Resistance to Mastitis (MAST)

MAST measures the resistance to Mastitis

Benefits of Trait

Introduced in April 2018 and April 2020, genetic and genomic evaluations for resistance to mastitis (MAST) are provided for Holstein and Jersey males and females, respectively. Evaluations are expressed in percentage points of resistance above and below the breed average.

Resistance to Mastitis (MAST)

The MAST predicted transmitting ability (PTA) represents the expected resistance of an animal’s offspring to clinical mastitis in a herd with average management conditions. Evaluations are expressed in percentage points of resistance above and below the breed average. Larger, positive values are more favorable.

Percentage points

The average resistance rate is equal to 89.8% in U.S. Holsteins. The resistance rate is equivalent to the incidence rate subtracted from 100.

Daughters of a Holstein bull with a MAST PTA of +3.0% are expected to have an average resistance rate to clinical mastitis of 93% (assuming the breed average resistance is approximately 90%). Daughters of a Holstein bull with a MAST PTA of -4.0% are expected to have an average resistance of 86%. Daughters from the bull with PTA -4.0% are expected to have twice the number of cases of mastitis as daughters from the bull with PTA of +3.0%

BreedMean GL for CowsMean GL for HeifersMean GL for the
current base year (2010)

Holsteins: April 3, 2018
Jerseys: April 7, 2020

MAST is currently available for Holsteins and Jerseys. As more health data become available, evaluations can be provided for additional breeds.

MAST is included in selection indices for Holsteins and Jerseys as part of the Heath $ sub-index.

Estimated heritability is 3.1% for resistance to clinical mastitis (observed scale).

Young genomic bulls are expected to have reliabilities averaging 49% for resistance to clinical mastitis, and progeny tested bulls are expected to have genomic reliabilities averaging 56%. As additional data is accumulated, reliabilities will increase.

BreedAverage Reliability for GL of
Young Genotyped Bulls by Breed

The only significant (P < 0.05) correlation with PTA for resistance to clinical mastitis was with CSC PTA at -0.68, indicating that SCS decreases as resistance to clinical mastitis increases. Additional significant correlations were 0.39 with productive life (PL) PTA, 0.22 with livability (LIV) PTA, 0.21 with cow conception rate (CCR) PTA, and 0.20 with daughter pregnancy rate (DPR) PTA.

CDCB MAST evaluations were developed using producer-recorded data collected through Dairy Herd Information (DHI) affiliates from herds across the U.S. Strict editing was applied to ensure only the most reliable data was included for the development of genetic evaluations. The edited data included a total of 2.5 million MAST records from 1.4 million cows. These health records are used in conjunction with lactation data available in the national cooperator database, stewarded by CDCB.

The standard deviation (variation) for MAST PTA is 1.6%. Because 1 and 2 standard deviations normally include 68% and 95% of observations, respectively, we assume about 68% of bulls will have a MAST PTA between -1.6% and +1.6% while 95% of the bulls will range from -3.2% to +3.2%.

MAST PTAs range from 5.9 percentage points below to 4.1 percentage points above average in evaluated Holstein bulls born since 1990 with reliabilities of ≥90% (December 2017).

Pre-release analysis indicates the active AI Holstein sires in December 2017 (614 bulls) range from -3.6% to +2.5%, with the average at approximately -0.1 percentage points.

Future Development

In the future, a bivariate analysis with SCS will be investigated. There is a large amount of historical data on SCS that may help to more accurately predict resistance to clinical mastitis given their significant correlation.

Related Publications
Martin, P., H.W. Barkema, L.F. Brito, S.G. Narayana, and F. Miglior. 2018. Symposium review: Novel strategies to genetically improve mastitis resistance in dairy cattle. J. Dairy Sci. 101:2724–2736. doi:10.3168/jds.2017-13554.
Donnelly, M. R., A. R. Hazel, B. J. Heins, & L. B. Hansen, 2018. Health treatment cost of Holsteins in 8 high-performance herds. J. Dairy Sci. (in preparation).
Liang, D., L.M. Arnold, C.J. Stowe, R.J. Harmon, & J.M. Bewley, 2017. Estimating US dairy clinical disease costs with a stochastic simulation model. J. Dairy Sci. 100(2): 1472–1486.

Information last updated March 2018.