Abstract

The study evaluated the effect of genetic and environmental factors on the Score of Somatic Cell Count [SCS = log₂ (SCC/100 000)+3] in Costa Rica dairy cattle. A total of 198 685 daily records from 43 535 lactations, 23 749 cows, 237 herds and 3 breeds were analyzed. A mixed animal model was assessed with zone, herd within zone, breed, number of lactation, month/year or recording, lactation period and daily milk yield as fixed effects; and permanent environment, herd×sire and cow as random effects. Heritability (^_h2 ) and repeatability (r) indices for SCS were estimated. Breeding values and reliabilities were estimated for all individuals in the population. The average value of SCS was 3.30 (SD=1.87). All fixed effects were significant. SCS among zones ranged from 3.4 (bh-p) to 3.7 (bmh-p). For herds within zone, SCS ranged from 1.6 to 5.5. SCS increased from 2.7 in first lactation to 4.0 for lactation ≥6. SCS was higher in January (3.6) and lower in April (3.4). A consistent downward trend was observed in SCS from year 2004 (4.3) to 2015 (3.3). A nonlinear pattern was observed in SCS throughout lactation, starting at 3.7, with a marked decrease to a minimum of 3.1 around day 60 postpartum, and a progressive increase to a maximum of 3.9 around day 365 postpartum. A reduction of 0.051 in SCS was observed for each increase of 1 kg in daily milk production. Values of ^_h2 and r for SCS were 0.08 (SD=0.008) and 0.33 (SD=0.004), respectively. Breeding values ranged from -1 to +1. Reliabilities of breeding values for cows and bulls were 0.40 (SD=0.16) and 0.44 (SD=0.20), respectively. These results can be used to improve SCS at the herd level, based on prevention in groups under higher risk, optimization of cow culling strategies and improved sire selection.

Keywords: Somatic cell count; heritability; udder health; dairy cows