From the NMC Newsletter "Udder Topics", June 1997
Genetic improvement in dairy cattle during the past 25 years has been one of the great success stories in agriculture. U.S. average lactation milk yield for Holsteins increased more than 6,500 pounds (lbs) from 1966 to 1991; more than 5,000 lbs of this increase was due to genetic improvement.
Due to the success of genetic improvement for yield, researchers are turning increased attention to developing genetic programs for improving health, reproduction and survival traits. This is important because improvement in yield is accompanied by slow increases, probably unnoticed, in health and reproductive problems. This article discusses what tools already exist for genetic improvement of these non-yield traits.
Genetic Evaluations for Health and Survival
In 1994, the U.S. Department of Agriculture (USDA) introduced three new Predicted Transmitting Ability (PTA) measures aimed at improving health and survival traits: Productive Life, Somatic Cell Score and New Merit Index. Previously, calving ease was the only health related information provided in genetic improvement programs. Although these evaluations have been available for three years, understanding and utilization of this information continues to lag. Producers can improve their herd's economic performance by making appropriate use of these evaluations.
Productive Life (PL) measures overall ability of the cow to stay in the herd. Cows that are disease-free with good reproductive performance and high production have longer productive life.; The PL is not simply a cow's age. In calculating a cow's PL, a cow is credited with up to 10 months per lactation and these months are accumulated over all lactations until the cow is culled or reaches 84 months of age.
Heritability of productive life is 8.5%. Due to this low heritability, the reliabilities of sire PTA's are somewhat lower for PL than for yield. PTA for PL of AI bulls ranges from a low of around -2 months to a high of +4 months. This means that daughters of the highest bulls will, on average, have about 6 months more PL than daughters of the lowest bulls.
Somatic Cell Score (SCS) is a measure of mastitis. Estimates of the genetic correlation between SCS and clinical mastitis vary around .7 indicating that SCS is a good indicator of clinical mastitis. Genetic correlations between milk yield and clinical mastitis vary around .3 indicating that genetic improvement for yield tends to increase genetic susceptibility to mastitis. Thus it is important to include SCS in genetic evaluation programs. Heritability of SCS is 10%. This means that about 10% of the difference in SCS between cows is due to genetics; the rest is attributed to management and environment. Research has shown that sire evaluations computed only in herds with low average SCS are virtually identical to sire evaluations computed only in herds with high average SCS. Therefore, SCS records from all herds are used in computing PTA SCS.
PTA for SCS of AI bulls ranges from a low of around 2.6 to a high of 3.8 -- a 1.2 point difference. In terms of somatic cell count, daughters of the highest bulls will have cell counts that are 2.3 (i.e. 2 raised to the power 1.2) times the daughters of the lowest bulls. For example, in a herd in which daughters of the lowest bulls average 150,000 cells/ml, daughters of the highest bulls are expected to average 345,000 cells/ml (i.e. 150 times 2.3). Research has found that herds with low average SCS will realize the same genetic improvement from selection against SCS as herds with high average SCS; therefore sire selection recommendations are the same for all herds. Because low SCS indicates the absence of mastitis, sire selection should favor bulls with low PTA SCS.
The yield traits provide much more opportunity for improving genetic value, so the selection emphasis on SCS should be relatively light. It is suggested that sires with PTA for SCS above 3.3 should be avoided unless the sire is truly outstanding for the most important economic traits. This will eliminate the worst 10% of bulls for SCS from consideration with minimal impact on opportunity to improve other traits. Sire selection against SCS should not be seen as a substitute for clean housing, good milking practices and other mastitis prevention methods, but it is a step toward improving genetic resistance to mastitis.
Net Merit index was developed by USDA to enable producers to easily incorporate PL and SCS into their sire selection procedures. Net Merit is in dollar units and combines the economic values of PTA for milk yield, fat yield, protein yield, PL and SCS. The yield traits are included in the form of MFP$, a combination of milk, fat and protein yields that reflects expected future market prices.
The formula for MFP$ is:
MFP$ = .031 (PTA Milk) + $.80 (PTA Fat) + $2.00 (PTA Protein).
The formula for Net Merit is:
NM$ = .7 (MFP$) + $11.30 (PTA PL) - $28.22 (PTA SCS - Breed Average SCS).
The economic weight for SCS includes mastitis treatment cost, discarded milk and milk quality premiums. Reduced milk yield due to mastitis is not included in the economic value for SCS because measured milk yield includes the effects of mastitis. The breed average SCS for Holsteins is 3.2. Sires with below breed average PTA SCS (this is good) will get a positive value added to their NM$ value.
In summary, PTA’s for Net Merit, Productive Life, and Somatic Cell Score are relatively new tools to aid in sire selection. Selection for Net Merit improves yield traits as well as survival and udder health. To the extent that cows with good functional type stay in the herd longer, selection for Net Merit indirectly also results in moderate improvement for the type traits.
Source: "Dairy Profit Report", University of Wisconsin-Madison Dairy Science Department Publication, May 1997.