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Evolution during the productive life and individual variability of milk emission at machine milking in Sardinian x Lacaune back-cross ewes

Casu, S., Marie-Etancelin, C., Robert-Granié, C., Barillet, F., Carta, A.
Small ruminant research 2008 v.75 no.1 pp. 7-16
dairy animals, ewes, milking, milking machines, milk ejection, kinetics, milk yield, quantitative traits, udders, lactation stage, parity (reproduction), temporal variation, longevity, genetic correlation, Sardinian (sheep breed), crossbreds, backcrossing
Breeders have always been interested in improving the ewes' milkability, since the milking is the most time-consuming task of dairy sheep farmers. This work was aimed at studying the evolution during the productive life and the individual variability of milk emission traits at machine milking in sheep. Data were recorded in an experimental flock of 889 Sardinian x Lacaune back-cross ewes, by using an automatic device developed by INRA. The analyzed variables (59,615 records) were total milk yield (TMY, ml), machine milk yield (MMY, ml), stripped milk yield (SMY, ml), machine milking time (MMT, s), latency time (LT, s), maximum (MMF) and average (AMF) milk flow (ml/s), collected fortnightly at a.m. and p.m. milking for 4 years. Univariate mixed models were used for estimating the evolution of traits with lactation stage and parity number and the relative weights of sire and individual variances within date and lactation and across lactations. As general trend, MMY and SMY decreased with lactation stage. SMY and its ratio with TMY increased from first to fourth parity. TL increased and MMF and AMF decreased with lactation stage and parity number. Machine milking time decreased with lactation stage and increased in adult ewes less than proportionally to the variation in machine milk yield. Most of milkability traits were highly repeatable within date. Within lactation repeatability was high for MMY, MMF and LT and moderate for TMY, SMY and AMF. Similar within date, within lactation and across lactations repeatability estimates for MMF and LT suggest that these traits are affected by random sources of variation, both environmental and genetic, which are constant in time. The proportion of the sire variance respect to the overall phenotypic variance ranged from 4% for MMT to 27% for MMF. These proportions were particularly high for MMY, LT, AMF and MMF and low for SMY and MMT. All the milk emission traits were favorably correlated between them and with milk yield. Only SMY showed low individual correlation with the other traits. Results suggest that milk emission traits are genetically determined and can be improved through selection. LT and MMF are the most relevant measurements to characterize the ewes' machine milking speed due to the high repeatability and the favorable correlation with MMT.