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Abstract
Abstract
The objectives of the current study were to estimate heritability of milk yield, milking characteristics, somatic cell count, and reproductive traits along with the genetic and environmental relationships among these traits in the first five lactations of Holstein-Friesian cows. Losses in milk yield as well as reduction in reproductive traits associated with elevating level of SCC in milk were also determined. The total number of dairy cows in this study was 953. Data were analyzed using random regression test-day animal model. Results showed that milk yield was significantly affected by cow, calving year, calving month, parity, DIM, and milking time. Means of most reproductive traits increased with increasing levels of SCC in milk from the lowest till the level > 500,000 to ≤ 750,000 cells/ml (17.1, 42.26, and 40.27 days for DFH, DO and CI, respectively), then declined thereafter. Calculated losses in milk yield were 0.42 kg from 1st to 2nd, 1.69 kg from 2nd to 3rd, and 1.39 kg from 3rd to 4th class. The overall h2 estimates for milk yield ranged from 0.02 to 0.52. Estimates of h2 for MD across DIM were low and ranged from 0.00 to 0.15. Estimates of heritability were low and ranged from 0.01 to 0.23 for most reproductive traits. h2 estimates for SCC across DIM were low and ranged from 0.00 to 0.13. Results indicated that susceptibility to mastitis infection differs depending on both level of milk production and stage of lactation.
Key words: Somatic cell count – Days in milk – Reproductive traits – Milking duration – Peak milk flow rate.