Impact of sire PTA_SCS on mastitis resistance and measures of daughter performance

Research to determine the impact of PTA_SCS on incidence of mastitis, daughter response to infection, and other measures of daughter performance was conducted using data on 1st, 2nd, and 3rd lactation Holsteins obtained from the Virginia Tech herd and from VA DHI herds. Overall correlation of PTA_SCS to lactation average SCS ranged from 0.13 to 0.17 across all data sets. Correlations between PTA_SCS and 1st lactation SCS measures were higher than those between PTA_SCS and SCS in later parities, but higher correlations were found between 2nd and 3rd lactation SCS measures than between 1st and later parities. Correlation of lactation average SCS and incidence of clinical mastitis was 0.41. Regression of lactation average SCS and averages of test day SCS measures on PTA_SCS was significant in 1st, 2nd, and 3rd lactations. All significant relationships were linear and equal or close to 1.0. Relationships between PTA_SCS and number of cases of clinical mastitis (.79), number of treatments (2.0), number of days treated (7.0), changes in SCS from beginning to end of a lactation (-.26), and the slope of changes in test day SCS with DIM (5.9x10-4) were significant only in 1st lactation. No significant relationships between PTA_SCS and measures of clinical mastitis or variation in test day SCS measures were found in 2nd or 3rd lactations. Heavy cull rates imposed on 1st lactation cows in the Virginia Tech herd explained lack of significance in the later parities in the herd study, but results in following analyses indicated that measures of SCS in 1st and later parities may be two different, but correlated, traits. The greatest impact of PTA_SCS on measures of daughter performance and profit was the negative relationship between PTA_SCS and herd life. Increased PTA_SCS resulted in the decreased ability to survive involuntary culling, and thus decreased opportunity for lifetime yield and profit. Selection on PTA_SCS should be an effective method of reducing incidence of clinical mastitis, lactation average SCS, and variation in SCS, or response to infection. The response, however, may be different in 1st lactation than in later parities.