daughters of the best and worst bulls will differ by up to 7% in 21-day pregnancy rate and nearly 30 days open per lactation.
• There is a slight negative correlation between milk yield and these health traits – sires that transmit high milk yield will tend to have daughters with more mastitis and poorer fertility, but there are many sire families that excel for production, fertility, and udder health.
