VARIATION BETWEEN MERINO EWES IN PASTURE INTAKE .1. BETWEEN FLOCK DIFFERENCES AND SOME ENVIRONMENTAL SOURCES OF VARIATION

Pasture intake of Merino breeding ewes was measured by marker dilution using chromium sesquioxide. Measurements were made on approx. 300 ewes in four seasons associated with stages of the annual reproductive cycle in each of 2 years, The ewes represented 15 separate randomly-bred bloodlines (flocks) in an experiment to determine the extent of environmental and genetic sources of variation. Mean digestible organic matter intakes (DOMI) were 623, 734, 1123 and 779 g/day in autumn (early pregnancy), winter (late pregnancy), spring (mid-lactation) and summer (post-weaning), respectively, The repeatability of daily estimates of DOMI within measurement periods varied from 0.32 to 0.47 across seasons, while the repeatability across periods was lower (0.09 to 0.27), Flock differences in DOMI were significant in all seasons except spring. In all seasons DOMI was positively related to liveweight and was negatively associated with ultrasonic fat depth. After adjusting for liveweight and fat differences, between-flock variation in DOMI was small relative to that of wool growth and liveweight.