Lactation length in Venezuelan dual-purpose cows

Lactation length was measured using 2158 records from 13 typical dual-purpo se farms in the Venezuelan lowlands. Cows were kept on grazing, milked by h and once daily with the calf present on nine farms and mechanically with or without calves on the rest. Cows were zebu-European breed crosses, classif ied according to grade of Bos taunts inheritance: low (L), medium (M) and h igh (H). Records of all lactations were included, whatever the length and c ause of termination. Data were analysed by linear models including the effe cts of farm, calving number, calving year, calving season, breed group, ter mination cause and the interactions involving breed group and termination c ause. The adjusted mean lactation length (+/- SD) was 150 +/- 209 days, wit h an unadjusted mean of 214 +/- 112 days. Termination cause was the most im portant source of variation. Significant reductions (p < 0.01) were associa ted with calf death (-104 days), death or culling of the cow (-109 days) an d removal of the cow for calf suckling (-147 days), which affected 7.8, 7.9 and 5.1% of all records, respectively. Constants for farms varied between -45 and 38 days (p < 0.01), and the proportions of 0-day lactations accordi ng to farm ranged from 1.1 to 25.3%. Neither calving number nor season affe cted length, but the difference between years was significant (p < 0.01). T he H cows milked for 30 days longer than the L group (p < 0.01), but there was a significant interaction (p < 0.05) between breed group and terminatio n cause, mainly because of the differential effect of calf death an lactati on length. The reduction in length due to calf death was higher (p < 0.01) in the case of the L cows, but still serious in the H dams (-26%). There wa s no difference between breed groups in the proportion of 0-day lactations. It was concluded that the unadjusted mean lactation length found in this s tudy, 214 days, provides a valid estimate of the time that tropical dual-pu rpose cattle are milked before going dry or Leaving the herd. The exclusion of records on the grounds df length or termination cause, seems unjustifia ble in studies of lactation length of tropical dual-purpose cattle.